In Technical 2 Mins Read

Power Change during add/remove of channels on filters

By MapYourTech August 24, 2016

Power Change during add/remove of channels on filters

The power change can be quantified as the ratio between the number of channels at the reference point after the channels are added or dropped and the number of channels at that reference point previously. We can consider composite power here and each channel at same optical power in dBm.

So whenever we add or delete number of channels from a MUX/DEMUX/FILTER/WSS following equations define the new changed power.

For the case when channels are added (as illustrated on the right side of Figure 1 ):

where:

A is the number of added channels

U is the number of undisturbed channels

For the case when channels are dropped (as illustrated on the left side of Figure 1):

where:

D is the number of dropped channels

U is the number of undisturbed channels

Figure 1

For example:

– adding 7 channels with one channel undisturbed gives a power change of +9 dB;

– dropping 7 channels with one channel undisturbed gives a power change of –9 dB;

– adding 31 channels with one channel undisturbed gives a power change of +15 dB;

– dropping 31 channels with one channel undisturbed gives a power change of –15 dB;

refer ITU-T G.680 for further study.

In Technical 1 Min Read

HD-FEC & SD-FEC differences

By MapYourTech August 11, 2016

Items	HD-FEC	SD-FEC
Definition	Decoding based on hard-bits(the output is quantized only to two levels) is called the “HD(hard-decision) decoding”, where each bit is considered definitely one or zero.	Decoding based on soft-bits(the output is quantized to more than two levels) is called the “SD(soft-decision) decoding”, where not only one or zero decision but also confidence information for the decision are provided.
Application	Generally for non-coherent detection optical systems, e.g., 10 Gbit/s, 40 Gbit/s, also for some coherent detection optical systems with higher OSNR	coherent detection optical systems, e.g., 100 Gbit/s,400 Gbit/s.
Electronics Requirement	ADC(Analogue-to-Digital Converter) is not necessary in the receiver.	ADC is required in the receiver to provide soft information, e.g., coherent detection optical systems.
specification	general FEC per [ITU-T G.975];super FEC per [ITU-T G.975.1].	vendor specific
typical scheme	Concatenated RS/BCH	LDPC(Low density parity check),TPC(Turbo product code)
complexity	medium	high
redundancy ratio	generally 7%	around 20%
NCG	about 5.6 dB for general FEC;>8.0 dB for super FEC.	>10.0 dB
Example(If you asked your friend about traffic jam status on roads and he replies)	maybe fully jammed or free	50-50 but I found othe way free or less traffic

In Interviews 1 Min Read

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

By MapYourTech June 2, 2016

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.

In Infographics 1 Min Read

What are main advantages and drawbacks of EDFAs?

By MapYourTech June 1, 2016

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.

In Technical 3 Mins Read

Invalid Data Flag or Suspect Interval in PM(Performance Monitoring) counters

By MapYourTech May 18, 2016

The ITU standards define a “suspect internal flag” which should indicate if the data contained within a register is ‘suspect’ (conditions defined in Q.822). This is more frequently referred to as the IDF (Invalid Data Flag).

PM is bounded by strict data collection rules as defined in standards. When the collection of PM parameters is affected then PM system labels the collection of data as suspect with an Invalid Data Flag (IDF). For the sake of identification; some unique flag is shown next to corresponding counter.

The purpose of the flag is to indicate when the data in the PM bin may not be complete or may have been affected such that the data is not completely reliable. The IDF does not mean the software is contingent.

Some of the common reasons for setting the IDF include:

a collection time period that does not start within +/- 1 second of the nominal collection window start time.
a time interval that is inaccurate by +/- 10 seconds (or more)
the current time period changes by +/- 10 seconds (or more)
a restart (System Controller restarts will wipe out all history data and cause time fluctuations at line/client module; a module restart will wipe out the current counts)
a PM bin is cleared manually
a hardware failure prevents PM from properly collecting a full period of PM data (PM clock failure)
a protection switch has caused a change of payload on a protection channel.
a payload reconfiguration has occurred (similar to above but not restricted to protection switches).
an System Controller archive failure has occurred, preventing history data from being collected from the line/client cards
protection mode is switched from non-revertive to revertive (affects PSD only)
a protection switch clear indication is received when no raise was indicated
laser device failure (affects physical PMs)
loss of signal (affects receive – OPRx, IQ – physical PMs only)
Control Plane is booted less than 15 min period for 15-min interval and less than 24 hour period for 24-hour interval.

