SDH/SONET Q&A

Interviews 15 Mins Read

Q. What is SDH ?

SDH stands for Synchronous Digital Hierarchy & is an international Standard for a high capacity optical telecommunications network.It is a synchronous digital transport system aimed at providing a more simple,economical,& flexible teleccommunication infrastructure.

Q. What is the difference between SONET and SDH?

A. To begin with there is no STS-1. The first level in the SDH hierarchy is STM-1 (Synchronous Transport Mode 1) is has a line rate of 155.52 Mb/s. This is equivalent to SONET’s STS-3c. Then would come STM-4 at 622.08 Mb/s and STM-16 at 2488.32 Mb/s. The other difference is in the overhead bytes which are defined slightly differently for SDH. A common misconception is that STM-Ns are formed by multiplexing STM-1s. STM-1s, STM-4s and STM-16s that terminate on a network node are broken down to recover the VCs which they contain. The outbound STM-Ns are then reconstructed with new overheads.

Q. What are the advantages of SDH over PDH ?

The increased configuration flexibility and bandwidth availability of SDH provides significant advantages over the older telecommunications system.
These advantages include:
A reduction in the amount of equipment and an increase in network reliability.
The provision of overhead and payload bytes – the overhead bytes permitting management of the payload bytes on an individual basis and facilitating centralized Fault sectionalisation.-nearly 5% of signal structure allocated for this purpose.
The definition of a synchronous multiplexing format for carrying lower-level digital signals (such as 2 Mbit/s, 34 Mbit/s, 140 Mbit/s) which greatly simplifies the interface to digital switches, digital cross-connects, and add-drop multiplexers.
The availability of a set of generic standards, which enable multi-vendor interoperability.
The definition of a flexible architecture capable of accommodating future applications, with a variety of transmission rates.Existing & future signals can be accomodated.

Q. What are the main limitations of PDH ?

The main limitations of PDH are:
Inability to identify individual channels in a higher-order bit stream.
Insufficient capacity for network management
Most PDH network management is proprietary
There’s no standardised definition of PDH bit rates greater than 140 Mbit/s
There are different hierarchies in use around the world. Specialized interface equipment is required to interwork the two hierarchies

Q. What are some timing/sync defining rules of thumb?

1. A node can only receive the synchronization referencesignal from another node that contains a clock ofequivalent or superior quality (Stratum level).
2. The facilities with the greatest availability (absence of outages) should be selected forsynchronization facilities.
3. Where possible, all primary and secondary synchronization facilities should be diverse, and synchronization facilities within the same cable should be minimized.
4. The total number of nodes in series from the stratum 1 source should be minimized. For example, the primary synchronization network would ideally look like a star configuration with the stratum 1 source at the center. The nodes connected to the star would branch out in decreasing stratum level from the center
5. No timing loops may be formed in any combination of primary

Q. What is meant by “Plesiochronous” ?

If two digital signals are Plesiochronous, their transitions occur at “almost” the same rate, with any variation being constrained within tight limits. These limits are set down in ITU-T recommendation G.811. For example, if two networks need to interwork, their clocks may be derived from two different PRCs. Although these clocks are extremely accurate, there’s a small frequency difference between one clock and the other. This is known as a plesiochronous difference.

Q. What is meant by “Synchronous” ?

In a set of Synchronous signals, the digital transitions in the signals occur at exactly the same rate. There may however be a phase difference between the transitions of the two signals, and this would lie within specified limits. These phase differences may be due to propagation time delays, or low-frequency wander introduced in the transmission network. In a synchronous network, all the clocks are traceable to one Stratum 1 Primary Reference Clock (PRC).

Q. What is meant by “Asynchronous” ?

In the case of Asynchronous signals, the transitions of the signals don’t necessarily occur at the same nominal rate. Asynchronous, in this case, means that the difference between two clocks is much greater than a plesiochronous difference. For example, if two clocks are derived from free-running quartz oscillators, they could be described as asynchronous.

Q. What are the various steps in multiplexing ?

The multiplexing principles of SDH follow, using these terms and definitions:

Mapping: A process used when tributaries are adapted into Virtual Containers (VCs) by adding justification bits and Path Overhead (POH) information.

Aligning: This process takes place when a pointer is included in a Tributary Unit (TU) or an Administrative Unit (AU), to allow the first byte of the Virtual Container to be located.

Multiplexing: This process is used when multiple lower-order path layer signals are adapted into a higher-order path signal, or when the higher-order path signals are adapted into a Multiplex Section.

Stuffing: As the tributary signals are multiplexed and aligned, some spare capacity has been designed into the SDH frame to provide enough space for all the various tributary rates. Therefore, at certain points in the multiplexing hierarchy, this space capacity is filled with “fixed stuffing” bits that carry no information, but are required to fill up the particular frame.

Explain 1+1 protection. In 1+1 protection switching, there is a protection facility (backup line) for each working facility At the near end the optical signal is bridged permanently (split into two signals) and sent over both the working and the protection facilities simultaneously, producing a working signal and a protection signal that are identical.At the Far End of the section, both signalsare monitored independently for failures. The receiving equipment selects either the working or the protection signal. This selection is based on the switch initiation criteria which are either a signal fail (hard failure such as the loss of frame (LOF) within an optical signal), or a signal degrade (soft failure caused by the error rate exceeding some pre-defined value).

Explain 1:N protection. In 1:N protection switching, there is one protection facility for several working facilities (the range is from 1 to 14). In 1:N protection architecture, all communication from the Near End to the Far End is carried out over the APS channel, using the K1 and K2 bytes. All switching is revertive; that is, the traffic reverts to the working facility as soon as the failure has been corrected.

In 1:N protection switching, optical signals are normally sent only over the working facilities, with the protection facility being kept free until a working facility fails.

Q. If voice traffic is still intelligible to the listener in a relatively poor communication channel, why isn’t it easy to pass it across a network optimized for data?

A. Data communication requires very low Bit-error Ratio (BER) for high throughput but does not require constrained propagation, processing, or storage delay. Voice calls, on the other hand, are insensitive to relatively high BER, but very sensitive to delay over a threshold of a few tens of milliseconds. This insensitivity to BER is a function of the human brain’s ability to interpolate the message content, while sensitivity to delay stems from the interactive nature (full-duplex) of voice calls. Data networks are optimized for bit integrity, but end-to-end delay and delay variation are not directly controlled. Delay variation can vary widely for a given connection, since the dynamic path routing schemes typical of some data networks may involve varying numbers of nodes (for example, routers). In addition, the echo-cancellers deployed to handle known excess delay on a long voice path are automatically disabled when the path is used for data. These factors tend to disqualify data networks for voice transport if traditional public switched telephone network (PSTN) quality is desired.

Q. How does synchronization differ from timing?

A. These terms are commonly used interchangeably to refer to the process of providing suitable accurate clocking frequencies to the components of the synchronous network. The terms are sometimes used differently. In cellular wireless systems, for example, “timing” is often applied to ensure close alignment (in real time) of control pulses from different transmitters; “synchronization” refers to the control of clocking frequencies.

Q. If I adopt sync status messages in my sync distribution plan, do I have to worry about timing loops?

A. Yes. Source Specific Multicasts (SSMs) are certainly a very useful tool for minimizing the occurrence of timing loops, but in some complex connectivities they are not able to absolutely preclude timing loop conditions. In a site with multiple Synchronous Optical Network (SONET) rings, for example, there are not enough capabilities for communicating all the necessary SSM information between the SONET network elements and the Timing Signal Generator (TSG) to cover the potential timing paths under all fault conditions. Thus, a comprehensive fault analysis is still required when SSMs are deployed to ensure that a timing loop does not develop.

Q. If ATM is asynchronous by definition, why is synchronization even mentioned in the same sentence?

A. The term Asynchronous Transfer Mode applies to layer 2 of the OSI 7-layer model (the data link layer), whereas the term synchronous network applies to layer 1 (the physical layer). Layers 2, 3, and so on, always require a physical layer which, for ATM, is typically SONET or Synchronous Digital Hierarchy (SDH); thus the “asynchronous” ATM system is often associated with a “synchronous” layer 1. In addition, if the ATM network offers circuit emulation service (CES), also referred to as constant bit-rate (CBR), then synchronous operation (that is, traceability to a primary reference source) is required to support the preferred timing transport mechanism, Synchronous Residual Time Stamp (SRTS).

Q. Most network elements have internal stratum 3 clocks with 4.6ppm accuracy, so why does the network master clock need to be as accurate as one part in 10^11?

A. Although the requirements for a stratum 3 clock specify a free-run accuracy (also pull-in range) of 4.6ppm, a network element (NE) operating in a synchronous environment is never in free-run mode. Under normal conditions, the NE internal clock tracks (and is described as being a traceable to) a Primary Reference Source that meets stratum 1 long-term accuracy of one part in 10^11.
This accuracy was originally chosen because it was available as a national primary reference source from a cesium-beam oscillator, and it ensured adequately low slip-rate at international gateways.
Note: If primary reference source (PRS) traceability is lost by the NE, it enters holdover mode. In this mode, the NE clock’s tracking phase lock loop (PLL) does not revert to its free-run state, it freezes its control point at the last valid tracking value. The clock accuracy then drifts elegantly away from the desired traceable value, until the fault is repaired and traceability is restored.

Q. What are the acceptable limits for slip and/or pointer adjustment rates when designing a sync network?

A. When designing a network’s synchronization distribution sub-system, the targets for sync performance are zero slips and zero pointer adjustments during normal conditions. In a real-world network, there are enough uncontrolled variables that these targets will not be met over any reasonable time, but it is not acceptable practice to design for a given level of degradation (with the exception of multiple timing island operation, when a worst-case slip-rate of no more than one slip in 72 days between islands is considered negligible). The zero-tolerance design for normal conditions is supported by choosing distribution architectures and clocking components that limit slip-rates and pointer adjustment rates to acceptable levels of degradation during failure (usually double-failure) conditions.

Q. Why is it necessary to spend time and effort on synchronization in telecom networks when the basic requirement is simple, and when computer LANs have never bothered with it?

A. The requirement for PRS traceability of all signals in a synchronous network at all times is certainly simple, but it is deceptively simple. The details of how to provide traceability in a geographically distributed matrix of different types of equipment at different signal levels, under normal and multiple-failure conditions, in a dynamically evolving network, are the concerns of every sync coordinator. Given the number of permutations and combinations of all these factors, the behavior of timing signals in a real-world environment must be described and analyzed statistically. Thus, sync distribution network design is based on minimizing the probability of losing traceability while accepting the reality that this probability can never be zero.

Q. How many stratum 2 and/or stratum 3E TSGs can be chained either in parallel or series from a PRS?

A. There are no defined figures in industry standards. The sync network designer must choose sync distribution architecture and the number of PRSs and then the number and quality of TSGs based on cost-performance trade-offs for the particular network and its services.

Q. Is synchronization required for non-traditional services such as voice-over-IP?

A. The answer to this topical question depends on the performance required (or promised) for the service. Usually, Voice-over-IP is accepted to have a low quality reflecting its low cost (both relative to traditional PSTN voice service). If a high slip-rate and interruptions can be accepted, then the voice terminal clocks could well be free-running. If, however, a high voice quality is the objective (especially if voice-band modems including Fax are to be accommodated) then you must control slip occurrence to a low probability by synchronization to industry standards. You must analyze any new service or delivery method for acceptable performance relative to the expectations of the end-user before you can determine the need for synchronization.

Q. Why is a timing loop so bad, and why is it so difficult to fix?

A. Timing loops are inherently unacceptable because they preclude having the affected NEs synchronized to the PRS. The clock frequencies are traceable to an unpredictable unknown quantity; that is, the hold-in frequency limit of one of the affected NE clocks. By design, this is bound to be well outside the expected accuracy of the clock after several days in holdover, so performance is guaranteed to become severely degraded.
The difficulty in isolating the instigator of a timing loop condition is a function of two factors: first, the cause is unintentional (a lack of diligence in analyzing all fault conditions, or an error in provisioning, for example) so no obvious evidence exists in the network’s documentation. Secondly, there are no sync-specific alarms, since each affected NE accepts the situation as normal. Consequently, you must carry out trouble isolation without the usual maintenance tools, relying on a knowledge of the sync distribution topology and on an analysis of data on slip counts and pointer counts that is not usually automatically correlated.

Q.How do you get value of an E1 as 2.048Mbps?

