Lecture – 23 X.25 and Frame Relay
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Lecture – 23 X.25 and Frame Relay

October 11, 2019


Hello and welcome to today’s lecture on
X.25 and frame relay. In the last couple of lectures we have discussed
about packet switching techniques and also discussed various issues related to packet
switching such as routing, congestion control, flow control and so on. In this lecture we shall discuss about two
important examples of packet-switched network such as X.25 which is the oldest one and frame
relay. Here is the outline of today’s lecture. First we shall discuss the basic features
of X.25 then consider the three layers of X.25 then we shall consider frame and packet
formats of X.25. We will see that it operates in two layers
data link as well as network layer. Then we shall consider the virtual circuits
that is being used in X.25 for data communication and also we shall consider the multiplexing
used in X.25. After that we shall focus our attention to
frame relay by first introducing the key features of frame relay why frame relay instead of
X.25 then we shall consider virtual circuits used in frame formats and we shall see the
congestion control is very important in the context of frame relay and we shall discuss
how congestion control is used in frame relay. Finally we shall conclude our lecture by comparing
X.25 and frame relay. And on completion of this lecture the student
will be able to understand the key features of X.25, explain the frame format of X.25,
understand the function of packet layer of X.25 and they will also understand the limitations
of X.25 and they will able to explain key features of frame relay and understand the
frame relay frame format and explain how congestion control is performed in the frame relay network. So let us start with X.25. X.25 is a packet-switched network developed
by ITU back in 1976. Of course subsequently several versions have
come up several editions by enhancing the features of X.25 but it is the one of the
oldest one developed in late 70s. And this particular protocol defines how a
packet mode terminal can be interfaced to a packet network for data communication. So here the user machine is termed as DTE
as we already know so this is the terminal DTE Data Terminal Equipment and packet switching
node to which this data terminal equipment is connected is termed as DCE. We are already familiar with these two terms
and as you can see DCE is part of the X.25 network and this particular specification
explains in detail how DTE working in packet mode can interface with DCE and perform packet
transmission. It has got three layers physical, frame and
packet layers. So if you are comparing it with the OSI model
it essentially occupies the three lower layers, the physical data link and network layer. And, the physical layer in X.25 is essentially
X.21 and then the frame layer uses a subset of HDLC which is known as LAPB, we already
mentioned about it and then the packet layer PLP. These are the three layers. We shall consider the functions of these three
layers in detail. Physical layer: as I mentioned it deals with
the physical interface between the attached station and the link that attaches that station
to the packet switching node. This interface designs the physical, electrical,
functional and procedural specification. The X.21 is the most commonly used physical
layer in standard what has been recommended for used with X.21. However, in absence of X.21 other standards
like RS-232 C can also be used which is analogue in nature and on the other hand X.25 is a
digital interface. But this RS-232 C can also be used in place
of X.21. Coming to the frame layer it facilitates reliable
transfer of data physically by transmitting the data as a sequence of frames. It uses a subset of HDLC known as Link Access
Protocol Balanced (LAPB) it is a bit-oriented protocol as seen in HDLC. Then the third layer is responsible for end-to-end
connection between two DTEs and functions performed are establishing connection, transferring
data, terminating a connection and it also performs error and flow control which is important
in the context of DTEs. Then with the help of X.25 packet layer data
is transmitted in packets over external virtual circuits. We have discussed different types of virtual
circuits, here external virtual circuits are used to perform data communication using the
X.25 network. Here it shows the X.25 interface. As you can see this is the DTE this is the
data terminal equipment, here is the data communication terminal equipment DCE which
is essentially part of the X.25 network. So this interface specifies that X.25 physical
interface. As I mentioned we can use RS-232 C in absence
of X.21. Then LAPB is used as the logical interface
between the link access layers which essentially works in the data link layer and it has got
multi-channel logical interface which allows several virtual circuits to be established
for communication of data in the packet layer. Then we have the user processors with which
you can communicate to remote users by using this interface. So here it specifies the X.25 interface the
interface between DTE and DCE. Let us look at the X.25 frame format. here the user data is used to form a packet
