Good day. Today we will talk about multiplexing. Multiplexing is about sharing a medium; that
means different users are sharing the same medium for communication at the same time.Under
the simplest condition, a medium can carry one signal at any moment because if there
are two signals over there, they are going to interfere and then the signal will get
garbled; but for multiple signals to share one medium, the medium must somehow be divided,
giving each signal a portion of the total bandwidth. If you remember, a particular frequency
range around one particular frequency is called bandwidth and this bandwidth is the most valuable
resource so far as communication is concerned. We try to use this bandwidth to facilitate
the communication between a number of pairs of senders and receivers. That is the idea
of multiplexing. There are various reasons we want to use multiplexing, and the chief
one is that transmission service is very expensive – leased line, packet switching networks,
etc.
For example, lying of lines is in itself fairly expensive and a complex proposition and once
you lay a line you like to utilize it to the maximum. If you can use that for the maximum
amount of communication, multiplexing and compression techniques are the techniques,
which we use for this purpose – it saves a lot of money for the business. When you
can send a lot of data through the same line, the data capacity of the line increases, it
becomes more cost-effective for the company; most data devices individually require modest
amount of data but when there are a number of users, their requirements are aggregated
together and the sum total may be of quite a substantial bandwidth.
The current techniques that can accomplish this include: frequency division multiplexing,
wavelength division multiplexing, time division multiplexing, and code division multiplexing.
We will look at some of these, at least.
This is the scheme of multiplexing; you have one multiplexer and then you have n inputs
on one side. These n inputs have come to the same multiplexer; they are getting mixed up
in some fashion and they are being sent over the same physical link. And on the other side,
depending on the fashion in which you have put them together, they have separated into
different lines. These different lines on the right can now go to different recipients.
So just as on the left we have different senders and we have different receivers, a number
of sender–receiver pairs are utilizing the same physical link in-between.
The alternative to multiplexing should be direct point-to-point connection; this has
a number of problems. The first problem is that you need those lines that we were talking
about, you need lines for each device, and you need a large amount of wiring, if they
are on different floors. Another important point is that you need a lot of I by O ports
on the computer side, which really is not feasible. You may have a few I by O ports,
but you cannot have hundreds of I by O ports, it’s really difficult to have hundreds of
I by O ports there.
That is also another bottleneck that we wish to address. Another approach could be, a somewhat
older approach, which is that of a multi-drop line. The host polls machines to see who wants
to send and then uses the same lines, saves I by O port; the total communications load
is not greater than the data rate of line.
These are actually the simple approaches. Using a multiplexer approach, all the devices,
their data is multiplexed on one side, sent through one line, and the number of lines
in is equal to the number of lines out; the link carries multiple channels of information.
The types of multiplexer we have are: FDM, that is, frequency division multiplexing;
TDM, that is, time division multiplexing; STDM, that is, statistical time division multiplexing.
We will look at these. Just quickly before we get into the details
of each of these, in time-division multiplexing, each user periodically gets the entire bandwidth.
That means the entire channel is dedicated to one user but only for a short period of
time, for a small burst of time. After that it is somebody else’s time. We will look
at the details of this later.
Another common approach is the frequency division multiplexing; here the frequency spectrum
is divided among the logical channels. Here we have only one physical channel, but we
want to have a number of logical channels, so the frequency spectrum of the channel is
divided, each user has exclusive access to his channel.
This FDM sends signals in several distinct frequency ranges: one of the oldest uses of
frequency division multiplexing is in radio. For example, we have this electromagnetic
field in our atmosphere, let us say, through which electromagnetic radiation can pass.
Electromagnetic radiation can be of very large range of bandwidth, the whole range of bandwidth
is there. Out of that, the so-called radio frequencies constitute one part of it, a fairly
important part of it. This whole bandwidth of radio frequencies is divided into small
channels and each channel is given to one particular station.
