Lecture -18 Switching Techniques Circuit Switching

Hello viewers welcome to today’s lectures
on switching techniques. So far we have assumed that two devices or equipment are directly
communicating with each other and for that purpose whatever protocol and techniques are
necessary we have discussed like the HDLC which we have discussed in detail. However,
there are many situations where two equipments are not directly connected so in such a case
they perform communication through a number of intermediate equipment or devices. So in
such a situation the technique that is adopted is known as switching technique and the switching
techniques can be broadly divided into two types; circuit switching and packet switching. In today’s lecture we shall discuss about
circuit switching. This is the outline of the lecture. First we shall discuss why circuit
switching, then in the context of circuit switching we shall introduce to you Switched
Communication Network that is the model that is being used for communication. Then I shall
discuss about the circuit switching fundamentals their advantages and disadvantages then various
concepts of switching that is used in circuit switching such as space division switching
and one application of that is crossbar switches and Time Division Switching. Of course both
of them have combine having space and Time Division Switching. Then we shall discuss
about how routing of signal takes place in circuit switched networks and finally we shall
discuss signaling in circuit switched networks. On completion of this lecture the student
will be able to understand the need for circuit switching, specify the components of a switched
communication network, understand how circuit switching takes place, understand how switching
takes place using space division and Time Division Switching, understand how routing
is performed and finally they will be able to understand how signaling is performed.
So, let us consider the first question how two devices perform communication when there
are many devices? Suppose you have got a number of equipments
or stations say A B C D so in such a case A wants to communicate with B C D may not
be simultaneously but at a time A wants to communicate with B or A wants to communicate
with C or A wants to communicate with D one possible alternative is to establish connection
from A to B, A to C and also from A to D. Similarly it is necessary to establish communication
from B to C, B to D and finally C to D so we have to provide a number of links and it
can be the shown that the total number of links required for n such stations is equal
to n into n minus 1 by 2 so that number is very large. For example, here you have got
four stations so you will require 4 into 3 by 2 is equal to 6 so if you count you will
find that total number of links is 6 that is 1 2 3 4 5 6 so you have got 1 2 3 4 5 and
6 so 6 links are necessary. Hence this is known as mesh topology. Obviously when you
have got a large number of stations this kind of mesh topology is not practical. for example,
if you have got hundred nodes then you will get 100 into 99 by 2 so many links that means
99 into 99 so many links will be necessary to establish a communication from any node
to any node or any station to any station so this is not a good choice. So what is the alternative? One alternative
is to use switched communication network. So whenever you have got large number of devices
then mesh topology is not practical so a better alternative is to use switching techniques
leading to switched communication network. So let me introduce to you what we mean by
switched communication network. In switched communication network we will
find end devices namely essentially computers, peripherals, communication equipments such
as cell phones, laptops PDAs and various other things and these are known as stations. The
switching devices are called nodes. That means we are using some additional devices intermediate
devices known as nodes and in such a situation some nodes connect to other nodes and some
nodes are attached to stations. Let me show you the network. Here as you see these are the end stations
A B C D E and on the other hand 1 2 3 4 5 6 are communication networks nodes so these
are the nodes used as switches as intermediate points for communication. How it is done?
As you can see network topology is not regular so as shown earlier there is the mesh topology
there are other topologies such as bass topology, ring topology, star topology etc. The bass
topology is like this say we have got a bass and on to that we can connect all the stations
this is your bass topology. So, in switched communication network you don’t have bass
topology you cannot use ring topology in which the stations are connected in the form a ring.
So you can see that each station is connected to the neighbor neither is star like this
so we find that in this case the topology is not regular. It uses FDM and TDM for node-to-node
communication. So here you find node-to-node communication
as you can see shown in different colors from this station to node communication link. Here
we find that this is narrower and this is wider. Actually this one to two or one to
three these are node-to-node links and this links are of higher bandwidth. Since these
links are with higher bandwidth and to make maximal use of the bandwidth we use FDM or
TDM techniques. We have already discussed about the FDM and TDM techniques. We have
also discussed how we can use the higher bandwidth of links so these links are used as FDM and
TDM. There exist multiple paths between a source
destination pair for better network reliability. Here we find it provides you a much higher
reliability. for example, if A wants to communicate with C it can communicate through the nodes
