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Single-mode fibre selection for Optical Communication System.....

 This is collected from article written by Mr.Joe Botha
Looking for a single-mode (SM) fibre to light-upyour multi-terabit per second system? Probably not, but let’s say you were - the smart money is on your well-intended fibre sales rep instinctively flogging you ITU-T G.652D  fibre. Commonlyreferred to as standardSM fibre and also known as Non-Dispersion-Shifted Fibre (NDSF) - the oldestand most widelydeployed fibre. Not a great choice,right? You bet.  So for now, let’s resist the notionthat you can do whatever-you-want using standardSM fibre. A varietyof  SM optical  fibres with  carefully optimisedcharacteristics are  available  commercially: ITU-T G.652, 653, 654, 655, 656 or 657 compliant.
Designs of SM fibre have evolvedover the decadesand present-day optionswould have us deploy G.652D, G.655 or G.656 compliantfibres. Note that G.657A is essentially a more expressive version of G.652D,with a superiorbending loss performance and should you start feeling a little benevolent towards deploying it on a longish-haul - I  can immediately confirm that this allows for a glimpse into the workingsof silliness. Dispersion Shifted Fibre (DSF) in accordance with G.653 has no chromatic dispersion at 1550 nm. However,they are limited to single-wavelength operation due to non-linear four-wave mixing. G.654 compliant fibres were developed specifically for underseaun-regenerated systems and since our focus is directed toward terrestrial applications - let’s leave it at that.
In the above context, the plan is to briefly weigh up G.652D,G.655 and G.656 compliantfibres against three parameters we calculate (before installation) and measure (after installation). I must just point-out that the fibre coefficients used are what one would expect from the not too shabby brands availabletoday.

Attenuation

G.652D compliant
G.655 compliant
G.656 compliant
λ
nm
ATTN
dB/km
λ
nm
ATTN
dB/km
λ
nm
ATTN
dB/km
1310
0.30
1310

1310

1550
0.20
1550
0.18
1550
0.20
1625
0.23
1625
0.20
1625
0.22

 
Attenuation is the reduction or loss of opticalpower as  light travels through an opticalfibre and is measured in decibelsper kilometer (dB/km). G.652D offers respectable attenuation coefficients, when compared with G.655 and G.656. It should be remembered, however, that even a meagre 0.01 dB/km attenuation

improvement would reduce a 100 km loss budget by a full dB - but let's not quibble. No attenuation coefficients for G.655 and G.656 at 1310? It was not, as you may immediately assume, an oversight. Both G.655 and G.656 are optimizedto support long-haul systems and thereforecould not care less about runningat 1310 nm. A cut-offwavelength is the minimum wavelength at which a particular fibre will support SM transmission. At 1260 nm, G.652 D has the lowest cut-off wavelength - with the cut-off wavelengths for G.655 and G.656 sittingat 1480 nmand 1450 respectively - which explainswhy we have no attenuation coefficient for them at 1310 nm.

PMD

G.652D compliant
G.655 compliant
G.656 compliant
PMD
ps / √km
PMD
ps / √km
PMD
ps / √km
0.06
0.04
0.04

 
Polarization-mode dispersion (PMD) is  an  unwanted effect caused by asymmetrical properties in an opticalfibre that spreads the optical pulse of a signal. Slight asymmetry in an

optical fibre causes the polarized modes of the light pulse to travel at marginally different speeds, distorting the signal and is reportedin ps / km, or "ps per root km". Oddly enough,G.652 and co all possess decent-looking PMD coefficients. Now then, popping a 40-Gbpslaser onto my fibre up againstan ultra-low 0.04 ps / km, my calculator reveals that the PMD coefficient admissible fibre length is 3,900 km and even at 0.1 ps / km, a distance of 625 km is achievable.
So far so good? But wait, there's more. PMD is particularly troublesome for both high data-rate-per-channel and high wavelength channel count systems, largely because of its random nature.Fibre manufacturer’s PMD specifications are accuratefor the fibre itself,but do not incorporate PMD incurred as a result of installation, which in many cases can be many orders of magnitude larger. It is hardly surprising that questionable installation practices are  likely  to  cause imperfect fibre symmetry - the obvious implications are incomprehensible data streams and mental anguish.Moreover, PMD unlikechromatic dispersion (to be discussednext) is also affectedby environmental conditions, making it unpredictable and extremelydifficult to find ways to undo or offset its effect.


CD

652D compliant
G.655 compliant
G.656 compliant
λ
nm
CD
ps/(nm·km)
λ
nm
CD
ps/(nm·km)
λ
nm
CD
ps/(nm·km)
1550
18
1550
2~6
1550
6.0~10
1625
22
1625
8~11
1625
8.0~13

 
CD (calledchromatic dispersion to emphasiseits wavelength-dependent nature) has zip-zero to do with the loss of light. It occurs because different wavelengths  of light travel at differentspeeds. Thus, when the allowable

CD is exceeded - light pulses representing a bit-stream will be renderedillegible. It is expressedin ps/ (nm·km). At 2.5- Gbps CD is not an issue - however, lower data rates are seldom desirable. But at 10-Gbps,it is a big issue and the issue gets even bigger at 40-Gbps.
What’s troubling is G.652D’shigh CD coefficient - which one glumly has to concede, is very poor next  to  the competition. G.655 and G.656, variants of non-zerodispersion-shifted fibre (NZ-DSF), comprehensively address G.652D’s shortcomings. It should be noted that nowadays some optical fibre manufacturers don’t bother with distinguishing between G.655 and G.656 - referring to their offerings as G.655/6 compliant.
On the face of it, one might suggest that the answer to our CD problemis to send light along an optical fibre at a wavelength where the CD is zero (i.e. G.653).The result? It turns out that this approach creates more problems than it is likely to solve - by unacceptably amplifying non-linear four-wave mixing and limiting the fibre to single-wavelength operation- in other words, no DWDM. That, in fact, is why CD should not be completely lampooned. Research revealed that the fibre-friendly CD value l i e s in the range of 6-11 ps/nm·km. Therefore, and particularly for high-capacity transport, the best-suited fibre is one in which dispersion is kept within a tight range, being neither too high nor too low.
NZ-DSFs are available in both positive(+D) and negative (-D) varieties. Using NZ-DSF -D, a reverse behavior of the velocity per wavelength is createdand therefore, the effect of +CD can be cancelled out. I almost forgot to mention,by the way, that short wavelengths travel faster than long ones with +CD and longer wavelengths travel faster than short ones with -CD. New sophisticated modulation techniques such as dual-polarized quadrature phase-shift keying (DP-QPSK) using coherent detection, yields high quality CD compensation. However, because of the addedsignal processing time (versussimple on-off keying) they require,this can potentially be a poor choice from a latency perspective.
WDM multiplies capacity
The use of Dense Wavelength Division Multiplexing (DWDM) technology and 40-Gbps(and higher) transmission rates can push the information-carrying capacityof a single fibre to well over a terabit per second.One example is EASSy’s (a 4-fibre submarine cable serving sub-Saharan Africa) 4.72-Tbps capacity. Now then, should my maffs prove to be correct, 118 x 40-Gbpslasers (popped onto only 4-fibres!) should give us an aggregatecapacity of 4.72-Tbps?
Coarse Wavelength Division Multiplexing (CWDM) is a WDM technology that uses 4, 8, 16 or 18 wavelengths for transmission. CWDM is an economically sensible option, often used for short-haul applications on G.652D,where signal amplification is not necessary. CWDMs large 20 nm channelspacing allows for the use of cheaper, less powerfullasers that do not require cooling.
One of the most important considerations in the fibre selectionprocess is the fact that optical signals may need to be amplified along a route. Thanks in no small part (get the picture?)to CWDM’s large channelspacing - typicallyspanning over several spectralbands (1270 nm to 1610 nm) - its signals cannot be amplifiedusing Erbium Doped-Fibre Amplifiers (EDFAs).You see, EDFAs run only in the C and L bands (1520 nm to 1625 nm). WhereasCWDM breaks the optical spectrum up into large chunks - by contrast,DWDM slices it up finely, cramming4, 8, 16, 40, 80, or 160 wavelengths (on 2-fibres)into only the C- and L-bands (1520nm to 1625nm) - perfectfor the use of EDFAs. Each wavelength can without any obvious effort support a 40-Gbps laser and on top of this, 100-Gbpslasers are chompingat the bit to go mainstream.

Making the right choice

On the whole, it is hard not to concludethat the only thing that genuinelyseparates fibre types for high-bit-rate systems is CD. The 3-things - the only ones that I can think of - that is good about G.652D - is that it is affordable, cool for CWDM and perfect for short-haul environments.Top  of the to-do lists of infrastructure providers pushing the boundaries of DWDM enabled ultra high-capacity transport over short, long or ultra long-haul networks - needlessto say, will be to source G.655/6compliant fibres. The cross-tabbelow indicates: Green for OK and oddly enough, Red for Not-OK

ITU-T Compliant
10-Gbps CWDM
40-Gbps CWDM
10-Gbps DWDM
40-Gbps DWDM
100-Gbps DWDM
G.652





G.655/6







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Comparing FWM-CPM-and-SPM non linearity in Optical Fiber in DWDM networks.

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FEC-Forward Error Correction for Optics Professionals

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What is Gridless Networking?

As the industry continues to push the boundaries of optical transmission speeds, the concept of “Gridless Networking” has emerged as a key requirement for tomorrow’s flexible and dynamic transport networks.  What is gridless networking?  Brian Lavallée, who heads up submarine networks industry marketing for Ciena, provides some insight.
Current optical transmission systems rely on fixed grid filtering to carve the available bandwidth into a number of channels that can be used to carry traffic. The filtering can be achieved using passive filters, active configurable MUX/DeMUX elements (e.g. Wavelength Selective Switch) or a combination of passive and configurable elements. The traditional terrestrial grid spacing is typically on theITU 50GHz or 100GHz grids. A tighter grid spacing of 25GHz or 33GHz is also used today, such as submarine application systems.
The goal is simple, squeeze as much information as possible into the available light spectrum on the fiber to improve the “spectral efficiency” of the optical network and achieve economies of scale.  Figure 1 illustrates the difference between a fixed grid spectrum and a gridless spectrum, where the latter squeezes the channels as close together as possible for maximum spectral efficiency.
Why do we want maximum spectral efficiency in the first place? To leverage a single line system and lower overall network costs.
The future of optical networks
As optical transmission technologies evolve, the bandwidth required for each optical channel, and the optimal spacing between them, will also evolve.  This is because some optical transmission technologies may require more or less channel space, or spectrum, than others. For instance, a 1Tb/s (1 Terabit) super channel will likely require more optical bandwidth than a 10Gb/s channel as they co-propagate down the optical line system. Today’s common use of fixed channel filter sizes can also lead to non-optimal channel spacing, resulting in potential “wasted” bandwidth and a lower resultant spectral efficiency.  As a result, the use of a gridless network will not only enable newer and higher bandwidth channels, but will allow more efficient spacing of today’s existing channels such as 10G.
Gridless Multiplexing
The WSS (Wavelength Selective Switch) can be used as the active MUX/DeMUX to enable remote control and automation of wavelength routing within a fiber plant. The WSS is a fixed grid product on either a 50GHz or 100GHz grid. To enable gridless transmission while still retaining remote network control and automation, a new type of active MUX/DeMUX is required and is referred to as an FSS (Flexible Spectrum Switch), which allows for variable channel spacing and the elimination of wasted bandwidth. In other words, various optical channel speeds can coexist, utilizing maximum spectral efficiency, on the same optical fiber network. This will achieve economies of scale and lower the overall network cost.  The deployment of the FSS works best in combination with the application of coherent detection, effectively reproducing the electrical baseband filtering centered exactly on the optical carrier.
The use of gridless technology results in the ability to provide flexible DWDM filtering for advanced modulation formats, such coherent detection. By being able to handle various bandwidth sizes per channel, this opens up the optical network for advances in modulation formats, as the network is no longer constrained by traditional fixed optical line filtering. The evolution of the transmission systems from today’s fixed grid structure to tomorrow’s gridless configuration allows the optical system to use channel bandwidth and pre-FEC error rates to dynamically select the best channel spacing while simultaneously eliminating potentially wasted bandwidth for optimized spectral efficiency.
The end result is a truly dynamic and intelligent optical transport infrastructure that can adapt to new optical technologies and bandwidth demands while ensuring the most efficient use of fiber spectrum as possible.