Suspect interval is determined by comparing nSamples to nTotalSamples on a counter PM. If nSamples is not equal to nTotalSamples then this period can be marked as suspect.

If any 15 minute is marked as suspect or reporting for that day interval is not started at midnight then it should flag that 24 Hr as suspect.

Some of the common examples are:

Interface type is changed to another compatible interface (10G SR interface replaced by 10G DWDM interface),
Line type is changed from SONET to SDH,
Equipment failures are detected and those failures inhibit the accumulation of PM.
Transitions to/from the ‘locked’ state.
The System shall mark a given accumulation period invalid when the facility object is created or deleted during the interval.
Node time is changed.

In Technical 4 Mins Read

A short discussion on 980nm and 1480nm pump based EDFA

By MapYourTech May 14, 2016

A short discussion on 980nm and 1480nm pump based EDFA

Introduction

The 980nm pump needs three energy level for radiation while 1480nm pumps can excite the ions directly to the metastable level .

(a) Energy level scheme of ground and first two excited states of Er ions in a silica matrix. The sublevel splitting and the lengths of arrows representing absorption and emission transitions are not drawn to scale. In the case of the 4 I11/2 state, s is the lifetime for nonradiative decay to the I13/2 first excited state and ssp is the spontaneous lifetime of the 4 I13/2 first excited state. (b) Absorption coefficient, a, and emission coefficient, g*, spectra for a typical aluminum co-doped EDF.

The most important feature of the level scheme is that the transition energy between the I15/2 ground state and the I13/2 first excited state corresponds to photon wavelengths (approximately 1530 to 1560 nm) for which the attenuation in silica fibers is lowest. Amplification is achieved by creating an inversion by pumping atoms into the first excited state, typically using either 980 nm or 1480 nm diode lasers. Because of the superior noise figure they provide and their superior wall plug efficiency, most EDFAs are built using 980 nm pump diodes. 1480 nm pump diodes are still often used in L-band EDFAs although here, too, 980 nm pumps are becoming more widely used.

Though pumping with 1480 nm is used and has an optical power conversion efficiency which is higher than that for 980 nm pumping, the latter is preferred because of the following advantages it has over 1480 nm pumping.

It provides a wider separation between the laser wavelength and pump wavelength.
980 nm pumping gives less noise than 1480nm.
Unlike 1480 nm pumping, 980 nm pumping cannot stimulate back transition to the ground state.
980 nm pumping also gives a higher signal gain, the maximum gain coefficient being 11 dB/mW against 6.3 dB/mW for the 1.48
The reason for better performance of 980 nm pumping over the 1.48 m pumping is related to the fact that the former has a narrower absorption spectrum.
The inversion factor almost becomes 1 in case of 980 nm pumping whereas for 1480 nm pumping the best one gets is about 1.6.
Quantum mechanics puts a lower limit of 3 dB to the optical noise figure at high optical gain. 980 nm pimping provides a value of 3.1 dB, close to the quantum limit whereas 1.48 pumping gives a value of 4.2 dB.
1480nm pump needs more electrical power compare to 980nm.

Application

The 980 nm pumps EDFA’s are widely used in terrestrial systems while 1480nm pumps are used as Remote Optically Pumped Amplifiers (ROPA) in subsea links where it is difficult to put amplifiers.For submarine systems, remote pumping can be used in order not to have to electrically feed the amplifiers and remove electronic parts.Nowadays ,this is used in pumping up to 200km.

The erbium-doped fiber can be activated by a pump wavelength of 980 or 1480 nm but only the second one is used in repeaterless systems due to the lower fiber loss at 1.48 mm with respect to the loss at 0.98 mm. This allows the distance between the terminal and the remote amplifier to be increased.

In a typical configuration, the ROPA is comprised of a simple short length of erbium doped fiber in the transmission line placed a few tens of kilometers before a shore terminal or a conventional in-line EDFA. The remote EDF is backward pumped by a 1480 nm laser, from the terminal or in-line EDFA, thus providing signal gain

Vendors

Following are the vendors that manufactures 980nm and 1480nm EDFAs

In Technical 1 Min Read

Basics of Coherent System

By MapYourTech February 19, 2016

What Is Coherent Communication?