in X.25 packet layer by putting header which we may call layer 3 header.as you can see
layer three header is attached to user data to form a packet then the packet is passed
on to the data link layer and the data link layer adds LAP-B header and LAP-B trailer
and this is how a data link layer frame looks like. A data link layer frame has this kind of format
as we have already discussed in the context of HDLC. it has got flags at the beginning
and at the end and then the address then the control and information. Then as you know there are three types of
frames allowed in HDLC. One is information frame which is used for
communication of user data, then S frame which is usually empty not used in the context X.25,
then U frame which is used to pass on control information because it uses in-band signaling. In-band signaling is used with the help of
these control frames. One point you should notice that here it is
essentially a point-to-point communication so you do not really require many addresses. Only two addresses are used. As you can see here 0000 and 0001 is used
by the command issued by the DTE and response to it. That means whenever some comment goes from
the DTE and some response to it is given with this address 0000 0001. On the other hand there is a common issued
by the DCE and response to it given by the DTE that is given in the address 0000 0011. These two addresses are only used in the context
of X.25 because it is a point-to-point communication and not a multipoint communication. However, as we shall see multiplexing can
be done in data link layer. We shall consider about it in detail. Here it shows how it works in the packet layer. here as you can see asynchronous packet mode
is sent and unnumbered acknowledgement comes from the other end for setting up the link
and then the data transfer can take place, information frame can go, here unnumbered
acknowledgement comes from the other side and not only that but several such data can
be transferred before it can be disconnected by sending a disconnect frame. Thus each time this kind of virtual circuit
is created by using this protocol. The virtual circuits are created at the packet
layer. It uses a virtual circuit identifier known
as the Logical Channel Number. So the virtual circuit identifier is used
to identify a particular virtual circuit. As you can see from this DTEA there are several
virtual circuits created. One is going to B, another is going to C and
another is going to D simultaneously. And this particular link that is this DTE
to DCE this link has three virtual circuits so through the same physical link you can
create several virtual circuits which are identified by a Logical Channel Number or
LCN. Also, several virtual circuits through the
same link using in-band signaling can be created. So this is the fundamental idea of virtual
circuit that is being used in packet layer of X.25. Now there are two types of virtual circuits. That is also used in X.25. One is known as PVC or Permanent Virtual Circuit
which is somewhat similar to leased line used in telephone network, that means line always
exists there is no need dial to set up the link so it differs from the dial-up link. So always there is a connection whether you
send data or not. Similarly here also the Private Virtual Circuit
is fixed similar to leased line established by the network or providing this link. Data transfer occurs with virtual calls and
packet transfer takes place one after the other and there is no need to have call set
up or termination in case of type of Permanent Virtual Circuit. However, in case of Switched Virtual Circuit
it is necessary to have link for each data transfer and the sequence of events to be
followed are given here. First of all links are set up between the
local DTE-DCE and the remote DTE-DCE. That means first a link is established between
the local DTE and DCE and also between the remote DTE and DCE that is the first thing
that is being done then a virtual circuit is set between the local and remote DTEs. So, after these two local and remote links
are established between the DTE and DCE the virtual circuit is set between the local and
remote DTEs then the data transfers are performed between the DTEs then the virtual circuit
is released and the link is disconnected. This is the sequence that has to be followed
for each session of data transfer using Switch Virtual Circuit in X.25. Now let us focus on the different types of
packets used in the packet layer. As I mentioned the packet can be of different
types; broadly it can be divided into two types; data packets and control packets. Data packets can be essentially used for sending
the user data. For user data obviously there is some maximum
limit on the size and that limit is used to form the data packets. On the other hand control packets can be used
for various purposes. One purpose is to perform flow control and
error control. This flow control and error control can be
done with the help of packets like RR which stands for Receive Request, RNR stands for
Receive Not Request and REJ stands for Reject Packets. With the help of these three types of packets
it is possible to perform flow control and error control but congestion control is not