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Evolution to flexible grid WDM

Evolution to flexible grid WDM


WDM networks operate by transmitting multiple wavelengths, or channels, over a fiber simultaneously. Each channel is assigned a slightly different wavelength, preventing interference between channels. Modern DWDM networks typically support 88 channels, with each channel spaced at 50 GHz, as defined by industry standard ITU G.694. Each channel is an independent wavelength.
The fixed 50-GHz grid pattern has served WDM networks and the industry well for many, many years. It helps carriers easily plan their services, capacity, and available spare capacity across their WDM systems. In addition, the technology used to add and drop channels on a ROADM network is based on arrayed-waveguide-grating (AWG) mux/demux technology, a simple and relatively low-cost technique particularly well suited to networks based on 50-GHz grid patterns.
FIGURE 1. The ITU's fixed 50-GHz grid and its 100-GHz variant form the foundation for today's optical networks.
WDM networks currently support optical rates of 10G, 40G, and 100G per wavelength (with the occasional 2.5G still popping up), all of which fit within existing 50-GHz channels. In the future, higher-speed 400-Gbps and 1-Tbps optical rates will be deployed over optical networks. These interfaces beyond 100G require larger channel sizes than used on current WDM networks. The transition to these higher optical rates is leading to the adoption of a new, flexible grid pattern capable of supporting 100G, 400G, and 1T wavelengths.

Current generation

The fixed 50-GHz grid pattern specified by ITU standards is shown in Figure 1. Any 10G, 40G, or 100G optical service can be carried over any of the 50-GHz channels, which enables carriers to mix and match service rates and channels as needed on their networks.
A look inside each channel reveals some interesting differences between the optical rates and resulting efficiency of the optical channel (see Figure 2). A 10G optical signal easily fits within the 50-GHz-channel size, using about half the available spectrum. The remaining space within the 50-GHz channel is unused and unavailable. Meanwhile, the 40G and 100G signals use almost the entire 50-GHz spectrum.
FIGURE 2. Optical rates and their spectral efficiency.
Spectral efficiency is one measure of how effectively or efficiently a fiber network transmits information and is calculated as the number of bits transmitted per Hz of optical spectrum. With 10G wavelengths the spectral efficiency is only 0.2 bit/Hz, while the 100G wavelength provides a 10X improvement in spectral efficiency to 2 bits/Hz. The more bits that can be transmitted per channel, the greater the improvement in spectral efficiency and increase in overall network capacity and the lower the cost per bit of optical transport.
While 100G wavelengths are becoming more common, carriers are already planning for higher-speed 400G and 1T channels on their future ROADM networks, with the expectation that spectral efficiency will at least remain the same, if not improve. New ways of allocating bandwidth will be needed to meet these expectations.

Superchannels

As mentioned, WDM networks currently transmit each 10G, 40G, and 100G optical signal as a single optical carrier that fits within a standard 50-GHz channel. At higher data rates, including 400G and 1T, the signals will be transmitted over multiple subcarrier channels (see Figure 3). The group of subcarrier wavelengths is commonly referred to as a "superchannel." Although composed of individual subcarriers, each 400G superchannel is provisioned, transmitted, and switched across the network as a single entity or block.
FIGURE 3. 400G modulation options and superchannels.
While 400G standards are still in preliminary definition stage, two modulation techniques are emerging as the most likely candidates: dual polarization quadrature phase-shift keying (DP-QPSK) using four subcarriers and DP-16 quadrature amplitude modulation (QAM) with two subcarriers. Due to the differences in optical signal-to-noise-ratio requirements, each modulation type is optimized for different network applications. The 4×100G DP-QPSK approach is better suited to long-haul networks because of its superior optical reach, while the 2×200G DP-16QAM method is ideal for metro distances.
Since 400G signals are treated as a single superchannel or block, the 400G signals shown in Figure 3 require 150-GHz- and 75-GHz- channel sizes, respectively. It's this transition to higher data rates that leads to the requirement for and adoption of new flexible grid channel assignments to accommodate mixed 100G, 400G, and 1T networks (see Figure 4).
A new flexible grid pattern has been defined and adopted by ITU G694.1. While commonly referred to as "gridless" channel spacing, in reality the newly defined flexible channel plan is actually based on a 12.5-GHz grid pattern. The new standard supports mixed channel sizes, in increments of n×12.5 GHz and easily accommodates existing 100G services (4×12.5 GHz = 50 GHz) and future 400G (12×12.5 GHz) and 1T optical rates.
FIGURE 4. Flexible grid channel plan.
One of the advantages of the flexible grid pattern is the improvement in spectral efficiency enabled by more closely matching the channel size with the signals being transported and by improved filtering that allows the subcarriers to be more closely squeezed together. As shown in Figure 5, four 100G subcarriers have been squeezed into 150-GHz spacing, as opposed to the 200 GHz (4×50 GHz) required if the subcarriers were transported as independent 50-GHz channels. The net effect of the flexible channel plan and closer subcarrier spacing is an improvement in network capacity of up to 30%.
One common "myth" in the industry is that legacy networks must be upgraded, or "flexible grid-ready," to support 400G optical rates and superchannels. While having flexible grid-capable ROADMs can improve spectral efficiency, they're not a requirement to support 400G or superchannels on a network. Since the subcarriers are fully tunable to any wavelength, they can simply be tuned to the existing 50-GHz grid pattern, allowing full backward compatibility with existing ROADM networks.
FIGURE 5. Transmitting 400G on legacy WDM networks.

CDC ROADMs

Closely associated with flexible grid channel spacing are colorless/directionless/gridless (CDG) and colorless/directionless/contentionless/gridless (CDCG) ROADM architectures. Along with gridless channel spacing, CDC ROADMs enable a great deal of flexibility at the optical layer.
Recall that existing ROADM systems are based on fixed 50-GHz-channel spacing and AWG mux/demux technology. The mux/demux combines and separates individual wavelengths into different physical input and output ports. While the transponder and muxponder themselves are full-band tunable and can be provisioned to any transmit wavelength, they must be connected to a specific port on the mux/demux unit. A transponder connected to the west mux/demux only supports services connected in the west direction. To reassign wavelengths – either to new channels or to reroute them to a different direction – requires technician involvement to physically unplug the transponder from one port on the mux/demux and plug it into a different physical mux/demux port.
CDC ROADMs enable much greater flexibility at the optical layer. The transponders may be connected to any add/drop port and can be routed to any degree or direction. Wavelength reassignment or rerouting can be implemented automatically from a network management system, or based on a network fault, without the need for manual technician involvement. The tradeoffs with CDC ROADMs are more complex architectures and costs.

Flexing network muscles

The existing 50-GHz-channel plan based on ITU G.694 has served the industry well for many years. But as the industry plans for the introduction of even faster 400G, and eventually 1T, optical interfaces, there's a need to adopt larger channel sizes and a more flexible WDM spacing plan.
These higher-speed optical interfaces rely on a new technique involving superchannels that comprise multiple subcarrier wavelengths. These subcarriers are provisioned, transported, and switched across a network as a single block or entity. Flexible grid systems enable the larger channel sizes required by 400G and 1T interfaces, but also allow the channel size to be closely matched to the signal being transported to optimize spectral efficiency.
No discussion of gridless ROADMs would be complete without including new next generation CDC ROADM architectures. These new ROADMs will enable a great deal more flexibility and efficiency at the optical layer.

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Optical Switching Comprehensive Article

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Gain: Input & output Photon relation...making it more simple.

What is a 30 dB gain means???

Puzzled..Earlier I use to be the same  but its really simple to understand.It means:-

For every input PHOTON there will be 1000 PHOTON's at output. That's what Gain is..

              X dB is 10x/10

You can verify the same with above formula...Make things Simpler.

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Optical Amplifiers..........its Payback or Trade-off.


Optical Amplifiers..........The Payback!!



Optical amplifiers alleviate that problem by amplifying all the channels together completely in the optical domain; therefore, optical amplifiers can enhance the transmission distance. So, does that mean that optical amplifiers can increase the amplifying distance as much as they wants? Not really! Amplifiers come at a price and induct a trade off; they enhance the signal power level, but at the same time, they add their own complement of noise. This noise is amplified spontaneous emission (ASE).




The noise is random in nature, and it is accumulated at each amplification stage.



Amplifier noise is a severe problem in system design. A figure of merit here is the optical signal-to-noise ratio (OSNR) requirement of the system. The OSNR specifies the ratio of the net signal power to the net noise power. It is a ratio of two powers; therefore, if a signal and noise are both amplified, system OSNR still tells the quality of the signal by calculating this ratio. System design based on OSNR is an important fundamental design tool.


_____________________________________________________________________________________________
OSNR is not just limited to optical amplifier-based networks. Other active and passive devices can also add noise and create an OSNR-limited system design problem. Active devices such as lasers and amplifiers add noise. Passive devices such as taps and the fiber can add components of noise. In the calculation of system design, optical amplifier noise is considered the predominant source for OSNR penalty and degradation. That does not imply unimportance to other sources of OSNR penalty.

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OTDR (Optical Time Domain Reflectomer)..Some Know-how!

How to Test a Fiber Optic System with an OTDR (Optical Time Domain Reflectomer)



>> The Optical Time Domain Reflectometer (OTDR)

OTDR is connected to one end of any fiber optic system up to 250km in length. Within a few seconds, we are able to measure the overall loss, or the loss of any part of a system, the overall length of the fiber and the distance between any points of interest. OTDR is a amazing test instrument for fiber optic systems.
1. A Use for Rayleigh Scatter
As light travels along the fiber, a small proportion of it is lost by Rayleigh scattering. As the light is scattered in all directions, some of it just happens to return back along the fiber towards the light source. This returned light is calledbackscatter as shown below.
image
The backscatter power is a fixed proportion of the incoming power and as the losses take their toll on the incoming power, the returned power also diminishes as shown in the following figure.
image
The OTDR can continuously measure the returned power level and hence deduce the losses encountered on the fiber. Any additional losses such as connectors and fusion splices have the effect of suddenly reducing the transmitted power on the fiber and hence causing a corresponding change in backscatter power. The position and degree of the losses can be ascertained.
2. Measuring Distances
The OTDR uses a system rather like a radar set. It sends out a pulse of light and ‘listens’ for echoes from the fiber.
If it knows the speed of light and can measure the time taken for the light to travel along the fiber, it is an easy job to calculate the length of the fiber.
image

3. To Find the Speed of the Light
Assuming the refractive index of the core is 1.5, the infrared light travels at a speed of
image
This means that it will take
image
This is a useful figure to remember, 5 nanoseconds per meter (5 nsm-1).
If the OTDR measures a time delay of 1.4us, then the distance travelled by the light is
image
The 280 meters is the total distance traveled by the light and is the ‘there and back’ distance. The length of the fiber is therefore only 140m. This adjustment is performed automatically by the OTDR – it just displays the final result of 140m.

4. Inside the OTDR
image
A. Timer
The timer produces a voltage pulse which is used to start the timing process in the display at the same moment as the laser is activated.
B. Pulsed Laser
The laser is switched on for a brief moment. The ‘on’ time being between 1ns and 10us. We will look at the significance of the choice of ‘on’ time or pulsewidth a little bit later. The wavelength of the laser can be switched to suit the system to be investigated.
C. Directive Coupler
The directive coupler allows the laser light to pass straight through into the fiber under test. The backscatter from the whole length of the fiber approaches the directive coupler from the opposite direction. In this case the mirror surface reflects the light into the avalanche photodiode (APD). The light has now been converted into an electrical signal.
D. Amplifying and Averaging
The electrical signal from the APD is very weak and requires amplification before it can be displayed. The averaging feature is quite interesting and we will look at it separately towards the end of this tutorial.
E. Display
The amplified signals are passed on to the display. The display is either a CRT like an oscilloscope, or a LCD as in laptop computers. They display the returned signals on a simple XY plot with the range across the bottom and the power level in dB up the side.
The following figure shows a typical display. The current parameter settings are shown over the grid. They can be changed to suit the measurements being undertaken. The range scale displayed shows a 50km length of fiber. In this case it is from 0 to 50km but it could be any other 50km slice, for example, from 20km to 70km. It can also be expanded to give a detailed view of a shorter length of fiber such as 0-5m, or 25-30m.
image
The range can be read from the horizontal scale but for more precision, a variable range marker is used. This is a movable line which can be switched on and positioned anywhere on the trace. Its range is shown on the screen together with the power level of the received signal at that point. To find the length of the fiber, the marker is simply positioned at the end of the fiber and the distance is read off the screen. It is usual to provide up to five markers so that several points can be measured simultaneously.
F. Data Handling
An internal memory or floppy disk can store the data for later analysis. The output is also available via RS232 link for downloading to a computer. In addition, many OTDRs have an onboard printer to provide hard copies of the information on the screen. This provides useful ‘before and after’ images for fault repair as well as a record of the initial installation.