And on the receiver side, on the radio side, what we do is that we tune our radio, let
us say, to one particular frequency, so that it receives signal from that particular station
only; although a number of stations are all transmitting at the same time. This is an
example of multiplexing. Another example of multiplexing, the same frequency division
multiplexing, we see nowadays in the cable TV. The cable TV providers give one cable,
one coaxial cable to the premise connected to the TV. That one coaxial cable apparently
is carrying a number of channels, may be hundreds of channels these days. What is done once
again is that, all the frequencies, which can travel down this cable, are broken into
a number of logical channels and each channel is dedicated to one particular station. That
is how we carry multiple video channels on a single cable. Of course I mean point-to-point
is out of question. Imagine the cost of stringing wire for each channel; you’ll have a hundred
cables coming into your building which is not really possible.
Each signal is modulated on to a different carrier frequency; carrier frequencies are
separated by guard bands. This guard band is important because one particular channel,
maybe a radio channel or a TV channel, the principle is the same everywhere. So, that
uses some middle frequency, maybe some range of frequencies. The next station maybe a TV
station or a radio station – whatever it is, that can occupy; first of all that has
to be non-overlapping. The range of frequencies which is assigned to the second station has
to be non-overlapping with the range of frequencies which is given to the first station and not
only that, between these two there must be some separation in frequencies. Otherwise
what will happen is these two signals are going to interfere with each other. So that
is a guard band.
The bandwidth of the transmission medium exceeds required bandwidth of all signals because
of these guard bands which have to come in-between. Usually for frequency division multiplexing,
analog signaling is used to transmit signals. Broadcast radio and television, cable television
and amps cellular phone system use frequency division multiplexing. Amps are an old cellular
phone system that was there in USA. Nowadays of course, the kind of Cellular technology
that we have in our country uses more complex multiplexing techniques; maybe I’ll mention
that later on.
These systems all use frequency division multiplexing. This technique is the oldest of the multiplexing
techniques. Assignment of non-overlapping frequency ranges to each user or signal on
a medium –all signals are transmitted at the same time, each using different frequencies.
A multiplexer accepts inputs and assigns frequencies to each device.
The multiplexer is attached to a high-speed communications line because all these frequency
bands on individual users are going to adapt to a fat range of frequencies. The communication
line must be able to handle this whole frequency range or in other words, the communication
line has to be high speed. A corresponding multiplexer or demultiplexer is on the end
of the high-speed line and separates the multiplexed signals, because on the other end you have
to separate them out. Since it involves analog signaling, it is more susceptible to noise.
Thus we have seen that the analog signals are more susceptible to noise. In digital
signals we can sort of clean the noise more easily.
This is a diagram. If you note that for a particular user, suppose user 1, we give channel
1; along the time this channel 1 is entirely dedicated to user 1 for all time. Similarly
channel 2, which is at a different frequency – frequency is on this side and time is
on this side – once again user 2 gets to use this frequency for the whole time. This
is the simplest kind of scheme, so that for this whole frequency band, frequency is being
divided, which is why it is called frequency division multiplexing.As was mentioned, the
frequency channel is divided into logical channels; each user hangs on to a particular
frequency, the radio spectrum and a radio is examples of the media and the mechanism
of extending information from the media.
This is another picture of the same thing, see channel 1 and then channel 2, channel
3 – they all send some signals. If you plot them across the frequency versus the signal
strength, you get a picture like this, so this is the first channel, this is the second
channel and this is the third channel. In-between this part here is the so-called guard band.
This frequency is not used by either of them. There is some overlapping due to noise and
other issues; we will not get into that; due to that this is what it looks like; the frequency
bands are all separated.
Here is another diagram which shows, let’s say you have the host and a 300 bps line;
a number of 300 bits per second lines are being multiplexed to 1200 bps line and then
there are some guard bands: maybe 300 to 1013 Hz is one and then 1433 to 2206 Hz is another
line; maybe 2686 to 3400 Hz is another line. These are the 300 bps sub-channels and you
have the guard bands in-between. On the other side again you have this multiplexer–demultiplexer,
which takes it to different users.