Definition of coherent light

A coherent light consists of two light waves that:

1) Have the same oscillation direction.

2) Have the same oscillation frequency.

3) Have the same phase or maintain a constant phase relationship with each other. Two coherent light waves produce interference within the area where they meet.

Principles of Coherent Communication

Coherent communication technologies mainly include coherent modulation and coherent detection.

Coherent modulation uses the signals that are propagated to change the frequencies, phases, and amplitudes of optical carriers. (Intensity modulation only changes the strength of light.)

Modulation detection mixes the laser light generated by a local oscillator (LO) with the incoming signal light using an optical hybrid to produce an IF signal that maintains the constant frequency, phase, and amplitude relationships with the signal light.

The motivation behind using the coherent communication techniques is two-fold.

First, the receiver sensitivity can be improved by up to 20 dB compared with that of IM/DD systems.

Second, the use of coherent detection may allow a more efficient use of fiber bandwidth by increasing the spectral efficiency of WDM systems

In Technical 2 Mins Read

Method to Resolve the Adjacent-Channel Interference in 100G Transmission.

By MapYourTech February 19, 2016

In a non-coherent WDM system, each optical channel on the line side uses only one binary channel to carry service information. The service transmission rate on each optical channel is called bit rate while the binary channel rate is called baud rate. In this sense, the baud rate was equal to the bit rate. The spectral width of an optical signal is determined by the baud rate. Specifically, the spectral width is linearly proportional to the baud rate, which means a higher baud rate generates a larger spectral width.

Baud (pronounced as /bɔ:d/ and abbreviated as “Bd”) is the unit for representing the data communication speed. It indicates the signal changes occurring in every second on a device, for example, a modulator-demodulator (modem). During encoding, one baud (namely, the signal change) actually represents two or more bits. In the current high-speed modulation techniques, each change in a carrier can transmit multiple bits, which makes the baud rate different from the transmission speed.

In practice, the spectral width of the optical signal cannot be larger than the frequency spacing between WDM channels; otherwise, the optical spectrums of the neighboring WDM channels will overlap, causing interference among data streams on different WDM channels and thus generating bit errors and a system penalty.

For example, the spectral width of a 100G BPSK/DPSK signal is about 50 GHz, which means a common 40G BPSK/DPSK modulator is not suitable for a 50 GHz channel spaced 100G system because it will cause a high crosstalk penalty. When the baud rate reaches 100 Gbaud/s, the spectral width of the BPSK/DPSK signal is greater than 50 GHz. Thus, it is impossible to achieve 50 GHz channel spacing in a 100G BPSK/DPSK transmission system.

(This is one reason that BPSK cannot be used in a 100G coherent system. The other reason is that high-speed ADC devices are costly.)

A 100G coherent system must employ new technology. The system must employ more advanced multiplexing technologies so that an optical channel contains multiple binary channels. This reduces the baud rate while keeping the line bit rate unchanged, ensuring that the spectral width is less than 50 GHz even after the line rate is increased to 100 Gbit/s. These multiplexing technologies include quadrature phase shift keying (QPSK) modulation and polarization division multiplexing (PDM).

In Technical 3 Mins Read

Measuring Coherent System OSNR using integral method

By MapYourTech February 19, 2016

For coherent signals with wide optical spectrum, the traditional scanning method using an OSA or inband polarization method (EXFO) cannot correctly measure system OSNR. Therefore, use the integral method to measure OSNR of coherent signals.

Perform the following operations to measure OSNR using the integral method:

1.Position the central frequency of the wavelength under test in the middle of the screen of an OSA.

2.Select an appropriate bandwidth span for integration (for 40G/100G coherent signals, select 0.4 nm).

3.Read the sum of signal power and noise power within the specified bandwidth. On the OSA, enable the Trace Integ function and read the integral value. As shown in Figure 2, the integral optical power (P + N) is 9.68 uW.

4.Read the integral noise power within the specified bandwidth. Disable the related laser before testing the integral noise power. Obtain the integral noise power N within the signal bandwidth specified in step 2. The integral noise power (N) is 29.58 nW.

5.Calculate the integral noise power (n) within the reference noise bandwidth. Generally, the reference noise bandwidth is 0.1 nm. Read the integral power of central frequency within the bandwidth of 0.1 nm. In this example, the integral noise power within the reference noise bandwidth is 7.395 nW.