A.As we know that voice signal is of frequency 3.3 Khz,and as per the Nyquist Rate or PCM quantization rate for transmission we required signal of >=2f(here ‘f’ is GIF [3.3]=4).and each sample of data is a byte. DS0: provides one 64kbps channel.E1: 32 DS0 or 32 channels with 64kbps

Also we know that voice signal frame consisits of 32 bytes .Hence value of an E1 will be

=2x4Khzx8bitsx32slots
=2.048Mbps

OR

PCM multiplexing is carried out with the sampling process, sampling the analog sources sequentially. These
sources may be the nominal 4-kHz voice channels or other information sources that have a 4-kHz bandwidth, such as data or freeze-frame video. The final result of the sampling and subsequent quantization and coding is a series of electrical pulses, a serial bit stream of 1s and 0s that requires some identification or indication of the beginning of a sampling sequence. This identification is necessary so that the far-end receiver knows exactly when the sampling sequence starts. Once the receiver receives the “indication,” it knows a priori (in the case of DS1) that 24 eight-bit slots follow. It synchronizes the receiver. Such identification is carried out by a framing bit, and one full sequence or cycle of samples is called a frame in PCM terminology.
Consider the framing structure of E1
PCM system using 8-level coding (e.g., 2^8= 256 quantizing steps or distinct PCM code words). Actually 256 samples of a signal will be sufficient to regenerate the original signal and each signal is made up of 1 or 0.

The E1 European PCM system is a 32-channel system. Of the 32 channels, 30 transmit speech (or data) derived from incoming telephone trunks and the remaining 2 channels transmit synchronization-alignment and signaling information. Each channel is allotted an 8-bit time slot (TS), and we tabulate TS 0 through 31 as follows:
TS TYPE OF INFORMATION
0 Synchronizing (framing)
1–15 Speech
16 Signaling
17–31 Speech
In TS 0 a synchronizing code or word is transmitted every second frame, occupying digits 2 through 8 as 0011011. In those frames without the synchronizing word, the second bit of TS 0 is frozen at a 1 so that in these frames the synchronizing word cannot be imitated. The remaining bits of TS 0 can be used for the transmission of supervisory information signals .Again, E1 in its primary rate format transmits 32 channels of 8-bit time slots. An E1 frame therefore has 8*32 =256 bits. There is no framing bit. Framing alignment is
carried out in TS 0.

The E1 bit rate to the line is:256 *8000 = 2, 048, 000 bps or 2.048 Mbps

Question for you Electrical E1 is ac or dc in nature????

Some Basic Interview Questions

Interviews 11 Mins Read

Tell me about yourself: – The most often asked question in interviews. You need to have a short statement prepared in your mind. Be careful that it does not sound rehearsed. Limit it to work-related items unless instructed otherwise. Talk about things you have done and jobs you have held that relate to the position you are interviewing for. Start with the item farthest back and work up to the present.

Why did you leave your last job? – Stay positive regardless of the circumstances. Never refer to a major problem with management and never speak ill of supervisors, co-workers or the organization. If you do, you will be the one looking bad. Keep smiling and talk about leaving for a positive reason such as an opportunity, a chance to do something special or other forward-looking reasons.

What experience do you have in this field? – Speak about specifics that relate to the position you are applying for. If you do not have specific experience, get as close as you can.

Do you consider yourself successful? – You should always answer yes and briefly explain why. A good explanation is that you have set goals, and you have met some and are on track to achieve the others.

What do co-workers say about you? – Be prepared with a quote or two from co-workers. Either a specific statement or a paraphrase will work. Jill Clark, a co-worker at Smith Company, always said I was the hardest workers she had ever known. It is as powerful as Jill having said it at the interview herself.

What do you know about this organization? – This question is one reason to do some research on the organization before the interview. Find out where they have been and where they are going. What are the current issues and who are the major players?

What have you done to improve your knowledge in the last year? – Try to include improvement activities that relate to the job. A wide variety of activities can be mentioned as positive self-improvement. Have some good ones handy to mention.

Are you applying for other jobs? – Be honest but do not spend a lot of time in this area. Keep the focus on this job and what you can do for this organization. Anything else is a distraction.

Why do you want to work for this organization? – This may take some thought and certainly, should be based on the research you have done on the organization. Sincerity is extremely important here and will easily be sensed. Relate it to your long-term career goals.

Do you know anyone who works for us? – Be aware of the policy on relatives working for the organization. This can affect your answer even though they asked about friends not relatives. Be careful to mention a friend only if they are well thought of.

What kind of salary do you need? – A loaded question. A nasty little game that you will probably lose if you answer first. So, do not answer it. Instead, say something like, That’s a tough question. Can you tell me the range for this position? In most cases, the interviewer, taken off guard, will tell you. If not, say that it can depend on the details of the job. Then give a wide range.

Are you a team player? – You are, of course, a team player. Be sure to have examples ready. Specifics that show you often perform for the good of the team rather than for yourself are good evidence of your team attitude. Do not brag, just say it in a matter-of-fact tone. This is a key point.

How long would you expect to work for us if hired? – Specifics here are not good. Something like this should work: I’d like it to be a long time. Or As long as we both feel I’m doing a good job.

Have you ever had to fire anyone? How did you feel about that? – This is serious. Do not make light of it or in any way seem like you like to fire people. At the same time, you will do it when it is the right thing to do. When it comes to the organization versus the individual who has created a harmful situation, you will protect the organization. Remember firing is not the same as layoff or reduction in force.

What is your philosophy towards work? – The interviewer is not looking for a long or flowery dissertation here. Do you have strong feelings that the job gets done? Yes. That’s the type of answer that works best here. Short and positive, showing a benefit to the organization.

If you had enough money to retire right now, would you? – Answer yes if you would. But since you need to work, this is the type of work you prefer. Do not say yes if you do not mean it.

Have you ever been asked to leave a position? – If you have not, say no. If you have, be honest, brief and avoid saying negative things about the people or organization involved.

Explain how you would be an asset to this organization – You should be anxious for this question. It gives you a chance to highlight your best points as they relate to the position being discussed. Give a little advance thought to this relationship.

Why should we hire you? – Point out how your assets meet what the organization needs. Do not mention any other candidates to make a comparison.

Tell me about a suggestion you have made – Have a good one ready. Be sure and use a suggestion that was accepted and was then considered successful. One related to the type of work applied for is a real plus.
What irritates you about co-workers? – This is a trap question. Think real hard but fail to come up with anything that irritates you. A short statement that you seem to get along with folks is great.

What is your greatest strength? – Numerous answers are good, just stay positive. A few good examples: Your ability to prioritize, Your problem-solving skills, Your ability to work under pressure, Your ability to focus on projects, Your professional expertise, Your leadership skills, Your positive attitude .

Tell me about your dream job – Stay away from a specific job. You cannot win. If you say the job you are contending for is it, you strain credibility. If you say another job is it, you plant the suspicion that you will be dissatisfied with this position if hired. The best is to stay genetic and say something like: A job where I love the work, like the people, can contribute and can’t wait to get to work.

Why do you think you would do well at this job? – Give several reasons and include skills, experience and interest.

What kind of person would you refuse to work with? – Do not be trivial. It would take disloyalty to the organization, violence or lawbreaking to get you to object. Minor objections will label you as a whiner.

What is more important to you: the money or the work? – Money is always important, but the work is the most important. There is no better answer.

What would your previous supervisor say your strongest point is? – There are numerous good possibilities: Loyalty, Energy, Positive attitude, Leadership, Team player, Expertise, Initiative, Patience, Hard work, Creativity, Problem solver

Tell me about a problem you had with a supervisor – Biggest trap of all. This is a test to see if you will speak ill of your boss. If you fall for it and tell about a problem with a former boss, you may well below the interview right there. Stay positive and develop a poor memory about any trouble with a supervisor.

What has disappointed you about a job? – Don’t get trivial or negative. Safe areas are few but can include: Not enough of a challenge. You were laid off in a reduction Company did not win a contract, which would have given you more responsibility.

Tell me about your ability to work under pressure – You may say that you thrive under certain types of pressure. Give an example that relates to the type of position applied for.

Do your skills match this job or another job more closely? – Probably this one. Do not give fuel to the suspicion that you may want another job more than this one.

What motivates you to do your best on the job? – This is a personal trait that only you can say, but good examples are: Challenge, Achievement, Recognition

Are you willing to work overtime? Nights? Weekends? – This is up to you. Be totally honest.

How would you know you were successful on this job? – Several ways are good measures: You set high standards for yourself and meet them. Your outcomes are a success.Your boss tell you that you are successful

Would you be willing to relocate if required? – You should be clear on this with your family prior to the interview if you think there is a chance it may come up. Do not say yes just to get the job if the real answer is no. This can create a lot of problems later on in your career. Be honest at this point and save yourself future grief.

Are you willing to put the interests of the organization ahead of your own? – This is a straight loyalty and dedication question. Do not worry about the deep ethical and philosophical implications. Just say yes.

Describe your management style. – Try to avoid labels. Some of the more common labels, like progressive, salesman or consensus, can have several meanings or descriptions depending on which management expert you listen to. The situational style is safe, because it says you will manage according to the situation, instead of one size fits all.

What have you learned from mistakes on the job? – Here you have to come up with something or you strain credibility. Make it small, well intentioned mistake with a positive lesson learned. An example would be working too far ahead of colleagues on a project and thus throwing coordination off.

Do you have any blind spots? – Trick question. If you know about blind spots, they are no longer blind spots. Do not reveal any personal areas of concern here. Let them do their own discovery on your bad points. Do not hand it to them.

If you were hiring a person for this job, what would you look for? – Be careful to mention traits that are needed and that you have.

Do you think you are overqualified for this position? – Regardless of your qualifications, state that you are very well qualified for the position.

How do you propose to compensate for your lack of experience? – First, if you have experience that the interviewer does not know about, bring that up: Then, point out (if true) that you are a hard working quick learner.

What qualities do you look for in a boss? – Be generic and positive. Safe qualities are knowledgeable, a sense of humor, fair, loyal to subordinates and holder of high standards. All bosses think they have these traits.

Tell me about a time when you helped resolve a dispute between others. – Pick a specific incident. Concentrate on your problem solving technique and not the dispute you settled.

What position do you prefer on a team working on a project? – Be honest. If you are comfortable in different roles, point that out.

Describe your work ethic. – Emphasize benefits to the organization. Things like, determination to get the job done and work hard but enjoy your work are good.

What has been your biggest professional disappointment? – Be sure that you refer to something that was beyond your control. Show acceptance and no negative feelings.

Tell me about the most fun you have had on the job. – Talk about having fun by accomplishing something for the organization.

Do you have any questions for me? – Always have some questions prepared. Questions prepared where you will be an asset to the organization are good. How soon will I be able to be productive? and What type of projects will I be able to assist on? are examples.

P.S: revert for new ideas and questions

LEGACY OPTICAL SYSTEM TIMING FAQ

Interviews 7 Mins Read

Some of the FAQs for Optical System Timing are:

Q. How do I time SONET NEs shelves in a central office environment?

A. Each SONET NEsshelf should be externally referenced to the BITS clock in the office. If a BITS clock is not available in the office, a traffic-carrying DS1 from the local switch may be bridged (for example, using a bridging repeater) as the reference to the SONET NEsshelf. Line timing may also be used, but at least one SONET NE shelf in the network must be externally timed.

Q. Where do I use the DS1 timing output feature?

A. The primary application is for supplying a timing reference to the office BITS clock. This allows the BITS clock to be slaved to a BITS clock in another office that is, in turn, traceable to the primary reference source (PRS). Typically, the SONET NE supplying the DS1 timing output will, in turn, be externally timed by the BITS clock. If there is no BITS clock, the DS1 timing output can be used to time a switch or switch remote (if the switch remote is equipped for that option) directly or even another SONET NE Multiplexer.

Q. How do I prevent my BITS clock from using a DS1 timing output when a failure in the network results in this DS1 being timed from a SONET NE in holdover?

A. SONET sync messaging informs the local SONET NE of this condition, and AIS is inserted on the DS1 timing output.

What is the advantage of using the DS1 timing output instead of a multiplexed DS1 as the timing reference?
The DS1 timing output is derived from the optical line rate and is superior because:

The DS1 is virtually jitter-free
Sync messages guarantee the traceability of the timing
Administration of traffic DS1s for timing is eliminated.