One problem with FDM is that it cannot utilize the full capacity of the cable. It is important
that the frequency bands do not overlap so that is why you have to give the guard bands,
so the full capacity of the cable is not utilized. The other disadvantage – we will summarize
these advantages and disadvantages later on – is that it is sort of bit more prone to
noise because we are dealing with analog signals. FDM is usually used to carry analog signals
although modulated digital signals can also be sent using this technique.
We have another type of multiplexing, which is wavelength division multiplexing. Wavelength
division multiplexing is the same as the frequency division multiplexing. Only thing is that
here the operating frequencies are much higher, actually they are in the optical range; that
is why somehow it is given different name like wavelength division multiplexing, you
know different wavelengths means different frequencies. If you are doing wavelength division
it is same as doing frequency division and this is used in optical fibers. This is the
same as FDM but applied to fibers.
There’s a great potential for fibers since the bandwidth is very huge. As I mentioned,
the operating frequencies are very high, even a slight percentage deviation from the mean
operating frequency gives you a very large bandwidth. So you can sort of compact or pack
a lot of channels at those high frequencies. Fibers have a large bandwidth capacity and
with different energy bands are passed through a diffraction grating prism combined on the
long distant link and then split at the destination. This is got high reliability as well as high
capacity, two very interesting properties.
So multiplexing and demultiplexing for a wavelength division multiplexing is of course very simple.
If you take a prism, you know if you send a white light through a prism it breaks up
into all the different colours, because the refractive index of the prism varies with
the frequency. So different frequency can come in, you can make the same light source
also to say in this and on the other side you can use a prism to break it up. Of course
I have shown a prism here for simplicity; usually something like a diffraction grating
is used. So if you look at the fiber spectrum, if you plot the power versus the frequency
so one fiber maybe giving at this frequency, another fiber maybe delivering power at this
frequency. If you put them together simply then you get this power versus ? or power
versus ?, kind of plot that you get Wavelength division multiplexing multiplexes multiple
data streams onto a single fiber optic line.
Different wavelength lasers called ?s; traditionally ? is used for denoting a wavelength, some
are quite extensively used in this optical communication domain. Different frequencies
are called different ?s and they transmit multiple signals. Each signal carried on the
fiber can be transmitted at a different rate from the other signals. This is one very interesting
thing of fiber that is, different colours although they may get mixed but finally again
they will be separated. What rate of data, what protocol etc. are they carrying? The
fiber system is really transparent to all that so that is a good thing – different
systems can be put to the same fiber. As I mentioned that fiber have a very high capacity
so you get to push in a lot of Channels into the same fiber. Dense wavelength division
multiplexing also called DWDM. They may combine as many as 30, 40, 50, 60 or more channels
these days into one fiber and each of these channels has a very high capacity; so DWDM
channel has a very high capacity and it is improving all the time. People are finding
ways of packing more ?s into the same fiber and it is supposed to be quite scalable into
the future; but of course if you want to pack those wavelengths in a very dense fashion,
your equipment becomes somewhat costly.
A cheaper alternative which has come up recently which are called CWDM so Dense Wavelength
Division Multiplexing is called DWDM, cheaper version is called Coarse Wavelength Division
Multiplexing or CWDM. In this CWDM the channels are more widely spaced on the spectrum. Multiplexer, demultiplexer,
and all these costs go down and become cheaper alternatives but still you can push in a number
of wavelengths into the same fiber. Now we come to the other very important multiplexing
technique called Time Division Multiplexing.
As a matter of fact this is extensively used in our computer communication or telecommunication.
Time division multiplexing is very extensively used especially on the service provider side;
we will look at these later on. Sharing of the signal is accomplished in time division
multiplexing by dividing the available transmission time on a medium among users; so time is being
divided. What is done is that, as I mentioned before, that each user gets a small burst
of time in which at that time the entire channel and the entire bandwidth is at his disposal;
but as soon as that short duration of time is over, then it is somebody else’s turn
to send and this maybe in a round-robin fashion after some time it will come back to the original
user, so that he can send his next burst of data or voice or whatever it is.