5. A Simple Measurement
If we were to connect a length of fiber, say 300m, to the OTDR the result would look as shown in the following figure.
image
Whenever the light passes through a cleaved end of a piece of fiber, a Fresnel reflection occurs. This is seen at the far end of the fiber and also at the launch connector. Indeed, it is quite usual to obtain a Fresnel reflection from the end of the fiber without actually cleaving it. Just breaking the fiber is usually enough.
The Fresnel at the launch connector occurs at the front panel of the OTDR and, since the laser power is high at this point, the reflection is also high. The result of this is a relatively high pulse of energy passing through the receiver amplifier. The amplifier output voltage swings above and below the real level, in an effect called ringing. This is a normal amplifier response to a sudden change of input level. The receiver takes a few nanoseconds to recover from this sudden change of signal level.

6. Dead Zones
The Fresnel reflection and the subsequent amplifier recovery time results in a short period during which the amplifier cannot respond to any further input signals. This period of time is called a dead zone. It occurs to some extent whenever a sudden change of signal amplitude occurs. The one at the start of the fiber where the signal is being launched is called the launch dead zone and others are called event dead zones or just dead zones.
image

7. Overcoming the Launch Dead Zone
As the launch dead zone occupies a distance of up to 20 meters or so, this means that, given the job of checking a 300m fiber, we may only be able to check 280m of it. The customer would not be delighted.
To overcome this problem, we add our own patch cord at the beginning of the system. If we make this patch cord about 100m in length, we can guarantee that all launch dead zone problems have finished before the customers’ fiber is reached.
image
The patch cord is joined to the main system by a connector which will show up on the OTDR readout as a small Fresnel reflection and a power loss. The power loss is indicated by the sudden drop in the power level on the OTDR trace.

8. Length and Attenuation
The end of the fiber appears to be at 400m on the horizontal scale but we must deduct 100m to account for our patch cord. This gives an actual length of 300m for the fiber being tested.
Immediately after the patch cord Fresnel reflection the power level shown on the vertical scale is about –10.8dB and at the end of the 300m run, the power has fallen to about –11.3 dB. A reduction in power level of 0.5 dB in 300 meters indicates a fiber attenuation of:
image
Most OTDRs provide a loss measuring system using two markers. The two makers are switched on and positioned on a length of fiber which does not include any other events like connectors or whatever as shown in the following figure.
image
The OTDR then reads the difference in power level at the two positions and the distance between them, performs the above calculation for us and displays the loss per kilometer for the fiber. This provides a more accurate result than trying to read off the decibel and range values from the scales on the display and having to do our own calculations.

9. An OTDR Display of a Typical System
The OTDR can ‘see’ Fresnel reflections and losses. With this information, we can deduce the appearance of various events on an OTDR trace as seen below.
image

A. Connectors
A pair of connectors will give rise to a power loss and also a Fresnel reflection due to the polished end of the fiber.
B. Fusion Splice
Fusion splices do not cause any Fresnel reflections as the cleaved ends of the fiber are now fused into a single piece of fiber. They do, however, show a loss of power. A good quality fusion splice will actually be difficult to spot owing to the low losses. Any signs of a Fresnel reflection is a sure sign of a very poor fusion splice.
C. Mechanical Splice
Mechanical splices appear similar to a poor quality fusion splice. The fibers do have cleaved ends of course but the Fresnel reflection is avoided by the use of index marching gel within the splice. The losses to be expected are similar to the least acceptable fusion splices.
D. Bend Loss
This is simply a loss of power in the area of the bend. If the loss is very localized, the result is indistinguishable from a fusion or mechanic splice.

10. Ghost Echoes (False Reflection)
In the following figure, some of the launched energy is reflected back from the connectors at the end of the patch cord at a range of 100m. This light returns and strikes the polished face of the launch fiber on the OTDR front panel. Some of this energy is again reflected to be re-launched along the fiber and will cause another indication from the end of the patch cord, giving a false, or ghost, Fresnel reflection at a range of 200m and a false ‘end’ to the fiber at 500m.
image
As there is a polished end at both ends of the patch cord, it is theoretically possible for the light to bounce to and fro along this length of fiber giving rise to a whole series of ghost reflections. In the figure a second reflection is shown at a range of 300m.
It is very rare for any further reflections to be seen. The maximum amplitude of the Fresnel reflection is 4% of the incoming signal, and is usually much less. Looking at the calculations, even assuming the worst reflection, the returned energy is 4% or 0.04 of the launched energy. The re-launched energy, as a result of another reflection is 4% of the 4% or 0.042 = 0.0016 x input energy. This shows that we need a lot of input energy to cause a ghost reflection.
A second ghost would require another two reflections giving rise to a signal of only 0.00000256 of the launched energy. Subsequent reflections die out very quickly as we could imagine.
Ghost reflections can be recognized by their even spacing. If we have a reflection at 387m and another at 774 then we have either a strange coincidence or a ghost. Ghost reflections have a Fresnel reflection but do not show any loss. The loss signal is actually of too small an energy level to be seen on the display. If a reflection shows up after the end of the fiber, it has got to be a ghost.

11. Effects of Changing the Pulsewidth
The maximum range that can be measured is determined by the energy contained within the pulse of laser light. The light has to be able to travel the full length of the fiber, be reflected, and return to the OTDR and still be of larger amplitude than the background noise. Now, the energy contained in the pulse is proportional to the length of the pulse so to obtain the greatest range the longest possible pulsewidth should be used as illustrated in the following figure.
image
This cannot be the whole story, as OTDRs offer a wide range of pulsewidths.
We have seen that light covers a distance of 1 meter every 5 nanoseconds so a pulsewidth of 100nm would extend for a distance of 20 meters along the fiber (see the following figure). When the light reaches an event, such as a connector, there is a reflection and a sudden fall in power level. The reflection occurs over the whole of the 20m of the outgoing pulse. Returning to the OTDR is therefore a 20m reflection. Each event on the fiber system will also cause a 20m pulse to be reflected back towards the OTDR.
image
Now imagine two such events separated by a distance of 10m or less as in the following figure. The two reflections will overlap and join up on the return path to the OTDR. The OTDR will simply receive a single burst of light and will be quite unable to detect that two different events have occurred. The losses will add, so two fusion splices for example, each with a loss of 0.2dB will be shown as a single splice with a loss of 0.4dB.
The minimum distance separating two events that can be displayed separately is called the range discrimination of the OTDR.
The shortest pulsewidth on an OTDR may well be in the order of 10ns so at a rate of 5nsm-1 this will provide a pulse length in the fiber of 2m. The range discrimination is half this figure so that two events separated by a distance greater than 1m can be displayed as separate events. At the other end of the scale, a maximum pulsewidth of 10us would result in a range discrimination of 1 km.
Another effect of changing the pulsewidth is on dead zones. Increasing the energy in the pulse will cause a larger Fresnel reflection. This, in turn, means that the amplifier will take longer to recover and hence the event dead zones will become larger as shown in the next figure.

12. Which Pulsewidth to Choose?
image
Most OTDRs give a choice of at least five different pulse length from which to select.
Low pulse widths mean good separation of events but the pulse has a low energy content so the maximum range is very poor. A pulse width of 10ns may well provide a maximum range of only a kilometer with a range discrimination of 1 meter.
The wider the pulse, the longer the range but the worse the range discrimination. A 1us pulse width will have a range of 40 km but cannot separate events closer together than 100 m.
As a general guide, use the shortest pulse that will provide the required range.

13. Averaging
The instantaneous value of the backscatter returning from the fiber is very weak and contains a high noise level which tends to mask the return signal.
As the noise is random, its amplitude should average out to zero over a period of time. This is the idea behind the averaging circuit.
The incoming signals are stored and averaged before being displayed. The larger the number of signals averaged, the cleaner will be the final result but the slower will be the response to any changes that occur during the test. The mathematical process used to perform the effect is called least squares averaging or LSA.
The following figure shows the enormous benefit of employing averaging to reduce the noise effect.
image
Occasionally it is useful to switch the averaging off to see a real time signal from the fiber to see the effects of making adjustments to connectors etc. This is an easy way to optimize connectors, mechanical splices, bends etc. Simply fiddle with it and watch the OTDR screen.

14. OTDR Dynamic Range
When a close range reflection, such as the launch Fresnel occurs, the reflected energy must not be too high otherwise it could damage the OTDR receiving circuit. The power levels decrease as the light travels along the fiber and eventually the reflections are similar in level to that of the noise and can no longer be used.
The difference between the highest safe value of the input power and the minimum detectable power is called thedynamic range of the OTDR and, along with the pulse width and the fiber losses, determines the useful range of the equipment.
If an OTDR was quoted as having a dynamic range of 36 dB, it could measure an 18km run of fiber with a loss of 2 dB/km, or alternatively a 72 km length of fiber having a loss of 0.5 dB/km, or any other combination that multiplies out to 36 dB.

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Four Wave Mixing (FWM) in WDM System...

Four Wave Mixing (FWM) in WDM System..



>> Nonlinear Effects in High Power, High Bit Rate Fiber Optic Communication Systems

When optical communication systems are operated at moderate power (a few milliwatts) and at bit rates up to about 2.5 Gb/s, they can be assumed as linear systems. However, at higher bit rates such as 10 Gb/s and above and/or at higher transmitted powers, it is important to consider the effect of nonlinearities. In case of WDM systems, nonlinear effects can become important even at moderate powers and bit rates.
There are two categories of nonlinear effects. The first category happens because of the interaction of light waves with phonons (molecular vibrations) in the silica medium of optical fiber. The two main effects in this category are stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS).
The second category of nonlinear effects are caused by the dependence of refractive index on the intensity of the optical power (applied electric field). The most important nonlinear effects in this category are self-phase modulation (SPM) andfour-wave mixing (FWM).
Four-Wave-Mixing-Products

>> Basic Principles of Four-Wave Mixing

1. How the Fourth Wave is Generated
In a WDM system with multiple channels, one important nonlinear effect is four-wave mixing. Four-wave mixing is an intermodulation phenomenon, whereby interactions between 3 wavelengths produce a 4th wavelength.
In a WDM system using the angular frequencies ω1, … ωn, the intensity dependence of the refractive index not only induces phase shifts within a channel but also gives rise to signals at new frequencies such as 2ωij and ωi + ωj – ωk. This phenomenon is called four-wave mixing.
In contrast to Self-Phase Modulation (SPM) and Cross-Phase Modulation (CPM), which are significant mainly for high-bit-rate systems, the four-wave mixing effect is independent of the bit rate but is critically dependent on the channel spacing and fiber chromatic dispersion. Decreasing the channel spacing increases the four-wave mixing effect, and so does decreasing the chromatic dispersion. Thus the effects of Four-Wave Mixing must be considered even for moderate-bit-rate systems when the channels are closely spaced and/or dispersion-shifted fibers are used.
To understand the effects of four-wave mixing, consider a WDM signal that is the sum of n monochromatic plane waves. Thus the electric field of this signal can be written as
inline-equation-1
The nonlinear dielectric polarization PNL(r,t) is given by
image
where χ(3) is called the third-order nonlinear susceptibility and is assumed to be a constant (independent of t).
Using the above two equations, the nonlinear dielectric polarization is given by
equation-2.28-to-2.35
Thus the nonlinear susceptibility of the fiber generates new fields (waves) at the frequencies ωi ± ωj ± ωk (ωi, ωj, ωknot necessarily distinct). This phenomenon is termed four-wave mixing.
The reason for this term is that three waves with the frequencies ωi, ωj, and ωk combine to generate a fourth wave at a frequency ωi ± ωj ± ωk. For equal frequency spacing, and certain choices of I,j, and k, the fourth wave contaminates ωi. For example, for a frequency spacing Δω, taking ω1, ω2, and ω3 to be successive frequencies, that is, ω2 = ω1 + Δω and ω3 = ω1 + 2Δω, we have ω123 = ω2, and 2ω213.
In the above equation, the term (28) represents the effect of SPM and CPM. The terms (29), (31), and (32) can be neglected because of lack of phase matching. Under suitable circumstances, it is possible to approximately satisfy the phase-matching condition for the remaining terms, which are all of the form ωi + ωj – ωk, I,j Not Equalk (ωi, ωj not necessarily distinct).
For example, if the wavelengths in the WDM system are closely spaced, or are spaced near the dispersion zero of the fiber, then β is nearly constant over these frequencies and the phase-matching condition is nearly satisfied. When this is so, the power generated at these frequencies can be quite significant.