Q. Can I ever use the SONET NE in the free-running timing mode?

A. If a PRS traceable external reference is available, it is the recommended timing mode for any/all CO applications. The free running timing mode can be used but a slight increase in jitter will result. If one SONET NE is provisioned for free running, all other SONET NEs in the network must be line timed and SONET interfaces to other equipment are not allowed. The DS1 timing output should not be enabled with a free running network.

Q. How do I provide timing to a central office host switch that does not have the option for an external reference?

A. DS1 carried over SONET may contain significant jitter/wander and be unacceptable to the switch as a timing reference. If the central office has a BITS clock, the recommendation is to use the output from the BITS clock into an unused DS1 traffic port on the switch. If the central office does not have a BITS clock, the recommendation is to use the DS1 timing output from the SONET NE as the line timing reference into an unused DS1 traffic port on the switch.

Q. Can a DS1 carried over SONET ever be used as a timing reference?

A. YES! In many applications there is no other choice. Most switch remotes, for instance, obtain their timing from a specific DS1 signal generated by their host switch, so these remotes must line time from the DS1 signal. In addition, DLC equipment, channel banks, and PBXs will not likely have external references and may be allowed to line time from a DS1 carried over SONET.

Q. Are there any specific concerns when using a DS1 carried over SONET to time equipment such as a switch remote or DLC?

A. Yes. The major concern is to make sure all the equipment is synchronous. The SONET NEs should be synchronous to each other to prevent pointer adjustments. This can be accomplished by having one source SONET that is externally timed. The other SONET NEs in the network should be line timed, or they should be externally timed to a clock to which they provide a DS1 timing output. The SONET NEs should also be synchronous to the switch to prevent excessive mapping jitter. This can be done by synchronizing the host switch to the BITS clock used to reference the SONET .

Q. Will I have any problems providing timing to a customer that has a high quality PBX or switch?

A. If the network is completely synchronous, as described in the previous answer, there should be no problems. If the PBX is sensitive to the jitter produced, even under the synchronous conditions, the DS1 timing output of SONET may be required to be used as a timing reference to this equipment.

Q. Why does Bellcore say that DS1s carried over SONET should not be used for timing?

A. Bellcore has provided this recommendation because there are several limitations. Bellcore says that DS1s carried over SONET must be used in applications such as switch remotes and will be acceptable, provided pointer adjustments are not created.

Q. Can pointer adjustments be prevented?

A. Neither random nor periodic pointer adjustments will occur if the SONET shelf is provisioned for line timing.

Q. How do I time SONET at a remote site?

A. Line time.

Q. How many SONET NEs can I chain together in an add/drop configuration before the timing becomes degraded?

A. The Stratum level traceability of the nth node in an add/drop chain is the same as that in the first node. Also, while timing jitter will theoretically increase as the number of nodes is increased, the high quality timing recovery and filtering on the SONET allows add/drop chains to be extended to any practical network limit without detectable increases in jitter levels. In practice, the only effects on timing at the nth node will occur whenever high-speed protection switches occur in any of the previous n-1 nodes. These effects should be rare.

Q. How do I time a SONET ring network?

A. An interoffice ring should have each node externally timed if BITS clocks are available. All other rings should have one node externally timed (two in some dual homing architectures) and the rest of the nodes line timed. Synchronization reconfiguration is automatic.

Q. Why are there more issues related to timing with SONET equipment than there are with asynchronous equipment?

A. SONET equipment was designed to work ideally in a synchronous network. When the network is not synchronous, mechanisms such as pointer processing and bit-stuffing must be used and jitter/wander increases.

Q. Can DS3 signals be used to carry DS1 timing signals without the worry of having the network synchronous?

A. Yes, although this option is more expensive.

Q. What are the limitations on automatic synchronization reconfiguration?

A. Automatic synchronization reconfiguration is only available when the SONET is provisioned for line timing mode. This allows the timing direction of an OC-n (OC-12, OC-48, or OC-192) ring network to change automatically in response to a failure. When the SONET is provisioned for external timing, automatic synchronization reconfiguration is not available. When an OC-n fault is detected in the timing direction, AIS is inserted on the derived DS1s which forces the BITS to switch to another good timing source or into holdover preventing timing loops.

Q. How do I synchronize a BITS clock and maintain automatic synchronization reconfiguration on a SONET ring?

A. Provision all but the host node (node with co-located PRS) for line timing. Provide each non-host BITS clock with a pair of derived DS1s. The SONET will detect faults and provide the BITS clocks with good inputs if available. Timing loops will be prevented. The host node should be set for external timing and get its timing from an externally timed BITS clock. To prevent a timing loop, the host BITS clock should get its timing from a PRS traceable source. The non-host nodes should not be timed from the co-located BITS clock since this would disable the automatic synchronization reconfiguration feature.

For more: visit:- http://www.sonet.com/

Great Answers to Tough Interview Questions

Interviews 16 Mins Read

The number one way to ace to an interview is to know yourself! Always complete your research before an interview: know the kinds of things you are good at, the kind of things that you may need to work on, the kinds of experiences you’ve had in the past, what kind of boss you’d like to work for, what kind of company you’d like to work for, etc.

Questions that revolve around potential negatives are always the toughest to answer. Prepare for them by thinking about answers ahead of time. Type up answers or write them out, and review them before going into the interview. It is like studying concise, and then stopping and seeing if the interviewer wants to pursue it further.

The following questions have been identified as some of the toughest in the interviewing business. These are the ones you need to prepare for ahead of time!

1. Tell me about yourself.
Probably the easiest difficult question you will face! It is easy to talk about you, but what does the employer really want to know? Try talking about personal characteristics and skills that translate into career strengths. Develop a one minute commercial for yourself. In one minute, tell someone from your high school days upward why you chose the college you did; what you did in college and why you made the decision to go into your particular field.

Typically the interviewer is looking for an overview of your related experience. Provide an answer that shows a logical progression throughout your educational/professional career. If your educational background led you in a certain direction, include that in the overview. While this may be seen as an easy question for some, quite a few solid candidates blundered on this simple question. We are not used to selling our abilities and competencies like a product. For those who have given it some thought, this is the right opportunity to talk about your strengths such as your enthusiasm, leadership, self-confidence and reliability, with a few real life anecdotes thrown in to support your points.

EXAMPLE: “I love to jump into projects with both feet. I can concentrate on solving a tech problem for hours, although I know to effectively use my time I need to consult my colleagues for their expertise, and often convene brief project team meetings bounce around a solution, plotting it out, and preparing a presentation for my boss. Object oriented technology [or any other skill] is my newest challenge.”

EXAMPLE: “My background to date has been centered on planning myself to become the very best I can become.

Let me tell you specifically how I’ve prepared myself…”

2. What are your greatest strengths?
You certainly have many positive qualities, but since you can only choose a few, be careful as your answer will also reflect your values. Some might say honesty, other reliability or a strong sense of leadership, but whatever it might be, be ready to make reference to a situation where you have demonstrated such strength. Examples help solidify and support your strengths.

3. What is your greatest weakness?
Admit that you have a weakness, and that you are successfully using a strategy to improve this weak are you have realized in your professional array of skills and experience. This technique turns the weakness into a strength by demonstrating your commitment to self improvement.

EXAMPLE: “I would say that my greatest weakness has been a lack of proper planning in the past. I would overcommit myself with too many variant tasks, then not b able to fully accomplish each as I would like. However, since I’ve come to recognize that weakness, I’ve taken steps to correct it. For example, I now carry my palm pilot at all times so that I can effectively plan my appointments and “to do list” items.”

4. What were your favorite subjects in school and why?
Of course, if your major is Computer Science, you want to mention some of your computer science classes. You can mention other related subjects as well. For example, if you’re interviewing at a financial services firm, you might discuss why you liked your accounting or finance classes. If you did any usual or special projects in that area bring it up now. Mention anything that shoes keen interests in this employer’s particular kind of work.

5. Why change jobs now?
Interviewers want to now your reasons for leaving a particular position/employer. It is important to stay positive about your past experiences. Nobody wants to hire a person that complains about past employersthey figure you may complain about them in the future. Talk about why it was time to move on, that you learned a lot, you are seeking new and more challenging opportunities, etc.

6. What makes you stand out when compared with your peers?
Employers want to know that they are considering someone who will go above and beyond the call of duty. It is easy to hire a person that will do that they are asked to do between nine and five. However, it is better to hire a person that meets all the

basic expectations and more. Provide examples of projects which you excelled with, ideas that helped streamline operations, or new sales/marketing techniques that increased revenues. If you put some thought into it, there are probably several things you have accomplishes in the past that makes you stand out.

7. What type of management / supervision do you prefer?
This question can come in many forms, but the meaning is always the same: are you able to be managed by people with different styles? You probably have reported to managers with vastly different approaches to management and supervision, and have learned which styles you prefer. Since you probably do not know the manager’s style at the prospective employer, talk about the positive aspects of each style you’ve encountered.

8. What do you like about your current position? What would you add to make it more challenging?
Interviewers are looking for people that have some relevant experience for the position offered. This question allows you to pick the responsibilities that you enjoyed that are also included in the available position. (I like the people is NOT an acceptable answer, unless you are taking them with you!) The second part of this question focuses on what you want to learn in your next job. If it were available in your present/last position, you would still be there. Think about what this position offers and what you can contribute. This should match well with that you want to learn/do.

9. Where do you see yourself in (X) years?
This may be the toughest question of them all! If you only had a crystal ball! Basically, however, Interviewers are doing a reality check here. They want to see if you are a realistic about your career goals and the steps you will need to take to attain them.

The best answers start in the present and work forward. Talk about your next move and what you hope to accomplish over the next year or two. Then take that one or two more steps. Most importantly, keep your dreams within the realm of reality.

10. Why should we hire you for this position?
This question is as straightforward as they come. If you’ve prepared for the interview ahead of time, this one should be easy. Your answer should focus on your experience, accomplishments, why you are different than your peers, and your work ethic, as they pertain to the specific job description. If you handled the interview well up to this point, you are just tying it all together.

11. What books/magazines do you read?
Obviously, a technical or trade journal is one answer they are looking for. The books you’ve read tell the manager something about you personality. Whatever you do, don’t say, “I don’t like to read.”

12. What interests you most about this position?
Your education, training and experience as well as your accomplishments should provide good references for answering that question. Do not hesitate to stress the fact that this would represent a dynamic move, while giving you the opportunity to grow, to increase your responsibilities and knowledge of the field, and may possibly help you define and reach new goals.

13. Do you work well under pressure?
Of course everyone will say yes, but it is more convincing if you could provide examples of situations when you have remained cool under fire. Be careful not to choose a situation of crisis for which you were the one responsible!

14. If you could change one thing about your personality, what would it be, and why?

Make the answer to this question positive by referencing your attitude and determination. Comments such as “I am sometimes impatient with slow performers” or “Being very demanding of myself” or “Sometimes I expect too much from others” are good. Keep in mind that most interviewers will use the information you give them to raise even more incisive questions.

15. How has your education prepared you for your career?
EXAMPLE: “As you will note on my resume, I’ve taken not only the required core classes in the field, I’ve also gone above and beyond. I’ve taken every class the college has to offer in the field and also completed an independent study project specifically in this area. But it’s not just taking the classes to gin academic knowledgeI’ve taken each class both inside and outside of my major, with this profession in mind. So when we’re studying in , I’ve viewed it from the perspective of . In addition, I’ve always tried to keep practical view of how the information would apply to my job. Not just theory, but how it would actually apply. My capstone course project in my final semester involved developing a realworld model of , which is very similar to what might be used within your company. Let me tell you about it…”

16. Are you a team player?
EXAMPLE: “Very much so. In fact, I’ve had opportunities in both athletics and academics to develop my skills as a team player. I was involved in at the intramural level, including leading my team in assists during the past yearI always try to help others achieve their best. In academics, I’ve worked on several team projects, serving as both a member and a team leader. I’ve seen the value of working together as a team to achieve a higher goal than any one of us could have achieved individually. As an example…”

17. Have you ever had a conflict with a boss or professor? How was it resolved?
EXAMPLE: “Yes, I have had conflicts in the past. Never major ones, but certainly there have been situations where there was a disagreement that needed to be resolved. I’ve found that when conflict occurs, it’s because of a failure to see both sides of a situation. Therefore, I ask the other person to give me their perspective and at the same time ask that they allow me to fully explain my perspective. At that point, I would work with the person to find out if a compromise could be reached. If not, I would submit their decision because they are my superior. In the end, you have to be willing to submit yourself to the directives of your superior, whether you’re in full agreement or not. An example of this was when…”

18. If I asked your professors to describe you, what would they say?
EXAMPLE: “I believe they would say I am very energetic person, that I put my mind to the task at hand and see to it that it’s accomplished. They would say that if they ever had something that to be done, I was the person who they could always depend on to see that it was accomplished. They would say that I always took a keen interest in the subjects that I was studying and always sought ways to apply the knowledge in real world settings. Am I just guessing that they would say these things? No, in fact, I am quite certain they would say those things because I have them with me in several letters of recommendation from my professors, and those are their very words.”