Usually we use digital signaling in time division multiplexing, almost always. Digital signaling
is used almost exclusively. Time division multiplexing comes in two basic forms: one
is synchronous time division multiplexing, the other is statistical or asynchronous time
division multiplexing. We will look at synchronous time division multiplexing first and because
that is the simpler scheme and then you will look at statistical multiplexing.
So the same multiplexing scheme now looks like this. If this is the frequency, the same
plot if you remember in the previous diagram when we were doing TDM we were slicing this
channel in this fashion. We are now slicing it in this fashion; that means time is divided;
channel 1 channel 2, channel 3, channel 4, channel 5, channel 6; they get the entire
frequency range for a small burst of time. And suppose only 6 channels are being multiplexed,
after 6 again it will be the turn of channel 1. So channel 1 sends the first burst of data
etc. using the entire frequency band during this time period and then during this time
period. During this time channel 1 is not sending anything so it is quiescent. Time
is getting divided in this fashion; this is why this is called time division multiplexing.
This is a schematic of a time division multiplexing system. Like FDM time division multiplexing
saves money by allowing more than one telephone call to use a cable at the same time. You
have terminal 1, terminal 2 and terminal 3. Terminal 1 is giving at this particular time
then terminal 2 and this terminal 3. And over here on the other side this sequence and the
way it goes that is known that has to be known; so whatever signal comes at one particular
slot; this multiplexer knows that this must be for receiver number 1 and next slot is
for receiver number 2 and next slot for receiver number 3. So on the receiver side; it just
knows if it is this time slot, it is for a particular user. This is a synchronous division
multiplexing because this clock and this clock have to be synchronized otherwise, there will
be a problem. Instead of dividing the cable frequency bands, TDM splits cable usage into
time slots. Each channel is given a regular time slot in which to send a PCM signal.
This is another view of the same time division multiplexing. Actually in actual practice,
we will go into the details of this later on. Actually this time division multiplexing
can be done in a hierarchy of manner. We will look at this hierarchy later on. If you remember
from our digital signals discussion, that our voice, because it is up to say 4 kHz is
our voice signal, because of Nyquist theorem, we require 8 kilo samples; that means double
the rate that means 8,000 samples/second for capturing voice. And for each sample, it has
to be converted. Each sample is an analog value; so that value has to be converted to
a digital signal, an 8 bit digital signal, which is called PCM – pulse coded modulation.
That means there are 256 different levels that this is distributing, which is very fine.
Our amplitude of voice if you divide it into 256 parts we are making a very fine distinction;
that gives you 8 bits per sample 8,000 samples by second; that comes to 64,000 bits per by
second. This is a very basic rate and this rate has got a name; we will go to these names
later on. A number of such channels may be multiplexed together. This 64,000 bits by
second is a very basic rate.
A number of such channels may be grouped together to form one particular higher level channel
and different higher lever channels may again join together to make a hierarchy of channels,
this is what is shown over here.These T1 etc., don’t bother about this figure at this moment
because we will look at this later on. So T1 is one particular rate at which some of
these signals come. There are worldwide standards actually there are two standards, we will
talk about it later on.
Standards of the rate at which these signals come; they sort of join together and form
a stream of T2 stream and different T2 streams, different waves have different names like
T1, T2, T3, etc., will form a T3 kind of frame and as you can see that the bandwidth is increasing
all the time. The T1 rate is 1.544 Mb by second. Time division multiplexing used for digital
signals or analog signals carrying digital data, data rate of transmission media exceeds
data rate of signals; so this has to be there otherwise your TDM system will not work. Because
in that small burst of time which is given to a particular user, in that burst of time
that user should be able to send whatever digital data it had accumulated during that
time, as well as the previous epoch during which it was quiescent; because at that time
it was other people’s turn. It may be a buffer or kind of thing where all these bits
are getting accumulated and when it gets its small burst of time, during that burst of
time it sends all the bits in that buffer, to make it empty.