6.Calculate OSNR. OSNR = 10 x lg{[(P + N) – N]/n}

In this example, OSNR = 10 x log[(9.68 – 0.02958)/0.007395] = 31.156 dB

We follow integral method because Direct OSNR Scanning Cannot Ensure Accuracy because of the following reason:

A 40G/100G signal has a larger spectral width than a 10G signal. As a result, the signal spectrums of adjacent channels overlap each other. This brings difficulties in testing the OSNR using the traditional OSA method, which is implemented based on the interpolation of inter-channel noise that is equivalent to in-band noise. Inter-channel noise power contains not only the ASE noise power but also the signal crosstalk power. Therefore, the OSNR obtained using the traditional OSA method is less than the actual OSNR. The figure below shows the signal spectrums in hybrid transmission of 40G and 10G signals with 50 GHz channel spacing. As shown in the figure, a severe spectrum overlap has occurred and the tested ASE power is greater than it should be .As ROADM and OEQ technologies become mature and are widely used, the use of filter devices will impair the noise spectrum. As shown in the following figure, the noise power between channels decreases remarkably after signals traverse a filter. As a result, the OSNR obtained using the traditional OSA method is greater than the actual OSNR..

In Technical 2 Mins Read

Basic understanding on Tap ratio for Splitter/Coupler

By MapYourTech February 17, 2016

Basic understanding on Tap ratio for Splitter/Coupler

Fiber splitters/couplers divide optical power from one common port to two or more split ports and combine all optical power from the split ports to one common port (1 × N coupler). They operate across the entire band or bands such as C, L, or O bands. The three port 1 × 2 tap is a splitter commonly used to access a small amount of signal power in a live fiber span for measurement or OSA analysis. Splitters are referred to by their splitting ratio, which is the power output of an individual split port divided by the total power output of all split ports. Popular splitting ratios are shown in Table below; however, others are available. Equation below can be used to estimate the splitter insertion loss for a typical split port. Excess splitter loss adds to the port’s power division loss and is lost signal power due to the splitter properties. It typically varies between 0.1 to 2 dB, refer to manufacturer’s specifications for accurate values. It should be noted that splitter function is symmetrical.

where IL = splitter insertion loss for the split port, dB

Pi = optical output power for single split port, mW

PT = total optical power output for all split ports, mW

SR = splitting ratio for the split port, %

Γe = splitter excess loss (typical range 0.1 to 2 dB), dB

Common splitter applications include

• Permanent installation in a fiber link as a tap with 2%|98% splitting ratio. This provides for access to live fiber signal power and OSA spectrum measurement without affecting fiber traffic. Commonly installed in DWDM amplifier systems.

• Video and CATV networks to distribute signals.

• Passive optical networks (PON).

• Fiber protection systems.

Example with calculation:

If a 0 dBm signal is launched into the common port of a 25% |75% splitter, then the two split ports, output power will be −6.2 and −1.5 dBm. However, if a 0 dBm signal is launched into the 25% split port, then the common port output power will be −6.2 dBm.

Calculation.

Launch power=0 dBm =1mW

Tap is 25%|75%

so equivalent mW power which is linear will be

0.250mW|0.750mW

and after converting them ,dBm value will be

-6.02dBm| -1.24dBm

Some of the common split ratios and their equivalent Optical Power is available below for reference.

In Technical 4 Mins Read

Q and dBQ : A Walkthrough

By MapYourTech January 13, 2016

Q is the quality of a communication signal and is related to BER. A lower BER gives a higher Q and thus a higher Q gives better performance. Q is primarily used for translating relatively large BER differences into manageable values.

Pre-FEC signal fail and Pre-FEC signal degrade thresholds are provisionable in units of dBQ so that the user does not need to worry about FEC scheme when determining what value to set the thresholds to as the software will automatically convert the dBQ values to FEC corrections per time interval based on FEC scheme and data rate.

The Q-Factor, is in fact a metric to identify the attenuation in the receiving signal and determine a potential LOS and it is an estimate of the Optical-Signal-to-Noise-Ratio (OSNR) at the optical receiver. As attenuation in the receiving signal increases, the dBQ value drops and vice-versa. Hence a drop in the dBQ value can mean that there is an increase in the Pre FEC BER, and a possible LOS could occur if the problem is not corrected in time.