19. What qualities do you feel a successful manager should have?
EXAMPLE: “The key quality should be leadership the ability to be the visionary for the people who are working under them; the person who can set the course and direction for subordinates. A manager should also be a positive role model for others to follow. The highest calling of a true leader is inspiring others to reach the highest of their abilities. I’d like to tell you about a person who I consider to be a true leader…”

20. If you had to live your life over again, what would you change?
EXAMPLE: “That’s a good question. I realize that it can be very easy to look back and wish that things had been different in the past. But I also realize that things in the past cannot be changed, that only things in the future can be changed. That’s why I continually strive to improve myself everyday and that is why I am working hard to become the very best your company has ever had to make a positive change. So in answer to your question, there isn’t anything in my past that I would change. I look to the future to make changes in my life.”

21. Why were you fired from your last job?
If you were fired wrongfully, say it matter of factly. Do not show a grudge against your old employer. If you were fired because something that was your fault, say it in a less damaging way.

EXAMPLE: “I cut prices on our product to make a customer happy without asking my superior. The company lost money and I was fired because of that. I agree it was my faultI should have asked my superior. I have learned my lesson and it will never happen again.”

22. It’s been 6 years since you started your bachelor’s degree, why aren’t you finished?
Be as positive as possible and explain your reasons calmly and logically. Never answer the following: you are having too much fun and did not have time, you do not think a degree is important, etc.

23. If you join our company, and another company offers you more money, will you leave? Or, how long will you stay with us?
If you say yes, you will probably not get the job. If you want, you can tell them that the salary will make you happy and keep you with this company. For example, you might say something like, “If I accept the job, then that means that I also accept the salary and I do not think I will leave.” Concerning your length of employment longer is better. You may want to discuss that you want to stay at your next job as long as the relationship is mutually beneficial.

24. What are the things in a job that make you more productive? Less productive?
Remember, no work environment is 100% perfect. You can’t please all of the people all of the time. However, we adapt and make the best of what we have. So make your answer practical.

25. Why have you been unemployed for the last two years?
You must have a sound reason for not working so long or having a big gap in your work history. Some examples of good reason would be: Caring for a sick family member; Raising children; Looking for the right job where you can really contribute (this will not work if the period of unemployment was an extremely long time); Trying out self employment or owning your own business; Returning to school for further training; Learning new skills; Exploring or traveling (for a short gap only).

26. At your last job, you worked at the same position for five years without a promotion. Why?
Avoid saying anything negative. Tell the truth in a positive way.Its human unconscious bias, negative things stuck in mind for long.

EXAMPLE: “There was not much growth in our department. None of the people working in my department had been promoted. This is one of the reasons I am looking for a new employment opportunity.”

27. What salary are you looking for? Or, what kind of salary do you think you are worth?
Don’t be too specific – a range is often most comfortable for everyone to work with. Even better, ask if the company has a salary scale and base your answer on that. Remember, salary is related to market value. Research your worth before you get into the interview. Also, interviews are not ignorant. Research your market value and give a range

EXAMPLE: “I checked a few sites on the internet and found that the average salary for a web designer ranges from 45K to 60K per year in this geographic area, depending on the responsibilities and duties involved. I am not set on a specific number and am willing to negotiate.”

28. How many players are there in a soccer game?
This type of question has nothing to do with your career. Right or wrong answers do not matter. Interviewers want to see if you can manage with nonrelated subjects, and think on your feet. They might be testing your ability to reason, problem solve, and utilize resources.

EXAMPLE: “I am not sure but my guess would be at least two players on each side, one in the goal and one to play defense. I will put the number between seven and twelve on each team. I would definitely want to consult a colleague of mine who plays soccer if this was a game show right now – using phone a friend is important sometimes in utilizing available resources.

29. Do you have any questions?
If you have done your homework, you’ll have several questions to ask about the position, company, or industry. Having questions already prepared makes you appear motivated to excel in the interview, and generally organized and put together. If all else fails, ask the interviewer if they need any further clarification about your qualifications.

These questions and answers are collected from internet pool.

Questions on WDM Technology

Interviews 6 Mins Read

Questions on WDM Technology

How to determine aggregate output power of a WDM?
Spectral width wavelength to frequency conversion.
Difference between CWDM and DWDM.
What is Red band and Blue band?
What is dark fibre and dim fibre?
What are dark fibre considerations?
How many types of optical fibres?
Specifications of Multimode Fibre?
How many types of Single mode Optical fibres.
What are dispersion and attenuation coefficient of NDSF.
If for same distance OTDR done and wavelength changed, will it show any difference in Losses?
What is micro bending and macro bending?
What is fibre characterization?
Types of patch cord.
What is Linear and nonlinear impairments
What is Stimulated Raman Scattering (SRS)?
What is Stimulated Brillouin Scattering (SBS)?
What is Four Wave Mixing (FWM)?
What is Intensity Modulation (MI)?
What is Self phase modulation (SPM)?
What is Cross phase Modulation (CPM or XPM)?
How to reduce FWM impact?
What is impact of linear and nonlinear effects in DWDM network?
What are 3R, 2R and 1R?
What are types of Pluggable Optics?
What is common optical specification of Optical Transceiver (Pluggable Optics or Transponder)?
What is fixed optics?
If for point to point fiber link, two routers are connected and LX SFP used. If losses increased in fibre, what are possibilities to keep the link working?Want to connect client who is 300 kms apart. I don’t want to use EoSDH. I have 1000base ZX SFP. Can I send my link using 5 amplifiers.
Types of Receivers?
Why Receiver Sensitivity is so important for optical module?
How many types of Amplifiers used in DWDM network?
Raman Amplifier specification Vs. EDFA Specification.
Difference between EDFA and Raman amplifier?
Typical application of Raman amplifier.
What is Dual or Double stage amplification?
What are probable causes when second amplifier is getting low input power or no power, while first amplifier is transmitting target power in Dual stage amplifier?
What is Hybrid Amplification?
What is difference between Pre- Amplifier and Post- Amplifier (Booster)?
What is power control mode and Gain control mode?
What is effect of Amplifier gain on OSNR while reducing or increasing flat gain? (Linkedin).
What are Specifications of Optical Amplifiers?
What is gain tilt and Gain ripple?
How amplifier positioned in Bidirectional Single fiber DWDM network?
How WDM Works?
What are typical specifications of DWDM MUX?
What is Maximum channel capacity of DWDM channel?
What are key attributes for selecting OADM?
What are optical specifications of OADM?
What is CWDM?
What is channel equalization? What is impact of non-uniform power spectrum?
What are probable causes when among 10 channels; one transponder at remote end is getting power below set threshold?
How equalization can be checked?
What is PMD?
What is DGD?
Is there any correlation between chromatic dispersion and PMD?
What is Linear and nonlinear impairments?
What is dispersion?
What are the types of dispersion?
What is Pilot Tone in WDM?
How CD and PMD limits vary with channels and bitrate?
What is FEC?
How does FEC impact OSNR and BER value of a link?
What is Q factor?
Why we simulate Q rather BER?
What is OSNR and what is its significance?
How to improve OSNR of a link?
At what point OSNR is max and min in a WDM link?
What is OSC in DWDM link? What are the wavelengths used for it?
What is the bandwidth of OSC signal?
Where is OSC signal introduced in a DWDM link?
What if OSC is inserted on MUX port?
Why 3dB is crucial factor in WDM and what does it mean?
What is dead zone and why it is created in Photonic’s spectrum?
What is centre frequency and how is it important?
What is channel width slot width, channel spacing and centre frequency?
What is the modulation techniques used in current for WDM transmission?
What is the difference between transponder and muxponder ?
What are the type of module used for transmitter and receiver in WDM link?
Why EDFA is considered as best for commercial use?
How RAMAN amplifier is used in the network?
What is the frequency band for EDFA and RAMAN?
Whether EDFA can be used in bidirectional WDM and if yes how?
What is composite power and how it is measured?
How to measure composite power when all input powers are different?
How monitoring ports work on WDM cards?
What are the causes and source of ASE generation in WDM link?
Do we place attenuator with EDFA amplifiers and if yes where?
Why we put DCM in between two AMP in a WDM link?
What is the maximum and minimum value of OSNR for a link?
What are the different types of amplifiers available?
What is Channel flatness and how it is important?
Why changing power of one channel affects whole band power or composite power?
Can you please tell me what the difference between, dB and dBm when you are trying to test fiber optic cable.
What is ORL value? How does it matters in a link?
What is BOL and EOL and what is the margin between them?
Why there is ALS system on AMPs or photonic systems?
Why is it required to create adjacencies or logical connection between MUX and Transponder and other elements in WDM network?
What is fiber fuse and what is the cause for the same?
What is chirp and what is it’s effect and how it affect a link?
Why Raman is not used in place of EDFA ?
What is Q factor ? and how it is calculated ?
Noise figure ?
How OSA tester works and what are different options in the test instrument ?
Commissioning of new OCH in the network?
Link budgeting and commisioning parameters ?
Braggs Grating & AWG difference?
How to test OSNR using OSA for 10G, 40G & 100G channels ?
Soft DECISION & Hard DECISION FEC?
What is osnr threshold for 10 / 40/ 100G?
what is the max no. OF repeater can be used in a link?
How alarm propogate when there is no frame structure in dwdm?
What are the general alarms observed in a DWDM network?
What are the performance parameters for DWDM elements.
How DCM is used in DWDM?
Can we use DCM in 100 G network?
What is the channel spacing for hybrid channels(10/40/100/200) coherent and non coherent?
Will a single mode connector work on multi-mode cable?
How an optical power meter differs from an OTDR.
Is it possible to convert optical fiber signal into cat5 and if yes how?
Why do some power meters have calibration at 1300 nm while others are 1310 nm?
What is flexgrid network?
What is difference between CDC networks?
What is WSS based technology?
What is ITL used for?
Why WDM network doesnot requires external SYNC ?
How Extended shelf communicates in rack ?
What are the passive and active components available in a DWDM network?
How different modulation techniques affect technical parameters in a link?
What are the various graphs generally used for representing WDM performance for a network?
What if very high power launch from transmitter/OTDR with another end open.

FIber Technology Questions

Interviews 4 Mins Read

FIber Technology Questions

1. What is optical fiber?
Optical fiber is a glass or plastic filament that guides a light wave along its path.

2. What is multimode fiber?
Multimode fiber is optical fiber that allows light to travel down multiple paths, also referred to as modes. It features a core diameter of 50 to 62.5 microns. Multimode fiber can be used to transmit AV signals in short to intermediate-distance applications, such as within a building.

3. What is singlemode fiber?
Singlemode fiber is optical fiber that allows light to travel down a single path known as the fundamental mode. It features a core diameter of 8 to 9 microns. Singlemode fiber can be used to transmit AV signals over extreme distances up to many miles or kilometers.

4. How is an AV signal transmitted down a fiber?
A fiber optic transmitter converts the AV signal into an optical signal, using a VCSEL or laser diode as a light source. A glass fiber guides the optical AV signal along its path. A photodetector in a fiber optic receiver at the far end of the fiber converts the optical AV signal back into an electrical AV signal.

5. What is a light-emitting diode?
A light-emitting diode — LED is a semiconductor device that emits light when an electrical current passes through it. An LED that emits visible light is used in a variety of applications, including signage, area lighting, numerical displays, and indicator lights on electrical equipment. In fiber optics, an LED is used as a light source for low-speed signals such as, TOSLINK or 100BASE-SX Ethernet, due to its low cost. An LED is not recommended for transmitting high speed video signals over fiber.

6. What is a laser diode?
A laser diode is a semiconductor device that emits a narrow beam of coherent light, such as the beam of light from a laser pointer. In AV fiber optic transmitters, laser diodes are used as the light source for transmitting video, audio, and control signals.