1 3 and 5 but if suppose one of the link is down or one of the nodes is down in such a
case if A can communicate through the node 1 2 6 and 4 or it can communicate through
1 2 4 5 so in this way several alternative routs are possible which increases the reliability
of the network and that is one objective of the switched network communication so that
it provides you higher reliability providing multiple paths. Here another important feature is the switching
nodes are not concerned with the contents of data. That means whatever data is being
sent by a station that is being communicated by the node. In other words nodes can be considered
as dumped devices. Whatever received is being transmitted to other node or to the destination
station node. So either ways it is possible, a station will send to a node and the node
will either send to another node or it will go to another station as it is shown here
and in such a case the nodes do not modify the information or data. Their purpose is
to provide a switching facility that will move data from node-to-node until they reach
the destination. So their purpose essentially to communicate the data which is received
from one end and sent to another end connecting to a node or a station. This is the basic
model of the switched communication network and as I mentioned various switching techniques.
These are circuit switching, message switching and packet switching so there are three alternatives. In this lecture as I mentioned we shall be
concerned with circuit switching and in circuit switching the communication via circuit switching
implies that there is a dedicated communication path. So I am high lighting this so it is
dedicated and this dedicated is important because without setting up a dedicated no
communication is possible in circuit switched network. So it is the dedicated communication
path between two stations. The path is a connected sequence of links between network nodes. So
it can be a connected sequence of links. That means it is not necessary that there will
be a direct link direct path there may be a path to a number of nodes. So it can be
a connected sequence of links between network nodes. And on each physical link a logical
channel is dedicated to the connection. This is another important feature. The circuit switching involves three important
phases for communication. First one is known as circuit establishment, then data transfer
and third is circuit disconnect. The circuit establishment phase is used to establish end
to end connection before any transfer of data. So before any transfer of data is performed
you have to set up a direct link and some segments of the circuit may be dedicated link
while some other segments may be shared. So there are two alternatives it can be dedicated
or it can be shared. Then in data transfer the transfer data is
from the source to the destination. Once the link is established whatever data is to be
transferred is transferred from the source to destination through the dedicated link
already established in that circuit establishment phase. then the data may be analog in nature
or digital in nature either is possible depending on the nature of network and the connection
is generally full-duplex. As I mentioned whenever two stations are connected
usually the communication is full-duplex in nature. And finally once the data transfer
is complete, circuit disconnect is performed that terminates the connection at the end
of data transfer; signals must be propagated to deallocate the dedicated resources. So
whatever link was established the deallocation is done for that. Here it is shown how it
is being performed. So here the call request signal is going from
A through node 1 to 2 then call request also goes from node 2 to node 4 then another call
request goes from node 4 to node 5 for establishing connection from A to C. then once the call
acknowledgement comes through the already established path that is after the call request
comes data is sent from A to C through nodes 1 2 4 and 5 so this is how data is sent and
after the data transfer is complete the acknowledgement signal sequence comes from the other end from
station C to station A and the circuit disconnect is performed. This is how the data transfer
takes place in circuit switching. This circuit switching technique was originally
developed for handling voice traffic but is now also used for data traffic. As I mentioned this circuit switching concept
was originally developed for public switch telephone network, for voice communication
but now it is also used for data transfer. And as I already mentioned I am emphasizing
that once the circuit is already established the network is transparent to the users. That
means what data is being sent will be sent and information is transmitted at a fixed
rate without no delay other than the required propagation through the communication medium.
That means after the link is established there is no other delay in fault except the preposition
time. Depending on the distance, depending on the medium that is being used there will
be some propagation time but apart from the propagation time there is no other delay involved
in this circuit switching technique. And as I mentioned best known example is the Public
Switched Telephone Network that is being used in communication network so Public Switched
Telephone Network (PSTN) is the best example of circuit switching technique. Now let us look at the advantages of this
circuit switching technique. First it is fixed bandwidth and guaranteed capacity. That means
there is an end-to-end link and since end-to-end link is there the bandwidth is fixed and it
does not change. And here the advantage is it gets guaranteed
capacity. That means after the establishment of the link is known then both the ends know