used in X.25. Essentially flow control and error control
are performed. Then the other packets are necessary essentially
for in-band signaling. As I have mentioned you have to set up the
virtual circuit, perform the data transfer, disconnect the link so all these links can
be done with the help of these packets. And as we know the user data are broken into
blocks of some maximum size and a 24-bit or 32-bit header is appended to each block to
form a data packet. Then it uses sliding-window protocol, piggybacking
for flow control. As I mentioned it performed flow control and
Go-back-N protocol for error control. So it performs flow control and error control
using sliding-window protocol and Go-back-N for error control. We have already discussed about these two
approaches. Now we shall see how this is being implemented
in X.25 network. X.25 also transmits control packets related
to establishment maintenance and transmission of virtual circuits as I mentioned. Each control packet includes the virtual circuit
number the packet type for example call request, call accepted, call confirm, interrupt, reset,
restart etc. These are the various control packets that
can be sent that can be communicated between the two DTEs and additional control information
specific to a particular type of packet. So, in addition to this type of packet some
additional information specific to that type may be added to the packet. Thus here is the packet format of X.25 system. Here you see the packets are divided into
two groups. First of all as I mentioned it is divided
into three different types; data packet, control packet and RR RNR REJ packets so three different
types. And as you can see here the data packet comprises
either 3 byte here it is 3 byte and here it is 4 byte and that depends on whether we are
using 3-bit sequence number or 7-bit sequence number. So whenever a 7-bit sequence number is used
it is necessary to have a 4 byte header instead of 3 byte header. On the other hand whenever 3-bit sequence
number is used that means you can have three bits for giving the sequence number or for
acknowledgement in the user data or in control packets. So here let me explain different functions
of different bits. This Q bit is actually not used by this layer,
it is left to the user to use it properly for some function. On the other hand this D bit is used as an
acknowledgement from the remote DTE. That means this bit as we know the acknowledgement
or flow control and error control can be performed between in the data link layer as well in
the packet layer. So whenever it is in the data link layer it
is essentially node-to-node or hop-by-hop basis. On the other hand it is end-to-end flow and
error control then it is essentially in the network layer and that is being specified
by D. Whenever D is equal to 0 then it is essentially between two adjacent nodes or
between the local DCE and DTE. On the other hand whenever D is equal to 1
then it is used for end-to-end flow and error control so this bit designates that. And essentially this 0 1 or 1 0 is used to
differentiate between the different sizes whether it uses 3-bit sequence or 7-bit sequence. Then 12-bit number which comprises of two
parts group number and channel number together forms the Logical Channel Number LCN and Logical
Channel Number is used to form a number of virtual circuits. Then the PR and PS these two are used for
acknowledgement and flow control. For example, PS is used by the sender, he
uses this as the sequence number of the packet and that sequence number can be from 0 to
6 if he uses a 3-bit sequence or it can be 0 to 127 if he uses a 7-bit sequence number. The number cannot be 128 or 8 here. Hence this number is used as sequence number
and PR is used essentially for acknowledgement in the piggyback form. That means it uses piggyback acknowledgement
using that Go-back acknowledgement using sliding-window protocol. The window size can be 7 or 128 depending
on the number of bits used in the PR and PS fields. That is why we will see that PS is not present
in this RR, RNR and REJ packets. When it is Received Request packet it specifies
the number the receiver is waiting for or is looking for. that means it specifies the number or whenever
it is RNR that means that packet has been corrupted and whenever it is rejected it is
using the error control that packet has to be rejected by using the Go-back-N ARQ technique. These are the functions. On the other hand whenever you are using a
large sequence of packets that end-to-end acknowledgement can be done either at the
end or for each of the packet. So it is a usual practice to send an end-to-end
acknowledgement after the end sequence of the packet is being specified by M bit. Whenever this bit is equal to 0 1 then essentially
it specifies that the end of sequence has occurred and end-to-end acknowledgement is
sent. On the other hand whenever it is 0 then you
are using the sequence and for each of the packet acknowledgement is being sent. So we have seen the function of different
fields of this packet layer. Now one point I should emphasize on is multiplexing
that is being used in case X.25. This is one of the most important services
provided by X.25. As we have seen DTE is allowed to have up
