Practice English Speaking&Listening with: Lecture - 9 SONET/SDH

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Good day. In this lecture we will discuss about SONET. The word SONET stands for Synchronous

Optical Network, SONET in the USA, Canada, and Japan, Synchronous Digital Hierarchy elsewhere.

For example in India we will be calling SDH. So this is a time division multiplexing system

that transmits a constant stream of information.

SDH is actually a successor of PDH. Few years back we used to have a PDH gear in our telecom

infrastructure in the wide area network part that is plesiochronous multiplexing (nearly

synchronous). This business of being a nearly synchronous introduces us a lot of problems

and complications. So, when from this nearly synchronous we went to synchronous, that was

a major achievement as well as improvement of services as we will see later on. In the

PDH multiplexing in which two or more signals are transmitted and nominally in the same

digital way and the significant instance occur at nominally the same time. This was PDH.

When SONET was introduced, it had a number of achievements to its credit. Firstly, it

is a standard multiplexing using multiples of 51.84 mbps that is an STS-1 rate and STS-N

we will look at these rates. This was used as building blocks. This is something

that is to be understood that why it is that particular rate value is so important. The

point is when you are multiplexing the original source may come from various sources, and

these signals will travel, will get together, will separate out and then mixed with others,

etc. That is possible only when we have in international multiplexing standard and this

international multiplexing standard was first achieved in SONET. What happened was previously

of course the rate which people used were must less and they had all kinds of differences.

As technology grew and different sort of countries on different things came together, they came

together at a certain rate of transmission and this is a basic building block of SONNET.

There is a standard multiplexing using multiples of this particular day. That in itself was

a big achievement.

Secondly, it also first stated the optical signal standard for interconnecting multiple

vendor equipment. the point was previously of course at lower rates they were all electrical

signal standards, SONET has both has electrical signal standard as well as optical signal

standard and in this optical signal standard it was possible to bring together multiple

vendors to agree on to some particular format.

And the third achievement in SONET was extensive OAM & P capabilities. So what are OAM & P?

O is for operation; a is for administration; and m is for maintenance; P is for protection.

Maintaining the system, administrating the system, operation of the system etc., are

much more flexible in SONET when compared to others. What kind of flexibility, etc.,

that we will see. Regarding protection also, as a matter of fact it is so strong in SONET,

that we will specifically discuss this aspect in a separate lecture when we talk about protection.

These are very strong points in SONET and thats not all.

The fourth one was multiplexing formats for existing digital signals. Its not that

such a development can take place in vacuumthat means they had some history and the

trouble was that different countries have different kinds of histories. It is not feasible

for a technology to come and say throw away whatever you have been doing and put this.

There will never be forklift upgrade; it is never possible because of cost, practical

considerations, and all kinds of things. So an evolving technology, in order to be successful,

has to bring together previous technologies so that they can merge into this new technology

and that was another SONET achievement. These existing digital signalsthese DS1, DS2,

etc., are different multiplexing standards at the low end. By the way there is also DS0

and DS0 rate is our venerable 64 kbps line rate. Do you remember once again that for

voice channels, we require a 64 kbps because of PCM, etc. we have already discussed. So

that is a DS0 rate and these several DS0 get together to form DS1 and so on, and that way,

there is a hierarchy of rates.

Then the fifth achievement of SONET was that it supports ITU hierarchy: E1, etc. so this

ITU hierarchy was more popular in Europe, India, etc. and they had rates like E1, E2,

E3, etc. E1 was something like 2 mbps, and then E2 was 1mbps, and E3 was 34 mbps, whereas

the development in USA was on a different track. They had a tone rate T1, T2, T3, etc.

Their rates were as above: DS1, DS2, etc. What happened was that when SONET got introduced,

these two sort of came together and although they are not perfectly identicalthese

SONET and SDHfor most of the part, they are identical; they interoperate with only

very slight modification at the boundaries, which is not very important. That is a great

thing and that means that the same standard is being adopted worldwide, so that any signal

can be transported in any way. If there is an infrastructure, we can transport in any

way to another part of world; there is no problem. So bringing together of these, that

means bridging the Atlantic Ocean of these two standards, which was another good achievement


The next is that it accommodates other applications. The other applications which were not a part

of this kind of hierarchy, like BISDN, that is broadband ISDN, also can be accommodated

in SONET and that way you see SONET was quite flexible; and how this flexibility is achieved

we will see that later on.