what is the possible transfer rate and there is no possibility of congestion. Since the
links are having dedicated link there is no possibility of sharing, there is no possibility
of congestion. And the second important advantage is there is low variance in end-to-end delay,
there is a constant delay. As I said this delay is essentially the propagation time
so there is no other delay involved in this communication. Now let us look at the disadvantages. In this
world nothing is one sided there will be some disadvantage. The disadvantages are; the circuit
establishment and circuit disconnect introduces extra overhead and delay. As I have shown
before any data transfer is performed it is necessary to perform circuit establishment
and at the end of transfer we have to perform disconnection. So, disconnection has to be
done. Both of these will take some extra overhead and as a result it will involve some delay
also particularly before any data transfer can be performed. It allows you constant data
from source to destination and channel capacity is dedicated for the duration of the connection
even if no data is transferred. That means after circuit establishment if data is not
transferred obviously the bandwidth is wasted. For example, you have a set of long distance
call and after setting up the call if you don’t talk even when you don’t talk you
pay for the link or bandwidth. This is one disadvantage. And it has been found that for
voice connection although the utilization is high the statistics show that 64 to 73%
time one speaker is speaking, 3 to 7% time both the speakers are speaking and 20 to 30%
of the time both are silent. Therefore as you can see even in voice communication
the utilization is not very high it is only 64 to 73% but for data communication which
is burst in nature this inefficiency can be very high. Particularly most of the time no
data is generated and only sometimes a burst of data is generated in data communication.
So, in a typical user host data connection the line utilization is very poor. And the
irony is that whenever the user is not using the bandwidth that others cannot use. This
is another important disadvantage of circuit switching. Now let us focus on the function of the switching
nodes which play a very important role in circuit switching. Let us consider the operation of a single
circuit switch node comprising a collection of stations attached to a central switching
unit which establishes a dedicated path between any two devices that wishes to communicate.
A single node network or a particular node will have these three functions, it will have
a digital switch which provides a transparent signal path between any pair of attached devices
usually full-duplex, it will provide the network interface, it represents the functions and
hardware needed to connect digital devices to the network like telephones so you have
to interface your devices like computers, telephones and modems to the network and for
that purpose some interface has been provided. And finally we require the control unit which
maintains and tears down a connection. Here is the block diagram or of a circuit
switch node. One node is shown here. Here as you can see these are the lines through
which the stations are connected so all the stations are connected through these links
and through this interface so this interface is used for linking a number of stations and
there is a control unit which performs the linking with the help of this digital switch.
So the control unit controls this digital switch with the help of which the interface
can be…….., for example in this case there is a link between say 1 and this is 2, this
is 3, this is 4, this is 5, this is 6, 7 and 8 you have eight lines and this 1 is connected
to 6, 2 is connected to 4 and 3 is connected to 1 and then 5 is connected to 7 so this
digital switch connects each pair with the help of the control unit. So this is the basic
function that is being performed by circuit switch node. Now switching can be done in a number of ways.
One technique is known as space division switching. This was originally developed for the analog
environment. You know that telephone network was originally used for voice communication
and as a consequence it was developed for analog environment but subsequently it was
carried over through digital domain as in nowadays. So, in a space division switch the
signal paths are physically separated. Therefore physically separate paths are provided for
each of these links or paths from one to another so it is divided in space essentially it is
a crossbar matrix. Let us have a look at the cross bar matrix. So here you see you have got inputs and outputs.
These are input lines and the output lines and it is organized in the form of a two dimensional
matrix and as you can see at the crossing of each particle on horizontal line there
is a cross and essentially at each cross you have got an electromechanical switch or micro
switch it is nothing but a micro switch. So with the help of a micro switch either a connection
can be established between a vertical line with a horizontal line. for example here this
is 6 and this is 8 so if this is closed that means if this cross here is closed this mirco
switch is closed then a path is established between 6 and 8 . So at any junction if the
switch is closed a path is established between the horizontal line and the vertical line
connecting that particular cross. So you see this is how the cross bar switch operates.
However, it uses electromechanical lineage or micro switches. The basic building block of the switch is
a metallic crosspoint or semiconductor gate that can be enabled or disable by a control
unit. That means the control unit performs the function. as I have shown earlier there
