X.25 - What is X.25 Networks?

X.25 – What is X.25 Networks?

• X.25 is a standard used by many older public networks specially outside the U.S.

• This was developed in 1970s by CCITT for providing an interface between public packet-switched network and their customers.

• The packet switching networks use X.25 protocol. The X.25 recommendations were first prepared in 1976 and then revised in 1978, 1980 and 1984.

• X.25 was developed for computerconnections, used for terminal/timesharing connection.

• This protocol is based on the protocols used in early packet switching networks such as ARPANET, DATAPAC, and TRANSPAC etc.

• X.25 Packet Switched networks allows remote devices to communicate with each other across high speed digital links without the expense of individual leased lines.

• A protocol X.21 which is a physical layer protocol is used to specify the physical electrical and procedural interface between the host and network.

• The problem with this standard is that it needs digital signal rather than analog signals on telephone lines.

• So not many networks support this standard. Instead RS 232 standard is defined.

• The data link layer standard has a number of variations. It is designed for error detection and corrections.

• The network layer protocol performs the addressing, flow control, delivery confirmation etc.

• It allows the user to establish virtual circuits and send packets on them. These packets are delivered to the destination reliably and in order.

• X.25 is a connection oriented service. It supports switched virtual circuits as well as the permanent circuits.


• Packet Switching is a technique whereby the network routes individual packets of HDLC data between different destinations based on addressing within each packet.


• A switched virtual circuit is established between a computer and network when the computer sends a packet to the network requesting to make a call to other computer.

• Packets can then be sent over this connection from sender to receiver.

• X.25 provides the flow control, to avoid a fast sender overriding a slow or busy receiver.

• A permanent virtual circuit is analogous to-a leased line. It is set up in advance with a mutual agreement between the users.

• Since it is always present, no call set up is required for its use.

• In order to allow the computers which do not use the X.25 to communicate with the X.25 network a packet assembler disassembler (PAD) is used.

• PAD is required to be installed along with each computer which does not use X.25.

• X.25 defines the interface for exchange of packets between a DTE and switch data subnetwork node.

Three Layers of X.25:

The X.25 interface is defined at three levels:


The three levels are:


(i) Physical layer (level 1)

(ii) Data link layer (level 2)

(iii) Packet layer (level 3).


• These three layers correspond to the three lower most layers of the ISO-OSI reference model. The physical layer takes care of the interface between a computer terminal and the link which attaches it to the packet switching node.

• The X.25 defines the interface for exchange of packets between the user’s machine (DTE) and the packet switching node to which this DTE is attached which is called as DCE.

• The three layers of X.25 interface are as shown in Fig.(a).

• At the physical level X.21 physical interface is being used which is defined for circuit switched data network. At the data link level, X.25 specifies the link access procedure-B (LAP-B) protocol which is a subset of HDLC protocol.

Different layers of X.25 and interface between DTE and DCE

• At the network level (3rd level), X.25 defines a protocol for an access to packet data subnetwork.

• This protocol defines the format, content and procedures for exchange of control and data transfer packets. The packet layer provides an external virtual circuit service.

• Fig.(b) shows the relationship between the levels of x’25. User data is passed down to X.25 level 3.

• This data then appends the control information as a header to form a packet. This control .information is then used in the operation of the protocol.

• The entire X.25 packet formed at the packet level is then passed down to the second layer i.e. the data link layer.

• The control information is appended at the front and back of the packet forming a LAP-B frame. The control information in LAP-B frame is needed for the operation of the LAP-B protocol.

• This frame is then passed to the physical layer for transmission.

Relationship between the levels of X.25

Virtual Circuit Service

• With the X25 packet layer, data are transmitted in packets over external virtual circuits, The virtual circuit service of X25 provides for two types of virtual circuits,

• The virtual circuit service of X25 provides for two types of virtual circuits i.e. “virtual call” and “permanent virtual circuit”.

• A virtual call is a dynamically established virtual circuit using a call set up and call clearing procedure.

• A permanent virtual circuit is a fixed, network assigned virtual circuit. Data transfer takes place as with virtual calls, but no call set up or clearing is required.

Characteristics of X.25

In addition to the characteristics of the packet switched network, X.25 has the following characteristics:


  1. Multiple logical channels can be set on a single physical line
  1. Terminals of different communication speeds can communicate
  1. The procedure for transmission controls can be changed.

Multiple Logical Channels can be set on a Single Physical Line

The terminal connected to the packet switched network can communicate with multiple terminals at the same time using a single physical line. This makes it possible to set multiple logical paths called logical channels on a single physical line. Multiple communications thus takes place through these logical channels. Based on the X.25 rules, 4096 logical channel can be set on a single physical line. To enable control of 4096 logical channels there are 16 logical channel groups. Each logical channel group is divided into 256 logical channels. These channel groups are known as LCGN (Logical Channel Group Number) and LCN (Logical Channel Number).

X.25 Representing Logical Channel

Terminals of Different Communication Speeds can communicate

As X.25 uses the store and forward method, therefore, the communication is possible. In other words, a terminal of 1.2 Kbits/s can communicate with a host computer at 9600 bits/s through the packet switched network. When the ‘telephone network or a leased line is used, this type of communication cannot be established. In other words, in these environments, the transmission speed of the sender should be the same as that of the receiver.


The reason that communication between terminals with different communication speeds is possible is that the senders and the receivers are not physically connected. Data transmission from a 1.2 Kbits/s terminal is temporarily stored in the receiving buffer of the packet switched network and the data is then passed through the network and transmitted to the host computer at 9600 bits/s.

Communication Using Different Speeds

By using the above 2 features the network can be established. By applying a higher line speed to the host computer than the terminal and setting multiple logical channels, the number of lines at the computer can be reduced.

Packet interleaved Multiplexing

The Procedure for Transmission Controls can be changed

It is possible to change the procedure for transmission control. As we know that X.25 uses the store and forward method, therefore, all data must be once stored in the packet switched unit. By implementing a protocol conversion function to the packet switched unit can connect the devices with different transmission control (basic procedure and X.25 protocol).With the help of this method, any terminal that cannot handle packets cannot be connected to the packet switched network. A terminal that cannot handle packets is called an NPT (Non-packet mode terminal).

Conversion of Transmission Procedure

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