And then it becomes quite again, so it will accumulate the bits again in the buffer and
then again when it gets the next burst of time it will send all the bits. Thus data
rate of transmission media has to exceed data rate of signals. It uses a frame; frame means
a number of bits and bytes put together to form one particular unit at which to send
it over; one slot for each slice of time. What would happen is that a slow device during
his slot will send a lower number of bits; very high-speed device will send a large number
of bits, but your slot will come periodically. One or more slots of each device is equal
to channel and time slots are transmitted whether source has data or not. This is one
particular problem with time division multiplexing. Think about this – that on the receiving
side, receiver side is blind; it does not know this stream of bits which has come is
for whom. The only way he knows that this is the time slot for receiver I, so he will
send those bits to receiver I and so on in a round robin fashion.
What happens is, suppose the senders are S1, S2 to Sn, receivers are also S1, S2 to Sn;
and then we have this. The SI will be sending to RI; what happens if SI has nothing to send
at that point of time? During that time it is not possible for the multiplexer or the
demultiplexer, in the normal scheme of things, to send somebody else maybe Sj’s signal
during that time although physically it would be possible; but then on the demultiplexing
side it would cause a problem because the demultiplexer would not know whether this
bit is for Ri or Rj. That is why even if Si has nothing to send, that slot will have to
run empty; you cannot utilize it any further. This is the weakness of time division multiplexing
which we try to sort of address in a different scheme, we will be talking about just now.
The synchronous time division multiplexing, remember that it is synchronous because the
multiplexer and the demultiplexer has to agree about the slot of time; the original time
division multiplexing, the multiplexer accepts input from attached devices in a round robin
fashion and transmit. T1 and ISDN telephone lines are common examples of synchronous time
division multiplexing.
If one device generates data at a faster rate than other devices, then the multiplexer must
either sample the incoming data stream from that device more often than it samples the
other devices or buffer the faster incoming stream. If a device has nothing to transmit
the multiplexer must still insert a piece of data from that device into the multiplexed
stream, it may be a dummy data.
Before I talk about the other kind of TDM, which is the statistical time division multiplexing;
I must mention that there are hybrids multiplexing schemes also. That means in the hybrid scheme,
what you do is that you take a frequency band; it is like frequency division multiplexing,
so you break up your entire available channels into a number of bands; but each of the bands
instead of in the pure FDM, we are giving it to only one user but this band is allotted
to may be a number of users who use TDM on this band. This is a combination of FDM and
TDM, so such kind of hybrid multiplexing systems are quite widely used in these days and one
good example is the cell phone communication that goes on; that may use hybrid kind of
multiplexing systems.
In hybrid multiplexing system, both FDM and TDM may be combined; the available channel
is broken up into frequency bands; in each band multiple channels are accommodated through
TDM. Now we talk about the other kind of time division multiplexing, namely, statistical
multiplexing. This is time division but on demand rather than fixed, so we reschedule
link on a per packet basis and packets from different sources are interweaved on the link.
Remember that I mentioned the disadvantage of time division multiplexing is that even
if the sender does not have to send anything, that time slot will go empty. We cannot utilize
it because otherwise on the demultiplexer side we will not be able to decipher who it
is in the pure time division multiplexing. This is to be addressed in statistical time
division multiplexing or STDM; what is done is that the packets are sent on demand; that
means if somebody has to send and the channel is free, it will be sent.
Of course if everybody has to send, then you have to once again do some kind of scheduling
in a round robin fashion in a simple vanilla case or you can do more fancy kind of scheduling.
I am not going into that. So this utilizes the channel in a much more efficient manner;
but you have a problem. We know what the problem is – the problem is on the demultiplexer
side. On the demultiplexer side, one particular time slot being allotted to one particular
sender or a receiver, that relationship is no longer there, and if that relationship
is no longer there, how do we make out on the other side whether this is meant for whom?
So you have to have that address inside that data stream. That means inside the data stream,
there must be something which tells the receiver and the demultiplexer that this is meant for
that receiver. So you have to have the address, at least the destination address in the packet
so to say, now I am calling it a packet that whatever the burst of data sent in a particular
time slot.