is a control unit and this control unit is controlling the digital switch and digital
switch in case of space division switching is this cross bar. So this is how the cross
bar switch works and originally these cross bar switches were having metallic crosspoint
or semiconductor gates that can be enabled or disabled by the control unit. Of course the early electromechanical switches
were not very reliable they were bulky they used to consume lots of power. But with the
advancement of VLSI technology nowadays the cross bar switches can be implemented with
the help of semi conductor devices. For example, the Xilinx crossbar switch using Field Programmable
Gate Arrays is available and it is based on reconfigurable routing infrastructure and
in this Field Programmable Gate Arrays based switches you can have the high capacity non-blocking
switches and it provides very high capacity varying from 64 by 64 or 1k by 1k so you can
have a very big cross bar switch such as o1k by 1k which operates at a very high speed
200 Mbps. Obviously the data rate is quite high and nowadays possible by using this Field
Programmable Gate Arrays and available from Xilinx. Let me introduce the concept of blocking and
non-blocking before I discuss the other functions of the cross bar switch. An important characteristic
of a circuit switch node is whether it is blocking or non-blocking. A blocking network is one which may be unable
to connect two stations because all possible paths between them are already in use. That
means you may have number of paths. For example, I have shown eight inputs and eight outputs
if you are not able to connect a particular pair because all the pairs are already used
in such a case you call it blocking. On the other hand a non-blocking network permits
all stations to be connected in pairs at once and grants all possible connection requests
as long as the called party is free. That means as long as the called party is free
the non-blocking switching allows connection between any pair of stations or inputs and
outputs. For a network which supports only voice traffics
a blocking configuration may be acceptable since most phone calls are of short duration. What I am trying to emphasize here is that
a blocking network is acceptable in voice environment because most of the time people
are not talking that means the usage is much less. On the other hand in data applications
the connection remains active for hours. Usually whenever we are talking over a telephone we
usually talk for a few minutes and then disconnect the call. On the other hand for data applications
the connection may be active for hours and non-blocking configuration is desirable. What
I am trying to tell is for voice application blocking network is acceptable or for data
communication the non-blocking network is desirable. Now let us focus on the crossbar switch and
what kind of limitations it has got. As you have seen the number of crosspoints switch
grows with the square of the number of attached stations. Obviously as the number of inputs
and outputs increase exponentially at the rate of square……. for example if you are
having let us assume 1000 input or 1000 output or 1000 by 1000 switch so in such a case we
will require one million crosspoints. That means a crosspoint is nothing but a electromechanical
relay or an electronic switch and in this case for 1k by 1k switch you will require
one million that means ten to the power six crosspoints so it is costly for a large switch.
And another important disadvantage is that the failure of a crosspoint prevents connection
between the two devices whose lines intersect at that crosspoint. Let us go back to the diagram of the cross
crossbar switch which will explain that. Suppose this particular crosspoint or the
switch has become faulty, if this becomes faulty it is not possible to connect five
with four so five and four cannot be connected if this becomes faulty. So even when four
is not busy the connection cannot be established between four and five if there is failure
fault here. That means the failure of a cross point prevents connection
between the two devices whose lines intersect at the crosspoint. Another important disadvantage is that the
cross points are inefficiently utilized. What do you mean by that? We have seen that for
a 1000 by 1000 network you will require one million crosspoints. But are the one million
crosspoints used efficiently? Unfortunately no, even when it is used heavily may be 40
to 50 crosspoints are used but all the other crosspoints remain idle. So only a small fraction
are engaged even if all of the attached devices are active that even when all the attached
devices are active. That means whenever you are active that means you have got n square
crosspoints but at a time when all pairs are connecting only n crosspoints remain busy
so you see n square minus n crosspoints remains idle so that is a large number. What is the
solution to this? What is the other alternative by which the efficiency can be increased?
One better alternative is to use multistage switches. What is being done here is by splitting the
cross bar switch into smaller units and interconnecting them it is possible to build multistage switches
with fewer crosspoints. Here that example is shown which is nothing but a small 8 by
8 switch we have got eight inputs and eight outputs. Instead of a single stage crossbar
switch here you have got three stage crossbar switches. As you can see here this is a 4
by 2 switch, this is a 2 by 2 switch these are all crossbar switches and they are internal
linked in this way. Now how many crosspoints are here? If it is
a single phase you will require 64 cross points. So in this particular case as we shall see
4 into 2 is equal to 8 plus 8 is equal to 16 plus 4 is equal to 20 and another 20 so
