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Questions on WDM

​Some Questions on WDM....keep patience for the answers...arriving soon

  1. How to determine aggregate output power of a WDM?
  2. Spectral width wavelength to frequency conversion.
  3. Difference between CWDM and DWDM.
  4. What is Red band and Blue band?
  5. What is dark fibre and dim fibre?
  6. What are dark fibre considerations?
  7. How many types of optical fibres?
  8. Specifications of Multimode Fibre?
  9. How many types of Single mode Optical fibres.
  10. What are dispersion and attenuation coefficient of NDSF.
  11. If for same distance OTDR done and wavelength changed, will it show any difference in Losses?
  12. What is micro bending and macro bending?
  13. What is fibre characterization?
  14. Types of patch cord.                                                                                                                                                                                                                                                                                                      
  15. What is Linear and nonlinear impairments
  16. What is Stimulated Raman Scattering (SRS)?
  17. What is Stimulated Brillouin Scattering (SBS)?
  18. What is Four Wave Mixing (FWM)?
  19. What is Intensity Modulation (MI)?
  20. What is Self phase modulation (SPM)?
  21. What is Cross phase Modulation (CPM or XPM)?
  22. How to reduce FWM impact?
  23. What is impact of linear and nonlinear effects in DWDM network?
  24. What are 3R, 2R and 1R?
  25. What are types of Pluggable Optics?
  26. What is common optical specification of Optical Transceiver (Pluggable Optics or Transponder)?
  27. What is fixed optics?
  28. 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.[LinkedIn]
  29. Types of Receivers?
  30. Why Receiver Sensitivity is so important for optical module?
  31. How many types of Amplifiers used in DWDM network?
  32.  Raman Amplifier specification Vs. EDFA Specification.
  33. Difference between EDFA and Raman amplifier?
  34. Typical application of Raman amplifier.
  35. What is Dual or Double stage amplification?
  36. 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?
  37. What is Hybrid Amplification?
  38. What is difference between Pre- Amplifier and Post- Amplifier (Booster)?
  39. What is power control mode and Gain control mode?
  40. What is effect of Amplifier gain on OSNR while reducing or increasing flat gain? (Linkedin).
  41. What are Specifications of Optical Amplifiers?
  42. What is gain tilt and Gain ripple?
  43. How amplifier positioned in Bidirectional Single fiber DWDM network?
  44. How WDM Works?
  45. What are typical specifications of DWDM MUX?
  46. What is Maximum channel capacity of DWDM channel?
  47. What are key attributes for selecting OADM?
  48. What are optical specifications of OADM?
  49. What is CWDM?
  50. What is channel equalization? What is impact of non-uniform power spectrum?
  51. What are probable causes when among 10 channels; one transponder at remote end is getting power below set threshold?
  52. How equalization can be checked?
  53. What is PMD?
  54. What is DGD?
  55. Is there any correlation between chromatic dispersion and PMD?
  56. What is Linear and nonlinear impairments?                                                                                                                                                                                                                                            
  57. What is dispersion?
  58. What are the types of dispersion?
  59. What is Pilot Tone in WDM?
  60. How CD and PMD limits vary with channels and bitrate?
  61. What is FEC?
  62. How does FEC impact OSNR and BER value of a link?
  63. What is Q factor?
  64. Why we simulate Q rather BER?
  65. What is OSNR and what is its significance?
  66. How to improve OSNR of a link?
  67. At what point OSNR is max and min in a WDM link?
  68. What is OSC in DWDM link? What are the wavelengths used for it?
  69. What is the bandwidth of OSC signal?
  70. Where is OSC signal introduced in a DWDM link?
  71. What if OSC is inserted on MUX port?
  72.  Why 3dB is crucial factor in WDM and what does it mean?
  73. What is dead zone and why it is created in Photonic’s spectrum?
  74. What is centre frequency and how is it important?
  75. What is channel width slot width, channel spacing and centre frequency?
  76. What is the modulation techniques used in current for WDM transmission?
  77. What is the difference between transponder and muxponder ?
  78. What are the type of module used for transmitter and receiver in WDM link?
  79. Why EDFA is considered as best for commercial use?
  80. How RAMAN amplifier is used in the network?
  81. What is the frequency band for EDFA and RAMAN?
  82. Whether EDFA can be used in bidirectional WDM and if yes how?
  83. What is composite power and how it is measured?
  84. How to measure composite power when all input powers are different?
  85. How monitoring ports work on WDM cards?
  86. What are the causes and source of ASE generation in WDM link?
  87. Do we place attenuator with EDFA amplifiers and if yes where?
  88. Why we put DCM in between two AMP in a WDM link?
  89. What is the maximum and minimum value of OSNR for a link?
  90. What are the different types of amplifiers available?
  91. What is Channel flatness and how it is important?
  92. Why changing power of one channel affects whole band power or composite power?
  93. Can you please tell me what the difference between, dB and dBm when you are trying to test fiber optic cable.
  94. What is ORL value? How does it matters in a link?
  95. What is BOL and EOL and what is the margin between them?
  96. Why there is ALS system on AMPs or photonic systems?
  97. Why is it required to create adjacencies or logical connection between MUX and Transponder and other elements in WDM network?
  98. What is fiber fuse and what is the cause for the same?
  99. What is chirp and what is it’s effect and how it affect a link?
  100. Why Raman is not used in place of EDFA ?
  101. What is Q factor ? and how it is calculated ?
  102. Noise figure ?
  103. How OSA tester works and what are different options in the test instrument ?
  104. Commissioning of new OCH in the network?
  105. Link budgeting and commisioning parameters ?
  106. Braggs Grating & AWG difference?
  107. How to test OSNR using OSA for 10G, 40G & 100G channels ?
  108. Soft DECISION & Hard DECISION FEC?
  109. What is osnr threshold for 10 / 40/ 100G?
  110. what is the max no. OF repeater can be used in a link?
  111. How alarm propogate when there is no frame structure in dwdm?
  112. What are the general alarms observed in a DWDM network?
  113. What are the performance parameters for DWDM elements.
  114. How DCM is used in DWDM?
  115. Can we use DCM in 100 G network?
  116. What is the channel spacing for hybrid channels(10/40/100/200) coherent and non coherent?
  117. Will a single mode connector work on multi-mode cable?
  118. How an optical power meter differs from an OTDR.
  119. Is it possible to convert optical fiber signal into cat5 and if yes how?
  120. Why do some power meters have calibration at 1300 nm while others are 1310 nm?
  121. What is flexgrid network?
  122. What is difference between CDC networks?
  123. What is WSS based technology?
  124. What is ITL used for?
  125. Why WDM network doesnot requires external SYNC ?
  126. How Extended shelf communicates in rack ?
  127. What are the passive and active components available in a DWDM network?
  128. How different modulation techniques affect technical parameters in a link?
  129. What are the various graphs generally used for representing WDM performance for a network?
  130. What if very high power launch from transmitter/OTDR  with another end open.