It allows connection of more nodes to the circuit than the capacity of the circuit.
How is this possible? This is because usually our communication is always bursting, for
example, when I am talking I may require 4 kHz, when I am making a sound; but then there
are periods when I am quiescent that means I’m not talking at all .So overall if I
can utilize that, maybe my effective rate of utilization of the channel will become
more than 4 kHz; because one user is using 4 kHz. And this way in each of the channels,
the same thing happens and much more dramatically, in the world of sending and receiving data.
Data traffic is inherently bursting, which means that one computer is communicating with
other computer, this will do so in bursts. It will want to send a lot of stuff; suddenly
it has something to send, so it will want to send it fast in a burst.
Then for a long time it will be quiet, as there will be nothing because it will go there,
maybe get processed over there, maybe some user sees it, sees that data, and things like
that. So it is quiet for a long time, then again it sends another burst. And in-between
this burst there is a lot of time. While the burst is there, I am using at a very high
speed but then other people can share it, so that is why in some sense it allows connection
of more nodes to the circuit than the capacity of the circuit.
This works on the premise that not all nodes will transmit at full capacity at all times;
must transmit a terminal identification, as I said the destination identification has
to be there; and may require storage because two different sources may want to send at
the same time and both of them land up hoping that the channel will be free. I am calling
it a multiplexer at this point, at the multiplexer end you have to store them and then forward
them over the channels, and then the channel becomes available.
This is a picture of once again getting multiplexed – they are getting multiplexed as they come.
This is the queue in-between and you have this buffer over here, this is the memory
where the incoming packets are getting stored and they are making a queue so there are a
number of packets here and from this queue one by one as they come, it is not necessary
that they have to go in a strict round robin fashion; as they arrive they have are being
pumped into this transmission channel. On this side, on the demultiplexing side, it
will look at some receiver address which is contained within this packet or cell, whatever
it is, and then send it to the proper receiver.
The buffer packets that are contending for the link, the packet queue may be processed
first in and out, but as I said this is the most vanilla kind of scheme; there are other
fancy schemes. For example you may have a high-priority queue for some of the users
who may be paying more – that queue is processed first before the other queues and all kinds
of possibilities are there. But of course since you have a buffer, it is theoretically
possible and since your capacity of the link cannot handle if all the senders decide to
send at the same time, which may happen, in which case it will be filling it in the buffer,
but the buffer may also overflow, which means that there is a congestion in the network
and then some data loss is there.
To summarize, a statistical multiplexer transmits only the data from active work stations. If
a work station is not active, no space is wasted on the multiplexed stream; a statistical
multiplexer accepts the incoming data streams and creates a frame containing only the data
to be transmitted and also as I said this frame will also contain the some kind of destination
address.
This is an example to identify each piece of data and the address is included. So the
data of c as well as address of c, data of a as well as address of a – the address
also has to go with the data. This of course is under the assumption that all these data
packets and these addresses, together all these packet lengths are fixed, which is the
case in some systems like ATM, etc. The size is fixed but the size may vary also, if the
size varies then you will have to include the length of a also, that means this is the
data of a; what is the total length of a or maybe the data part, if this part is fixed
depending on the scheme then the address to which this is destined. All these are put
in the form of a frame and then it is sent. Finally I would like to summarize the advantages
and disadvantages of each of the schemes. By the way, in the beginning I mentioned something
called code division multiplexing, which is a very interesting scheme.
We will talk about code division multiplexing when we talk about multiple accesses. There
are actually two terms; one is multiplexing and the other is multiple accesses. Usually
this is how we mean it but distinction is getting blurred these days; usually when you
are multiplexing, the sources are sort of converged to the multiplexer in some sense.
The sources are converged to the multiplexer in some sense and it goes to the demultiplexer
where it diverges. There is a specific convergence which is there, that means a number of telephone
lines are getting multiplexed; a number of telephone lines will come to the local telephone
exchange. There these may be all destined for some other exchange, so all these lines
will be multiplexed together so they are happening physically in one place. If the problem is
this, then once again you have a shared medium but the users are distributed. In that case
we call it a multiple access system.