you will require 40 crosspoints so this is the reduction. If it is large crossbar switch
and if you use multistage switches then the reduction is more significant. So in the previous
example as I have explained the number of crosspoints needed reduces from 64 to 40. Another important advantage is that there
is more than one path through the network to connect two endpoints and this increases
the reliability. For example, in this case suppose one wants to communicate with five
how connection can be done? One can be connected through in this path using this path or it
can be connected using this path so multiple paths are existing that increases the reliability.
So if there is a failure in one path another path can be used for establishing the connection
so this is an important advantage. Unfortunately there is a disadvantage because multistage
switches may lead to blocking but this problem can be tackled by increasing the number or
size of the intermediate switches which also increases the cost. That means although the
probability of blocking is reduced it will remain in case of multistage switches. Let
us see how it happens. For example we would like to establish connection
from 1 to three, two to four three to sox and four to eight which is shown here. So
1, 2, 3 connection is established here then 2 to 4 connection is established in this manner
but now how the connection can be done for 3 to 6. Now we find that 3 and 4 cannot be
connected to 6 and 8 the reason is from this switch there is no other path available at
this moment for connecting 3 and 4 to 6 and 8 because from this crossbar switch there
are two outputs and both the outputs are now engaged for linking 1 to 3 and 2 to 4 and
no other bars are available so it leads to blocking. So, whenever we are using multistage
switches essentially it becomes blocking although it reduces the number of crosspoints. Now
let us consider another implementation based on Time Division Switching. We have already discussed about TDM or Time
Division Switching which is essentially based on Time Division Multiplexing and here both
voice and data can be transferred using digital signals and all modern circuit switches use
digital Time Division Multiplexing techniques for establishing and maintaining circuits
and synchronous TDM allows multiple low speed bit streams to share a high speed line. We
shall explain how exactly this is being done. A set of inputs is sampled in a round robin
manner; the samples are organized serially into slots to form a recurring frame of slots
as we saw in TDM synchronous Time Division Multiplexing. Then during successive time
slots different I by O pairings are enabled allowing a number of connections to be carried
over the shared bus. Let us see how exactly it is being done. To keep up with the input lines the data rate
on the bus must be high enough so that the slots recur sufficiently frequently. That
means we have to use high speed for Time Division Multiplexing switch. For example, for 100
full-duplex lines at the rate of 19.200 Kbps the data rate on the bus must be greater than
1.92 Mbps. So the source destination pairs corresponding to all active connections are
stored in the control memory I shall explain how exactly this is being done thus these
slots need not specify the source destination address because it is stored in the control
memory. So here for example we have a simple TDM where
the switching is not done. So in this case it is 1 2 3 4 where 1 is sending
in data in slot A, 2 is sending in slot B, 3 is sending in slot C and 4 is sending slot
D and slot A is again taken to 1 and slot B data goes to 2, slot C data goes to 3 and
slot D data goes to….. this is the simple Time Division Multiplexing. Obviously this
will not serve our purpose so we have to use something in between. The first thing that
can be done is TSI. TSI stands for Time Slot Interchange. What Time Slot Interchange does is it essentially
does the interchange of the slot. for example, the connection is necessary from one to four
so the data from 1 should go to 4 so this data is now in slot 4 then what comes from
2 goes to 1 so B goes to slot 1 data, then 3 to 3 here there is no change and from 4
it goes to 1 so B goes to 1 so it is 1 to 4, 2 to 1, 3 to 3 and you can see D to A so
this is how it is being interchanged so this data now goes in the proper form. You can
see here BDCA so you see all the data is going in this manner. So you find that this data
communication is taking place in this manner. Now how exactly it is being done? It is being
done in this manner. So here you can see the data is coming in
slots then it is stored in some memory so in this memory the writing is done in a sequential
manner that means 1 2 3 4 and then reading is done selectively. That means here the writing
takes place sequentially and reading takes place sequentially that’s how the Time Slot
Interchange takes place and data goes from 1 to 4, 2 to 1, 3 to 3 and 4 to 1. This is
how it takes place in Time Slot Interchange. Another alternative is the TDN bus switching.
So here we use a high speed bus and here the data comes at low speed and then is connected
to a high speed bus. And for example at slow speed whenever this switch is closed then
this switch is closed 1 to 4, whenever this switch 2 is closed then this 1 is closed and
from the high speed bus the data goes from this input to the output. So, for example
here whatever data rate is here on this bus it will be four times of that because it is
doing the Time Division Multiplexing of data and it is being read on 1 2 3 and 4 in this
fashion. So it is essentially some kind of Asynchronous Time Division Multiplexing that
is being performed but however it is done in a different way. From a high speed bus
with the help of the control unit the reading is taking place at different times. Therefore
