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.

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.

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.

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

This means that it will take

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

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

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.

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.

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.

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.

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:

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.

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.

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.

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.

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.

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?

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.

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.

Why is BER difficult to simulate or calculate? 

For a given design at a BER (such as 10-12 and a line rate of OC-3, or 155 Mbps), the network would have one error in approximately 10 days. It would take 1000 days to record a steady state BER value. That is why BER calculations are quite difficult. On the other hand, Q-factor analysis is comparatively easy. Q is often measured in dB. The next question is how to dynamically calculate Q. This is done from OSNR.

In other words, Q is somewhat proportional to the OSNR. Generally, noise calculations are performed by optical spectrum analyzers (OSAs) or sampling oscilloscopes, and these

measurements are carried over a particular measuring range of Bm. Typically, Bm is approximately 0.1 nm or 12.5 GHz for a given OSA. From Equation 4-12, showing Q in dB in

terms of OSNR, it can be understood that if B0 < Bc, then OSNR (dB )> Q (dB). For practical designs OSNR(dB) > Q(dB), by at least 1–2 dB. Typically, while designing a high bitrate system, the margin at the receiver is approximately 2 dB, such that Q is about 2 dB smaller than OSNR (dB).

Why Receiver Sensitivity is so important for optical module?

For Optical communication to happen, a receiver (essentially a photodetector, either a PIN or APD type) needs a minimum amount of power to distinguish the 0s and 1s from the raw input optical signal.

The minimum power requirement of the receiver is called the receiver sensitivity

The optical power at the receiver end has to be within the dynamic range of the receiver;
otherwise, it damages the receiver (if it exceeds the maximum value) or the receiver cannot
differentiate between 1s and 0s if the power level is less than the minimum value.

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.

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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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What,why and How of Pilot Tone in DWDM system?

There have been many efforts to use pilot tones (i.e., small sinusoidal components added to WDM signals) for the monitoring of WDM signals directly in the optical layer. For example, it has been reported that pilot tones could be used to monitor various optical parameters of WDM signals such as optical power, wavelength, and optical signal-to-noise ratio (OSNR), and so on .

The pilot-tone-based techniques could be used to monitor these parameters without the expensive demultiplexing filters (such as tunable optical filter and diffraction grating). Thus this technique could be extremely cost-effective. In addition, this technique is well suited for use in a dynamic WDM network, since the pilot tones are bound to follow their corresponding optical signals anywhere in the network. Thus the optical path of each WDM signal could be monitored simply by tracking its tone frequency

Although the pilot-tone-based monitoring technique has many advantages, it also has some limitations owing to the following problems.

First, the pilot tone could impose unwanted amplitude modulation on the data signal and degrade the receiver sensitivity .

Second, the performance of the pilot-tone-based monitoring technique could be deteriorated by ghost tones caused by cross-gain modulation (XGM) and stimulated Raman scattering (SRS) . These problems could be mitigated by using proper amplitudes and frequencies of pilot tones.  However, for use in a long-haul network with a large number of channels, it may still be necessary to restrict the number ofWDM channels to be monitored at a time (by using an optical bandpass filter).

Operating Principle

Figure shows the operating principle of the pilot-tone-based monitoring technique .We assume that an optical signal is transmitted from node A to node C via node B. A small sinusoidal component (i.e., pilot tone) is added to the optical signal at node A. This pilot tone can be extracted at node B by use of a simple electronic circuit and can be used for monitoring various optical parameters such as optical power, wavelength, OSNR, and so on. Pilot tones can also be used to monitor the optical paths of WDM signals even in a dynamically reconfigurable network. This is because once the pilot tone is attached, it is bound to follow the optical signal throughout the network. Thus we can monitor the optical path of each WDM signal simply by tracking its corresponding tone frequency.