Finally it allows quick recovery from failure, talking about protection, etc. So if there

is a failure like a line failure or if there is a terminal equipment failure you can deploy

a SONET in a particular fashion and SONET can recover from this failure and this retransmission,

etc., can take place in a short period of time.

That is very important when you want to give the so-called career great service, where

arbitrary down time is absolutely not acceptable. As I said, we will discuss this separately

in another lecture.

Some of the broad features of SONET and SDH: it was first standardized by ANCI/ECSA, SDH

by ITUT. So SONET was by this ANCI/ECSA and SDH by this ITUT. SONET is time division

multiplexing, pure. We know what time division multiplexing is, and we will see later on

how frames, etc. are made up. It is a pure time division multiplexing system. SONET encompasses

optical and electrical specifications, so there are optical specifications as well as

electrical specifications. You know that usually at the user end, quite often things start

at the electrical level and the rates are low.

But as you go more towards the backbone of the network, the rates that are needed at

the backbone start becoming higher and higher and finally at the real backbone it has to

be very high-speed network, and such high-speed networks are only possible through optical

communication and optical networking. Once again, we will see about optical networking

in the next couple of lectures. Our specification, the SONET specification, spans both the electrical

side as well as the optical side, and that is a very good feature of SONET.

SONET uses octet multiplexing, octet means the same thing as a byte that means 8 bits,

so sonnet uses octet multiplexing. They are multiplexed byte by byte. SONET uses extremely

precise timing, something like in 30 years, maybe; SONET has very precise timing and that

is why things are synchronous. And if things become synchronous, then we derive a lot of

advantages out of that. And SONET provides support for operation, maintenance, and administration

(OAM) as we have already mentioned

SONET is actually superior to T3 and T4, etc. with improvements over the T carriers; these

T3, T4 are still in use but they feed into SONET nowadays.

But earlier, they were used to feed into this PDH and these T3, T4 have particular rates

which existed, and their specification left something to be desired. Because of this lack

of synchronicity, handling the signals from different sources is not easy. What could

happen is that when things are not synchronous, but just almost synchronous, then to handle

thisalmostpart, you have to do something; you have to incur some overhead; and you have

to incur some complexity. That was the difficulty with PDH; in SDH or SONET, this is eliminated,

and we get better transport performance. Then, we have the ability to identify sub streams.

This was another advantage of SONET over PDH, which is that a particular user uses may be

using a very small kind of bandwidthsmall in relative senseand then, as more and

more users, as I said as all these data streams or communications streams come towards the

backbone of the network, the pipes tend to get fatter. That means, we need faster and

faster communication. So between say two points in the backbone, there may be a very fast

communication going on and then after going to some other hops, this will again diverge.

SONET has this ability that different streams can get together, travel for some time, and

then again diverge. So the ability to identify sub streams is very important, and that is

also allowed in SONET, which was more difficult in the RDR system. And of course international

connectivity, as I said that it breached Atlantic and that was great. It enhanced control and

administrative function that was also very good from the point of view of service providers.

We have talked about this seven-layer OSI protocol; where does a SONET SDH really fit

in? SONET SDH goes to the bottom of this. If you remember, starting from the application

layer, we go right up to the physical layer. There are several layers in OSI model, and

there are other models. Anyway, usually the bottom-most layer is always the physical layer.

So SONET really fits into the physical layer in some sense.

So what would happen is that the layer just above the physical is the data link layer,

may be, or layer two. So after all this encapsulation, etc. is over through all these six other layers

including the data link layer, SONET takes it over for transporting it from one point

to another. So SDH is placed at the bottom of the protocol stack in the physical layer

along with the fiber. Any IP traffic even if it is the IP traffic of a packet oriented

trafficand remember that SONET is a TDM systemit can sort of travel within a

sort of TDM transport as they quite often do. So any IP traffic that is destined to

be transmitted across a fiber-based SDH network will be framed by a layer two protocol before

being ready to take its orders from the SDH equipment.