7. What is a VCSEL?
VCSEL stands for Vertical Cavity Surface Emitting Laser. A VCSEL is a special type of laser diode that has lower manufacturing costs than other types of laser diodes. It can be mass-produced with high yield rates and has a smaller PCB footprint, making it ideal for use in fiber optic transmitters to send high resolution video, audio, and control signals.

8. What is a photodetector?
A photodetector is a semiconductor device that converts an optical signal into an electrical signal. A photodetector is used in a fiber optic receiver to convert optical AV signals.

9. What wavelengths are used with multimode fiber?
Multimode fiber is capable of transmitting a wavelength at or around 850 nm, 1300 nm, or 1550 nm. The most common wavelengths are 850 nm and 1300 nm due to the availability of low cost semiconductor light sources and photodetectors.

10. What wavelengths are used with singlemode fiber?
The most common wavelengths are 1310 nm and 1550 nm. At 1310 nm, chromatic dispersion is near zero, and at 1550 nm, attenuation is near its minimum. In OS1 singlemode fiber, wavelengths around 1390 nm should be avoided due to high attenuation caused by absorption. OS2 singlemode fiber is capable of transmitting any wavelength above its cutoff wavelength, which is typically around 1250 nm.

11. What is the cutoff wavelength for singlemode fiber?
The cutoff wavelength for singlemode fiber is the minimum wavelength that supports one mode of propagation. Above the cutoff wavelength, singlemode fiber propagates only one mode. Below the cutoff wavelength, singlemode fiber propagates more than one mode, similar to multimode fiber.

We know that pointers give beginning location of Payload in SONET/SDH frame.So how to get end of payload info?

Interviews 2 Mins Read

Actually SPE(synchronous payload envelope) can start anywhere within the SONET payload, which necessitates the need for a pointer to point to the beginning of the SPE.

Lets consider that the SPE begins on byte 276 (fourth row, sixth column) of frame i, and ends at byte 275 (fourth row, fifth column) of the next frame i + 1. The next SPE starts immediately, on byte 276 of frame i + 1, and so on. In general, SONET assumes that the SPE can be floated within the payload of the frame, and it provides a pointer in the overhead section for locating its beginning.

As we know that the beginning location of frame is given by H1H2 value and also that H1+H2 indicates the offset (in bytes) from H3 to the SPE (i.e. if 0 then J1 POH byte is immediately after H3 in the row). Similarly end can be found because each SPE has a fixed number of bytes.

In H1+H2, 4 MSBs are New Data Flag, 10 LSBs are actual offset value (0 – 782).

So, When offset=522 the STS-1 SPE is in a single STS-1 frame. And In all other cases the SPE straddles two frames When offset is a multiple of 87, the SPE is rectangular. This all happens with ideal synchronization conditions else H3 byte take cares further.
Some fixed start and end locations for various payloads are:-
Pointer Range: 0-103 (VT1.5 type)
Pointer Range: 0-139 (VT2 type)
Pointer Range: 0-211 (VT3 type)
Pointer Range: 0-427 (VT6 type)
Pointer Range: 0-782 (SPE,STS-1 type)
Pointer Range: 0-2339 (STS-3c type)

Brief introduction on Interviewing at Amazon Web Services (AWS)

Interviews 17 Mins Read

Amazon Web Services (AWS) is a subsidiary of Amazon providing on-demand cloud computing platforms and APIs to individuals, companies, and governments on a metered pay-as-you-go basis. Read more on https://aws.amazon.com/

Amazon keeps on hiring all round the year as they are continuously striving to look for candidates with great leadership and technical skills. The intent of this article to enable candidates to understand the process better and prepare themselves accordingly.

There are multiple ways of finding a job at AWS, and a few of my recommendations are:-

Amazon Job Portal: This job portal is the best way to explore jobs according to your skills. Just search whatever kind of job and at what locations you are looking for, and you can see the multiple posted jobs that you are always free to apply for. Some FAQs

Approach for referral: The second best way is to look for your former colleagues or acquaintance on Linkedin Contacts who can refer you for the correct job. Ask them to refer you, providing them with the Job ID mentioned on the application. This process helps you in getting some insights into the profile and the job nature.

Once your application is reviewed and found successful, somebody from HR will approach you regarding the job and ask you for your interview availability. They may ask few dates which suit your availability. Candidate is free to provide available date as per their choice.

Once the date is finalized, HR will confirm the next step, usually Phone Screen/Phone Interview.

I will explain here about Network Development Professionals. We use NDE(Network Development Engineers) as technical roles term. I will collect most of the information which is available in the public domain. This article is for experienced candidates with experience in relevant roles for some time.

Interviewing Steps :

Phone Screen/Phone Interview:

This will be a 45-60 min interview on the telephone. At scheduled time, candidate needs to join a chime bridge and you probably need to download Amazon Chime, our video-conferencing tool (the step-by-step guide can be found here). If you’re presenting, you will need to download Chime onto your desktop. The meeting ID# will be emailed to you by your recruitment point of contact. For optimal sound quality, use a headset with a microphone or with the details already provided in the email from HR.

Personal Recommendations:
-Visit Amazon Leadership Principles and go through each and recall if you have countered a situation either in your past roles or in your current role that depicts the particular leadership skills. The more the better it will lead you closer to Amazon.

– I will recommend choosing good technical experiences if you are applying for technical roles as it will be easy for both candidate and interviewer to probe more and conclude the skill.

AWS Recommendations:-

Our interviews are based on behavioural questions which ask about past situations or challenges you’ve faced and how you handled them, using the Leadership Principles to guide the discussion. We don’t use brain-teasers (e.g., “How many windows are in Manhattan?”) during the interview process. We’ve researched this approach and have found that those types of questions are unreliable when it comes to predicting a candidate’s success at Amazon. Here are some examples of behavioural questions:

Tell me about a time when you were faced with a problem that had a number of possible solutions. What was the problem and how did you determine the course of action? What was the outcome of that choice?
When have you ever taken a risk, made a mistake or failed? How did you respond and how did you learn from that experience?
Describe a time when you took the lead on a project
What did you do when you needed to motivate a group of individuals or encourage collaboration during a particular project?
How have you leveraged data to develop a strategy?
Keep in mind, Amazon is a data-driven company. When you answer questions, you should focus on the question asked, ensure your answer is well-structured and provide examples using metrics or data if applicable. Refer to recent situations whenever possible.

You should bear in mind that your interviewers won’t be evaluating your ability to memorise all the details of each of these topics. They will be analysing your ability to apply what you know and solve problems efficiently and effectively. Given that you sometimes have only limited time to prepare for a technical interview, I will recommend to go through the basic fundamentals with your technological stuffs .It is always recommended to looks for the lasted and most promising technology that are well proven in the technical community or different forums. This will likely yield the best results in the available time.

Other Tips for a smooth Phone interview:

Some teams at Amazon incorporate role-specific exercises or online assessments into the interview process.

You will be notified if you are required to take a test.
For your phone interview, find a quiet and comfortable place with no distractions.
Use a computer with a reliable connection and access to email.
If you’re using a mobile phone, make sure you’re in a place with good signal.
Have a copy of your CV on hand.
Have paper and a pen readily available.
Come prepared with questions that you would like to explore in more detail (e.g. on initiatives/projects, the team culture, the scope of the role).
If you need clarification on anything or have any additional follow-up questions, reach out to your recruiting point of contact.
If you have any special requirements, questions or concerns, please reach out to us as we are committed to making reasonable provisions for all individuals.

Virtual interviews:

If you’ve been asked to do a virtual interview, you will probably need to download Amazon Chime, our video-conferencing tool (the step-by-step guide can be found here). If you’re presenting, you will need to download Chime onto your desktop. The meeting ID# will be emailed to you by your recruitment point of contact. For optimal sound quality, use a headset with a microphone

What to Expect after Phone Screen/Phone Interview?

-Amazon HR will get back to you within two business days after your phone interview. If you haven’t heard from HR by then, feel free to give them a nudge via email.

Outcome:

–Next interview scheduling because you definitely cleared Phone Interview. Expect a call or email from HR.
–Next Phone Screen Scheduling as First Phone interview was not sufficient enough to collect data about you or due to short of time(sometimes this happens too). Second Interview Procedure is exactly same as the first Phone Interview. Expect a call or email from HR.
–Your application is not successful during this time email from HR. You are free to ask for some feedback and HR can provide some over phone maybe.

Once you are eligible for the next interview which is called On-site interview;HR will reach you again for scheduling your interviews and will send you a questionnaire with few questions . Major is Current salary (include base, bonus and equity) and Expected salary (include base, bonus and equity).Do your research and let them know the answers.

You can take your time and reply to HR with your availability. HR will send confirmation email for the interview along with the interviewers name(most of the time). Seeing current COVID-19 scenarios or sometime based on your geographical location or availability of Amazon office near your location you can opt for Video conference/In person visit to nearly Amazon office.This is generally faster else you need to wait for visa if in different country or travel arrangements.Everything is up to your choice and best available method combined.

Personal recommendation:
If you can manage a good internet speed ,do on video conference from home i.e your most comfort zone is best.

If internet speed is not reliable ,approaching nearest Amazon office is best choice.

Last should be the travel option. If job location is nearer , nothing can beat than sharing human to human interaction live.

Structure of Onsite-Interview:
Mostly Five rounds of interview schedule (45-60min each) called Loop.

Manager’s Interview-1
Members from your Team or cross team Interview -3
Bar raiser interview-1

If you are visiting the interview location there will be a lunch buddy assigned to you who will go on a quick lunch between interview with you .

Tips for the interview.

Exactly same tips as for Phone interview but expect yourself for being more patient here and you may need to sit for long hours.
Sometime it takes multiple days for the interview to complete, specially during global vacation time because of less availability of interviewers. In this case 5 interviews may get fragment to 2-3 interviews/day .

Go through Leadership principles
We use our Leadership Principles every day, whether we’re discussing ideas for new projects or finding the most effective solution to a problem. It’s just one of the things that makes Amazon peculiar. All candidates are evaluated based on our Leadership Principles. The best way to prepare for your interview is to think about how you’ve applied the Leadership Principles in your previous professional experience.

Prepare for Behavioural Interviewing
-Our interviews are based on behavioural questions which ask about past situations or challenges you’ve faced and how you handled them, using the Leadership Principles to guide the discussion. We don’t use brain-teasers (e.g., “How many windows are in Manhattan?”) during the interview process. We’ve researched this approach and have found that those types of questions are unreliable when it comes to predicting a candidate’s success at Amazon.

Here are some examples of behavioural questions:

Tell me about a time when you were faced with a problem that had a number of possible solutions. What was the problem and how did you determine the course of action? What was the outcome of that choice?
When have you ever taken a risk, made a mistake or failed? How did you respond and how did you learn from that experience?
Describe a time when you took the lead on a project
What did you do when you needed to motivate a group of individuals or encourage collaboration during a particular project?
How have you leveraged data to develop a strategy?

Keep in mind, Amazon is a data-driven company. When you answer questions, you should focus on the question asked, ensure your answer is well-structured and provide examples using metrics or data if applicable. Refer to recent situations whenever possible.

Use best proven STAR answering format:

The STAR method is a structured manner of responding to a behavioural interview question by discussing the specific situation, task, action and result of what you’re describing. Here’s what it looks like:

SITUATION

Describe the situation that you were in or the task that you needed to accomplish. Give enough detail for the interviewer to understand the complexities of the situation. This example can be from a previous job, school project, volunteer activity or any relevant event.

TASK

What goal were you working towards?

ACTION

Describe the actions you took to address the situation with an appropriate amount of detail and keep the focus on you. What specific steps did you take? What was your particular contribution? Be careful that you don’t describe what the team or group did when talking about a project. Let us know what you actually did. Use the word “I,” not “we,” when describing actions.

RESULT

Describe the outcome of your actions and don’t be shy about taking credit for your behaviour. What happened? How did the event end? What did you accomplish? What did you learn? Provide examples using metrics or data if applicable.

Consider your own successes and failures in relation to the leadership principles. Have specific examples that showcase your expertise and demonstrate how you’ve taken risks, succeeded, failed and grown over the course of your career. Bear in mind that some of Amazon’s most successful programmes have risen from the ashes of failed projects. Failure is a necessary part of innovation. It’s not optional. We understand that and believe in failing early and iterating until we get it right.