For practical applications, pilot tones should be added into and extracted from the optical

signal anywhere in the network.

dBm:- A mathematical Interpretation.

dBm definition

How to convert mW to dBm

How to convert dBm to mW

How to convert Watt to dBm

How to convert dBm to Watt

How to convert dBW to dBm

How to convert dBm to dBW

dBm to Watt, mW, dBW conversion table

Introduction To The dB

Describing Power

Signal stages are cascaded, so powers are multiplied by gain or loss. This yields a lot of multiplications. This suggests the need for a logarithmic representation of power. A logarithmic scale is used to Condense wide range of numbers Ease multiplication

Logarithms

Log(x) = power to which base must be raised to give x. The base is chosen to be 10. Log(x) = y means that  x = 10y Log(A x B) = Log(A) + Log(B) Hence: Log(xN) = N x Log(x) Some example logarithm values: Log(100) = 2 because 102 = 100. Log(1000) = 3 because 103 = 1000. Log(1000000) = 6 because 106 = 1000000. Log(10) = 1 because anything to the power of 1 is itself. Log(1) = 0 because anything to the power of 0 is 1. Log(1/10) = -1 because 10-1 = (1/10) Log(1/1000) = -3

The deciBel

Represent gains or attenuations logarithmically (base 10) (the Bel) But to make numbers more convenient, scale by a factor of 10 (the deciBel or dB) Then, G = 10Log(Pout / Pin) in dB Examples: An amplifier has a power gain of 1000. What is this in dB?G = 10Log(1000) = 10 x 3 = 30 dB An attenuator has its output power 1/10th of its input. What is its transfer function in dB?G = 10Log(1/10) = 10 x -1 = -10 dB. (Note – dB can be negative) Since Log(A x B) = Log(A) + Log(B) we can add gains and losses. PR = PT + 20 – 1 + 30 – 2 – 204 + 30 -1 + 60 = PT – 68 dB For converting from a power ratio to dB, first work out powers of 10, e.g:   Ratio dB 1000 = 103 30 dB 1 = 100 0 dB 1/1000000 = 106 -60 dB   Then note the smaller factors: Factor of 2 is 3 dB (remember this!) Factor of 4 = 2 x 2 is 3 + 3 = 6 dB etc.   Ratio dB 20 2 x 10 is 3 + 10 13 dB 1/400 4 x 100 is 6 + 20 -26 dB   Examples of converting from dB to a Ratio (or more generally, ratio = 10dB/10):   dB Ratio 23 3 + 20 is 2 x 100 200 -3 1/2 -63 -60 – 3 is 1/106 x 1/2 1/2000000 -160 10-16 -167 -170 + 3 is 10-17 x 2 2 x 10-17 7 10 – 3 is 10/2 5 9 3 + 3 + 3 is 2 x 2 x2 8 1 10-9 is 10/8 1.25

Applying dB to Other Units

By default, dB is a power ratio. But it can be other things, for example, dB banana = dB relative to 1 banana. dBW = dB relative to 1 watt, so: 3 dBW = 2 W -30 dBW = 1/1000 W = 1 mW (1 milli-watt) = 0 dBm (m here – milliwatt) -60 dBW = 1 µW (1 micro-watt) = -30 dBm Bandwidth in Hz can be expressed in dB-Hz 1 MHz = 60 dB-Hz Similarly, Noise Temperature: 200 K =  23 dB-K By default, with dBs we are dealing with power. P = V2 / R where  V is the root mean square voltage, VRMS Thus a change in power (e.g. due to amplification) can be represented by: 10Log(P2 / P1) = 10Log(V22 / V12) = 20Log(V2 / V1) since Log(xN) = NLog(x) TIP: Take care with “Voltage gain in dB” which is usually a power gain, i.e 20Log(V2 / V1)

How Big Is A dB?

Examples of BER vs. Eb/No in dB: 1dB is approximately 25% change in power 1 dB is approximately the smallest detectable audio power change 0.1 dB is a practical measurement limit  But 1 dB is significant in digital demodulation

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 10^x/10

^{You can verify the same with above formula…Make things Simpler.}

Synchronization and Timing Loop Presentation–mapyourtech

Switching Conditions in SDH

Attenuation

Because the core of a fiber is made of glass, impurities (such as iron, magnesium, or even water) and irregular structures can cause the light irradiance to decrease, a condition known as attenuation, as the light travels through kilometers of the core. The attenuation factor is well known for all the types of glass used in long-haul fiber-optic cables; you can find it in the fiber manufacturer’s catalog.