These are some of the multiplexing standardsI have not given all of them I just indicate

some of them. If you remember as I mentioned DS0 is a 64 kbps channel and 24 of them constitute

a T1 line. So T1 rate is approximately about 1.5 mbps; 4 T1 gives T2 and 6 T2 gives T3

and so on. Similarly 30 DS0this is a European systemgives E1 line. So E1, if you remember,

is about say 2 mbps: 4 E1 gives E2; E3 is a 34 mbps line. And then I suddenly jump right

up to this thing called OC3; this o is for optical. So this way, this 155 mbps is 3 of

the basic STS 1 rates that I mentioned earlier; I will come to this later on. So these are

some of the standards. There is a whole hierarchy of standards; for example, this name SDH is

also synchronous digital hierarchy, this is a hierarchy. For the SONET, the basic rate

is STS 1 that is synchronous transport signal level 1, and the speed is 51.84 mbps. This

is designed to carry what was DS3 RDR or a combination of DS1 c, and DS2 etc. As I said

a combination of different streams can flow through a SONET pipe or SONET infrastructure.

So that is good and that means DS3 is a fat pipe or DS3 is almost the same as STS-1. So

it is a fat pipe through which multiple pipes, say may be DS2 or DS1, etc. may travel.

And this net goes up to STS-N, whereas synchronous transport signal level is N; so this has a

speed of N into 51.84 mbps designed to carry multiple STS -1. I mentioned that these are

byte multiplexed STS-1 means 1 byte from one source and another byte from another source

and so on.

Fundamental SDH frame is STM -1; SDH if you remember is the other standard, which came

from Europe and they sort of came together and that is what we are talking about. SDH

frame is STM -1 synchronous transport module and the SONET version is OC -3, that is, optical

container, each providing 155 mbps. So when we come to this rate these 155 mbps

OC 13, different rates etc. and different systems are culminated here, at this 155 mbps,

almost 155. STM 4 provides four times the STM -1 capacity, STM 16 provides a further

fourfold increase, which means STM4 may be about 620 mbps, and then, if you go to STM16,

which is four times that of about 2.5 giga bit by s, then you have STM 64, which is about

10giga bit by s. So all these rates are there; that means, from this point onwards, these

two streams have converged and we are going to higher and higher rates in a sort of universal

fashion, which makes things easy across the world.

It is worth noting that the internet working between SDH and SONET systems is possible

at matched bit rates; for example STM4 and OC12; so they interoperate. A slight modification

to the overhead is required as they are structured little differently so there will always be

a little something; but anyway that is not very serious. So they do interoperate.

We have seen the SONET electrical hierarchy; now we look at the SONET optical signal hierarchy:

OC-1 is the optical career, level 1; it carries STS-1; OC 3 carries STS-3 or STM -1 at 155

mbps; OC-N optical career level N.

OC - N as I mentioned is an optical carrier, which uses N into 51.84 mbps, so OC - 48 is

about 2.4 gbps; overhead percentage is about 3.45%. OC signal is sent after scrambling

to avoid a long string of 0s and 1s to enable clock recovery. This is a small technical

point; that means in order to keep the whole thing synchronized, the SDH units use the

transitions which happen when there is a 1. So the point is that if there is no 1 for

a very long period in the data stream, then the clock on one side may drift relative to

the clock on the other side; that is always possible. So we try to avoid long streams

of 0s in this SONET or SDH, and we do that by scrambling the data from various streams,

etc., or descrambling them. The idea is that even if one of them is sending a long stream

of 0s, there will be quite a few 1s from the other streams and then the clock will be maintained.

An STS -N is synchronous transport signal electronic equivalent of optical carriers.

OC 3, OC12, OC 24 and OC 48 rates are common in telecom circuitsif you remember OC

48 is 16 times of OC 3; that is, 16 times 155 mbps, which is about 10 gbps. Up to 10

gbps is very common these days. Actually right now, with DWDM systems, OC 192 rate is already

in operation, and OC 768, which is 40 gbps, is being talked about. So that was another

disadvantage earlier that this digital hierarchy of standard rates did not exist beyond a very

small rateI mean small in todays comparison. But now we have an extended and open system

where, as technology improves, we can always go for higher and higher rates; so from OC

3, which is 155 mbps, we can go to maybe OC 192, which is 10 gbps or OC 768, which is

40 gbps that we are talking about now.