The Quality of an Optical Rx signal can be measured by determining the number of “bad” bits in a block of received data. The bad bits in each block of received data are removed and replaced with “good” zero’s or one’s such that the network path data can still be properly switched and passed on to its destination. This strategy is referred to as Forward Error Correction (FEC) and prevents a complete loss of traffic due to small un-important data-loss that can be re-sent again later on. The process by which the “bad” bits are replaced with the “good” bits in an Rx data block is known as Mapping. The Pre FEC are the FEC Counts of “bad” bits before the Mapper and the FEC Counts (or Post FEC Counts) are those after the Mapper.

The number of Pre FEC Counts for a given period of time can represent the status of the Optical Rx network signal; An increase in the Pre FEC count means that there is an increase in the number of “bad” bits that need to be replaced by the Mapper. Hence a change in rate of the Pre FEC Count (Bit Erro Rate – BER) can identify a potential problem upstream in the network. At some point the Pre FEC Count will be too high as there will be too many “bad” bits in the incoming data block for the Mapper to replace … this will then mean a Loss of Signal (LOS).

As the normal number of Pre FEC Counts are high (i.e. 1.35E-3 to 6.11E-16) and constantly fluctuate, it can be difficult for an network operator to determine whether there is a potential problem in the network. Hence a dBQ value, known as the Q-Factor, is used as a measure of the Quality of the receiving optical signal. It should be consistent with the Pre FEC Count Bit Error Rate (BER).

The standards define the Q-Factor as Q = 10log[(X1 – X0)/(N1 – N0)] where Xj and Nj are the mean and standard deviation of the received mark-bit (j=1) and space-bit (j=0) ……………. In some cases Q = 20log[(X1 – X0)/(N1 – N0)]

For example, the linear Q range 3 to 8 covers the BER range of 1.35E-3 to 6.11E-16.

Nortel defines dBQ as 10xlog10(Q/Qref) where Qref is the pre-FEC raw optical Q, which gives a BER of 1E-15 post-FEC assuming a particular error distribution. Some organizations define dBQ as 20xlog10(Q/Qref), so care must be taken when comparing dBQ values from different sources.

The dBQ figure represents the dBQ of margin from the following pre-FEC BERs (which are equivalent to a post-FEC BER of 1E-15). The equivalent linear Q value for these BERs are Qref in the above formula.

Pre-FEC signal degrade can be used the same way a car has an “oil light” in that it states that there is still margin left but you are closer to the fail point than expected so action should be taken.

In Standards 2 Mins Read

What an OTDR can do for you?

By MapYourTech September 30, 2014

The Optical Time Domain Reflectometer (OTDR) is useful for testing the integrity of fiber optic cables. An optical time-domain reflectometer (OTDR) is an optoelectronic instrument used to characterize an optical fiber. An OTDR is the optical equivalent of an electronic time domain reflectometer. It injects a series of optical pulses into the fiber under test. It also extracts, from the same end of the fiber, light that is scattered (Rayleigh backscatter) or reflected back from points along the fiber. The strength of the return pulses is measured and integrated as a function of time, and plotted as a function of fiber length.

Using an OTDR, we can:

1. Measure the distance to a fusion splice, mechanical splice, connector, or significant bend in the fiber.

2. Measure the loss across a fusion splice, mechanical splice, connector, or significant bend in the fiber.

3. Measure the intrinsic loss due to mode-field diameter variations between two pieces of single-mode optical fiber connected by a splice or connector.

4. Determine the relative amount of offset and bending loss at a splice or connector joining two single-mode fibers.

5. Determine the physical offset at a splice or connector joining two pieces of single-mode fiber, when bending loss is insignificant.