The unit decibel describes the ratio of the optical power input into the fiber to the optical power measured at the output of the fiber of some length; it helps gauge attenuation. Power, which is the rate at which the light carries energy, is a more-convenient, more easily measured quantity than irradiance for characterizing the behavior of light in a fiber, so most fiber systems characterize the light-carrying capabilities of fibers by noting the effect on the power.

The decibel is a log base 10 scale, so the number of decibels is equal to –10log(Pout/Pin), which means that for a power ratio of 1/10, the measure is 10 decibels; 1/100 is 20 decibels, 1/1,000 is 30 decibels, and so on.

The equation that tells you how much power you can get out of a fiber of a certain length is In this equation,

✓ Pout is the power of the light exiting the fiber.
✓ Pin is the power input into the fiber
✓ α is the attenuation of the fiber, in units of decibels/kilometer.
✓ L is the length of the fiber, in units of kilometers.

This equation is important for designing a fiber-optic link when sending data over long distances (tens of kilometers) because it helps you plan where you need to place signal amplifiers, called repeaters, in the fiber to make sure that the signal sent is still usable. Flip to the later section “Repeaters” for more on these devices.

Relation Between DWDM Channel Spacing and Wavelength by Sanjay Yadav

The format of the label for SDH and/or SONET TDM-LSR link is:
https://docs.google.com/file/d/0BwE3NGerAe3tbXZ0NHhfUHpQbkk/edit?usp=sharing
0                    1                    2                    3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|               S            |   U    |   K    |   L      |   M      |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
For SDH, this is an extension of the numbering scheme defined in
G.707 section 7.3, i.e. the (K, L, M) numbering. For SONET, the same
signaling scheme is used in order to provide easy interworking between
SDH and SONET signaling. For the S field a STS-3 group, which
corresponds with the SDH AUG-1 level is introduce. The U field indicates
the position of the STS-3c-SPE or STS-1-SPE within the STS-3 group

1. S is the index of a particular AUG-1/STS-3 group. S=1->N
indicates a specific AUG-1/STS-3 group inside an STM-N/STS-3xN
multiplex. For example, S=1 indicates the first AUG-1/STS-3 group,
and S=N indicates the last AUG-1/STS-3 group of this multiplex.
S is not significant for STM-0/STS-1.

2. U indicates a specific VC/STS-SPE inside a given AUG-1/STS-3
group or STM-0/STS-1. U=1 indicates a single VC-4/STS-3c-SPE in a
AUG-1/STS-3 group or the single VC-3/STS-1-SPE in a STM-0/STS-1, while U=2->4
indicates a specific VC-3/STS-1-SPE inside the given AUG-1/STS-3
group.

3. K is only significant for VC-4/STS-3c and must be ignored for
higher order VC-3/STS-1-SPE. For SDH it indicates a specific branch of a VC-4.
K=1 indicates that the VC-4 is not further subdivided and
contains a C-4. K=2->4 indicates a specific TUG-3 inside the VC-4.
For a SONET STS-3c-SPE it is fixed to K=1 as SONET doesn’t support
substructured STS-3c-SPE.

4. L indicates a specific branch of a TUG-3, VC-3 or STS-1 SPE.
It is not significant for an unstructured VC-4/STS-3c-SPE. L=1
indicates that the TUG-3/VC-3/STS-1 SPE is not further
subdivided and contains a VC-3/C-3 in SDH or the equivalent in
SONET. L=2->8 indicates a specific TUG-2/VT Group inside the
corresponding higher order signal.

5. M indicates a specific branch of a TUG-2/VT Group. It is not
significant for an unstructured VC-4, STS-3c-SPE, TUG-3, VC-3 or STS-1 SPE.
M=1 indicates that the TUG-2/VT Group is not further subdivided
and contains a VC-2/VT-6 SPE. M=2->3 indicates a specific VT-3
inside the corresponding VT Group, these values MUST NOT be used
for SDH since there is no equivalent of VT-3 with SDH. M=4->6
indicates a specific VC-12/VT-2 SPE inside the corresponding
TUG-2/VT Group. M=7->10 indicates a specific VC-11/VT-1.5 SPE
inside the corresponding TUG-2/VT Group. Note that M=0 denotes
an unstructured VC-4, VC-3 or STS-1 SPE (easy for debugging).