2. Power Penalty Due to Four-Wave Mixing
From the above discussion, we can see that the nonlinear polarization causes three signals at frequencies ωi, ωj, and ωkto interact to produce signals at frequencies ωi ± ωj ± ωk. Among these signals, the most troublesome one is the signal corresponding to
ωijk = ωi + ωj – ωk,       i Not Equalk, j Not Equalk
Depending on the individual frequencies, this beat signal may lie on or very close to one of the individual channels in frequency, resulting in significant crosstalk to that channel. In a multichannel system with W channels, this effect results in a large number (W(W-1)2) of interfering signals corresponding to i ,j,k varying from 1 to W. In a system with three channels, for example, 12 interfering terms are produced, as shown in the following figure.
four-wave-mixing-5.29
Interestingly, the effect of four-wave mixing depends on the phase relationship between the interacting signals. If all the interfering signals travel with the same group velocity, as would be the case if there were no chromatic dispersion, the effect is reinforced. On the other hand, with chromatic dispersion present, the different signals travel with different group velocities. Thus the different waves alternately overlap in and out of phase, and the net effect is to reduce the mixing efficiency. The velocity difference is greater when the channels are space farther apart (in systems with chromatic dispersion).
To quantify the power penalty due to four-wave mixing, we can start from the following equation
image
This equation assumes a link of length L without any loss and chromatic dispersion. Here Pi, Pj, and Pk denote the powers of the mixing waves and Pijk the power of the resulting new wave, image is the nonlinear refractive index (3.0x 10-8 μm2/W), and dijk is the so-called degeneracy factor.
In a real system, both loss and chromatic dispersion are present. To take the loss into account, we replace L with the effective length Le, which is given by the following equation for a system of length L with amplifiers spaced l km apart.
image
The presence of chromatic dispersion reduces the efficiency of the mixing. We can model this by assuming a parameter ηijk, which represents the efficiency of mixing of the three waves at frequencies ωi, ωj, and ωk. Taking these two into account, we can modify the preceding equation to
image
For on-off keying (OOK) signals, this represents the worst-case power at frequency ωijk, assuming a 1 bit has been transmitted simultaneously on frequencies ωi, ωj, and ωk.
The efficiency ηijk goes down as the phase mismatch Δβ between the interfering signals increases. We can obtain the efficiency as
image
Here, Δβ is the difference in propagation constants between the different waves, and D is the chromatic dispersion. Note that the efficiency has a component that varies periodically with the length as the interfering waves go in and out of phase. In this example, we will assume the maximum value for this component. The phase mismatch can be calculated as
Δβ = βi + βj – βk – βijk
where βr represents the propagation constant at wavelength λr.
Four-wave mixing manifests itself as intrachannel crosstalk. The total crosstalk power for a given channel ωc is given as
inline-equation-2
Assume the amplifier gains are chosen to match the link loss so that the output power per channel is the same as the input power. The crosstalk penalty can therefore be calculated from the following equation.
image
Assume that the channels are equally spaced and transmitted with equal power, and the maximum allowable penalty due to Four-Wave Mixing (FWM) is 1 dB. Then if the transmitted power in each channel is P, the maximum FWM power in any channel must be < εP, where ε can be calculated to be 0.034 for a 1 dB penalty using the above equation. Since the generated FWM power increases with link length, this sets a limit on the transmit power per channel as a function of the link length. This limit is plotted in the following figure for both standard single mode fiber (SMF) and dispersion-shifted fiber (DSF) for three cases
(1) 8 channels spaced 100 GHz apart
(2) 32 channels spaced 100 GHz part
(3) 32 channels spaced 50 GHz apart
For standard single mode fiber (SMF) the chromatic dispersion parameter is taken to be D = 17 ps/nm-km, and for DSF the chromatic dispersion zero is assumed to lie in the middle of the transmitted band of channels. The slope of the chromatic dispersion curve, dD/dλ, is taken to be 0.055 ps/nm-km2.
four-wave-mixing-power-penalty-5.30
We can get several conclusions from the above power penalty figure.
1). The limit is significantly worse in the case of dispersion-shifted fiber than it is for standard single mode fiber. This is because the four-wave mixing efficiencies are much higher in dispersion-shifted fiber due to the low value of the chromatic dispersion.
2). The power limit gets worse with an increasing number of channels, as can be seen by comparing the limits for 8-channel and 32 channel systems for the same 100 GHz spacing. This effect is due to the much larger number of four-wave mixing terms that are generated when the number of channels is increases. In the case of dispersion-shifted fiber, this difference due to the number of four-wave mixing terms is imperceptible since, even though there are many more terms for the 32 channel case, the same 8 channels around the dispersion zero as in the 8 channel case contribute almost all the four-wave mixing power. The four-wave mixing power contribution from the other channels is small because there is much more chromatic dispersion at these wavelengths.
3) The power limit decreases significantly if the channel spacing is reduce, as can be seen by comparing the curves for the two 32-channel systems with channel spacing of 100 GHz and 50 GHz. This decrease in the allowable transmit power arises because the four-wave mixing efficiency increases with a decrease in the channel spacing since the phase mismatch Δβ is reduced.  (For SMF, though the efficiencies at both 50 GHz and 100 GHz are small, the efficiency is much higher at 50 GHz than at 100 GHz.)

3. Solutions for Four-Wave Mixing
Four-wave mixing is a severe problem in WDM systems using dispersion-shifted fiber but does not usually pose major problem in systems using standard fiber. In face, it motivated the development of None-Zero Dispersion-Shifted Fiber (NZ-DSF). In general, the following actions alleviate the penalty due to four-wave mixing.
1) Unequal channel spacing. The positions of the channels can be chosen carefully so that the beat terms do not overlap with the data channels inside the receiver bandwidth. This may be possible for a small number of channels in some cases but needs careful computation of the exact channel positions.
unequal-channel-spacing-for-four-wave-mixing
2) Increases channel spacing. This increases the group velocity mismatch between channels. This has the drawback of increasing the overall system bandwidth, requiring the optical amplifiers to be flat over a wider bandwidth, and increases the penalty due to Stimulated Raman Scattering (SRS).
3) Using higher wavelengths beyond 1560nm with DSF. Even with DSF, a significant amount of chromatic dispersion is present in this range, which reduces the effect of four-wave mixing. The newly developed L-band amplifiers can be used for long-distance transmission over DSF.
4) As with other nonlinearities, reducing transmitter power and the amplifier spacing will decrease the penalty
5) If the wavelengths can be demultiplexed and multiplexed in the middle of the transmission path, we can introduce difference delays for each wavelength. This randomizes the phase relationship between the different wavelengths. Effectively, the FWM powers introduced before and after this point are summed instead of the electric fields being added in phase, resulting in a smaller FWM penalty.

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Optical modulation for High Baud Rate networks...40G/100G Speed Networks......

Optical modulation for High Baud Rate networks.... 40G/100G  Speed Networks......



>> What is Higher-Order Modulation Method?

A range of newly developed fundamental communications technologies must be employed in order to reliably transmit signals of 40/43 Gbps and even 100 Gbps in the near future using telecommunications networks.
One of these technologies involves the use of higher-level modulation methods on the optical side, similar to those which have been used for many years successfully on the electrical side in xDSL broadband access technology, for example.
Until now, just one modulation method was used for transmission rates of up to 10 Gbps, namely on/off keying or OOK for short.
Put simply, this means that the laser light used for transmission was either on or off depending on the logical state 1 or 0 respectively of the data signal. This is the simplest form of amplitude modulation.
Additional external modulation is used at 10 Gbps. The laser itself is switched to give a continuous light output and the coding is achieved by means of a subsequent modulator.

>> Why Do We Need Higher-Order Modulation Methods?

Why are higher-level modulation methods with their attendant complexity needed at 40/43 Gbps? There are many reasons for this.
1. Greater Bandwidth and Noise Power Level
Every method of modulation broadens the width of the laser spectrum. At 10 Gbps this means that about 120 pm bandwidth is needed for OOK. If the transmission rate is quadrupled to 40 Gbps, the necessary bandwidth also quadruples, i.e. to around 480 pm. The greater bandwidth results in a linear increase in the noise power level in the communications channel. A four-fold increase in the noise power level corresponds to 6 dB and would result in a decrease in the minimum sensitivity of the system by this same factor. This results in a much shorter transmission range at 40 Gbps, and a consequent need for more regenerators.
Increasing the laser power in sufficient measure to compensate for the missing balance in the system compared to 10 Gbps is not possible. Nonlinear effects in the glass fiber, such as four-wave mixing (FWM), self-phase modulation (SPM), and cross-phase modulation (XPM) would also adversely affect the transmission quality to a significant degree.
Higher-level modulation methods reduce the modulation bandwidth and thus provide a way out of this dilemma.
2. Integrate 40/43 Gbps into Existing DWDM Infrastructure
One absolute necessity is the need to integrate the 40/43 Gbps systems into the existing DWDM infrastructure. The bandwidth required by OOK or optical dual binary (ODB) modulation only allows a best case channel spacing of 100 GHz (= approx. 0.8 nm) in a DWDM system. Systems with a channel spacing of 50 GHz (= approx. 0.4 nm) have long been implemented in order to optimize the number of communications channels in the DWDM system.
For both technologies to be integrated into a single DWDM system, the multiplexers/demultiplexers (MUX/DEMUX) would have to be reconfigured back to a channel spacing of 100 GHz and the corresponding channel bandwidths, or hybrid MUX/DEMUX would have to be installed. Both these solutions are far from ideal, since they either result in a reduction in the number of communications channels or the loss of flexibility in the configuration of the DWDM system.
Here, too, the answer is to use higher-level modulation methods that reduce the required bandwidth.
3. Other Factors
As well as other factors, the transmission quality of a communications path also depends on polarization mode dispersion (PMD) and chromatic dispersion (CD).
CD depends on the fiber and can be compensated for relatively simply by switching in dispersion-compensating fibers. However, this once again degrades the loss budget. This is within acceptable limits for realizing the usual range distances in 10 Gbps systems. But this is not the case with 40 Gbps, where the system budget is already reduced anyway. For this reason, other compensation methods must be used, subject to the additional requirement for exact compensation at all wavelengths of a DWDM system because the maximum acceptable value for CD is a factor of 16 lower than that for 10 Gbps.
The maximum acceptable PMD value for 40 Gbps is reduced by a factor of four. The PMD value is largely affected by external influences on the fiber, such as temperature and mechanical stress, and is also dependent on the quality of manufacture of the fiber itself. A requirement for any new modulation method would be a corresponding tolerance to PMD and CD.