at different times it is coming from here 1 2 3 4 and here at different times it is
being read by 1 2 3 4 in a different sequence. This is how the TDM bus switching operates. However, you can combine the space and Time
Division Switching. we have already discussed the Space Division Switching and Time Division
Switching and it can be combined here. So these are the typical TDM switches Time Division
Switches and here it is Space Division Switches which is nothing but a crossbar. Therefore
as you can see here this interchange can be done with the help of this crossbar switch. Earlier we have seen the interchange was done
with the help of memory or a first bus here the cross bars switch does the necessary interchange.
That means from any one of the switch it will come here and then it can be taken to anyone
of these multiplexes. So here it is a multiplexer and here it is a demulitplexer. So, from any
one of these multiplexers it will come here and with the help of this cross bar switch
it can be taken to one of the demultiplexer and as you can sequence it can go to anyone
of these lines. This is how the space and time division switching are combined in implementing
the circuit switch nodes. And as I mentioned the most important application of circuit
switching is in telephone, the telephone network. I have already discussed about the telephone
network which is organized in a hierarchical manner. As you know you have got end offices
and with the help of local loop these are connected to the end stations. So here you
have the end stations which are connected through the local loop with the end offices
and the end offices are connected to the Tandem offices and you have got trunk lines these
are lines where we shall use the FDM or TDM techniques and then Tandem offices are connected
to regional offices. So there is some kind of hierarchy here. That
means say one end office will connect to a number of stations so a number of stations
will be connected. Similarly, a Tandem office will connect to a number of end offices. These
are end offices so these are stations or telephones you can say and this is your end office and
these are all local loops and these offices are connected to the Tandem offices and the
Tandem offices in turn are connected to the regional offices so several such Tandem offices
are connected to regional offices. Therefore in this way there is a hierarchical network
used in Public Service Telephone Network (PSTN). Now let us consider the routing operation
performed in circuit switch networks. As I mentioned in large circuit switch network
connections often require a path through more than one switch that is the typical property
of the switched communication network. And here the basic objective is the efficiency
and resilience. What do you really mean by efficiency? Here we would like to utilize
the minimum number of switches. Obviously we would like to optimize on the hardware
cost. On the other hand the resilience wants high reliability. High reliability means for
example you may decide the switch capacity on the basis of average traffic. However,
there are situations where the traffic will be above average. So traffic handling with
the help of this network actually decides the resilience. Whenever there is some failure
in the network even in such a situation the network should be able to handle with some
degraded performance. If there is a catastrophic situation that occurs like flood or earthquake
or something else then the traffic on the network suddenly increases or even when some
exam results are out obviously it leads to lot of telephone calls. So network should
be able handle that which requires resilience. For routing purpose there are two basic approaches.
One is known as static routing and other is known as dynamic routing. in static routing
routing function in public switch telecommunication networks has been traditionally quite simple
and static, normally it is very static in nature and as we have already seen the switches
are organized as a tree structure. Since it is organized in a tree structure there is
a fixed well defined path from one point to another point through a number of switches.
However, to add some resilience to the network additional high usage trunks are added to
cut across the tree structure to connect exchanges with high volumes of traffic between them. That means to have some resilience some additional
high usage trunks added some redundancies are added so that in case of failure in case
of heavy traffic this can be tackled. However, it cannot adapt to changing conditions. As
I mentioned there may be some changing conditions because of some catastrophic situations or
some special situations. So for these special situations this static routing cannot perform
and obviously this leads to congestion in case of failure. Whenever there is some failure
will lead to congestion. That means the connection cannot be established between two users. To overcome the limitations and to cope with
growing demands all providers presently use dynamic approach and routing decisions are
influenced by current traffic conditions. In dynamic routing it is not fixed and static
it is based on the current traffic and conditions. So, switching nodes have a peer relationship
with each other rather than a hierarchical one. That means in such a case it is not the
hierarchical network. The nodes or switching nodes have peer relationship among all other
nodes, they exchange information to find out traffic conditions and other things and as
a result based on that the routing is done. Routing is more complex and more flexible
and obviously it will be complex. Since it is not fixed it is based on the current conditions
and it has to tackle the catastrophic or special situations. However, it has to be more flexible