to 4095 simultaneous virtual circuits with other DTEs over the single DTE-DCE link as
we have already seen with the help of the 12 bits in the packet, this 12-bit group number
and channel number and with the help of these bits up to 4095 virtual circuits can be created. Each packet contains a 12-bit virtual circuit
number expressed as four bit logical group number plus an 8-bit Logical Channel Number
as I have mentioned. Individual virtual circuits could correspond
to application, processes, terminals etc so you can divide them as per the application
or processes or terminals. The DTE-DCE link provides full-duplex multiplexing. That means data communication can be done
in both directions and not just in one direction through the same link by using in-band signaling. So we have discussed in nutshell the various
functions of X.25 and we have seen how it really works. Now X.25 was developed back in 1976 where
the speed of the telephone network was not very high, only 64 kilobits were sent was
the standard, only 64 kilobit it is not very high number in today’s context moreover
the links were very error prone, as a result it was necessary to have elaborate error control
and flow control mechanism which however is not ready not really required in present day
context that assured the development of a faster packet switched network known as frame
relay. We shall discuss about it now. X.25 cannot satisfy the present day requirement
such as higher data rate at lower cost. For example, if we use X.25 network since
it is a point-to-point network communication if four stations want to communicate to each
other we have to set up this kind of point-to-point link. So, if there are five nodes which wants to
communicate with each other you will require ten different links. This can be avoided in frame relay network. So in a frame relay network as you can see
each DTE is connected to a data communication network and the frame relay network does the
remaining thing. So here it is not point-to-point communication. Here it works like a switched communication
network, the nodes can communicate with each other and it does the switching. So, as a consequence it is much more efficient. Moreover that data rate is much higher in
the case of frame relay. For example, compared to 64 Kbps used in X.25
the frame relay uses minimum of 1.544 Mbps. nowadays higher data rates can also be used
but that was the minimum value with which it was started so speed is significantly higher. Another important point is the X.25 does not
support bursty nature of data. It establishes a point-to-point communication
and the data flow has to be streamed or continuous. But unfortunately the present day context
it is necessary to have bursty nature of data because the computer communication uses bursty
nature of data. For example, this is the stream data flow
and this is the bursty nature of data flow. And the frame relay has been designed to support
the bursty nature of data. So the data can be sent in burst as you can
see within some time say 8 may be millisecond the data is coming in say 3 bursts compared
to stream data that is used in X.25 so here frame relay support this by buffering the
data. And as we shall see, if the total value of
data does not exceed some limit then the packets should be delivered without error. Third important factor is, as i mentioned
in the era of X.25 network was not reliable. The links were error prone, slow and as a
consequence it was necessary to perform flow control and error control not only between
end-to-end but between each link as you can see here. So here as you can see the data is going from
station A to node 1 and then acknowledgement comes which are used for flow control and
error control then if it is correct then the data goes to the next hop and again acknowledgement
is sent from that particular side. Similarly from node 2 to node 3 data goes
and acknowledgement comes from node 3 to node 2 to perform flow control and error control
then node 3 to station B data goes and acknowledgement comes. The story does not end there. As we have seen there should be end-to-end
flow and error control and that is being performed by sending acknowledgement from the DTE of
the other side and again the same is repeated between each hop as you can see. Then the acknowledgement goes between each
hop and the acknowledgement frame goes from node 2 to node 1 again acknowledgement of
that comes node 1 to node 2 and finally acknowledgement reaches the DTE. So here it completes the end-to-end flow and
error control. Therefore what you see is, to send a single
packet how many packets have been transferred, it increases the load in the network although
it makes it very reliable but increases the load significantly. So in the era of X.25 this was necessary because
the links were not reliable. However, in the context of frame relay this
is not necessary. So what was done was the flow and error control
was completely removed not only the data link layer but also in the frame layer. So as you can see the traffic significantly
reduced here so data goes from a station A to node 1 and again data goes from one node
1 to node 2 and data goes from node 2 to node 3 and finally it is delivered to station B.