>> A Brief Tutorial on Higher-Order Modulation Methods

When you take a look at the data sheets issued by systems manufacturers or in other technical publications, it is easy to be confused by the number of abbreviations used for new modulation methods.
How do these methods differ, and which of them are really suitable for future transmission speeds? Unfortunately, there is no easy answer to that either. Apart from the technical requirements, such as
  • Significant improvement in minimum OSNR by reducing the signal bandwidth
  • Compatibility with the 50 GHz ITU-T channel spacing or at least with a spacing of 100 GHz
  • Coexistence with 10 Gbps systems
  • Transmission in networks that use ROADMs
  • Scalable for 100 Gbps
The degree of technical difficulty and hence the economic viability also have to be taken into account.
>> Categories of Modulation Methods
The modulation methods can be basically divided into different categories (see figure 1 below).
1. Amplitude Modulation
- NRZ/RZ on/off Keying (OOK) —- Baud Rate = Bit Rate
image
2. Single Polarization State Phase Modulation (DPSK)
Normalized phase and amplitude at the bit center. DPSK differential phase shift keying. —–  Baud Rate = Bit Rate.
image
3. Differential Quadrature Phase Shift Keying (DQPSK). —- Baud Rate = 1/2 Bit Rate
image
4. Dual Polarization State Phase Modulation (DP-QPSK)
Absolute phase and amplitude at the bit center. 3D phase constellation diagram. —–  Baud Rate = 1/4 Bit Rate.
image
OOK amplitude modulation and optical dual binary (ODB) modulation can only be used in a very restricted sense for 40/43 Gbps for the reasons described above. Higher-level phase modulation methods represent the next category.
DPSK improves the system balance by means of a much reduced OSNR limit value. In all the other aspects mentioned, this modulation method has similar characteristics to OOK. This modulation method can therefore only be used for DWDM systems with 100 GHz channel spacing because of the bandwidth it requires. It can only be employed with restrictions in ROADM based networks.
Reconfigurable optical add/drop multiplexers allow routing of individual wavelengths in a DWDM system at network nodes. The basic components of a ROADM are multiplexers and demultiplexers with wavelength-selective filter characteristics and a switch matrix. The cascading of bandpass filters unavoidably leads to a narrowing of the communications channel pass band, with the resultant truncation of the DPSK modulated signal.
Adaptive DPSK takes account of these restrictions and results in clear improvements when used in complex network structures. Improvements in all areas are brought about by modulation methods in the next category, that of quadrature phase shift keying QPSK.
Return-to-zero DQPSK (RZ-DQPSK) has been around for some time now. The RZ coding requires slightly higher complexity on the modulation side compared with the more usual non-return-to-zero (NRZ) coding, but it considerably reduces the susceptibility to PMD and nonlinear effects.
QPSK modulated signals use four possible positions in the constellation diagram. Each phase state now encodes two bits. The baud rate (symbol rate) is therefore halved, so the bandwidth requirement is also halved. Use in systems with 50 GHz channel spacing and in ROADM based networks is assured, with a simultaneous improvement in susceptibility to PMD.
image
The technical complexity required in the realization of this modulation method is admittedly greater. The figure above shows the principle of modulation and demodulation of a QPSK signal and outlines the technical outlay on the optical side.
Systems using dual polarization state DP-QPSK modulation methods have been tried out recently. This opens the way towards a coherent system of transmission and detection. Although this is by far the most complex method, the advantages are significant. Using a total of eight positions in what is now a three-dimensional constellation diagram, the baud rate is thus reduced by a factor of four. Each state encodes four bits.
This makes the method ideally suited for 100 Gbps, and the bandwidth requirement is within a range that would fit within existing DWDM structures. An additional forward error correction (FEC) is applied to 100 Gbps signals, so the actual transmission rate is more likely to be around 112 Gbps. The symbol rate using DP-QPSK modulation would be in the range of 28 GBaud, which requires a bandwidth of about 40 GHz.
The table below compares the characteristics of the different modulation methods.
image

>> Current Higher-Order Optical Modulation Methods Status

Implementation of higher-level modulation methods for optical communications is still in the early stages. It is to be expected that further innovations will be triggered by the next level in the transmission rate hierarchy.
In order to be as widely useful as possible, the measurement equipment would have to include facilities for testing the complete range of modulation methods. It is true that there will always be standardized interfaces on the client side of the network; these are 40 Gbps in SDH and 43 Gbps in OTN according to ITU-T Recommendation G.709 for the 40G hierarchy.
However, there is an increase in the diversity of non-standardized solutions on the line side. Not only do the optical parameters vary, but manufacturer-specific coding is being used more and more frequently for FEC. Use of through mode in an analyzer for insertion into the communications path has so far been an important approach:
It is important to check that the payload signal is correctly mapped into the communications frame on the line side, that the FEC is generated correctly, and that alarms are consistent. Or that the correct signaling procedure is followed in the receiver when an error message is received, and that error-free reception is possible in the presence of clock offsets or jittered signals.
It is now the time to decide quickly on using just a few modulation methods, otherwise the cost of investment in measuring equipment will rise to astronomical levels. In contrast to the wide variety in electrical multiplexers for 10 Gbps, optical modulation methods each require a corresponding optical transponder. The cost of these transponders largely determines the price of the test equipment. The greater the diversity, the less likely it is that investment will be made in a tester for a particular optical interface. This will mean that important tests will be omitted from systems using the latest technology.
Access to the line side is probably the easiest route for network operators who in any case have had to keep up with a diversity of systems manufacturers over the years. The most important tests on installed communications systems are end-to-end measurements. Fully developed test equipment for such measurements is available for 40/43 Gbps.

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Optical Transport Network (OTN):A comprehensive study

Optical Transport Network (OTN)
ITU-T Recommendations on the OTN Transport Plane
The following table lists all of the known ITU-T Recommendations specifically related to the OTN Transport Plane.
Topic
Title
Publ.*
Definitions
G.870 Definitions and Terminology for Optical Transport Networks (OTN
2004
Framework for Recommendations
G.871/Y.1301 Framework for Optical Transport Network Recommendations
10/00
Architectural Aspects
G.872 Architecture of Optical Transport Networks
11/01
G.872 Amend. 1 Architecture of Optical Transport Networks
12/03
G.872 Living List

Control Plane
ASTN/ASON recommendations are moved to specific ASTN/ASON standards page

Structures Mapping
G.709/Y.1331 Network node interface for the optical transport network (OTN)
03/03
G.709/Y.1331 Addendum 1
12/03
G.709 Living List

G.975 Forward Error Correction
10/00
Functional Characteristics
G.681 Functional characteristics of interoffice long-haul line systems using optical amplifiers, including optical multiplexing
10/96
G.798 Characteristics of optical transport network (OTN) equipment functional blocks
01/02
G.798 Amendment 1
06/02
G.798 Living List
G.806 Characteristics of transport equipment - Description Methodology and Generic Functionality
10/00
G.7710/Y.1701 Common Equipment Management Requirements
11/01
Protection Switching
G.808.1 (G.gps) Generic protection switching - Linear trail and subnetwork protection
12/03
G.873.1 Optical Transport network (OTN) - Linear Protection
03/03
G.873.1 Errata 1 Optical Transport network (OTN) - Linear Protection
10/03
Management Aspects
G.874 Management aspects of the optical transport network element
11/01
G.874.1 Optical Transport Network (OTN) Protocol-Neutral Management Information Model For The Network Element View
01/02
G.875 Optical Transport Network (OTN) management information model for the network element view
Data Communication Network (DCN)
G.7712/Y.1703 Architecture and specification of data communication network
03/03
G.dcn living list
Error Performance
G.8201 (G.optperf) Error performance parameters and objectives for multi-operator international paths within the Optical Transport Network (OTN)
09/03
G.optperf living list
M.2401 (M.24otn) Error Performance Limits and Procedures for Bringing-Into-Service and Maintenance of multi-operator international paths and sections within Optical Transport Networks
12/03
Jitter & Wander Performance
G.8251(G.otnjit) The control of jitter and wander within the optical transport network (OTN)
11/01
G.8251 Amendment 1 The control of jitter and wander within the optical transport network (OTN)
06/02
G.8251 Corrigendum 1 The control of jitter and wander within the optical transport network (OTN)
06/02
Physical-Layer Aspects
G.664 General Automatic Power Shut-Down Procedures for Optical Transport Systems
06/99
G.691 Optical Interfaces for single-channel SDH systems with Optical Amplifiers, and STM-64 and STM-256 systems
10/00
G.692 Optical Interfaces for Multichannel Systems with Optical Amplifiers
10/98
G.693 Optical interfaces for intra-office systems
11/01
G.694.1 Spectral grids for WDM applications: DWDM frequency grid
06/02
G.694.2 Spectral grids for WDM applications: CWDM wavelength grid
06/02
G.695 Optical interfaces for Coarse Wavelength Division Multiplexing applications
2003
G.696.1(G.IaDI) Intra-Domain DWDM applications
2004
G.697(G.optmon) Optical monitoring for DWDM system
2004
G.959.1 Optical Transport Networking Physical Layer Interfaces
02/01
Sup.39 (Sup.dsn) Optical System Design and Engineering Considerations
2003
Fibres
G.651 Characteristics of a 50/125 um multipmode graded index optical fibre cable
02/98
G.652 Characteristics of a single-mode optical fibre cable
03/03
G.653 Characteristics of a dispersion-shifted single mode optical fibre cable
12/03
G.654 Characteristics of a cut-off shifted single-mode fibre cable
06/02
G.655 Characteristics of a non-zero dispersion shifted single-mode optical fibre cable
03/03
Components & Sub-systems
G.661 Definition and test methods for the relevant generic parameters of optical amplifier devices and subsystems
10/98
G.662 Generic characteristics of optical fibre amplifier devices and subsystems
10/98
G.663 Application related aspects of optical fibre amplifier devices and sub-systems
04/00
G.671 Transmission characteristics of passive optical components
06/02
  

COMPARISON OF SDH AND OTN










1       Abbreviations

This document uses the following abbreviations:
0xYY                 YY is a value in hexadecimal presentation
3R                    Re-amplification, Reshaping and Retiming
ACT                  Activation (in the TCM ACT byte)
AI                     Adapted Information
AIS                    Alarm Indication Signal
APS                   Automatic Protection Switching
BDI                   Backward Defect Indication
BEI                    Backward Error Indication
BIAE                  Backward Incoming Alignment Error
BIP                    Bit Interleaved Parity
CBR                  Constant Bit Rate
CI                     Characteristic Information
CM                    Connection Monitoring
CRC                  Cyclic Redundancy Check
DAPI                 Destination Access Point Identifier
EXP                   Experimental
ExTI                  Expected Trace Identifier
FAS                   Frame Alignment Signal
FDI                   Forward Defect Indication
FEC                   Forward Error Correction
GCC                  General Communication Channel
IaDI                  Intra-Domain Interface
IAE                   Incoming Alignment Error
IrDI                   Inter-Domain Interface
JOH                  Justification Overhead
LSB                   Least Significant Bit
MFAS                 Multi-Frame Alignment Signal
MFI                   Multi-frame Indicator
MS                    Maintenance Signal
MSB                   Most Significant Bit
MSI                   Multiplex Structure Identifier
NNI                   Network Node Interface
OCh                  Optical channel with full functionality
OCI                   Open Connection Indication
ODU                  Optical Channel Data Unit
ODUk                Optical Channel Data Unit-k
ODTUjk             Optical channel Data Tributary Unit j into k
ODTUG              Optical channel Data Tributary Unit Group
ODUk-Xv            X virtually concatenated ODUk's
OH                    Overhead
OMS                  Optical Multiplex Section
OMS-OH                        Optical Multiplex Section Overhead
OMU                  Optical Multiplex Unit
ONNI                 Optical Network Node Interface
OOS                  OTM Overhead Signal
OPS                   Optical Physical Section
OPU                  Optical Channel Payload Unit
OPUk                Optical Channel Payload Unit-k
OPUk-Xv            X virtually concatenated OPUk's
OSC                  Optical Supervisory Channel
OTH                  Optical Transport Hierarchy
OTM                  Optical Transport Module
OTN                  Optical Transport Network
OTS                  Optical Transmission Section
OTS                  -OH Optical Transmission Section Overhead
OUT                  Optical Channel Transport Unit
OTUk                Optical Channel Transport Unit-k
PCC                  Protection Communication Channel
PM                    Path Monitoring
PMI                   Payload Missing Indication
PMOH                Path Monitoring OverHead
Ppm                  parts per million
PRBS                 Pseudo Random Binary Sequence
PSI                    Payload Structure Identifier
PT                    Payload Type
RES                   Reserved for future international standardization
RS                     Reed-Solomon
SAPI                  Source Access Point Identifier
Sk                     Sink
SM                    Section Monitoring
SMOH                Section Monitoring OverHead
So                     Source
TC                    Tandem Connection
TCM                  Tandem Connection Monitoring
TS                     Tributary Slot
TxTI                 Transmitted Trace Identifier
UNI                   User-to-Network Interface
VCG                  Virtual Concatenation Group
VCOH                Virtual Concatenation Overhead
vcPT                 virtual concatenated Payload Type


2       What is OTN/OTH


The Optical Transport Hierarchy (OTH) is a new transport technology for the OTN developed by the ITU. It is based on the network architecture defined in ITU G.872 “Architecture for the Optical Transport Network (OTN)”.

G.872 defines an architecture that is composed of the Optical Channel (OCh), Optical Multiplex Section (OMS) and Optical Transmission Section (OTS). It then describes the functionality that is needed to make OTN work. However, it may be interesting to note the decision made during G.872 development:

“During the development of ITU-T Rec. G.709, (implementation of the Optical Channel Layer according to ITU-T Rec. G.872 requirements), it was realized that the only techniques presently available that could meet the requirements for associated OCh trace, as well as providing an accurate assessment of the quality of a digital client signal, were digital techniques….”

“For this reason ITU-T Rec. G.709 chose to implement the Optical Channel by means of a digital framed signal with digital overhead that supports the management requirements for the OCh. Furthermore this allows the use of Forward Error Correction for enhanced system performance. This results in the introduction of two digital layer networks, the ODU and OTU. The intention is that all client signals would be mapped into the Optical Channel via the ODU and OTU layer networks.”

Currently there are no physical implementations of the OCh, OMS and OTS layers. As they are defined and implemented, they will be included in this document.