as well. There are two basic techniques is used one
is known as alternate routing another is known as adaptive routing. Let us see how they work.
First let us consider the alternate routing approach. Here the possible routes between
two end points are predetermined. So it is the responsibility of the originating switch
to select the appropriate route for each call. That means in this case based on busy traffic
hours the routes are decided. For example, may be in the morning hours all people are
going to office or schools and in the evening hours people are returning from offices or
schools and based on that you know that traffic control is done, the route timings can be
set in normal traffic control. Here also something similar thing can be done that is based on
the statistics of history the direction of traffic can be decided at a particular time
instance. In this case in practice usually a different set of preplanned routes are used
for different time periods. For example, in morning period one route is established, in
the evening period one route can be established or depending on the traffic condition and
different paths and that can be based on history and statistics. So it takes advantage of different
traffic patterns in different time zones and at different time of the day. This is more appropriate in USA. For example,
it has got different time zones and based on different time zones the office hours are
different in different places so accordingly the routes can be set up in a predetermined
manner and the routing can be done depending on the date and time and hour of the day. Then we have the alternate routing approach.
In alternate routing approach several alternate routes are predetermined which are used for
communication. For example, you can have a fixed alternate route, for example from A
to D it can be that is from A to node 1 then to 2 then to 6 then it goes to D that means
it is through the nodes 1 2 and 6 so this is fixed alternate routing. But it can be
dynamic alternate routing. For example, another alternate route instead of 1 to 6 it can be
1, 3, 5 and 6 so this is the alternative. These alternative routes are available may
be initially the shortest route is tried and if there is blocking then the alternate routes
are explored in alternative approach. On the other hand adaptive routing approach
is designed to enable switches to react to changing traffic conditions on the network
and it allows you better management overhead, switches must exchange information and it
has potential for more effectively optimizing the use of network resources. For example dynamic traffic management is
being used by some telephone companies. A central controller collects data at the interval
of ten seconds to determine preferred alternate routes. So it gathers statistics then the
routes are decided. So apart from routing the nodes must send control signal to manage
the network by which calls are established, maintained and terminated. So, to perform
these functions; establishment, maintenance and termination various types of signals are
to be generated. For example, supervisory signals which essentially
gives you the availability of sources then at risk for example at different stations
a particular node has some telephone number that telephone number has to be sent so this
is the address that has to be communicated and the call information as whether it is
busy or something has happened or network management has to be done which is used for
the purpose of maintenance and termination. And signaling can be done in two different
ways; inchannel, it can be in band or out of band and same band frequencies used by
voice signals are used to transmit control signals and in out of band it uses different
part of the frequency band but uses the same facilities as the voice signal. On the other hand the common channel is dedicated
signaling are used to transmit control signals and are common to a number of voice channels.
So we have discussed by the circuit switching technique and it is time now to give you review
questions. Here are the five review questions. 1) What are the three steps involved in data
communication through circuit switching? 2) Mention the key advantages and disadvantages
of circuit switching technique 3) Why data communication through circuit
switching is not efficient? 4) Compare the performance of space division
single stage switch with multistage switch 5) Distinguish between inchannel and common
channel signaling techniques used in circuit switched networks. Now I shall quickly give you the answers of
the questions of the lecture minus 17. 1) Why are asynchronous protocols losing popularity? Asynchronous protocols are losing popularity
because of their high overhead of more than twenty percent as we will explain in details
on the other hand synchronous protocols provide significantly lower overhead that is why asynchronous
protocols are losing popularity. 2) Why bit-oriented protocols are gaining
popularity? Bit-oriented protocols allow packing more
information compared to character-oriented protocols that is why bit-oriented protocols
are gaining popularity. 3) In HDLC what is bit stuffing and why is
it needed? The presence of the bit sequence 01111110
used as flag to indicate the start and end of a frame may lead to division of a single
frame into more than one frame wrongly. This problem is overcome by using bit stuffing.
A 0 is introduced after each occurrence of five consecutive ones in the information field
of a frame. 4) What is piggybacking and how it is being
used in HDLC? As i have mentioned the acknowledgement can
be sent along with the information from the other end. That is being used in the piggybacking.
So with this we come to the end of the lecture, thank you.

