These slides cover a topic on X.25, Frame relay and ATM in Data Communication. All the slides are explained in a very simple manner. It is useful for engineering students & also for the candidates who want to master data communication & computer networking.

1. Introduction to Data communication
Topic: X.25 and Frame Relay
Lecture # 12
Dr Rajiv Srivastava
Director
Sagar Institute of Research & Technology (SIRT)
Sagar Group of Institutions, Bhopal
http://www.sirtbhopal.ac.in

2. Unit 2
Lecture 12
Course Lecture 29
X.25and Frame Relay

3. • Definition : X.25 is a standard suite of
protocols used for packet switching across
computer networks. The X.25 protocols works
at the physical, data link, and network layers
(Layers 1 to 3) of the OSI model.
• While X.25 has, to a large extent, been
replaced by less complex protocols, especially
the Internet protocol (IP), the service is still
used and available in niche and legacy
applications.

4. PSE : Packet Switch Exchange
DTE : Data Terminal Equipment
DCE : Data Circuit Terminating
Equipment
PAD : Packet Assembler/
TE
NT

5. • Its a virtual-circuit switching network
• It uses a data transfer speed of 64 kbps
• It uses extensive error & flow controls at both
data link & network layer
• X.25 defines how a packet-mode terminal can be
connected to a packet network for exchange of
data. It describes procedure necessary for
establishing, maintaining & terminating
connections.
• It also describes set of services, called facilities, to
provide functions such as reverse charge, call
direct & delay control

6. • X.25 is also known as a subscriber network
interface(SNI) protocol.
• It defines how user’s DTE communicates with
the network & how packets are sent over that
networks using DCEs.
• It uses a virtual circuit approach to packet
witching (SVC & PVC) & use
asynchronous(statistical) TDM to multiplex
packets.

7. X.25 Layers
• X.25 protocol specifies three layers:
1. Physical layer
2. Frame layer
3. Packet layer
• These layers defines functions of physical
layer, data link layer & network layer of OSI
model.
• Given figure explains the situation

8. X.25 Layers in Relation to the OSI Layers

10. Physical Layer
• X.25 specifies a protocol named as X.21 at
physical layer. It is also called as X.21 bis.
• X.21 has been designed by ITU-T
• X.21 is quite similar to other physical layer
protocols such as EIA-232 which is also
supported by X.25

11. Frame Layer
• X.25 uses a bit oriented protocol at frame layer.
• This protocol is called as Link Access Procedure,
Balanced (LAPB)
• LAPB is subset of High-level Data Control (HDLC),
(which is bit oriented protocol used in point to
point & multi point links)
• Figure shows general format of the LAPB packet.

12. Format of a Frame

13. Frames are of three categories :
• I-Frames : I-frames are used to encapsulate PLP
packets from the network layer.
• S-Frames : S-frames are for flow and error control in
the frame layer.
• U-Frames : U-frames are used to set up and
disconnect the links between a DTE and a DCE. The
three packets most frequently used by LAPB in this
category are
• SABM (or ESABM if the extended address mode is used)
• UA &
• DISC

14. Frame Layer Phases
In the frame layer, communication between a DTE and a
DCE involves three phases :
1. link setup
2. packet transfer, and
3. link disconnect

15. Packet Layer
• The network layer in X.25 is called the packet layer
protocol (PLP).
• This layer is responsible for establishing the connection,
transferring the data, and terminating the connection.
• In addition, it is responsible for creating the virtual circuits
and negotiating network services between two DTEs.
• While the frame layer is responsible for making a
connection between a DTE and a DCE, the packet layer is
responsible for making a connection between two DTEs
(end-to-end connection).
• Note that X.25 uses flow and error control at two levels
(frame layer and packet layer).

16. Advantages of X.25
• The main advantages of the switching and
routing technique are as follows –
• Fast response times
• No blocking except when the network storage
is completely flooded.
• High availability because of distributed
routing.
• High-speed data bursts can be handled as well
as low-speed requirements.

17. Disadvantages of X.25
Disadvantages of X.25 packet switching network
are as follows :
(i)X.25 has a low data rate i.e. 64-kbps.
(ii)X.25 has extensive flow and error control at
both the data link layer and the network layer.
(iii)Flow and error control at both layers create a
large overhead and slow down transmissions.

18. (iii) X.25 was designed for private use, not for the
Internet. X.25 has its own network layer which
means that the user data are encapsulated in the
network layer packets of X.25. Although, the Internet
has its own network layer, which means if the
Internet wants to use X.25, it has to deliver its
network layer packet, known as datagram, to X.25
for encapsulation in the X.25 packet. Thus, the
overhead doubles.
(iv) Limitations of X.25 has given way to another
technologies for WAN like Frame Relay & then ATM
(Asynchronous Transfer Mode).

19. Frame Relay
Frame relay is a wide area network with the following
features:
1. Frame Relay operates at a higher speed (1.544 Mbps and
recently 44.376 Mbps). This means that it can easily be
used instead of a mesh of T-1 or T-3 lines.
2. Frame Relay operates in just the physical and data link
layers. This means it can easily be used as a backbone
network to provide services to protocols that already have
a network layer protocol, such as the internet.
3. Frame Relay allows bursty data.
4. Frame Relay allows a frame size of 9000 bytes, which can
accommodate all local area network frame sizes.

20. 5. Frame Relay is less expansive than other
traditional WANs.
6. Frame Relay has error detection at data link
layer only. There is no flow control or error
control.
7. There is not even a retransmission policy if a
frame is damaged; it is silently dropped.
Frame Relay was designed in this way to
provide fast transmission capability for more
reliable media and for those protocol that
have flow and error control at the higher
layers.

21. ATM
• Short for Asynchronous Transfer Mode, a network
technology based on transferring data in cells or packets of
a fixed size. The cell used with ATM is relatively small
compared to units used with older technologies. The small,
constant cell size allows ATM equipment to transmit video,
audio, and computer data over the same network, and
assure that no single type of data hogs the line.
• ATM (asynchronous transfer mode) is a dedicated-
connection switching technology that organizes digital
data into 53-byte cell units and transmits them over a
physical medium using digital signal technology.
Individually, a cell is processed asynchronously relative to
other related cells and is queued before being multiplexed
over the transmission path.

22. • Because ATM is designed to be easily
implemented by hardware (rather than
software), faster processing and switch speeds
are possible.
• The prespecified bit rates are either 155.520
Mbps or 622.080 Mbps. Speeds on ATM
networks can reach 10 Gbps.
• Along with Synchronous Optical Network
(SONET) and several other technologies, ATM is
a key component of broadband ISDN (BISDN).

23. Thank You
Dr Rajiv Srivastava
Director
Sagar Institute of Research & Technology (SIRT)
Sagar Group of Institutions, Bhopal
http://www.sirtbhopal.ac.in