The M encoding is summarized in the following table:

M    SDH                          SONET
———————————————————-
0    unstructured VC-4/VC-3  unstructured STS-1 SPE
1    VC-2                    VT-6
2    –                       1st VT-3
3    –                       2nd VT-3
4    1st VC-12               1st VT-2
5    2nd VC-12               2nd VT-2
6    3rd VC-12               3rd VT-2
7    1st VC-11               1st VT-1.5
8    2nd VC-11               2nd VT-1.5
9    3rd VC-11               3rd VT-1.5
10   4th VC-11               4th VT-1.5

In case of contiguous concatenation, the label that is used is the
lowest label of the contiguously concatenated signal as explained
before. The higher part of the label indicates where the signal
starts and the lowest part is not significant. For instance, when
requesting an VC-4-16c the label is S>0, U=0, K=0, L=0, M=0.

Examples of labels:

Example 1: S>0, U=1, K=1, L=0, M=0
Denotes the unstructured VC-4/STS-3c-SPE of the Sth AUG-1/STS-3 group.

Example 2: S>0, U=1, K>1, L=1, M=0
Denotes the unstructured VC-3 of the Kth-1 TUG-3 of the Sth AUG-1.

Example 3: S>0, U>1, K=0, L=1, M=0
Denotes the Uth unstructured VC-3/STS-1 SPE of the Sth AUG-1/STS-3 group.

Example 4: S>0, U>1, K=0, L>1, M=1
Denotes the VC-2/VT-6 in the Lth-1 TUG2/VT Group in the Uth VC-3/STS-1 SPE of the Sth AUG-1/STS-3 group.

Example 5: S>0, U>1, K=0, L>1, M=9
Denotes the 3rd VC-11/VT-1.5 in the Lth-1 TUG2/VT Group in the Uth VC-3/STS-1 SPE of the Sth AUG-1/STS-3 group.

Example 6: S>0, U=1, K>1, L>1, M=5
Denotes the 2nd VC-12 in the Lth-1 TUG2 in the Kth TUG3 in the VC-4 of the Sth AUG-1.

This Topic is legacy now but just to keep reader aware about the basic concepts, compiling  all together SONET/SDH here.

• Bellcore  defines GR.253 (SONET) and  ITU-T defines G.691 (SDH)
• SONET means Synchronous Optical NETwork
• First North American Fiber-Optic Telecommunications Standard to overcome the limitations of the traditional asynchronous network.
• Formulated by the Exchange Carriers Standards Association for the American National Standards Institute
• Incorporated into the Synchronous Digital Hierarchy (SDH) recommendations of the International Telecommunications Union (ITU).
• SDH is mostly used outside North America and principally in Europe
• Synchronous systems (e. g. SONET) :
• — Average frequency of all clocks in the system are same or nearly—Clocking provided by a highly stable reference supply

  —Allows many STS- 1s to be stacked together without of bit- stuffing

  —STS- 1s as well as VTs easily accessed from higher rates signals

  —Pointers accommodate differences in reference source frequencies, and phase wander

  • STS-1: Synchronous Transport Signal – level 1
  • STM-1: Synchronous Transport Module – level 1
• The telecommunications industry adopted the Synchronous Optical Network (SONET) or Synchronous Digital Hierarchy (SDH) standard for optical transport of TDM data. SONET, used in North America, and SDH, used elsewhere, are two closely related standards that specify interface parameters, rates, framing formats, multiplexing methods and management for synchronous TDM  over fiber.
• SONET/SDH takes “n” bit streams, multiplexes them and optically modulates the signal, sending it out using a light emitting device over fiber with a bit rate equal to (incoming bit rate) multiplied by “n.” Traffic arriving at the SONET/SDH multiplexer from four places at 2.5 Gigabits per second will go out as a single stream at 4 times 2.5 Gigabits per second, or 10 Gigabits per second.  