How do you use these high-speed links? These high-speed links of course have to be on fiber

we can look at details of fiber later on, but please note that in practical application,

an SDH line system will have a multiplexer that takes its inputs from a variety of sources

in different layer 2 data formats. So here we are talking about these different signals

coming in the electronic domain, and they are coming from a variety of sources, may

be coming with different layer 2 data formats. These are aggregated up to form frames at

a line rate of system, for example up to STM 64 for a 10 gbps bit rate system.

Now these frames at 10 gbps cannot be pumped anywhere. It is very difficult to pump it

on a copper. So these frames are transmitted out onto optical fiber links. There is a possibility

of multiple SDH multiplexers to each give out one wavelength of a WDM system. As we

will see later on, this WDM stands for Wave Length Division multiplexing; this is some

form of frequency division multiplexing. I mentioned about it when I talked about frequency

division multiplexing. In fiber optics, we talk about wavelength multiplexing so it is

possible that one multiplexer is feeding into one wavelength, another multiplexer is feeding

into another wavelength, and all these different wavelengths are traveling together in the


At the end of the system, there will be an SDH demultiplexer on the other end, just as

we have a multiplexer on one side. Naturally, you have to have a demultiplexer on the other

side that now accesses the individual data streams from the STM 64 frames as required.

So STM 64 is carrying lots of frames in a very short time; they are sort of separated

out and then fed into slower streams down the line. So there may also be an SDH add

drop multiplexer with the ability to remove and insert lower bit rate streams from the


Alternatively a digital cross connect may be present with the ability to switch individual

VC4s. Well, this is virtual container four, which is another concept, we will talk about

later. So between different fiber links there is a digital cross connect; if you have the

digital cross connect in the optical level, the advantage is that you need not go into

the electronic domain at all. So the advantage of not going into electronic domain is that

you are handling a huge, very fat, pipe; that means, a large number of channels, and you

can just switch them from one fiber to another fiber simply in the optical domain without

doing any kind of processing; and that is always an advantage.

We will talk about some SONET terms now; for example, envelope. This envelope is the payload.

Basically, after all encapsulation, etc., you remember that finally near the bottom

we have this layer 2 and this layer 2 protocol will encapsulate it and then hand it over

to SONET at the lower level, maybe at the physical level. So whatever this layer 2 hands

over to SONET is the payload; the rest of it are kind of system overheadspayload

plus some end system overhead also goes into this payload. So these together form what

is known as the envelope; this is a SONET term. Other bits and bytes which are used

for management, that means OAM and P portion, goes as the overhead of SONET. Then there

is the concept of concatenation; that means, unchannelized envelope can carry super rate

data payload, for example, ATM, etc. So, the method of concatenation is different from

that of T carrier hierarchy; we need not bother about it at the moment.

Then there are some nonstandard functional names in SONET, like

TM is for terminal multiplexer, also known as line terminating equipment or LTE. These

are ends of point-to-point links. ADM is for add drop multiplexer; we have mentioned this.

DCC is for digital cross connect wideband and broadband; MN is for matched nodes and

D plus R means drop and repeat, etc. Anyway, these are just some terms.

Now let us come to some important concepts in SONET namely: section, line, and path.

What is a section? I will just show you figure first and then come back to this.

Please look at this figure: we have some multiplexers. So as the figure shows, we have a multiplexer

in this side, another is an output that fits to another multiplexer. This multiplexer is

going in this direction and after some time, the signal becomes weak. So we want a repeater;

what is a repeater? A repeater is something which boosts the signal strength.

So there is a repeater, then it travels some more distance then there is a repeater again

and then it travels some more distance and then on other side we have the corresponding

demultiplexer and then it fits into the other de-multiplexer. From repeater to repeater,

we call it a section. So from repeater to multiplexer, this is also a section. So multiplexer

to repeater, repeater to repeater, these are called sections. And then, from multiplexer

to multiplexer, we call it a line. At the repeater, nothing happens excepting the signal

is cleaned up.