2.1      References

ITU-T Rec. G.709 (2009)            “Interfaces for the Optical Transport Network (OTN)”
ITU-T Rec. G.798 (2010)            “Characteristics of optical transport network hierarchy equipment functional blocks”
ITU-T Rec. G.872 (2001)            “Architecture of optical transport networks”
ITU-T Rec. G.873.1 (2006)         “Optical Transport Network: Linear protection”
ITU-T Rec. G.874 (2010)            “Management aspects of the optical transport network element”
ITU-T Rec. G.874.1 (2002)         “Optical transport network: Protocol-neutral management information model for the network element view”
ITU-T Rec. G.959.1 (2009)         “Optical transport network physical layer interfaces”
ITU-T Rec. G.8251 (2011)          “The control of jitter and wander within the optical transport network (OTN)”



3       Optical transport network interface structure

3.1      Basic signal structure

Figure 1 Structure of the OTN interfaces

3.1.1     OCh substructure

The optical channel layer is further structured in layer networks in order to support the network management and supervision functionalities:

·         The optical channel with full (OCh) or reduced functionality (OChr), which provides transparent network connections between 3R regeneration points in the OTN.

·         The optical channel transport unit (OTUk/OTUkV) which provides supervision and conditions the signal for transport between 3R regeneration points in the OTN.



·         The optical channel data unit (ODUk) which provides:
·         tandem connection monitoring (ODUkT)
·         end-to-end path supervision (ODUkP)
·         adaptation of client signals via the optical channel payload unit (OPUk)
·         adaptation of OTN ODUk signals via the optical channel payload unit (OPUk)

3.1.2     Full functionality OTM-n.m (n ≥ 1) structure

The OTM-n.m (n ≥ 1) consists of the following layers:
·         optical transmission section (OTSn)
·         optical multiplex section (OMSn)
·         optical channel (OCh)
·         optical channel transport unit (OTUk/OTUkV)
·         one or more optical channel data unit (ODUk)

3.1.3     Reduced functionality OTM-nr.m and OTM-0.m structure

The OTM-nr.m and OTM-0.m consist of the following layers:
·         optical physical section (OPSn)
·         reduced functionality optical channel (OChr)
·         optical channel transport unit (OTUk/OTUkV)
·         one or more optical channel data unit (ODUk)



3.2      Information structure for the OTN interfaces



Figure 2 Principal information containment relationships

The following layers are defined in OTN:
·         OPUk: Optical channel payload unit k (k = 0, 1, 2, 3, 4)
·         ODUk: Optical channel data unit k (k = 0, 1, 2, 3, 4)
·         OTUk: Optical channel transport unit k (k = 1, 2, 3, 4)
·         OCh: Optical channel, a single wavelength
·         OMSn: Optical multiplex section of order n (Capacities for n = 0 and n = 16 are defined)
·         OTSn: Optical transmission section of order n (Capacities for n = 0 and n = 16 are defined)


·         OTM-n.m: Optical transport module of rate m with n optical channels. Possible values for m are:
 1: 2.5 Gb/s
 2: 10 Gb/s
 3: 40 Gb/s
 4: 100 Gb/s

Figure 3 shows how they are being used in a network.





However for all intents and purposes there are only 4 layers



Figure 4 OTN Hierarchy

The OPUk, ODUk, and OTUk are in the electrical domain. The OCh is in the optical domain. There are more layers in the optical domain than just the OCh, but they are not being used now.



4       Multiplexing/mapping principles and bit rates


Figure 5 shows the relationship between various information structure elements and illustrates the multiplexing structure and mappings (including wavelength and time division multiplexing) for the OTM-n.



Figure 5 OTM multiplexing and mapping structure

The OTS, OMS, OCh and COMMS overhead is inserted into the OOS.

4.1      Mapping

The OPUk encapsulates the Client signal (e.g. SDH) and does any rate justification that is needed. It is analogous to the path layer in SDH in that it is mapped at the source, de-mapped at the sink, and not modified by the network.
The OPUk is mapped into an ODUk. The ODUk performs similar functions as the path overhead in SDH.
The ODUk is mapped into an OTUk[V]. The OTUk[V] contains the FEC and performs similar functions as the section overhead in SDH.
After the FEC are added, the signal is then sent to a serializer/ deserializer to be converted to the optical domain. The OTUk[V] is mapped into an OCh[r] and the OCh[r] is then modulated onto an OCC[r].

4.2      Wavelength division multiplex

Up to n (n ≥ 1) OCC[r] are multiplexed into an OCG-n[r].m using wavelength division multiplexing. The OCC[r] tributary slots of the OCG-n[r].m can be of different size.

The OCG-n[r].m is transported via the OTM-n[r].m. For the case of the full functionality OTM-n.m interfaces the OSC is multiplexed into the OTM-n.m using wavelength division multiplexing.

4.3      Bit rates and capacity

The data rates were constructed so that they could transfer SDH and Ethernet signals efficiently. The bit rates are shown in the following tables:




Table 1 OTU types and capacity


Table 2 ODU types and capacity



Table 3 OPU types and capacity



4.4      ODUk Time-Division Multiplex

Figure 6 and Figure 7 show the relationship between various information structure elements and illustrate the multiplexing structure and mappings (including wavelength and time division multiplexing) for the OTM-n. In the multi-domain OTN any combination of the ODUk multiplexing layers may be present at a given OTN interface.

Figure 6 shows that a (non-OTN) client signal is mapped into a lower order OPU, identified as "OPU (L)". The OPU (L) signal is mapped into the associated lower order ODU, identified as "ODU (L)". The ODU (L) signal is either mapped into the associated OTU[V] signal, or into an ODTU. The ODTU signal is multiplexed into an ODTU Group (ODTUG). The ODTUG signal is mapped into a higher order OPU, identified as "OPU (H)". The OPU (H) signal is mapped into the associated higher order ODU, identified as "ODU (H)". The ODU (H) signal is mapped into the associated OTU[V].

The OPU (L) and OPU (H) are the same information structures, but with different client signals. The concepts of lower order and high order ODU are specific to the role that ODU plays within a single domain.





Figure 6 OTM multiplexing and mapping structure



Figure 7 shows that an OTU[V] signal is mapped either into an optical channel signal, identified as OCh and OChr, or into an OTLk.n. The OCh/OChr signal is mapped into an optical channel carrier, identified as OCC and OCCr. The OCC/OCCr signal is multiplexed into an OCC group, identified as OCG-n.m and OCG-nr.m. The OCG-n.m signal is mapped into an OMSn. The OMSn signal is mapped into an OTSn. The OTSn signal is presented at the OTM-n.m interface.
The OCGnr.m signal is mapped into an OPSn. The OPSn signal is presented at the OTM-nr.m interface.
A single OCCr signal is mapped into an OPS0. The OPS0 signal is presented at the OTM-0.m interface. The OTLk.n signal is mapped into an optical transport lane carrier, identified as OTLC. The OTLC signal is multiplexed into an OTLC group, identified as OTLCG. The OTLCG signal is mapped into an OPSMnk. The OPSMnk signal is presented at the OTM-0.mvn interface.





Figure 7 OTM multiplexing and mapping structure




5       OTUk, ODUk, OPUk Frame Structure

Figure 8 shows the overall frame format for an OTUk signal. The various fields will be explained in the following sub-sections.



Figure 8 OTN frame format



5.1      OPUk Overhead and Processing

The OPUk (k = 1,2,3) frame structure is organized in an octet-based block frame structure with 4 rows and 3810 columns.



Figure 9 OPUk frame structure

The two main areas of the OPUk frame are:
·         OPUk overhead area
·         OPUk payload area

Columns 15 to 16 of the OPUk are dedicated to OPUk overhead area.
Columns 17 to 3824 of the OPUk are dedicated to OPUk payload area.

NOTE – OPUk column numbers are derived from the OPUk columns in the ODUk frame

OPUk OH information is added to the OPUk information payload to create an OPUk. It includes information to support the adaptation of client signals. The OPUk OH is terminated where the OPUk is assembled and disassembled.


 



Figure 10 OPUk frame

5.1.1     Payload Structure Identifier (PSI)

The 256-byte PSI signal is aligned with the ODUk multi-frame (i.e. PSI[0] is present at ODUk multi-frame position 0000 0000, PSI[1] at position 0000 0001, PSI[2] at position 0000 0010, etc.).

PSI[0] contains a one-byte payload type. PSI[1] to PSI[255] are mapping and concatenation specific.

5.1.2     Payload Type (PT)

A one-byte payload type signal is defined in the PSI[0] byte of the payload structure identifier to indicate the composition of the OPUk signal.


5.2      ODUk Overhead and Processing

The ODUk (k = 1,2,3) frame structure is organized in an octet-based block frame structure with 4 rows and 3824 columns.



Figure 11 ODUk frame structure

The three main areas of the ODUk frame are:
·         OTUk area
·         ODUk overhead area;
·         OPUk area.

Columns 1 to 14 of rows 2-4 are dedicated to ODUk overhead area.

Columns 1 to 14 of row 1 are reserved for frame alignment and OTUk specific overhead.

Columns 15 to 3824 of the ODUk are dedicated to OPUk area.

ODUk OH information is added to the ODUk information payload to create an ODUk. It includes information for maintenance and operational functions to support optical channels. The ODUk OH consists of portions dedicated to the end-to-end ODUk path and to 6 levels of tandem connection monitoring. The ODUk path OH is terminated where the ODUk is assembled and disassembled. The TC OH is added and terminated at the source and sink of the corresponding tandem connections, respectively.



Figure 12 ODUk overhead


5.2.1     Path Monitoring (PM)





Figure 13 ODUk path monitoring overhead

5.2.1.1     Trail Trace Identifier (TTI)

The TTI is a 64-Byte signal that occupies one byte of the frame and is aligned with the OTUk multi-frame. It is transmitted 4 times per multi-frame.

5.2.1.2     BIP-8

This byte provides a bit interleaved parity-8 (BIP-8) code.

The ODUk BIP-8 is computed over the bits in the OPUk (columns 15 to 3824) area of ODUk frame i, and inserted in the ODUk PM BIP-8 overhead location in the ODUk frame i+2.

5.2.1.3     Backward Defect Indication (BDI)

This is defined to convey the “Signal Fail” status detected at the path terminating sink function, to the upstream node.

5.2.1.4     Backward Error Indication and Backward Incoming Alignment Error (BEI/BIAE)

This signal is used to convey in the upstream direction the count of interleaved-bit blocks that have been detected in error by the corresponding ODUk path monitoring sink using the BIP-8 code. This count has nine legal values, namely 0-8 errors. The remaining seven possible values represented by these 4 bits can only result from some unrelated condition and are interpreted as 0 errors.

5.2.1.5     Path Monitoring Status (STAT)

They indicate the presence of a maintenance signal.

5.2.2     Tandem Connection Monitoring (TCM)

There are 6 TCM’s. They can be nested or overlapping.




Figure 14 ODUk tandem connection monitoring #i overhead

5.2.2.1     Trail Trace Identifier (TTI)

The TTI is a 64-Byte signal that occupies one byte of the frame and is aligned with the OTUk multi-frame. It is transmitted four times per multi-frame.

5.2.2.2     BIP-8

This byte provides a bit interleaved parity-8 (BIP-8) code.

Each ODUk BIP-8 is computed over the bits in the OPUk (columns 15 to 3824) area of ODUk frame i, and inserted in the ODUk TCM BIP-8 overhead location (associated with the tandem connection monitoring level) in ODUk frame i+2.

The BIP-8 is only overwritten at the start of a Tandem Connection. Any existing TCM is not overwritten.

5.2.2.3     Backward Defect Indication (BDI)

This is defined to convey the “Signal Fail” status detected at the path terminating sink function, to the upstream node.

5.2.2.4     Backward Error Indication and Backward Incoming Alignment Error (BEI/BIAE)

This signal is used to convey in the upstream direction the count of interleaved-bit blocks that have been detected as being in error by the corresponding ODUk tandem connection monitoring sink using the BIP-8 code. It is also used to convey in the upstream direction an incoming alignment error (IAE) condition that is detected in the corresponding ODUk tandem connection monitoring sink in the IAE overhead.

During an IAE condition the code "1011" is inserted into the BEI/BIAE field and the error count is ignored. Otherwise the error count (0-8) is inserted into the BEI/BIAE field. The remaining 6 possible values represented by these 4 bits can only result from some unrelated condition and are interpreted as 0 errors and BIAE not active.