So if any flow and error control is necessary that has to be supported by upper layers,
the frame relay does not supported it because it is not necessary, the links are much more
reliable and flow and error control may not be required and also the speed is high. But as we shall see it will be necessary to
perform congestion control because of higher speed. In frame relay also the virtual circuit is
identified by a number called data link connection number. Here also it is based on virtual circuit,
creation and there are two types of virtual circuits as we can see the Permanent Virtual
Circuit and the DLCIs the Data Link Connection Identifiers are permanent and assigned by
the network provider. So the network provider provides these DLCIs
and there is a Permanent Virtual Circuit created from this station A to station B or DTE A
to DTE B and this particular link from A node 1 the DLCI is 75 and between these links DLCI
is 85. So these numbers will be different for different
links. On the other hand whenever the Switched Virtual
Circuits are created the DLCIs are temporary. The data link connection identifiers are temporary
and are assigned by the frame relay that means the network does the assignment during the
connection phase. As you can see for communication from A to
B these two DTEs the DLCI here is 75, here from 1 to 2 it is 48, from 2 to 3 it is 65,
from 3 to 5 it is 98 and from 5 to B is 85. This is being set up in a dynamic manner as
you can see. A set of phase is involved for setting up
the virtual circuit and connection is established after that data transfer is performed, several
packets can be communicated and finally the link is released as it is done in case of
X.25. So this part somewhat similar to X.25. Now let us see how the switching takes place
within the network. Here each node maintains some kind of table
somewhat similar to the routing table. As you have seen in case of fixed routing
the routing tables are stored in each node so a similar thing is done in this case also. Here you see this node 2 this node or the
frame relay switch is shown in an expanded form and here you see the table that is being
stored. That means it has got three interfaces coming
from one, then another is two and another is three. This is the node or switch two. So, interface one this is the incoming so
when it goes from one to two the DLCI is 48 and the outgoing DLCI is 65 and whenever it
is going to three the DLCI is different that is 62 and the interface that goes is 3 DLCI
is 98. When it comes from the other direction that
means comes from two to one when it goes in the reverse direction the DLCI numbers are
different, that is how the full-duplex communication can be done. So here it is 75 and it is 85 similarly from
2 to 3 it is 82 and 92, from 3 to 1 this is 52 and 76 these are the DLCI numbers and from
3 to 2 is 42 and 96. Therefore you can see how the DLCI numbers
are used to do the switching and using this particular table the switching is done in
case of this virtual circuit switch. So within the network each node performs this
kind of switching. The frame relay operates in two layers compared
to three layers used in X.25. So you can see here, this is the frame relay
it got two layers physical and data link and the upper layers are provided by others may
be by IP protocol. On the other hand X.25 also supports the network
layers, it has got three layers as we have already discussed. With the help of these two layers the communication
is performed. And the frame format used in frame relay is
shown here. It has got flag at both ends then the address,
control, information and frame check sequence. Here the DLCI comprises of ten bits so DLCI
comprises of ten bits. So you can have up to 1k DLCI numbers. Then you have got another bit which says whether
it is a command or response, this bit specifies whether a particular frame corresponds to
command or a response then this EA stands for Extended Address. It is not necessary that the DLCI number has
to be restricted to a 10-bit number but it can have more number of bits. In that case the EA bit is used; it is set
to 1 whenever it has got a larger address. Then FECN, BECN and DE these are used in the
context of congestion control. So although a frame relay does not perform
flow control and error control it performs elaborate congestion control. Let us see later on about how it is done. The congestion control that is performed is
explained here. Because of higher data rate and no use of
flow control frame relay network is prone to congestion. The basic reason for congestion is bursty
nature of data and the frame relay has been designed to support bursty nature of data,
the link speed is high so these two parameters has made it vulnerable to congestion and as
a result it necessary to have to congestion control mechanism developed for frame relay. It is done in this manner. It uses two bits for congestion control. One is Backward Explicit Congestion Notification
BECN bit and another is Forward Explicit Congestion Notification FECN. And also another bit is used; it uses another
bit that is DE Discard Eligibility bit which is used for packet discarding. That means whenever other things do not work
then this discarding is performed. Let us see how it is being done. Thus the