The signal may be boosted or there may be other cleaning operation, synchronizing operation,

etc., that may be done at the repeater; but as such, the signals which are traveling here,

the same set of signals are traveling here. At the multiplexer, of course, some of the

signals may go off in another direction; some signals may go in some other direction, etc.

So at the multiplexer, there may be a convergence or divergence, depending on which way the

signal is flowing. That may happen at the multiplexer, so from multiplexer to multiplexer,

we call it a line; and then from the end user point to end user point, we call it a path.

Look at this once; the portion from a multiplexer to a repeater is known as a section or it

could be a repeater to a repeater also; the portion from a multiplexer to another multiplexer

is a line. The portion from source to destination multiplexer is a path; below path line and

section is the photonic sub layer; that means photonic sub layer is whatever is happening

in the optical domain, and we are not discussing that at the moment.

Sections are bounded by repeaters or multiplexers that terminate the line; lines may carry several

tributary signals and are bounded by multiplexers, a path goes end to end between terminating


Each STH frame lasts 125 microseconds. As I mentioned, this 125 microseconds time period,

time epoch, is sort of sacred in this whole domain because 125 microseconds is what is

required for a DS0 channel. Remember this is a time division multiplexing, which means

that if you have a 125 microsecond kind of slot, then some of the DS0 bytes can take

these bytes. Actually if you have to take it as 8 kbps

and if it is 8 kbps, inverse of that is 125 microsecond. So if you have a 125 microsecond

slot, if 1 byte travels in this frame, then that is enough for 1 DS0 channel. In SONET

we have very sophisticated and very fast equipment; that means this is a time division multiplexing

system; within this 125 microseconds, not only 1 byte can go but lot of other bytes

can go. That means a lot of channels can travel together in this 125 microseconds frame. This

is the idea. So each STH frame lasts 125 microseconds; how many bytes are going in there depends

on whether it is STS -1 or STS -2 or STSN, etc.

So 125 microseconds as I mentioned is 8000 frames/s. STS -1 frame has 6480 bits or 810

bytes. That means in one, 125 microsecond slot or frame, we are putting in 810 bytes.

Theoretically, of course, that means it can carry 810 DS0 or voice signals; actually it

is not 810, it is lesser than that because a number of these bytes are used for different

types of overheads. We will talk about this. We have these 810 bytes; the octets are understood

in terms of a table of 9 rows and 90 columns; so let us look at this figure.

We have a SONET frame or an SDH frame, which has 9 rows; you can see the 9 rows on this

side and then 90 columns, total 90 columns. Out of these 90 columns, 3 columns have been

shown in yellow. These are sort of used for overhead and these 87 columns are used for

payload or for envelope. If you remember, the envelope contains the payload as well

as little bit of overhead, which we will come to later on. This is how after every 90 bytes,

we come back to again another 3 bytes of this overhead. This is how it is to be understood:

the first 3 columns contain transport overhead and TOH has 9 rows by 3 columns, that means

27 bytes, which is subdivided into section overhead SOH (section overhead), 9 bytes,

3 rows of 3 columns; LOH, that is, line overhead, which is 18 bytes, that is, 6 rows of 3 columns.

So we have section overhead and we have a line overheadremember we have these three

concepts like section, line, and path. We have not talked about path overhead.

There is some path overhead and it goes into the envelope; so there is some path and as

far as these things as line and section are concerned, these are the overhead bytes. Just

to clarify why do we require the over bytesthe point is that the multiplexers or

the repeaters have to have some communication between them in the control plain so as to

give you this OAM capability. For that some information needs to be sent or exchanged

between the two points; anywhere there is a section, the section overhead would consider

those things which are central to the section about the signal strength and other kind of

things; line overhead maybe would contain something else and similarly path overhead

would contain something else. But these are required for these OAM capabilities that we

have in SONET.

Let us look at these overheads separately; first section overhead, which defines and

identifies frames and monitors section errors and communication between sections terminating

equipment. So these are its functions: it identifies frames; monitors section errors

if there are errors, it monitors section errors; and communication between section

terminating equipment, maybe two repeaters or a repeater and multiplexer, and so on.