5.2.2.5     TCM Monitoring Status (STAT)

For each tandem connection monitoring field three bits are defined as status bits (STAT). They indicate the presence of a maintenance signal (if there is an incoming alignment error at the source TCM, or if there is no source TCM active).

5.2.2.6     Tandem Connection Monitoring ACTivation/deactivation (TCM-ACT)

Its definition is for further study.

5.2.3     General Communication Channels (GCC1, GCC2)

Two fields of two bytes are allocated in the ODUk overhead to support two general communications channels between any two network elements with access to the ODUk frame structure (i.e., at 3R regeneration points). These are clear channels. The bytes for GCC1 are located in row 4, columns 1 and 2, and the bytes for GCC2 are located in row 4, columns 3 and 4 of the ODUk overhead.

5.2.4     Automatic Protection Switching and Protection Communication Channel (APS/PCC)

Up to 8 levels of nested APS/PCC signals may be present in this field. The APS/PCC bytes in a given frame are assigned to a dedicated level depending on the value of MFAS as follows:

MFAS bit 678
APS/PCC channel applies to connection level
Protection scheme using the APS/PCC channel
000
ODUk Path
ODUk SNC/N
001
ODUk TCM1
ODUk SNC/S, ODUk SNC/N
010
ODUk TCM2
ODUk SNC/S, ODUk SNC/N
011
ODUk TCM3
ODUk SNC/S, ODUk SNC/N
100
ODUk TCM4
ODUk SNC/S, ODUk SNC/N
101
ODUk TCM5
ODUk SNC/S, ODUk SNC/N
110
ODUk TCM6
ODUk SNC/S, ODUk SNC/N
111
OTUk Section
ODUk SNC/I

Table 4 Multi-frame to allow separate APS/PCC for each monitoring level

For linear protection schemes, the bit assignments for these bytes and the bit-oriented protocol are given in ITU-T Recommendation G.873.1. Bit assignment and byte oriented protocol for ring protection schemes are for further study.

5.2.5     Fault Type and Fault Location reporting communication channel (FTFL)

One byte is allocated in the ODUk overhead to transport a 256-byte fault type and fault location (FTFL) message. The byte is located in row 2, column 14 of the ODUk overhead.


5.3      OTUk Overhead and Processing

The OTUk (k = 1,2,3) frame structure is based on the ODUk frame structure and extends it with a forward error correction (FEC). 256 columns are added to the ODUk frame for the FEC and the overhead bytes in row 1, columns 8 to 14 of the ODUk overhead are used for OTUk specific overhead, resulting in an octet-based block frame structure with 4 rows and 4080 columns.

The OTUk forward error correction (FEC) contains the Reed-Solomon RS(255,239) FEC codes. If no FEC is used, fixed stuff bytes (all-0s pattern) are inserted.

OTUk OH information is part of the OTUk signal structure. It includes information for operational functions to support the transport via one or more optical channel connections. The OTUk OH is terminated where the OTUk signal is assembled and disassembled.



Figure 15 OTUk Overhead





Figure 16 OTUk overhead

5.3.1     Scrambling

The OTUk signal needs sufficient bit timing content to allow a clock to be recovered. A suitable bit pattern, which prevents a long sequence of "1"s or "0"s, is provided by using a scrambler.

The operation of the scrambler is functionally identical to that of a frame synchronous scrambler of sequence length 65535 operating at the OTUk rate.

The generating polynomial is 1 + x + x3+ x12 + x16.

The scrambler is reset to "FFFF" (HEX) on the most significant bit of the byte following the last framing byte in the OTUk frame, i.e. the MSB of the MFAS byte. This bit and all subsequent bits to be scrambled are added modulo 2 to the output from the x16 position of the scrambler. The scrambler runs continuously throughout the complete OTUk frame. The framing bytes (FAS) of the OTUk overhead are not scrambled.

Scrambling is performed after FEC check bytes computation and insertion into the OTUk signal.

5.3.2     Frame Alignment Overhead

5.3.2.1     Frame Alignment Signal (FAS)

A 6 byte OTUk-FAS signal is defined in row 1, columns 1 to 6 of the OTUk overhead.
OA1 is "1111 0110". OA2 is "0010 1000".



Figure 17 Frame alignment signal overhead structure

5.3.2.2     Multi-Frame Alignment Signal (MFAS)

Some of the OTUk and ODUk overhead signals span multiple OTUk/ODUk frames. A single multi-frame alignment signal (MFAS) byte is defined in row 1, column 7 of the OTUk/ODUk overhead The value of the MFAS byte will be incremented each OTUk/ODUk frame and provides as such a 256 frame multi-frame.

Individual OTUk/ODUk overhead signals use this central multi-frame to lock their 2-frame, 4-frame, 8-frame, 16-frame, 32-frame, etc. multi-frames to the principal frame.

5.3.3     Section Monitoring (SM)




Figure 18 OTUk section monitoring overhead

5.3.3.1     Trail Trace Identifier (TTI)

The TTI is a 64-Byte signal that occupies one byte of the frame and is aligned with the OTUk multi-frame. It is transmitted four times per multi-frame.

5.3.3.2     BIP-8

This byte provides a bit interleaved parity-8 (BIP-8) code.

The OTUk BIP-8 is computed over the bits in the OPUk (columns 15 to 3824) area of OTUk frame i, and inserted in the OTUk BIP-8 overhead location in OTUk frame i+2.

Note: The OPUk includes the Justification Bytes, thus an OTN signal can not be retimed without de-mapping back to the client signal.

5.3.3.3     Backward Defect Indication (BDI)

This is defined to convey the “Signal Fail” Status detected at the Section Terminating Sink Function, to the upstream node.

5.3.3.4     Backward Error Indication and Backward Incoming Alignment Error (BEI/BIAE)

This signal is used to convey in the upstream direction the count of interleaved-bit blocks that have been detected in error by the corresponding OTUk section monitoring sink using the BIP-8 code. It is also used to convey in the upstream direction an incoming alignment error (IAE) condition that is detected in the corresponding OTUk section monitoring sink in the IAE overhead.

During a IAE condition the code "1011" is inserted into the BEI/BIAE field and the error count is ignored. Otherwise the error count (0-8) is inserted into the BEI/BIAE field. The remaining six possible values represented by these four bits can only result from some unrelated condition and are interpreted as 0 errors and BIAE not active.

5.3.3.5     Incoming Alignment Error (IAE)

A single-bit incoming alignment error (IAE) signal is defined to allow the ingress point to inform its peer egress point that an alignment error in the incoming signal has been detected.

IAE is set to "1" to indicate a frame alignment error; otherwise it is set to "0".

The egress point may use this information to suppress the counting of bit errors, which may occur as a result of a frame phase change of the OTUk at the ingress of the section.

5.3.4     General Communication Channel 0 (GCC0)

Two bytes are allocated in the OTUk overhead to support a general communications channel between OTUk termination points. This is a clear channel. These bytes are located in row 1, columns 11 and 12 of the OTUk overhead.


6       OTN Maintenance Signals

6.1      OTUk maintenance signals

6.1.1     OTUk alarm indication signal (OTUk-AIS)

The OTUk-AIS is a generic-AIS signal. Since the OTUk capacity (130 560 bits) is not an integer multiple of the PN-11 sequence length (2047 bits), the PN-11 sequence may cross an OTUk frame boundary.

The PN-11 sequence is defined by the generating polynomial 1 + x9 + x11.

6.2      ODUk maintenance signals

Three ODUk maintenance signals are defined: ODUk-OCI, ODUk-LCK and ODUk-AIS.

6.2.1     ODUk Open Connection Indication (ODUk-OCI)

ODUk-OCI is specified as a repeating "0110 0110" pattern in the entire ODUk signal, excluding the frame alignment overhead (FA OH) and OTUk overhead (OTUk OH).

The presence of ODUk-OCI is detected by monitoring the ODUk STAT bits in the PM and TCMi overhead fields.

The insertion of this is under management control. There is no defect that inserts ODUk-OCI.

6.2.2     ODUk Locked (ODUk-LCK)

ODUk-LCK is specified as a repeating "0101 0101" pattern in the entire ODUk signal, excluding the Frame Alignment overhead (FA OH) and OTUk overhead (OTUk OH).

The presence of ODUk-LCK is detected by monitoring the ODUk STAT bits in the PM and TCMi overhead fields.

The insertion of this is under management control. There is no defect that inserts ODUk-LCK.

6.2.3     ODUk Alarm Indication Signal (ODUk-AIS)

ODUk-AIS is specified as all "1"s in the entire ODUk signal, excluding the frame alignment overhead (FA OH), OTUk overhead (OTUk OH) and ODUk FTFL.

The presence of ODUk-AIS is detected by monitoring the ODUk STAT bits in the PM and TCMi overhead fields.

ODUk-AIS is generated if the OTUk input signal fails or it detects ODUk-OCI or ODUk-LCK on the input signal.

6.3      Client maintenance signal

6.3.1     Generic AIS for constant bit rate signals

The generic-AIS signal is a signal with a 2047-bit polynomial number 11 (PN-11) repeating sequence.
The PN-11 sequence is defined by the generating polynomial 1 + x9 + x11.

During a signal fail condition of the incoming CBR2G5, CBR10G or CBR40G client signal (e.g. in the case of a loss of input signal), this failed incoming signal is replaced by the generic-AIS signal, and is then mapped into the OPUk.

During signal fail condition of the incoming ODUk/OPUk signal (e.g. in the case of an ODUk-AIS, ODUk-LCK, ODUk-OCI condition) the generic-AIS pattern is generated as a replacement signal for the lost CBR2G5, CBR10G or CBR40G signal.Maintenance Signal Insertion

6.3.2     Client source ODUk-AIS

During a signal fail condition of the incoming ODUj client signal (e.g. OTUj-LOF), this failed incoming signal will be replaced by the ODUj-AIS signal. This ODUj-AIS is then mapped into the respective timeslot in the ODUk.

6.3.3     Client source ODUk-OCI

For the case the ODUj is received from the output of a fabric (ODUj connection function), the incoming signal may contain (case of open matrix connection) the ODUj-OCI signal This ODUj-OCI signal is then mapped into the respective timeslot in the ODUk.

Not all equipment will have a real connection function (switch fabric) implemented; instead the presence/absence of tributary interface port units represents the presence/absence of a matrix connection.
If such unit is intentionally absent or not installed, the associated timeslot in the ODUk shall carry an ODUj-OCI signal.
If such unit is installed but temporarily removed as part of a repair action, the associated timeslot in the ODUk shall carry an ODUj-AIS signal.



7       Defect detection

There are no defects detected in the multiplexer. There are defects detected in the de-multiplexer.

7.1      Default PLM (Payload Mismatch)

Default PLM is declared if the accepted payload type (AcPT) is not equal to the expected payload type(s) as defined by the specific adaptation function. Default PLM is cleared if the accepted payload type is equal to the expected payload type(s).

A new payload type PT (AcPT) is accepted if a new consistent value is received in the PSI[0] byte in 3 consecutive multi-frames.

7.2      Default MSIM (Multiplex Structure Identifier Mismatch supervision)

Default MSIM is declared if the accepted MSI (AcMSI) is not equal to the expected multiplex structure identifier (ExMSI). dMSIM is cleared if the AcMSI is equal to the ExMSI. ExMSI is configured via the management interface. A new multiplex structure identifier MSI (AcMSI) is accepted if a new consistent value is received in the MSI bytes of the PSI overhead (PSI[2...5] for ODU2, PSI[2...17] for ODU3) in 3 consecutive multi-frames.

7.3      Default LOFLOM (Loss of Frame and Multi-frame)

If the frame alignment process is in the out-of-frame (OOF) state for 3 ms, default LOFLOM is declared. To prevent from the case of intermittent OOFs, the integrating timer is reset to 0 until an in-frame (IF) condition persists continuously for 3 ms. Default LOFLOM is cleared when the IF state persists continuously for 3 ms.

The ODUj frame and multi-frame alignment is found by searching for the framing pattern (OA1, OA2 FAS bytes) and checking the multi-frame sequence (MFAS byte) contained in the ODUj frame.

In the out-of-frame state the framing pattern searched for is the full set of the OA1 and OA2 bytes. The in-frame (IF) is entered if this set is found and confirmed one frame period later and an error-free multi-frame sequence is found in the MFAS bytes of the two frames.

In the in-frame state (IF) the frame alignment signal is continuously checked with the presumed frame start position and the expected multi-frame sequence. The framing pattern checked for is the OA1OA2 pattern (bytes 3 and 4 of the first row of the ODUj[/i] frame). The out of frame state (OOF) is entered if this subset is not found at the correct position in 5 consecutive frames or the received MFAS does not match with the expected multi-frame number in 5 consecutive frames.