packet is going from this sender in this direction and it has been found that this part that
is beyond three in this direction there is congestion. So this particular switch is identified by
some means. This identifies whether congestion has taken
place in the forward direction so each switch or node does. So whenever it identifies that congestion
has occurred then it sends one explicit packet BECN to the source. So it goes towards the source and the sender
in that case takes suitable measure which reduces the traffic reduces the rate of packet
that is being introduced to the network to come out of congestion. This is the use of BECN bit for controlling
congestion. Then comes the efficient bit. So here you see there is this Forward Explicit
Congestion Notification. This bit is used to inform the receiver. So, whenever there is congestion in this part
of the network then this bit is set, this is the direction of congestion in both cases
as you can see, then the receiver is alerted about the congestion. Now it is up to the receiver to inform the
sender by sending a packet that it is necessary to perform flow control so that it can come
out of congestion. Therefore by using these two bits BECN FECN
you have got four possibilities, when these two bits are 0 0 it means there is no congestion
in either direction. Then whenever it is 0 1 or 1 0 there is congestion
only one in one direction. On the other hand whenever these two bits
are 1 1 there is congestion in both directions. So, by using these two bits the congestion
control can be done. However, whenever this does not work then
this Discard Eligibility DE bit is used. How it is used? This bit gives the priority which means whenever
it is set to 0 it is set to 0 for some packets and it is set to 1 for some packets. That means whenever it is using this particular
bit whenever it is set 1 it means they have lower priority where these packets can be
discarded. So it essentially sets a priority in the packet. In case of congestion the packet has to be
discarded and based on this bit the packets are discarded by the switches. And as you already know the packet discarding
is a very important mechanism used for congestion control particularly when you have to come
out of congestion. Now you may be asking how do you find out
that congestion has taken place or when to send FECN or BECN bits by a packet? For that purpose it is necessary to perform
traffic control measurements. Each switch performs some kind of measurement
on the network and based on that it will decide when to send FECN or BECN and for that purpose
four attributes are used. First one is known as access rate. It is decided by the bandwidth of the channel
that means it is the maximum rate of introduction of packet. For example, if the T1 line is used then the
maximum rate is 1.544 Mbps. That means if a particular node sends a packet
at a rate faster than this then we can say that access rate has been violated. The second parameter is committed burst size
Bc which is the maximum number of bits in a predetermined period. It does not say that you have to send at this
rate. It may be necessary, for example, in 5 millisecond
or may be 5 seconds how many bytes can be introduced by a particular source node that
is specified by this committed burst size. So may be this is the duration and within
this duration it can be introduced in the form of bursts. And as long as the total size does not exceed
the committed burst size it is guaranteed that the packets will be delivered without
error. Then the third parameter that is being used
is Committed Information Rate. It defines the average number of bits per
second that means it performs the averaging and finds out whether the average number of
bits is essentially related to the access rate and this parameter is also specified
in the beginning and if the average number of bits per second is not exceeded then it
is guaranteed that the packets will be delivered without error. On the other hand it is exceeded then the
switch will inform the sender that the Committed Information Rate has been exceeded. Another parameter is excess burst size. This is the maximum number of bits in excess
of the Bc where already the committed burst size is there. The network allows in addition to Be some
more before it decides to send FECN or BECN. That means in addition on top of BECN, suppose
this is the limit of the BECN, so this is Bc and this is the Be so this much of excess
is allowed by the frame relay network. But this is loosely bound on the sides but
usually the committed burst size is more important than the excess burst size but in any case
the burst size should not exceed this excess burst size. Thus we have discussed how the traffic control
is performed and how it is used to decide when to send the congestion control particularly
by sending FECN and BECN. Now frame relay has been designed not only
to support or send frame relay packets but it can also send packets coming from other
networks. For example, X.25, ATM or PPP point-to-point
so for that purpose special functionalities known as FRAD Frame Relay Assembler Disassembler
is introduced because the frames of other types of standards or network like X.20 or
ATM or PPP may not be the same as that of frame relay. So what the FRAD function does is, it disassembles