22 comments on “Lecture -18 Switching Techniques Circuit Switching”

  1. friendlyuser1 says:

    thanku 4 posting!

  2. Anthony Benites says:

    Excellent post! you are the best!

  3. Aashish Mohan says:

    thank u for posting…………

  4. Edo says:

    at 40:37, 4 goes to 2 its probably a mistake

  5. Abhishek Verma says:

    Best way of teaching…

  6. Gaurav Mantri says:

    worst lecture ever by nptelhrd…just reading out a ppt..dnt wst ur tym on dis guys…

  7. Fun With Electronics says:

    generally nptel lectures are informative,but this lecture lacks some spark

  8. Suranjan Kumar says:

    I am unable to find lecture 19.plz provide me a link of lecture 19

  9. SOUVIK SARDAR says:

    thank you sir

  10. Daniel Angali says:

    Thank you sir.

  11. Nishant Singh says:

    Awesome Sound Quality of the video it is pleasing to years. Please make all nptel lectures with same sound quality

  12. Antika says:

    Great …

  13. Nandu Mishra says:

    Could you please elaborate on blocking and non-blocking switches.
    I came to know that non-blocking can be obtained by introducing more switches in the network.
    But with in a particular switch whether intermediate or exterior switch if there is an established connection, then that particular path would be completely blocked. How non-blocking passage is obtained between two exterior nodes, when there are multiple intermediate nodes.

    Thanks and Regards,
    Nandu Mishra

  14. Nandu Mishra says:

    Thank you very much for posting these kinds of informative videos.

  15. Omkar More says:

    shittiest lecture. couldn't even spot errors in ppt. he was just reading the ppt. dont waste your time here

  16. Abhishek Malakar says:


  17. Preeti Sharma says:

    thnx sir

  18. mritunjay sharma says:

    hindi bol liyae kro sir

  19. mritunjay sharma says:

    jitnae bhee nptl pr lecture hai sab non energetic lagtae hai ……….energy lagta hee nhi kise mae ……….it seem that any time they can sleep

  20. Cheedella Eswari says:

    Please improve the sound quality

  21. 1-minute Tips says:


  22. kushagra deep says:

    excellent slides and explanantion ….Thnq Prof Pal 🙂

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