Some Basic Interview Questions

  1. 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.
  2. 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.
  3. 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.
  4. 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.
  5. 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.
  6. 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?
  7. 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.
  8. 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.
  9. 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.
  10. 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.
  11. 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.
  12. 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.
  13. 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.
  14. 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.
  15. 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.
  16. 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.
  17. 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.
  18. 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.
  19. 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.
  20. 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.
  21. 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.
  22. 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 .
  23. 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.
  24. Why do you think you would do well at this job? - Give several reasons and include skills, experience and interest.
  25. 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.
  26. 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.
  27. 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
  28. 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.
  29. 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.
  30. 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.
  31. 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.
  32. 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
  33. Are you willing to work overtime? Nights? Weekends? - This is up to you. Be totally honest.
  34. 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
  35. 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.
  36. 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.
  37. 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.
  38. 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.
  39. 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.
  40. 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.
  41. Do you think you are overqualified for this position? - Regardless of your qualifications, state that you are very well qualified for the position.
  42. 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.
  43. 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.
  44. 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.
  45. What position do you prefer on a team working on a project? - Be honest. If you are comfortable in different roles, point that out.
  46. 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.
  47. 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.
  48. Tell me about the most fun you have had on the job. - Talk about having fun by accomplishing something for the organization.
  49. 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

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How to Answer Any Technical Interview Question

For most of us, technical interviews are the worst part of getting a new job: Not only does the interviewer ask challenging questions, but sometimes those questions don’t even make sense in the context of the job you want.

No matter how tough the question, however, remember that bluffing is rarely your best option. Instead of trying to fake an answer, honesty is usually best. Saying something like, “I am less familiar with [X skill], but I am familiar with [Y skill], and can share my thoughts in that context” can move the conversation forward in a productive way. (If a skill is important to the job and you really don’t have it, it’s always better to be honest from the beginning.)