CDMA is a multiple access system as well as you can in some sense call it a multiplexing
system also. As I said this distinction is slowly getting blurred these days; for example,
you are doing some kind of multiplexing when you come to cell phones. We are doing some
hybrid kind of multiplexing; that means we have got a combination of FDM and TDM; i.e.,
we have some bands of frequencies which are being used by a number of users; but these
users are not at the same place. They are carrying some wireless station.
That means they are carrying their mobile handsets and are at various locations. Even
for the same base station, under the same base station, they are at various locations
in that area so to say. Thus the distinction between multiplexing and multiple accesses
is getting a little blurred these days. Anyway we will discuss code division multiplexing
or multiple accesses when we talk about other kinds of multiple accesses. As you can guess,
there is time division multiple access and then there is frequency division multiple
access and then of course this code division multiple access is also there; so we will
discuss it there.
Coming back to the scheme that we discussed, namely, FDM, what are the advantages and good
things about each of these schemes? FDM is of course simple and being simple means that
it is cheap and is very popular, and as I mentioned this is the oldest multiplexing
technique. Your radio uses this and lots of other systems use this. TDM is specifically
for digital signals, so for digital signals TDM is very nice and then you can have a multiplexing
hierarchy; that is another advantage of TDM. Why it is an advantage? Well the advantage
is because of this: at different places we may require different levels of bandwidth
and speed. Just think about this – suppose we have this telephone network, you may have
a small exchange; the small exchange will naturally have a small number of users and
only a few of those users will want to communicate or make an STD call or to communicate with
some other telephone, which is under some other exchange.
So the few will have to be multiplexed together at the small exchange and it will be sent
to the trunk exchange and from there it may go to another trunk exchange and so on and
finally reach. At the trunk exchange what is happening is that once again various such
connections from different local exchanges are coming and converging to the same trunk
exchange. When this trunk exchange is communicating with another big trunk exchange, what is happening
is that there are a lot of tributaries flowing into it; there are a lot of channels. So basically
in the same signal, we are doing time division multiplexing.
That means the rate at which the receiver at this end is sending, that same rate has
to be maintained throughout; but then instead of flowing in small tributaries, it is moved
into a very fat channel. We require a multiplexing hierarchy. So such multiplexing hierarchies
are easy to implement using time division multiplexing; we will see more of this when
we talk about synchronous digital hierarchy later on.
STDM’s advantages: we are only talking about advantages. We will talk about disadvantages
later on. In statistical time division multiplexing, this has certainly more efficient bandwidth
use. It is much more efficient because as I mentioned, that especially data usage and
usage in general may be very bursty and if it is bursty, then during the quiescent period,
ordinary TDM is just wasting the channel whereas STDM is making full use of it.
So it has more efficient bandwidth use; that is a good thing about STDM. Another advantage
of STDM is that the frame can contain control information; since you are putting a frame,
may be with the address of the receiver or may be other kinds of control information
can be put in the frame. And then, the packets can be of varying sizes. These are the advantages
of STDM; not all STDM systems will allow this varying sizes; for example, ATM will not allow
varying size cells. In ATM, the packets are called cells, they will have a fixed size
but some systems may allow. WDM: we are writing it separately because although as I said this
is some form of FDM, this works with a very high capacity, is scalable, and this has got
low noise sensitivity.
That is another good thing about the communication in the optical domain. Suppose you are doing
it in a electrical domain through a wire, may be, if there is some kind of electrical
noise, somewhere some motor has started or very bad if there is some lightning somewhere
nearby, lot of noise will get into it and this electrical wire will catch all this noise.
Whereas an optical fiber is not going to catch any of these noises, it is very immune to
noise, which is why it’s got a very low noise sensitivity, which is very good and
very high capacity and you can scale it to still higher capacity using DWDM.