Tips for great answers
Practise using the STAR method to answer the behavioural interview questions listed above and incorporate the Amazon Leadership Principles

Ensure each answer has a beginning, middle and end. Describe the situation or problem, the actions you took and the outcome

Prepare short descriptions of a handful of different situations and be ready to answer follow-up questions in greater detail. Select examples that highlight your unique skills

Give specific examples that showcase your experience and demonstrate that you’ve taken risks, succeeded, failed and grown over the course of your career

Specifics are key: avoid generalisations. Give a detailed account of one situation for each question you answer and use data or metrics to support your example

Be forthcoming and straightforward. Don’t embellish or omit parts of the story.

Personal recommendation for technical Interviewing:

I will recommend to choose good technical experiences if you are applying for technical roles as it will be easy for both candidate and interviewer to probe more and conclude the skill.

e.g for Optical Engineers

-Prepare and revise DWDM fundamentals.
-Network Architecture and designs.
-How to solve problems through Automation skills.You can read more on my article https://mapyourtech.com/automation-in-optical-networkingif-you-are-reading-this-you-can-do-this/

-Read about latest CFPs/QSFPs/ZR etc working and fundamentals.
-Read about best problem solving techniques for issues in Network.
-Read about key performance metrices/indicators that can help you solve a problem in network.
-Read about how to collaborate with other technologies or cross platforms teams/devices to isolate and fix the problems.
-Recording solutions and creating MOP(Method of Procedure/Process) to fix issue in future.
-Think about innovative solutions that can help manage network smoothly.

Tips before you head in
Be prepared to explain what interests you about the role you’re being interviewed for and the team (or teams) you’ll be meeting with

When answering questions, be concise but detailed. We realise it’s hard to gauge how much information is too much versus not enough. An effective test is pausing after your succinct response to ask if you’ve provided enough detail or if the interviewer would like you to go into more depth

Follow-up if you need clarification. If you’re asked a question, but are not given enough information to provide a solid answer, don’t be shy about asking for more information. If additional context is not available, focus on how you would attempt to solve the problem with only limited information

For some roles, we may ask you to complete a writing sample. Why? At Amazon, we don’t use PowerPoint or any other slide-oriented presentations. Instead we write narratively structured memos and silently read one at the beginning of each meeting. These documents generally range from one to six pages and articulate the project goal(s), approach to addressing it, outcome and next steps. Given this unique aspect of our culture and the impact these papers have on what decisions we make as a company, being able to articulate your thoughts in written form is a necessary skill

We aim to hire intelligent, thoughtful and customer-obsessed people. Reflect on what motivated you to pursue a career with Amazon and be prepared to share your thought process. Although “Why Amazon?” is a standard question, it’s not just a formality for us. We genuinely want to understand what inspired you to explore an opportunity with us so we get a better sense of who you are

We try to leave a few minutes at the end of each interview to answer questions you might have, but if we don’t get to all of them, please don’t hesitate to ask your recruitment point of contact.

Interview day
Check-in: arrive 15 minutes early to check in for your interview. Have your government-issued photo ID ready (e.g. driving licence, passport)

Location: detailed instructions will be sent to you via email. Some of our offices are dog-friendly. Let us know if you have any special requirements or allergies

Dress code: comfortable and casual. While safety clothing – such as closed-toed shoes – is required for some positions in our fulfilment centres, most of our office staff wear everyday clothing. We’re interested in what you have to say, not what you’re wearing

What to expect: interviews will be a mixture of questions and discussions concerning your previous experience and the challenges you’ve encountered. Come armed with detailed examples — concise, structured answers are ideal

Interviewers: depending on the role, you will meet with anywhere from two to seven Amazonians. They will likely be a mix of managers, team members, key stakeholders from related teams and a “Bar Raiser” (usually an objective interviewer from another team). All interviewers will assess potential for growth beyond the position you’re being interviewed for and focus on evaluating how well your background and skills meet core competencies, along with how they relate to Amazon’s Leadership Principles. We recommend approaching each of your interviews the same way rather than trying to tailor answers to the interviewer’s role. Interviewers will often be taking notes on their laptops. It’s important that they have precise notes of their time with you to share with other interviewers

CV: interviewers will have a copy, but feel free to bring one as well

Duration: each interview usually lasts from 45 minutes to an hour

Lunch: We will provide lunch if your interview is scheduled during the lunch hour. Let recruitment or your lunch buddy know if you have any dietary preferences

Amazon Non-Disclosure Agreement: All candidates must sign our standard Non-Disclosure Agreement. If you’re unable to print and sign prior to your arrival, we’ll have a copy available for you

Technical roles: If you’re being interviewed for a technical role, be prepared to use a whiteboard

Virtual interviews: If you’ve been asked to do a virtual interview, you will probably need to download Amazon Chime, our video-conferencing tool (step-by-step guide here). If you’re presenting, you will need to download Chime onto your desktop. The meeting ID# will be emailed to you by your recruitment point of contact. For optimal sound quality, use a headset with a microphone.

Before and after the interview
Some teams at Amazon incorporate role-specific exercises or online assessments into the interview process. You will be notified if you are required to take a test

Confirm or book arrangements if your interview requires travel. Your recruiting point of contact will either set up your travel arrangements or put you in touch with our travel agency to help you coordinate travel details and hotel stay.

An expense report should be submitted after your visit. Your recruitment point of contact will provide details on where and how to submit your report. Fill in your form clearly and ensure scanned receipts are legible – this will help prevent reimbursement delays

If you have any special requirements, questions or concerns, please contact us: we are committed to making reasonable provisions for all individuals

After your interview, be on the lookout for a quick post-interview survey via email. It is important for us to know how we did so we can continually improve our interview process. We really value your input

Expect to hear back from recruitment within five business days following your interview. If you don’t, feel free to give us a nudge.

Outcome:

Successful :
Great you have made it. Lot of Amazonians are looking forward to meet you to learn and share the best with you and from you.

Next Time:

Dont worry, you can still apply for different opportunities and its good that you got the first hand experience of interview and next time you can prepare yourself in a better way. Remember that the decision is taken in consideration with multiple parameters like current available candidates skill sets in same role ,current available racks that meets your skill sets, your future growth aspects etc.

Offer

HR will give call to explain the offer.Based on your experience, job level and skill sets there will be an competitive compensation offered to you.There is always a scope for negotiations if the explanations are convincing and agreed between candidate and the HR.

Offer consists of base salary +RSU stocks+joining bonus and other relocation benefits.It is recommended to know your worth and expectation and for that candidate can refer to online websites/different forums etc.

Recommended sites for more exploration:

https://www.levels.fyi/?compare=Amazon,Google,Facebook,Microsoft&track=Software%20Engineer

https://www.glassdoor.com/Interview/Amazon-Interview-Questions-E6036.htm?countryRedirect=true

https://dev.to/fallenstedt/three-steps-i-took-to-get-a-job-offer-from-amazon-1p23

https://www.zippia.com/advice/amazon-interview-questiond/

https://www.quora.com/How-long-does-it-take-Amazon-to-make-a-job-offer

https://fearlesssalarynegotiation.com/amazon-salary-negotiation/

https://6figr.com/in/salary/amazon

What is the reason behind most of the EDFAs (Erbium Doped Fiber Amplifier) using only 980nm and 1480nm as their pumping wavelengths and not any other wavelengths?

Interviews 1 Min Read

The pump wavelength used is either 980nm or 1480 nm due to the availability of these laser sources. In the course of the explanation let see the energy level diagrams for the (Erbium) Er³⁺ ion, the absorption band of Er+, and the pump efficiency.

(a) Energy levels of erbium ions and (b) gain and attenuation spectra.

Excited State Absorption (ESA)

Ground-state absorption (GSA)

There are several states to which the erbium ions can be pumped using sources of different wavelengths, such as those operating at 1480, 980, and 800 nm. However, in practical optical systems, the pump wavelength must provide a high power to achieve a high gain per pump.

The commonly available laser diodes can operate at 800, 980, and 1480 nm, but the pump efficiency can go more than 1 dB/mW with low attenuation depending on the pump frequency.

The only pump wavelength laser sources that can give a high pumping efficiency with lower attenuations are those operating at 980 and 1480 nm.

In practice, the 980 nm pumping source is commonly used due to its high gain coefficient (4 dB/mW). The difference in the effects of these two wavelength sources is mainly caused by the absorption and emission factors.

OSNR: What does this mean;Why do we need and How to take care of it?

Interviews 20 Mins Read

Signal to Noise Ration (SNR) is not an unknown terminology for Engineers and Tech professionals who are dealing with Digital or Analog form of Communication.Here we will explore the aspect of SNR in Optical Fiber Communication space known as Optical Signal to Noise Ratio(OSNR).

Warning! Keep patience while scrolling down to read as this article may seem long as this is important topic.Whole content is collected from free available trusted sources and can be downloaded or shared and covers content for everyone from beginner to professional so reader can absorb what he wants.Bingo! Let’s start it.

Some handy definition of OSNR to pick :-

OSNR [dB] is the measure of the ratio of signal power to noise power in an optical channel .

OSNR is the short form of Optical Signal to Noise Ratio. It is key parameter to estimate performance of Optical Networks. It helps in BER calculation of Optical System.
OSNR is important because it suggests a degree of impairment when the optical signal is carried by an optical transmission system that includes optical amplifiers.
If we know the OSNR and the bandwidths, we can find Q and the BER
It can be seen as the QoS at the physical layer of optical networks. OSNR is directly related to bit-error rate, which will lead to packet losses seen by higher layers.
OSNR indirectly reflects BER and can provide a warning of potential BER deterioration.
OSNR has long been recognised as a key performance indicator for amplified high-speed transmission networks to ensure network performance and reliability and it is related to many design parameter such as number or repeater/amplifiers ,reach ,available modulation formats etc.
OSNR is a metric for the quality assessment of received signals that are corrupted by the ASE noise of EDFAs OSNR is defined as the ratio of the average optical signal power to the average optical noise power For a single EDFA with the output power Pout and the noise power NASE, OSNR is computed as

where
NF is the noise figure G is the amplifier gain
hf is the photon energy Δf is the optical measurement bandwidth

When addressing an OSNR value, it is important to define an optical reference bandwidth for the calculation of OSNR A bandwidth Δf of 12 5 GHz (or Δλ = 0 1 nm) is the typical reference bandwidth for calculating OSNR values

Now let’s explore it in more detail:-

Optical signal-to-noise ratio (OSNR) is used to quantify the degree of optical noise interference on optical signals. It is the ratio of service signal power to noise power within a valid bandwidth.When the signal is amplified by the optical amplifier (OA), like EDFA, its optical signal to noise ratio (OSNR) is reduced, and this is the primary reason to have limited number of OAs in a network.

The OSNR values that matter the most are at the receiver, because a low OSNR value means that the receiver will probably not detect or recover the signal. The OSNR limit is one of the key parameters that determine how far a wavelength can travel prior to regeneration.

OSNR serves as a benchmark indicator for the assessment of performance of optical transmission systems. DWDM networks need to operate above their OSNR limit to ensure error – free operation. There exists a direct relationship between OSNR and bit error rate (BER), where BER is the ultimate value to measure the quality of a transmission.

The value of OSNRout that is needed to meet the required system BER depends on many factors such as the bit rate, whether and what type of FEC is employed, the magnitude of any crosstalk or non-linear penalties in the DWDM line segments, etc.

Below picture highlights OSNR as one of the important parameter in a DWDM link.

Optical amplifiers such as erbium-doped fiber amplifiers (EDFAs) are normally employed in optical networks to compensate for the transmission losses over long distances. However, besides providing optical gain, EDFAs also add unwanted amplified spontaneous emission (ASE) noise into the optical signal. Furthermore, the cascading of EDFAs results in accumulation of ASE noise. ASE noise is typically quantified by OSNR and is one of the most important parameters to be monitored in optical networks since the BER is directly related to the signal OSNR Furthermore, it also plays a pivotal role in fault diagnosis and as a measure of general health of links in an optical network.

OSNR may change due to signal power changes or higher repeater noise levels due to aging.

Now lets read the Legacy method to measure it:-

The traditional method to measure OSNR is defined in the IEC 61280-2-9 standard and is known as the interpolation or out-of-band method, as shown in Figure-1 below

OSNR for a Point-to-Point Link

NFstage is the noise figure of the stage, h is Plank’s constant (6.6260 × 10-34), ν is the optical frequency 193 THz, and Δf is the bandwidth that measures the NF (it is usually 0.1 nm).