Line overhead locates first octet of SPE and monitors line errors and communication between

terminating equipment. We will come back to this locating of the first octet of SPE. This

is a very interesting feature and we will talk about this separately. Previously we

talking about section errors; so line errors and communication between terminating equipment,

etc., is taken care of by the line overhead. Apart from that, line overhead contains this

pointer, really, which points to the first byte of the SPE.

And then there is a path overhead; and as I said path overhead is really inside the

envelope and we will look at all these later. Path overhead verifies connection path; you

remember path means from end to end; that means from the end to end multiplexer is a

path. Whether the connection has been established or not, it monitors path errors, receivers

status, communication between path termination equipment, and so on. This is the POH . We

talked about the synchronous payload envelope or SPE that I was talking about. That is,

the other 87 columns hold the SPE (synchronous payload envelope). So SPE has 9 waves by 87

columns, which are divided into path overhead and payload, which means the path overhead

goes along with the envelope that is in the SPE, whereas other overheads have separate

bytes or separate columns associated with them as shown.

Now this SPE does not necessarily start in the column 4, which means that the SPE does

not necessarily stay within one frame; these are two very important points in SONET. The

point is that although you have these 87 columns, actually the data may start getting transmitted

at some arbitrary points inside those 87 columns. What is the idea? I mean why do you want to

leave something and then only start from the middle? The point is that if there are some

kind of mismatches of late, etc., if everything in the world were absolutely synchronous,

all activities and all equipments, etc., then you could have started from the beginning.

But that is not the case and this is where we absorb this kind of variation and this

gives great flexibility to SONET, which was not there earlier. And the other interesting

thing is that the SPE does not necessarily stay within one frame, which means that the

SPE may start in one frame and then end in another. We will just look at a diagram of

this; let us have a diagram of this.

You see the SPE in light green color; it really starts from somewhere. I mean somewhere after

leaving some of the rows: it starts here, and the path overhead is somewhere here, and

there are two frames here. So SPE is really spanning both the frames.

SPE is not frame aligned; it overlaps multiple frames; avoids buffer management complexity

and artificial delays. Whenever there is something to send, you can just send it in the envelope;

just put that pointer to that yellow edge, so that yellow edge will point to the first

byte of the SPE. It allows direct access to byte synchronous

lower level signals, for example, DS1, with just one frame recovery procedure.

These are the advantages of the SONET frames. This is one frame coming in may be 125 microseconds;

this is the next frame; and SPE, as I have already shown, can overlap. I mean it may

start somewhere within the first frame and then continue in the second frame in this

fashion and then be over here. Actually after this, some other envelope may come in over


Now of course where is the path overhead? There are two fields, H1 and H2 in LOH; LOH

means line overhead, which points to the beginning of the path overhead. Path overhead beginning

floats within the frame; 9 bytes that is one column may span frame along with the SPE;

it is originated and terminated by all path devices; and this gives you end-to-end support.

These are the features of path overhead. The point is that if you remember the path is

end to end, that means it is close to the end users; just as the end user may start

somewhere arbitrarily in-between, a path overhead also goes along with the SPE and it starts

over there and at LOH, we keep a pointer to this path overhead.

Just as some of the equipment that we use in SONET, one of the most important of these

is the add drop multiplexer. They are important because at certain point in the network, what

might happen is that there are some sources which want to send into the network. They

will sort of go so there is this SONET equipment, which is ADM let us say, and SONET stream

is flowing let us say like this. There may be something that wants to upload and travel

along with this thing. At the same time, this may be the destination location for some of

the other signals which originated elsewhere; they have to be dropped here. So some signals

have to be dropped, some signals have to be added. So this multiplexer can handle that

and that is very important.

That is why they are called add drop multiplexers. This stream is itself of course flowing at

a tremendous rate, whatever that rate is. So SONET SDH is a synchronous system with

the master clock accuracy of 1 in 109, which you will see is highly accurate. It shows

when you come in some kind of CCM clock somewhere and then there is a protocol for distributing

and maintaining this clock over the entire network. Frames are sent byte by byte and

ADMs can add drop smaller tributaries into the main SONET SDH stream and I have explained

how that is done. Within that frame you can send lot of bytes; you can take out some of

the bytes and add some of the bytes. That is how you take out some of the smaller tributaries

and add some of the smaller tributaries.