The frame and multi-frame start are maintained during the OOF state.

There is one of these defects for each tributary.



8       Synchronization

8.1      Introduction

OTN is transparent to the payload it transports within the ODUk. The OTN layer does not need to transport network synchronization since network synchronization can be transported within the payload, mainly by SDH client tributaries.

Two types of mapping have been specified for the transport of CBR payload, e.g. SDH.
The first one is the asynchronous mapping, which is the most widely used, where the payload floats within the OTN frame. In this case, there is no frequency relationship between the payload and the OTN frame frequencies, thus simple free running oscillators can be used to generate the OTN frame.
The second is the synchronous mapping where the timing used to generate the OTN frame is extracted from a CBR client tributary, e.g. SDH. In case of LOS of the input client, the OTN frequency that does not transport payload is generated by a free running oscillator, without need for a holdover mode.

This specification allows for very simple implementation of timing in OTN equipments compared to SDH.
An OTN NE does not require synchronization interfaces, complex clocks with holdover mode nor SSM processing. Another difference with SDH is that there is no geographical option for the timing aspects of OTN.

OTN transports client signals into a G.709 frame, OTUk that is transported by an OCh on one lambda of the Optical Transport Module (OTM). Each lambda carries its G.709 frame with its own frequency; there is no common clock for the different OTUk of the OTM.

A trail through OTN is generated in an OTN NE that maps the client into an ODUk and terminated in another OTN NE that de-maps the client signal from the ODUk. Between the 2 OTN trail terminations, there might be 3R regenerators, which are equipments that perform complete regeneration of the pulse shape, clock recovery and retiming within required jitter limits.
The number of 3R regenerators that can be cascaded in tandem depends on the specification of this regenerator and on the jitter and wander generation and tolerance applicable to the OTUk interfaces; it is stated to be at least 50.

ODUk multiplexing has been standardized; its implication on timing has been taken into account in the relevant recommendations.

8.2      Network requirements

In an OTN, jitter and wander accumulate on transmission path according to the generation and transfer characteristics of interconnected equipments, 3R regenerators, client mappers, de-mappers and multiplexers, de-multiplexers. In order to avoid the effects of excessive jitter and wander, the ITU-T Recommendation G.8251 recommendation specifies the maximum magnitude of jitter and wander, and the minimum jitter and wanders tolerance, at OTN network interfaces.

The OTN generates and accumulates jitter and wander on its client signals due to the buffers of the mapping into ODUk and due to the ODUk multiplexing. The limits for such accumulation are given in the ITU-T Recommendation G.825 for SDH signal clients.
Jitter and wander is also accumulated on the OTN signals itself due to the ODUk multiplexing and 3R jitter generation. The network limits for this are given in the ITU-T Recommendation G.8251.



The ITU-T Recommendation G.8251 specifies the jitter and wander tolerance. As OTN clocks do not generate wander, no wander limit has been defined for OTN.

The ITU-T Recommendation G.8251 specifies the different type of clocks that are required to perform the following functions: the accuracy of these clocks depends on the definition of the G.709 frame and on the accuracy specified for the clients.

·         Asynchronous mapping of a client into an ODUk and ODUk multiplexing: this ODCa clock is a free- running clock with a frequency accuracy of ± 20 ppm.

·         Synchronous mapping of a client into an ODUk: this ODCb clock is locked on the client frequency.

·         3R regeneration: this ODCr clock is locked on an OCh input frequency which must be within ± 20 ppm.

·         De-mapping a client signal from an ODUk and ODUk de-multiplexing: this ODCp clock is locked on an OCh input frequency which must be within ± 20 ppm.

The ITU-T Recommendation G.8251 specifies the jitter generation of these clocks and, when applicable, noise tolerance, jitter transfer and transient response.

All these clock functions are used for clock recovery and clock filtering of a particular signal. They never serve as an equipment synchronization source. Therefore there is no holdover mode specified for these clocks since there is no need for an accurate clock when the input signal disappears.

The ITU-T Recommendation G.8251 provides a provisional adaptation of the SDH synchronization reference chain to include OTN islands. This is an amendment of the reference chain being defined in the ITU-T Recommendation G.803. Considering that SDH may be transported by OTN islands, the SEC will no longer be present but replaced by OTN NEs. This leads to the definition of a reference chain where all SECs located between 2 SSUs are replaced by an OTN island. The local part of the reference chain, after the last SSU can still support 20 SECs in tandem. Each of these islands may be composed of OTN NEs performing mapping/de-mapping or multiplexing/de-multiplexing operations. This adaptation of the reference chain raises a buffer size constraint for the OTN NEs in order to keep the overall network wander performance within specified limits. Predominantly the mapping and the de-mapping functions of the OTN contribute to wander accumulation due to the buffers being involved in these functions. The size limit of these buffers is specified in the ITU-T Recommendation G.798. This allows inserting up to 10 mapping/ multiplexing nodes per OTN island. A total of 100 mapping/de-mapping functions can be performed on this synchronization reference chain.

The ITU-T Recommendation G.8251 presents a Hypothetic Reference Model for 3R regenerator jitter accumulation: according to this model, at any OTUk interface the jitter will remain within network limits in a chain of one mapping clock and up to 50 cascaded 3R regenerators plus a de-mapping clock. It reports the results of extensive simulations showing that it is possible to have 50 OTN regenerators without exceeding the network limits of OTUk interfaces, assuming the regenerators comply with the model defined in this Recommendation.



9       Why use OTN

OTN offers the following advantages relative to SDH:
·         Stronger Forward Error Correction (FEC)
·         More levels of Tandem Connection Monitoring (TCM)
·         Transparent transport of client signals
·         Switching scalability

OTN has the following disadvantages:
·         Requires new hardware and management system

9.1      Forward Error Correction (FEC)

Forward error correction is a major feature of the OTN.

Already SDH has a FEC defined. It uses undefined SOH bytes to transport the FEC check information and is therefore called an in-band FEC. It allows only a limited number of FEC check information, which limits the performance of the FEC.

For the OTN a Reed-Solomon 16 byte-interleaved FEC scheme is defined, which uses 4 x 256 bytes of check information per ODU frame.



Figure 19 Error correction in OTN

According to ITU-T Recommendation G.709, an Reed-Solomon (255, 239) code with a symbol size of 8 is used for FEC. 239 input bytes are encoded in 255 output bytes. This code enables the detection of 2t = (n – k) = 16 errors in a codeword and the correction of t = (n – k)/2 = 8 of them.

FEC has been proven to be effective in OSNR limited systems as well as in dispersion limited systems. As for non-linear effects, reducing the output power leads to OSNR limitations, against which FEC is useful. FEC is less effective against PMD, however.



G.709 defines a stronger Forward Error Correction for OTN that can result in up to 6,2 dB improvement in Signal to Noise Ratio (SNR). Another way of looking at this is that to transmit a signal at a certain Bit Error Rate (BER) with 6,2 dB less power than without such a FEC.

The coding gain provided by the FEC can be used to:
·         Increase the maximum span length and/or the number of spans, resulting in an extended reach. (Note that this assumes that other impairments like chromatic and polarization mode dispersion are not becoming limiting factors.)

·         Increase the number of DWDM channels in a DWDM system which is limited by the output power of the amplifiers by decreasing the power per channel and increasing the number of channels. (Note that changes in non-linear effects due to the reduced per channel power have to be taken into account.)

·         Relax the component parameters (e.g. launched power, eye mask, extinction ratio, noise figures, and filter isolation) for a given link and lower the component costs.

·         but the most importantly the FEC is an enabler for transparent optical networks:
Transparent optical network elements like OADMs introduce significant optical impairments (e.g. attenuation). The number of transparent optical network elements that can be crossed by an optical path before 3R regeneration is needed is therefore strongly limited. With FEC an optical path can cross more transparent optical network elements.
This allows evolving from today’s point-to-point links to transparent, meshed optical networks with sufficient functionality.



9.2      Tandem Connection Monitoring

SDH monitoring is divided into section and path monitoring. A problem arises when you have “Carrier’s Carrier” situation where it is required to monitor a segment of the path that passes another carrier network.



Figure 20 Tandem Connection Monitoring

Here Operator A needs to have Operator B carries his signal. However he also needs a way of monitoring the signal as it passes through Operator B’s network. This is what a “Tandem connection” is. It is a layer between Line Monitoring and Path Monitoring. SDH was modified to allow a single Tandem connection. G.709 allows 6.

TCM1 is used by the User to monitor the Quality of Service (QoS) that they see. TCM2 is used by the first operator to monitor their end-to-end QoS. TCM3 is used by the various domains for Intra domain monitoring. Then TCM4 is used for protection monitoring by Operator B.

There is no standard on which TCM is used by whom. The operators have to have an agreement, so that they don’t conflict.

TCM’s also support monitoring of ODUk (G.709 w/o FEC) connections for one or more of the following network applications (refer to ITU-T G.805 and ITU-T G.872):
·         optical UNI to UNI tandem connection monitoring; monitoring the ODUk connection through the public transport network (from public network ingress network termination to egress network termination);
·         optical NNI to NNI tandem connection monitoring; monitoring the ODUk connection through the network of a network operator (from operator network ingress network termination to egress network termination);
·         sub-layer monitoring for linear 1+1, 1:1 and 1:n optical channel sub-network connection protection switching, to determine the signal fail and signal degrade conditions;
·         sub-layer monitoring for optical channel shared protection ring (SPRING) protection switching, to determine the signal fail and signal degrade conditions;
·         Monitoring an optical channel tandem connection for the purpose of detecting a signal fail or signal degrade condition in a switched optical channel connection, to initiate automatic restoration of the connection during fault and error conditions in the network;
·         Monitoring an optical channel tandem connection for, e.g., fault localization or verification of delivered quality of service.

A TCM field is assigned to a monitored connection. The number of monitored connections along an ODUk trail may vary between 0 and 6. Monitored connections can be nested, overlapping and/or cascaded.



Figure 21 ODUk monitored connections

Monitored connections A1-A2/B1-B2/C1-C2 and A1-A2/B3-B4 are nested, while B1-B2/B3-B4 are cascaded.



Overlapping monitored connections are also supported.



Figure 22 Overlapping ODUk monitored connections

9.3      Transparent Transport of Client Signals

G.709 defines the OPUk which can contain the entire SDH signal. This means that one can transport 4 STM-16 signals in one OTU2 and not modify any of the SDH overhead.
Thus the transport of such client signals in the OTN is bit-transparent (i.e. the integrity of the whole client signal is maintained).

It is also timing transparent. The asynchronous mapping mode transfers the input timing (asynchronous mapping client) to the far end (asynchronous de-mapping client).

It is also delay transparent. For example if 4 STM-16 signals are mapped into ODU1’s and then multiplexed into an ODU2, their timing relationship is preserved until they are de-mapped back to ODU1’s.

9.4      Switching Scalability

When SDH was developed, its main purpose was to provide the transport technology for voice services. Two switching levels were therefore defined. A low order switching at VC-12 level supports directly the E1 voice signals and a high order switching level at VC-4 level is used for traffic engineering. Switching levels at higher bit rates were not foreseen.



Over time the line rate increased while the switching rate was fixed. The gap between line rate and switching bit rate widened. Furthermore new services at higher bit rates (IP, Ethernet services) had to be supported.

Contiguous and virtual concatenation were introduce in order to solve part of the services problem as they allow to support services above the standard SDH switching bit rates.

The gap between line or service bit rate and switching bit rate however still exists as even with concatenation switching is performed at the VC-4 level.

For a 4 x 10G to 40G SDH multiplexer this means processing of 256 VC-4. This will result not only in efforts in the equipment hardware, but also in management and operations efforts.

For efficient equipment and network design and operations, switching at higher bit rates has to be introduced.

One could now argue that photonic switching of wavelengths is the solution. But with photonic switching the switching bit rate is bound to the bit rate of the wavelength and as such would be the service. An independent selection for service bit rates and DWDM technology is not possible.

A operator offering 2,5 Gbit/s IP interconnection would need a N x 2G5 DWDM system. When adding 10G services he has to upgrade some of its wavelengths to 10G. This would lead to inefficient network designs.

OTN provides the solution to the problem by placing no restrictions on switching bit rates. As the line rate grows new switching bit rates are added.

An operator can offer services at various bit rates (2G5, 10G …) independent of the bit rate per wavelength using the multiplexing and inverse multiplexing features of the OTN.


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