Most technical questions are designed to reveal how you think, communicate, and solve problems. That means two candidates can arrive at the same (correct) solution to a problem, yet still be judged differently based on how they arrived at that answer.

Having sat on the other side of the table and interviewed over 700 software engineering candidates, I have some strong opinions on good and bad answers. I want to share with you the best strategy for handling technical questions, especially when you don’t know the answer.

Make Sure You Understand the Question

Sometimes when you are given a challenging question, it is difficult to answer because you don’t fully understand what the interviewer wants. (To be fair, sometimes interviewers are intentionally ambiguous, in order to get you to ask clarifying questions.) When faced with a hard question, verify you understand it.

For example, if you’re asked to “traverse a tree,” you really don’t know enough yet to solve the problem. What is the structure of the tree? Should you traverse it in a particular way? Ask questions to make sure you really understand what you’re trying to solve, before you actually start solving the question.

Work Through Examples

Once you understand the question, use any available examples to help solve the problem; examples allow you to pick up on patterns and generalizations that apply to the question at hand. You should work through basic, simple examples and avoid things that could be edge cases that could throw you astray (i.e. null, 0, 1, -1, etc.).

Come Up With the Easiest, Most Obvious Solution

Sure, we’d all like to come up with brilliant solutions to problems—but if you’re on a tight deadline, sometimes the most direct one is best. The latter also shows the interviewer that you can come up with a baseline solution, atop which you can further iterate.

For example, let’s say the interviewer asks you how many golf balls would fit inside a 747. (Yes, many employers still ask those sorts of crazy questions, just to see you work the problem.) Coming up with a quick solution, just to show you’re capable of thinking things through logically, also buys you time to further optimize it. Speaking of which…

Always Look for Improvements

Never leave a “bad” solution in place. Think about how you can refine it, hopefully while doing less work computationally. Can you store intermediate results so you don’t have to recompute them? Can you use other information to your advantage so you can do less work?

Arriving at an initial solution is a great point to ask more questions of the interviewer. Are there things you might be missing? Resources you should be taking advantage of? Most interviewers are interested in whether you can ask intelligent questions.

Work Through More Examples

If you get stuck, or aren’t seeing a better way, try working through more examples—pick smaller and bigger ones to see if you can notice new patterns that can help you solve the main question.

Don’t Give Up

If you get stuck, keep iterating through more examples, and thinking about how the problem at hand (hopefully) bears similarities to ones you’ve solved in the past. If you’re given a brainteaser that stumps you, consider asking the interviewer whether you can do a similar problem to which you actually know the answer.

Even if you never land on an ideal solution, showing that you are calm and resourceful under pressure is still a good thing to demonstrate to the hiring manager.

After you arrive at a solution, talk the interviewer through your work. If you had to figure out a solution while actually on the job, what would you do? How would you verify the answer was correct?

Do your best not to get frustrated or discouraged. (Given the length of job interviews these days, you may still have a long day ahead of you.) Take a deep breath and keep trying. Most interviewers will admire your tenacity, which is a great trait in any employee.

Follow Up

If you don’t get the best answer, make a note in your notebook (you brought something to take notes on during the interview, right?) and come back to it after the interview. Don’t be afraid to keep working on it and send in the right answer that evening if you figure it out. This may not help you get the job, but it will help you learn and become a better interviewee (and if you were close, it can help you get your foot over the edge into the door of a great new job).

The key to answering hard technical questions is to focus on what you know. Do you understand the question? Can you work through some examples? Can you come up with a solution you know would work (even if it isn’t optimal)? Hopefully this strategy helps you ace your next interview.

 

 

for more visit :http://news.dice.com/2015/02/05/answer-technical-interview-question/?icid=ON_DN_UP_JS_AV_OG_RA_3

TIMING FAQ

Q. How do I time SONET NEsshelves 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.

  1. 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

Great Answers to Tough Interview Questions

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 one­minute 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, self­confidence and reliability, with a few real­life 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. Object­oriented 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 self­improvement.

 

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 had­to 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 matter­of­factly. 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 self­employment 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.

 

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 put­together. If all else fails, ask the interviewer if they need any further clarification about your qualifications.

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.

SDH Q&A


 


 

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 ?

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????