Just a quick look at the disadvantages: FDM, as I mentioned earlier, is susceptible to
noise, wasted bandwidth, because of this guard band, etc., and of course one frequency is
whatever the small band is being permanently given to one user and if his usage is very
bursty, then a lot of bandwidth is wasted. Then, we have a limited frequency range. In
TDM we have wasted bandwidth for the same reason.
STDM takes care of that problem in a TDM but this is very expensive; traditionally we did
not have any other alternative so we used STDM a lot when high speed was necessary especially
in the service provider side, we used STDM. WDM is somewhat more costly than TDM and FDM
but since this is a developing technology, this cost is coming down and that means the
WDM cost is coming down but it is still somewhat more costly than TDM and FDM but may be less
costly than STDM. We have looked at the different multiplexing techniques today; in the next
lecture we will look at one kind of network, which is very widely used for data and all
kinds of communication, namely, the telecom network, the telecommunication network. Because
remember that although it is called telecommunication network and it was traditionally for people
talking over telephones, for voice, etc., that is the network which carries most of
the wide area data traffic today, and that is a very important. We will talk about it
in the next lecture.
Today we will talk about telecom networks, means the kind of network that is used by
telephones and of course it is by host of other things as we will discuss. This telecom
network is very important in the sense that first of all, it is one of the earliest networks
that we had and that is one thing that is quite old and secondly, the telecom network
even today is mostly used for a wide area communication. Comparatively, we have much
fewer, I mean entirely or exclusive data networks. We will look at the evolution of telecom networks.
Well when Alexander Graham Bell developed the first telephone, it was a couple of telephone
instruments connected by wire. Now if you are trying to connect more than one people
that means more than one subscribers are there; taking this principle forward you will have
to connect everybody to everybody else through a wire which of course becomes very unwieldy
very soon; because connecting or stringing a wire between every pair of telephones, that
might want to communicate was not a good long term strategy at all. As you can see that
very soon we’ll have veritable jungle of wires and of course it’s costly and it is
very difficult to manage.
At the subscriber end you cannot handle so many wires coming into your premise, so better
idea was to connect all the telephones to a central switching office. There an operator
could connect one telephone to another via switchboard. That is the next figure, you
see that the number of wires have come down. As a matter of fact in the figure at the top
if you have n subscribers; naturally you have n (n -1) by 2 links which is approximately
n2 kind of links. Whereas in the latter case you have only n links so the number of links
as n becomes large, and you know today there are so many telephone subscribers so there
is no question of connecting them individually, you have to connect them to a central switching
office.
Even then once, again I mentioned this earlier in the lecturer, you must remember that it
is not that we have a whole parallel network for data from computers etc. all over the
globe, it is not there. The telecom network has been around for a long time. It had a
long and very excellent history so that is why even today most of the wide area network
traffic still rides on this telecom network. Originally it might have been a packet then
it might be sort of made into some kind of a session for which there is some part of
the connection is dedicated and all kinds of technology has come in but even today most
of the wide area networks are based on the telecom networks and telecom. That is one
side of the story, the other side of the story is that telecom networks the telecom people
are actually in the process of routing some of the voice calls and may be some of the
other kinds of calls into this packet kind of technology.
So DSL at subscriber end- voice traffic is transmitted as standard analog telephony signals;
data traffic is transmitted over the same line via a DSL modem that transmits the data
as high frequency digital broadband signals on the same line. If you have DSL modem you
can have both your voice as well as data at the same time, unlike very ordinary analog
lines.
So DSL at the central office end, CO end, and the signal passes through a splitter as
I have shown and a local loop management system. The subscriber line access multiplexer is
there in this figure we have already seen. In this figure, we have DSLAM and the loop
management over there, the broadband digital signals is directed to a DSLAM that terminates
and coordinates traffic from multiple local loop lines. From the DSLAM, the traffic is
sent to a router and then to the internet. And finally we have the loop management system,
which may be in front of the splitter, or behind the splitter? This comes to various
functions like giving the connections and testing and all this service kind of testing
are done through the local loop management. Thank you. 496