the packets sends through the frame relay network, and then at the other end it performs
the assembly. This is your frame relay network . Thus both
assembly and disassembly are performed by two ends so that you can send packets coming
from other protocols and that can be carried using the frame relay frames. Thus the Frame Relay Assembler Disassembler
assembles and disassembles frames coming from other protocols which can be carried by frame
relay frames. This function relate is provided and which
shows that it can be made compatible with X.25 ATM and other packet switched networks. Before I conclude today’s lectures it is
time to compare the functionalities of X.25 and frame relay packet switched network. We shall compare the features. First one is connection establishment. As we know in X.25 connection establishment
is done by network layer. On the other hand it is not done by the frame
relay it is done upper layers. Then the flow and error control is performed
in X.25 both at data link as well as network layer hop-by-hop and end-to-end. On the other hand frame relay does not perform
any flow and error control and for X.25 the data rate is fixed on the other hand frame
relay supports bursty nature of data. The multiplexing performed by X.25 is in the
network layer on the other hand frame relay performs the multiplexing in the data link
laye. as we have seen it is done by using DLCI and here it is done by Logical Channel
Number. Congestion control is not necessary in X.25
because of elaborate use of flow control and error whereas it is necessary in frame relay. So it compare these two packet switched network
we have discussed today. Now it is time to give you the review questions. 1) In what layer X.25 operates? 2) what are the key functions of X.25 protocol
3) What limitations of X.25 are overcome in frame relay protocol? 4) Distinguish between permanent virtual and
switched virtual connections used in frame relay protocol
5) How congestion control is performed in frame relay network? Now it is time to give the answers to the
questions of lecture – 22. 1) What is congestion? Why congestion occurs? When the offered load crosses certain limit
then there is sharp fall in the throughput and increase in delay. This phenomenon is known as congestion. As there is sudden increase in the load congestion
arises. In that situation whenever link utilization
exceeds beyond 80% then it is used as rule of thumb for identifying congestion. Congestion may occur due to certain increase
in traffic in the network, it may also arise because of slow processors and slow bandwidth
lengths. 2) What are the two basic mechanisms of congestion
control? As we know there are two mechanisms of congestion
control. One is preventive based on open loop technique
such as the Leaky bucket algorithm or token bucket
algorithm where precautions are taken so that congestion cannot occur in the first place. On the other hand another approach is based
on the recovery from congestion by using close loop congestion control techniques when congestion
has already taken place. We have discussed various techniques such
as choke packet and so on. 3) How congestion control is performed by
Leaky bucket algorithm? In Leaky bucket algorithm a buffering mechanism
is introduced between the host computer and the network in order to regulate the flow
of traffic. Bursty traffic is generated by the host computer
and introduced in the network by Leaky bucket mechanism in the following manner. Packets are used in the network in one per
tick. In case of buffer overflow packets are discarded. 4) In what way token bucket algorithm is superior
to Leaky bucket algorithm? Leaky bucket algorithm controls the rate at
which the packets have been introduced in the network, but it is very conservative in
nature. Some flexibility is introduced in token bucket
algorithm. In token bucket algorithm tokens are generated
at each tick up to certain limit obviously based on the size of the counter. For an incoming packet to be transmitted it
must capture a token and transmission takes place at the same rate. Hence some of the busty packets are transmitted
at the same rate if tokens are available and thus introduce some amount of flexibility
in the system, this also improves performance. 5) What is choke packet? How it is used for congestion control? As we know choke packet scheme is a close
loop mechanism where each link is monitored to examine how much utilization is taking
place. If the utilization goes beyond a certain threshold
limit the link goes to a warning state and special packet called choke packet is sent
to the source. On receiving the choke packet the source reduces
the traffic in order to overcome congestion. With this we come to the end of today’s
lecture. In the next lecture we shall discuss about
another packet switch network that is ATM, thank you.

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  1. there is but it seems only on right channel. If you have monaural equipment it may be equivalent to no sound as monaural uses left channel…

  2. X.25 protocol allows computers on different public network to communicate through an intermediary computer at the network layer level.

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