OSNR =158.9+ Pin.dBm −NF−10log(Br )

where

OSNRdB = optical signal to noise ratio of the optical amplifier, dB
Pin.dBm = average amplifier input signal power (DWDM systems use single-channel power), dBm
NF = amplifier noise figure, dB
h = Planck’s constant 6.626069 ◊ 10−34, Js
f = signal center frequency, Hz
Br = optical measurement bandwidth RBW, Hz

If the measurement optical bandwidth can be assumed to be 0.1 nm (12.48 GHz),

OSNR = 58 + Pin.dBm − NF

OSNRF.dB =158.9+Psource.dBm −Γ−NF−10log(Br)−10logN

where

OSNRF.dB = final OSNR seen at the receiver, dB
Psource.dBm = average source signal power into the first span(DWDM systems use single-channel power),dBm NF =amplifier noise figure, the same for all EDFAs, dB

Br = optical measurement bandwidth, Hz
N = number of amplifiers in the fiber link excluding the booster
Γ = span loss, the same for all spans, dB

Above equation provides the actual mathematical calculation of OSNR. This calculation method has quite a few approximations in which we can still find the system OSNR to a great degree of accuracy. In a multichannel WDM system, the design should consider OSNR for the worst channel (the one that has the worst impairment). The worst channel is generally the first or last channel in the spectrum.

we can see that the EDFA gain factor G is not considered. That is because OSNR is a ratio, and the gain acts equally on signal and noise, canceling the gain factor in the numerator and denominator. In other words, although EDFAs alleviate the upper bound on transmission length due to attenuation, by cascading EDFAs in a series, the OSNR is continuously degraded with transmission length and ASE (from EDFAs). This degradation can be lessened somewhat by distributed Raman amplifiers (DRAs).

Addition of Raman and OSNR change:-

As we can see from above equation the factor GRA in the numerator actually enhances the OSNR of the system.(stages) could be considered as EDFA hops here.

OSNR-based design essentially means whether the OSNR at the final stage (at the receiver) is in conformity with the OSNR that is desired to achieve the required BER. This also guarantees the BER requirement that is essential for generating revenue.

Below context is taken from article published by Jean-Sébastien

This method works well for networks up to 10G, without any Reconfigurable Optical Add-Drop Multiplexers (ROADM).

But traditional way of measurement don’t work anymore in High Speed Communications:-

However, IEC 61280-2-9 isn’t feasible for 100G+ signals as well as ROADM networks.

Figure 2 illustrates 100G channels spaced 50GHz apart, which is a common spacing in modern submarine (and terrestrial) networks. Polarization-Multiplexed (Pol-Mux) 100G+ signals are typically wider (require more optical spectrum) than legacy On-Off-Keyed (OOK) 10G signals, meaning they could overlap with neighboring channels. Accordingly, the midpoint between channels no longer consists only of noise, but rather of signal plus noise. Thus, the IEC method applied to 100G+ Pol-Mux signals will therefore lead to an overestimation of noise and inaccurate measurement date leading to incorrect decisions.

Figure 2: IEC 61280-2-9 Method Fails with Dense Pol-Mux 100G+ signals

Figure 3 illustrates a 100G signal that has gone through a ROADM, with the green area showing the channel bandwidth. Given filters inside a ROADM, the noise at the midpoint between channels will be carved (or filtered), leading to an underestimation of the noise level, if the IEC 61280-2-9 method is used, meaning this method is not feasible in ROADM-enabled coherent submarine networks.

Figure 3: IEC 61280-2-9 Method Fails in ROADM 100G+ Pol-Mux Networks

To address the issues described in Figures 2 and 3, in-band OSNR was introduced around 2009 to support OSNR measurements of 10G signals in ROADM networks and 40G OOK signals. However, this method can’t be applied to coherent, Pol-Mux 100G+ signals, because of technical reasons beyond the scope of this blog. Consequently, Pol-Mux OSNR techniques have been introduced to support 100G+ signals, which is the topic of the latest standards.

Appropriate standards for Pol-Mux OSNR measurements?

There are two standards providing relevant guidelines for OSNR measurements of Pol-Mux signals. They are the China Communications Standards Association (CCSA) YD/T 2147-2010 standard and the IEC 61282-12 standards, which was recently introduced in February 2016. Both standards provide a future-proof definition of OSNR, which can be applied to any type of signal, at any data rate, including super-channels and flexible-grid signals. Specifically, the IEC-61282-12 standard specifies that:

where:

s(λ) is the time-averaged signal spectral power density, not including ASE, expressed in W/nm
ρ(λ) is the ASE spectral power density, independent of polarization, expressed in W/nm
Br is the reference bandwidth expressed in nm (usually 0.1nm if not otherwise stated)
and the integration range in nm from λ1to λ2is chosen to include the total signal spectrum

The only drawback of these two standards is that a careful application of their formulae requires turning off channels, to access the Amplified Spontaneous Emission (ASE) noise floor, which isn’t possible on an in-service lest we upset end-users! Fortunately, in-service Pol-Mux OSNR methods have been introduced.

below table summarizes the correct OSNR method for each type of signal.

Data Rate	ROADM Present?	OSNR Method	Works on in-service network?
≤10G signals	No	IEC 61280-2-9	Yes
≤10G signals	Yes	In-band OSNR	Yes
Non-coherent 40G signals	Yes or No	In-band OSNR	Yes
Coherent 100G+ signals	Yes or No	Pol-Mux OSNR (IEC and CCSA standards)	No
Coherent 100G+ Signals	Yes or No	In-Service Pol-Mux OSNR	Yes

Table 1: OSNR Measurement Methods for Various Signal Types

Using the wrong OSNR measurement method for a given signal can have a significant impact on results, as it can lead to errors ranging from a few dBs up to 10dB, potentially leading to future outages. Using the proper method guarantees the right OSNR measurements are achieved leading to accurate network modeling, link simulation, and maintenance of ongoing submarine cable network performance.

Summarizing the importance of OSNR and its proper measurement method?

Some of the benefits of OSNR testing, including avoiding network outages, optimizing troubleshooting times, and ensuring optimal terrestrial and submarine cable performance. OSNR will become even more critical at data rates beyond 100G, because of the more stringent OSNR thresholds that will be required. Several OSNR methods have been introduced over the years, so the key takeaway is that the right OSNR measurement method must be used on a specific signal type to get accurate results.

Introduction to OSNR for high speed communication

The OSNR is the signal-to-noise ratio (SNR) measured in a reference optical bandwidth, where frequently a bandwidth Bref of 12.5 GHz is used corresponding to 0.1 nm wavelength. The OSNR relates to the Es ∕N0 and Eb ∕N0 as

where Bref is the previously introduced reference bandwidth, RS corresponds to the symbol rate of the transmission, r is the mentioned rate of the code with r = k∕n, and q corresponds to the number of bits mapped to each modulation symbol.

The use of high-order modulation formats requires a higher optical signal-to-noise ratio (OSNR), which may result in a significantly reduced achievable transmission distance .Following diagram shows the OSNR estimation stage for High Speed Optical Communication.

Lets talk about OSNR Penalty now:-

OSNR penalty is obtained from the BER curves and determined at a particular BER. A value of the OSNR penalty is obtained by comparing the values of OSNR before and after the change of the parameters, which are under test, as given by

Gripple = penalty due to DWDM amplifier gain ripples
OSNRpenalties = various transmission penalties due to CD, PMD, PDL, etc. (note these penalties maybe different for 100G vs. 400G)

Calculation of Q-Factor from OSNR

The OSNR is the most important parameter that is associated with a given optical signal. It is a measurable (practical) quantity for a given network, and it can be calculated from the given system parameters. The following sections show you how to calculate OSNR. This section discusses the relationship of OSNR to the Q-factor.

The logarithmic value of Q (in dB) is related to the OSNR

In the equation, B0 is the optical bandwidth of the end device (photodetector) and Bc is the electrical bandwidth of the receiver filter.

Q is somewhat proportional to the OSNR

BER FORMULAS FOR THE MOST COMMON QAM SYSTEMS

Gray coding is assumed for all formats. For PM-BPSK the exact formula is:

For PM-QPSK the exact formula is:

For PM-16QAM and PM-64QAM, respectively, the following formulas are approximate, but their accuracy is better than ±0.05 dB of OSNRNL over the range 10−1 and 10−4:

EDFA Noise – Why Input Power Matters

Optical signal suffers more than only attenuation. In amplitude, spectrally, temporally signal interaction with light- matter, light- light, light-matter-light leads to other signal disturbances

such as :-

Power reduction
Dispersion
Polarization
Unbalanced amplification

Thus leading to random noise, which causes misalignments, jitter and other disturbances resulting in erroneous bits, the rate of which is known as bit-error-rate

Because of all possible influences outlined bits transmitted by source and bits arriving at the receiver may not have the same value. In actuality a threshold value is set at the receiver, above the threshold refers to a logic “one” and below threshold refers to a logic “zero”.

In order to measure BER in photonic regime, the optical signal is converted to electrical signal.

Example: Assuming a confidence level of 99%, BER threshold set at 10-10 and a bit rate of 2.5 Gb/s the required number n is 6.64 x 1010

Given the OSNR, the empirical formula to calculate BER for single fiber is

Log10 (BER) = 10.7-1.45 (OSNR)

Some mathematical aspects of OSNR:-

Assume that OSNR = 14.5 dB
ThenLog10 (BER) = 10.7-1.45 (14.5) = -10.30

Therefore BER = 10(-10.30) BER is approx 10- 10

In an experimental environment where factors such as loss, dispersion, and non-linear effects are excluded, if the OSNR is less than the specified threshold, the pre-FEC BER will be excessively large and uncorrectable bit errors will be generated. The OSNR threshold in this case is called B2B OSNR tolerance.

Calculating OSNR from OSA:

As can be seen from the definitions above, two quantities must be known to compute OSNR: the Total Signal Power, and the amount of ASE Noise Power present in a 0.1nm bandwidth.

Measuring the ASE power:

When the ASE noise floor is clearly visible left and right of the optical signal, the ASE Noise Power at the signal wavelength can be interpolated from two measurements made left and right of the signal.

Alternately, when the noise floor is not visible left and right, the optical signal needs to be removed temporarily in order to allow the measurement of the ASE Noise Power at the signal wavelength.

▪ This is the case when some filtering devices implemented somewhere in line are removing some of the noise between channels (Example = WSS).

▪ This could also be the case if the modulated signal bandwidth is so large so that the tail of adjacent signals overlaps the open space between them – masking the noise floor.

Note that power values are frequently provided in dBm by the OSA – whether the measurements is made using integrated power function or a user-specified resolution bandwidth. To convert the values in dBm to mW, the following relation must be used:

Measuring the Total Signal Power:

When making this measurement it is important to use a bandwidth that is large enough to capture the entire signal:

▪ If using an OSA with variable resolution bandwidth, this means that the resolution bandwidth has to be set larger than the width of the signal.

▪ If using a integrated- power function between vertical markers, the markers have to be set to include the entire signal bandwidth.

Note that when measuring a DWDM spectrum, the power of each DWDM signal cannot be measured independent of ASE noise present in the measurement bandwidth.

Ie., the value that is actually measured is: Total Signal Power + ASE Noise Power.

To get the Total Signal Power only, the ASE noise content (measured separately in the previous step) must be subtracted from the measurement. Since the measurement bandwidth used to measure the noise on its own (ASE BW) may be different from the bandwidth used to measure the signal (Signal BW), a factor is added to the equation. This removes the correct amount of noise from the measurement.

where:
Signal BW is the bandwidth used to measured the signal,
ASE BW is the bandwidth used to measure the ASE Noise Power only (e.g., 0.1nm).

Calculating OSNR from measurements:

Since OSNR must be reported as signal power with respect to 0.1nm worth of ASE noise, the denominator of the OSNR equation also includes a factor. This adjusts the amount of ASE noise measured to an amount expected inside a 0.1nm bandwidth:

Thus
In a transmission chain, the relative evolution of the optical signal and noise levels is usually characterized by the optical signal-to-noise ratio (OSNR). The OSNR, in a given optical bandwidth, is defined as

The optical signal may be polarized, but the noise is usually not and, depending on the receiver polarization sensitivity, the noise must be considered with a single or two polarizations. Any active (or passive) linear optical device amplifies (or attenuates) simultaneously the incoming signal and the incoming noise

Accordingly with the fluctuation dissipation theorem, it also adds noise, making its output OSNR lower than its input one. This OSNR degradation is expressed in terms of noise factor (NF) defined as:

where G is the optical power gain (or the attenuation coefficient) of the device which is larger (or smaller) than 1. NF is denoted noise figure when it is expressed in dB. Because the relative importance of the added noise strongly depends on the input noise level, NF is only an intrinsic parameter of the optical device (i.e. independent of the input signal and noise) when an input reference noise is defined.

