X.25, an ITU-T protocol for WAN communications, is a packet switched data network
protocol which defines the exchange of data as well as control information between a
user device, called Data Terminal Equipment (DTE) and a network node, called Data
Circuit Terminating Equipment (DCE).
X.25 is designed to operate effectively regardless of the type of systems connected to the
network. X.25 is typically used in the packet-switched networks (PSNs) of common
carriers, such as the telephone companies. Subscribers are charged based on their use of
the network. X.25 utilizes a Connection-Oriented service which insures that packets are
transmitted in order.
X.25 sessions are established when one DTE device contacts another to request a
communication session. The DTE device that receives the request can either accept or
refuse the connection. If the request is accepted, the two systems begin full-duplex
information transfer. Either DTE device can terminate the connection. After the session is
terminated, any further communication requires the establishment of a new session. X.25
uses virtual circuits for packets communications. Both switched and permanent virtual
circuits are used.
X.75 is the signaling protocol for X.25, which defines the signaling system between two
PDNs. X.75 is essentially an Network to Network Interface (NNI).
X.25 protocol suite comes with three levels based on the first three layers of the OSI
seven layers architecture.
The Physical Level: describes the interface with the physical environment. There are
three protocols in this group: 1) X.21 interface operates over eight interchange circuits; 2)
X.21bis defines the analogue interface to allow access to the digital circuit switched
network using an analogue circuit; 3) V.24 provides procedures which enable the DTE to
operate over a leased analogue circuit connecting it to a packet switching node or
concentrator.
The Link Level: responsible for the reliable communication between the DTE and the
DCE. There are four protocols in this group: 1) LAPB, derived from HDLC and the most
commonly used, enables to form a logical link connection besides all the other
characteristics of HDLC; 2) Link Access Protocol (LAP) is an earlier version of LAPB
and is seldom used today; 3) LAPD, derived from LAPB and used for ISDN, enables data
transmission between DTEs through D channel, especially between a DTE and an ISDN
node; 4) Logical Link Control (LLC), an IEEE 802 LAN protocol, enables X.25 packets
to be transmitted through a LAN channel.
The Packet Layer Protocol (PLP): describes the data transfer protocol in the packet
switched network at the network layer (layer 3). PLP manages packet exchanges between
DTE devices across virtual circuits. PLPs also can run over Logical Link Control 2
implementations on LANs as well as over ISDN interfaces running LAPD. The PLP
operates in five distinct modes: call setup, data transfer, idle, call clearing, and restarting.
Protocol Structure - X.25: ITU-T Protocol for WAN CommunicationsX.25 PLP has
many control messages. The control packet as well as all X.25 packets begins with a 3-
byte header. Bytes 1,2 contain the Group and the Channel fields that together form a 12
bit virtual circuit number. The additional information for each message is different.
1. Control Packet
2. The additional information of the Call Request Packet is as follows:
Other Control Packets are:
3. The format of data packet is as follows:
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