Digital cross connect, which is an optical layer equipment, is also very important. It

cross connects thousands of streams and software control, so it replaces patch panel; that

is a good thing about the digital cross connect and a software control is coming where the

control is coming from the control plane of the switches. You can connect the streams

from may be one fiber to another; it handles performance monitoring, PDH SONET streams,

and also provides ADM functions; that means add drop multiplexing functions.

Finally we have this concept of grooming in SONET. Grooming means, we group the traffic

in some format. So you want to keep this group in one particular way; it could be that there

is a one group of streams for whom you want to give higher priority or you want to give

higher quality of service. So you have to group them together. Similarly there may be

multiple groups; so it enables grouping traffic with similar destination, Quos, etc., which

is a part of grooming. It enables multiplexing or extracting streams alsothat is also

part of grooming. Narrow wider broadband and optical cross connects may be used for grooming.

If you look at this figure, you have this narrow band, this SONET layer and optical

layer. In the narrow band, we have this DS0 grooming and then in the DS1 grooming, there

is a white band and then the broadband DS3 groomingso the rates are going up, starting

from the 64 kbps, it is going up. When you are going up for the STS 48, you are in optical

domain; that means STS 48 is STM 16, so that is a high rate. The point is that, at that

rate, most probably, you are well in the optical domain. Then, finally, you can go to all optical

domain; that means wavelength, waveband, and fiber groomingthere are different levels

of grooming, depending on what you want to do. Lastly we will just talk a little bit

about virtual tributaries or containers. We have already talked a little bit about it.

This is the opposite of STM; actually in some sense this is called sub multiplexing; that

is, different streams coming together to form one very fat or very fast stream.

This is the other thinghow do we, sort of, differentiate these sub streams within

this, which has to do with sub multiplexing? STS -1 is divided into 7 virtual tributary

groups, SDH uses the term virtual containers or VCs. We talked about VCs; we just mentioned

what are called VTs or virtual tributaries in SONET lingo. So we have 7 virtual tributaries,

12 columns each, which can be subdivided further. You see that there are 12 columns each, with

7 virtual tributary groupswe have got 84 columns and these 84 columns are out of

the 87 you have in STS -1.

VT groups are byte interleaved to create a basic SONET SPE. So this VT groups are byte

interleaved. They may be again extracted from each other. VT 1.5 is the most popular, quickly

accessed, T1 line within the STS-1 frame. So the idea is that you have a T1 line, which

is approximately 1.5 mbps line, which is coming out of your small business, and you have a

1.5 mbps line. So that is your bandwidth requirement, you want to connect it to a distant location

somewhere. And you do not want your thing to get mixed up with others. At the same time,

as a small business you cannot have infrastructure of connecting to another location which is

wide apart. So you will go with this public infrastructure or public switched tele PSDN

network or whoever is maintaining this communication equipment.

Usually telecom people maintain it in most of the places. Anyway, they have a sort of

fiber going from one place to another, which contains very high-speed links. What you want

is your T1 line should join them, sort of get transported over the distance and then

go and feed into another T1 line at the destination. That is what you want. You want your T1 line

to sort of have a separate sort of existencejust like in a compartment, we have different

passengers. Passengers have their own individual entity but together they are packed into one

compartment and then they travel. Similarly your T1 line is going to ride onto to this

very fast stream and travel to the destination. So VT 1.5 gives your T1 line.

How do you find out about the difference? How do you separate them in the SP? The point

is, you require one more level of pointer used to access it. You can access a T1 with

just a 2-pointer operation, first from the LOHyou remember, you go to the SP, just

like that. Similarly, you go to the different tributaries or different containers using

just one more level of pointer. This flexibility was not there earlier; so it was very complex

to do the same function in DS3. For example, accessing DS0 within DS3 requires full demultiplexing,

stacked multiplexing, etc. So you require full demultiplexing; that is not required

in SONET. The point is that the other streams may go; you know where in that frame your

bytes really are traveling for the stream or for the container or for the tributary

that you are interested; you just extract it, others keep on traveling as they are.