For polarization insensitive devices, input noise and output noise usually do depend on polarization, making noise figure independent of polarization considerations.

Single amplifier noise factor

OSNR at the output of an optical amplifier with an output power Pout and for an optical bandwidth B0 is expressed as:

where m=1 or 2 is the number of polarization modes contributing to noise. It is usual to consider the two polarization modes (m=2) of the noise and to make reference to an optical bandwidth equal to 0.1 nm corresponding to Bo = 12.5GHz at a wavelength of 1550 nm. In this particular situation, the OSNR is expressed in dB as:

NFdB, Noise factor of a cascade of fibers and amplifiers

Let us consider now the cascade of spans displayed in Figure and including Namp optical fiber spans with an attenuation coefficient A and Namp lumped linear phase insensitive amplifiers with a gain net G and a noise factor NF. For each span a near compensation of the signal attenuation by the gain is assumed, making its net gain GSPAN 5 GA close to 1. Each fiber reduces the sig- nal level and reduces the noise level nearly in the same way. Since the input noise level is far above the appropriated reference level, the attenuation noise of the fiber is negligible and the OSNR is kept nearly unchanged. Each amplifier output POUT restores the input noise of the link but adds the amplifier noise.

The accumulated noise is Namp times larger than single amplifier and we have:

To bridge transoceanic distances while keeping a high OSNR (optical SNR), it is crucial to limit the noise contribution added by the successive amplifiers. The impact of the added noise on the output OSNR can be calculated with:

This equation can also be expressed in a more physical manner:

where N is the amplifier count, and Δλ the width of the filter where the OSNR is expressed. The number “30” at the end of Equation corresponds to the conversion of signal input power from units in watts into units scaled in milliwatts.

Few Key Concepts to remember

The OSNR values that matter the most are at the receiver, because a low OSNR value means that the receiver will probably not detect or recover the signal.

The OSNR limit is one of the key parameters that determine how far a wavelength can travel prior to regeneration.

OSNR serves as a benchmark indicator for the assessment of performance of optical transmission systems. DWDM networks need to operate above their OSNR limit to ensure error – free operation.

There exists a direct relationship between OSNR and bit error rate (BER), where BER is the ultimate value to measure the quality of a transmission. Given the OSNR, the empirical formula to calculate BER for single fiber is:

Log10 (BER) = 10.7-1.45 (OSNR)

In DWDM links a rule of thumb would be to target an OSNR value greater than 15 dB to 18 dB at the receiver.

OSNR requirements depend on:

Location: The required OSNR will be different for different locations in the light path. The OSNR requirement will be higher closer to the transmitter and lower closer to the receiver. This is because optical amplifiers and reconfigurable add/drop modules (ROADMs) add noise, which means that the OSNR value degrades after going through each optical amplifier or ROADM. To ensure that the OSNR value is high enough for proper detection at the receiver, the number of optical amplifiers and ROADMs needs to be considered when designing a network.
Type of Network: For a metro network, an OSNR value of >40 dB at the transmitter might be perfectly acceptable, because there are not many amps between the transmitter and the receiver.For a submarine network, the OSNR requirements at the transmitter are much higher.
- - - - Data Rate: With the increase in the data rate for a specific modulation format, the OSNR requirement also increases.
        
        Target BER: A lower target BER calls for a higher OSNR value.

The exact requirements at the receiver will vary from one manufacturer to another. Table displays a few average OSNR figures to guarantee a BER lower than 10-8 at the receiver

Note: Now try to utilise the above concepts and equations whatever way you want .

Calculating Channel Capacity for DWDM links

Infographics 1 Min Read

The maximum data rate (maximum channel capacity) that can be transmitted error-free over a communications channel with a specified bandwidth and noise can be determined by the Shannon theorem. This is a theoretical maximum data transmission rate for all possible multilevel and multiphase encoding techniques.

As can be seen below that the maximum rate depends only on channel bandwidth and the ratio between signal power to noise power. There is no dependence on modulation method.

Rmax =Bolog2(OSNR+1)

where

Rmax = maximum data rate for the channel (also known as channel capacity), Gbps

Bo = optical channel passband, GHz

OSNR = channel optical signal to noise ratio

Example:-

For a 62 GHz channel passband (for standard 200 GHz DWDM channel spacing) and an OSNR of 126 (21 dB) the maximum possible channel capacity is 433 Gbps.

As channel bandwidth decreases so does maximum transmission rate. For a 30 GHz channel passband (100 GHz DWDM channel spacing) and OSNR of 126 (21 dB) the maximum possible channel capacity is 216 Gbps.

What is Optical Power Requirement and margin for a optics module’s power?

Interviews 1 Min Read

Optical power tolerance: It refers to the tolerable limit of input optical power, which is the range from sensitivity to overload point.

Optical power requirement: If refers to the requirement on input optical power, realized by adjusting the system (such as adjustable attenuator, fix attenuator, optical amplifier).

Optical power margin: It refers to an acceptable extra range of optical power. For example, “–5/ + 3 dB” requirement is actually a margin requirement.

What are main advantages and drawbacks of EDFAs?

Infographics 1 Min Read

The main advantages and drawbacks of EDFAs are as follows.

Advantages

Commercially available in C band (1,530 to 1,565 nm) and L band (1,560 to 1,605) and up to 84-nm range at the laboratory stage.
Excellent coupling: The amplifier medium is an SM fiber;
Insensitivity to light polarization state;
Low sensitivity to temperature;
High gain: > 30 dB with gain flatness < ±0.8 dB and < ±0.5 dB in C and L band, respectively, in the scientific literature and in the manufacturer documentation
Low noise figure: 4.5 to 6 dB
No distortion at high bit rates;
Simultaneous amplification of wavelength division multiplexed signals;
Immunity to crosstalk among wavelength multiplexed channels (to a large extent)

Drawbacks

Pump laser necessary;
Difficult to integrate with other components;
Need to use a gain equalizer for multistage amplification;
Dropping channels can give rise to errors in surviving channels:dynamic control of amplifiers is necessary.

Linkedin query on OSC/GCC/DCC…answered by me:-

Interviews 2 Mins Read

Query on Linkedin:Wondering about the difference in use of the OSC and GCC in OTN+WDM systems

First thing I would like to share is that transport equipment typically contains integrated external DCN ports for standalone management. However, as the network infrastructure equipment continues growth with remote locations, it may not be feasible to have a local external DCN network at each and every remote site. Thus OSC/GCC/DCC can provide a cost-effective in-band/Outband data communications link back to a gateway NE (GNE) and back to a centralize network management stations for remote maintenance from customer NMS/OSS.

Now brief on GCC.
GCC are general Communication channel used in OTN.

In OTN, the GCC0 is used on the OTU overhead while GCC1 and GCC2 are used from the ODU overhead. GCC0 provides terminal-regen and regen-regen communications. GCC1/GCC2 provides direct terminal-terminal communications.It allows an in-band management solution between various elements in network.As they are inband communication mechanism,bytes inside frame are responsible for the communication as different levels as in case of DCC in SDH/SONET.

Now brief on OSC

ITU-T G.692,defines OSC.As we donot have framing concept for DWDM rather it acts as a carrier we require some outband communication systems for maintenace and remote operation.OSC is generated from special supervisory cards that are capable to transmit communication information over bandwidth say 2Mbps,155Mbps etc.Bandwidth rate is vendor specific.

Again question arises what is OSC?

The OSC is carried on a separate wavelength, different from the wavelengths carrying the actual traffic.It is separated from the other wavelengths at each amplifier stage and received, processed, and retransmitted,

The OSC provides an out-of-band, full-duplex communications channel for remote node management, monitoring and control similar to the Data Communications Channel (DCC) of SONET/SDH and the General Communications Channel (GCC) of OTN. The OSC optically segregates network management and control from user data, so even if the OSC is lost, data forwarding continues uninterrupted.

For WDM systems operating in the C-band, the popular choices for the OSC wavelength include 1310 nm, 1480 nm, 1510 nm, or 1620 nm. Using the 1310 nm band for the OSC precludes the use of this band for carrying traffic.

Both functionality/working principles are part of ITU-T standards.

SDH Q&A:helpful to crack SDH based interviews…………….

Interviews 15 Mins Read

Q. What is SDH ?

SDH stands for Synchronous Digital Hierarchy & is an international Standard for a high capacity optical telecommunications network.It is a synchronous digital transport system aimed at providing a more simple,economical,& flexible teleccommunication infrastructure.

Q. What is the difference between SONET and SDH?
A. To begin with there is no STS-1. The first level in the SDH hierarchy is STM-1 (Synchronous Transport Mode 1) is has a line rate of 155.52 Mb/s. This is equivalent to SONET’s STS-3c. Then would come STM-4 at 622.08 Mb/s and STM-16 at 2488.32 Mb/s. The other difference is in the overhead bytes which are defined slightly differently for SDH. A common misconception is that STM-Ns are formed by multiplexing STM-1s. STM-1s, STM-4s and STM-16s that terminate on a network node are broken down to recover the VCs which they contain. The outbound STM-Ns are then reconstructed with new overheads.

Q. What are the advantages of SDH over PDH ?

The increased configuration flexibility and bandwidth availability of SDH provides significant advantages over the older telecommunications system.
These advantages include:
A reduction in the amount of equipment and an increase in network reliability.
The provision of overhead and payload bytes – the overhead bytes permitting management of the payload bytes on an individual basis and facilitating centralized Fault sectionalisation.-nearly 5% of signal structure allocated for this purpose.
The definition of a synchronous multiplexing format for carrying lower-level digital signals (such as 2 Mbit/s, 34 Mbit/s, 140 Mbit/s) which greatly simplifies the interface to digital switches, digital cross-connects, and add-drop multiplexers.
The availability of a set of generic standards, which enable multi-vendor interoperability.
The definition of a flexible architecture capable of accommodating future applications, with a variety of transmission rates.Existing & future signals can be accomodated.

Q. What are the main limitations of PDH ?

The main limitations of PDH are:
Inability to identify individual channels in a higher-order bit stream.
Insufficient capacity for network management
Most PDH network management is proprietary
There’s no standardised definition of PDH bit rates greater than 140 Mbit/s
There are different hierarchies in use around the world. Specialized interface equipment is required to interwork the two hierarchies

Q. What are some timing/sync defining rules of thumb?
A.
1. A node can only receive the synchronization referencesignal from another node that contains a clock ofequivalent or superior quality (Stratum level).
2. The facilities with the greatest availability (absence of outages) should be selected forsynchronization facilities.
3. Where possible, all primary and secondary synchronization facilities should be diverse, and synchronization facilities within the same cable should be minimized.
4. The total number of nodes in series from the stratum 1 source should be minimized. For example, the primary synchronization network would ideally look like a star configuration with the stratum 1 source at the center. The nodes connected to the star would branch out in decreasing stratum level from the center
5. No timing loops may be formed in any combination of primary

Q. What is meant by “Plesiochronous” ?

If two digital signals are Plesiochronous, their transitions occur at “almost” the same rate, with any variation being constrained within tight limits. These limits are set down in ITU-T recommendation G.811. For example, if two networks need to interwork, their clocks may be derived from two different PRCs. Although these clocks are extremely accurate, there’s a small frequency difference between one clock and the other. This is known as a plesiochronous difference.

Q. What is meant by “Synchronous” ?

In a set of Synchronous signals, the digital transitions in the signals occur at exactly the same rate. There may however be a phase difference between the transitions of the two signals, and this would lie within specified limits. These phase differences may be due to propagation time delays, or low-frequency wander introduced in the transmission network. In a synchronous network, all the clocks are traceable to one Stratum 1 Primary Reference Clock (PRC).

Q. What is meant by “Asynchronous” ?

In the case of Asynchronous signals, the transitions of the signals don’t necessarily occur at the same nominal rate. Asynchronous, in this case, means that the difference between two clocks is much greater than a plesiochronous difference. For example, if two clocks are derived from free-running quartz oscillators, they could be described as asynchronous.

Q. What are the various steps in multiplexing ?