 SONET/SDH Digital Hierarchy

 SONET/SDH Tributaries

Optical Fiber Communication ..Once More by Sanjay Yadav

Optical Transport Network (OTN)

COMPARISON OF SDH AND OTN

1       Abbreviations

2       What is OTN/OTH

2.1        References

3       Optical transport network interface structure

3.1        Basic signal structure

3.1.1         OCh substructure

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

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

3.2        Information structure for the OTN interfaces

4       Multiplexing/mapping principles and bit rates

4.1        Mapping

4.2        Wavelength division multiplex

4.3        Bit rates and capacity

4.4        ODUk Time-Division Multiplex

5       OTUk, ODUk, OPUk Frame Structure

5.1        OPUk Overhead and Processing

5.1.1         Payload Structure Identifier (PSI)

5.1.2         Payload Type (PT)

5.2        ODUk Overhead and Processing

5.2.1         Path Monitoring (PM)

5.2.2         Tandem Connection Monitoring (TCM)

5.2.3         General Communication Channels (GCC1, GCC2)

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

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

5.3        OTUk Overhead and Processing

5.3.1         Scrambling

5.3.2         Frame Alignment Overhead

5.3.3         Section Monitoring (SM)

5.3.4         General Communication Channel 0 (GCC0)

6       OTN Maintenance Signals

6.1        OTUk maintenance signals

6.1.1         OTUk alarm indication signal (OTUk-AIS)

6.2        ODUk maintenance signals

6.2.1         ODUk Open Connection Indication (ODUk-OCI)

6.2.2         ODUk Locked (ODUk-LCK)

6.2.3         ODUk Alarm Indication Signal (ODUk-AIS)

6.3        Client maintenance signal

6.3.1         Generic AIS for constant bit rate signals

6.3.2         Client source ODUk-AIS

6.3.3         Client source ODUk-OCI

7       Defect detection

7.1        Default PLM (Payload Mismatch)

7.2        Default MSIM (Multiplex Structure Identifier Mismatch supervision)

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

8       Synchronization

8.1        Introduction

8.2        Network requirements

9       Why use OTN

9.1        Forward Error Correction (FEC)

9.2        Tandem Connection Monitoring

9.3        Transparent Transport of Client Signals

9.4        Switching Scalability

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.

Figure 3 OTN Network Layers

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:

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.

 OTN ALARM FLOW

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>> What is PMD versus Differential Group Delay (DGD)?

>> What is fiber PMD versus cable PMD?

>> What is a Link Design Value?

>> What is system PMD?

>> How much PMD can be tolerated in my system?

>> How do I guard against the transient nature of PMD?

>> Can I fix or compensate for PMD?

>> Are there standards for PMD?

>> There is so much ambiguity in PMD specifications. What should I be looking for?

>> What should I ask of my fiber cable supplier in terms of a PMD specification?

>> Is the sensitivity to PMD, fiber design – or manufacturer dependent?

>> How do I test an incoming cable or fiber for PMD?

>> Why is the minimum measurable PMD length dependent?

>> What are the limitations to measurement of low PMD?

>> What is a “low mode coupled” PMD measurement?

>> Is it possible to measure PMD with an OTDR?

>> I am not running high bit rates, like 10 or 40 Gbps, now, should I be concerned about PMD?

>> Is there any correlation between chromatic dispersion and PMD?

>> Should I worry about PMD in a multimode fiber?

source:http://www.fiberoptics4sale.com/wordpress/what-is-wavelength-selective-switchwss/

1. What Is a Wavelength Selective Switch (WSS)?

2. How Does a WSS Work?

1. The common input fiber enters the switch at point A where light is collimated by a microlens.
2. The following lens image the collimated beam on the diffraction grating at point C.
3. The wavelength dispersed beams fall then onto the MEMS device plane D
4. On MEMS device plane D, the beams are reflected with certain tilt angle depending on micromirrors’ setting.
5. All reflected beams are focused on point B again, where the angle to space conversion section will image the beam on the output fiber. Each output corresponds to a specific tilt angle of the micromirrors.

3. WSS Switching Engine Technologies

4. Switch Engine Technologies and Minimum Achievable Spot Sizes

2012-Infonetics-CRS-Optical-Leadership-SP-Survey….Know the Optical Product Manufacturer