So you do not demultiplex the whole thing and that gives a great advantage of add drop


This is just a figure showing that you can have various types of lines, all feeding into

the same infrastructure. You may have what we have put over here: DS1, which is 1.544

mbps, E1, 2.048 mbps, DSIC DS2, DS3, ATM .48.384, E4, which is 139.264 mbps, ATM is about 150

mbps, etc. They are sort of traveling; they are getting in different containers. From

VT 1.5, different tributaries, that is 1. 5236 etc., form a VT group and ride on a higher

strength or higher speed stream.

Just as I said, these are sort of identified through a pointer; so we have this transport

overhead. We use some bytes for that out of those 87 columns we have. So we use some columns

of that and then we put a pointer, which gives to the STS payload pointer. Then there is

a VT pointer, virtual tributary pointer, and this much is the VT SPE within the overall

STS-1 SPE, which is the payload. Even now SONET is the most widely used technology in

wide area networking that is existing today. Of course, as you know, as technology grows,

maybe we will go out of SONET.

People are already talking about going out of SONET because one disadvantage of SONET

is that its equipment tends to be expensive. Well, expensive compared to what we think

today. What is cheap today and what we think is cheap today may sound very expensive tomorrow;

that is how the technology grows. So people are talking about direct transport over the

optical layer, etc. May be we will touch those aspects later on. But all that is still in

a sort of experimental stage and on the field, actually, SDH or SONET equipment is almost

everywhere; all types of telephone companies are connected through that and major service

providers use this as a means of transport. Thank you.

Good day, so today we will be speaking about fiber optic components and fiber optic communication

as might of heard this lecture as well as the next couple of lectures, we will concentrate

on fiber optic components. We have looked at some of the physical layer components of

fiber optic systems before so we will sort of quickly review that, some of the stuff

we will be talking about today is going to be common and then from that point.

we will take out take it up into WDM systems, how wavelength division multiplexing is done

and how systems are handled in fiber optic domain, this fiber optic domain happens to

be very crucial because a lot of traffic in terms of volume may be as much as forty to

fifty percent, actually goes through the fiber as days are going by and as more and more

demand for bandwidth is coming up fiber optics is becoming more and more important , we will

be talking about fiber optic components today.

In fiber optic component of course the basic fiber is there we have already talked about

it, so we will talk little bit more about this then we have light source and receivers

on two end because we know that in fiber optic cables light is the carrier of information

then we require these different components like amplifiers, couplers, modulator, multiplexer

and switches so we will look up at these components one by one and then we will start our discussion

on wavelength division multiplexing.

The next set of components are multiplexers filters gratings, just talk little bit about

it ,if you look at this wavelength , these are all; wavelength selective , devices multiplexers,

filters , these are wavelength selective devices in a wavelength filter and what we want is

suppose ?1, ?2 etc so many are coming, I want only ? 1 out 2 ?3 ? 4 etc are absorbed or

something where as if you are a multiplexer I want the difference this ? coming in different

lines, I want all to be mixed together and use the same line, these are wavelength multiplexer

so application could be particular wavelength or a particular wave band selection.

Wave band is nothing but some contiguous operating wavelengths which all are side by side, if

you remember that in the operating window what ever be that 1550 what ever may be the

window you are using there you can have a number of ? all side by side, there is a guard

band between each of these operating ? so where the guard band that is given by the

i q t has specified, how much guard band etc you have to have but so you can have large

number of ? all group together in the same window. Aband out of that means a bunch of

sequence is out of that you can short select instead of selecting only, that is wavelength

band selection static wavelength cross connects and OAM is optical add drop multiplexers,

you have come across this term optical add drop multiplexers in the context of Sonet

but optical domain we require optical add drop multiplexers, we will come to that. Equalization

of gain so that is another application filtering of noise ideas used in laser operation and

dispersion compensation modules etc, these are the different applications one of the

standard wavelength selective component is arrayed waveguide gratings ,we have seen this


The Description of Lecture - 9 SONET/SDH