6. Measure the optical return loss of discrete components, such as mechanical splices and connectors.

7. Measure the integrated return loss of a complete fiber-optic system.

8. Measure a fiber’s linearity, monitoring for such things as local mode-field pinch-off.

9. Measure the fiber slope, or fiber attenuation (typically expressed in dB/km).

10. Measure the link loss, or end-to-end loss of the fiber network.

11. Measure the relative numerical apertures of two fibers.

12. Make rudimentary measurements of a fiber’s chromatic dispersion.

13. Measure polarization mode dispersion.

14. Estimate the impact of reflections on transmitters and receivers in a fiber-optic system.

15. Provide active monitoring on live fiber-optic systems.

16. Compare previously installed waveforms to current traces.

In Standards 1 Min Read

OTN maintenance signal interaction

By MapYourTech September 27, 2014

The maintenance signals defined in [ITU-T G.709] provide network connection status information in the form of payload missing indication (PMI), backward error and defect indication (BEI, BDI), open connection indication (OCI), and link and tandem connection status information in the form of locked indication (LCK) and alarm indication signal (FDI, AIS).

Interaction diagrams are collected from ITU G.798 and OTN application note from IpLight

In Standards 4 Mins Read

How OTN supercedes over SDH/SONET

By MapYourTech June 24, 2014

Here we will discuss what are the advantages of OTN(Optical Transport Network) over SDH/SONET.

The OTN architecture concept was developed by the ITU-T initially a decade ago, to build upon the Synchronous Digital Hierarchy (SDH) and Dense Wavelength-Division Multiplexing (DWDM) experience and provide bit rate efficiency, resiliency and management at high capacity. OTN therefore looks a lot like Synchronous Optical Networking (SONET) / SDH in structure, with less overhead and more management features.

It is a common misconception that OTN is just SDH with a few insignificant changes. Although the multiplexing structure and terminology look the same, the changes in OTN have a great impact on its use in, for example, a multi-vendor, multi-domain environment. OTN was created to be a carrier technology, which is why emphasis was put on enhancing transparency, reach, scalability and monitoring of signals carried over large distances and through several administrative and vendor domains.

The advantages of OTN compared to SDH are mainly related to the introduction of the following changes:

Transparent Client Signals:

In OTN the Optical Channel Payload Unit-k (OPUk) container is defined to include the entire SONET/SDH and Ethernet signal, including associated overhead bytes, which is why no modification of the overhead is required when transporting through OTN. This allows the end user to view exactly what was transmitted at the far end and decreases the complexity of troubleshooting as transport and client protocols aren’t the same technology.

OTN uses asynchronous mapping and demapping of client signals, which is another reason why OTN is timing transparent.

Better Forward Error Correction:

OTN has increased the number of bytes reserved for Forward Error Correction (FEC), allowing a theoretical improvement of the Signal-to-Noise Ratio (SNR) by 6.2 dB. This improvement can be used to enhance the optical systems in the following areas:

Increase the reach of optical systems by increasing span length or increasing the number of spans.
Increase the number of channels in the optical systems, as the required power theoretical has been lowered 6.2 dB, thus also reducing the non- linear effects, which are dependent on the total power in the system.
The increased power budget can ease the introduction of transparent optical network elements, which can’t be introduced without a penalty. These elements include Optical Add-Drop Multiplexers (OADMs), Photonic Cross Connects (PXCs), splitters, etc., which are fundamental for the evolution from point-to-point optical networks to meshed ones.
The FEC part of OTN has been utilised on the line side of DWDM transponders for at least the last 5 years, allowing a significant increase in reach/capacity.

Better scalability:

The old transport technologies like SONET/SDH were created to carry voice circuits, which is why the granularity was very dense – down to 1.5 Mb/s. OTN is designed to carry a payload of greater bulk, which is why the granularity is coarser and the multiplexing structure less complicated.

Tandem Connection Monitoring:

The introduction of additional (six) Tandem Connection Monitoring (TCM) combined with the decoupling of transport and payload protocols allow a significant improvement in monitoring signals that are transported through several administrative domains, e.g. a meshed network topology where the signals are transported through several other operators before reaching the end users.

In a multi-domain scenario – “a classic carrier’s carrier scenario”, where the originating domain can’t ensure performance or even monitor the signal when it passes to another domain – TCM introduces a performance monitoring layer between line and path monitoring allowing each involved network to be monitored, thus reducing the complexity of troubleshooting as performance data is accessible for each individual part of the route.

Also a major drawback with regards to SDH is that a lot of capacity during packet transport is wasted in overhead and stuffing, which can also create delays in the transmission, leading to problems for the end application, especially if it is designed for asynchronous, bursty communications behavior. This over-complexity is probably one of the reasons why the evolution of SDH has stopped at STM 256 (40 Gbps).

References: OTN and NG-OTN: Overview by GEANT