X. 25 is packet switched wide area network(WAN) common interface protocol between public data networks. It is a packet network access. It is used for establishing connections and and exchanging data as well as control information between network devices. It is mainly used for network terminal operation in packet mode.
3 types of X. 25 devices are there :
- DTE (Data terminal equipment) Are end systems which communicate with each other on the network with the help of DCEs.
- DCE(Data Circuit Terminal Equipment) Are communicating devices such as modems, switches which provide interface between DTEs and a PSE.
- PCE (Packet Switching Exchanges) Are switches that constitute majority of the network.
PSEs transfer data from one DTE to another DTE with the help of X. 25 layer network. PAD(Packet Assembler/Disassembler) is a device found in between DTE and DCE. PAD provides the following services:
- It acts as a packet buffer
- It assembles the packets
- It disassembles the packets.
At the source end PAD converts the source’s packet format into X.
5 data format and at the receiver’s end PAD converts from X. 25 data format to receiver’s data format, because sender’s and receiver’s data format may not be the same. X. 25 is a connection-oriented end-to-end protocol and it uses virtual circuit approach of packet switching. The standards supported by X. 25 are V. 35,RS-232. ITU-T designed X. 21 interface to make all digital communication between DTEs and DCEs.
X. 21 specifies the 15 pin DB-15 connector as the physical connector and defines the functionalities of each pin. Pinno. Function 1Shield ground 2Transmit data or control 3Control 4Receive data or signal 5Indication 6Signal element timing 7Byte timing 8Signal ground Shield ground 9Transmit data or control 10Control 11Receive data or signal 12Indication 3Signal element timing 14Byte timing 15Reserved for future use X.
25,layer2(Frame layer): X. 25 layer2 or frame layer transfers data between DTE and DCE as a sequence of frames using bit-oriented protocol called link access procedure,balanced(LAPB) and LAPB is a subset of HDLC. The frame layer provides the following functions:
- Transfer data in a efficient way maintaining the time properly.
- Frame layer takes into account whether the sender and receiver is synchronized.
- Provides error detection and recovery.
- Identify and report procedural errors to a higher level for recovery.
There are several frame layer protocols that may be used :
- LAPB currently most popular logical link connection protocol as well as it has some features of HDLC.
- LAPD(Link access protocol,D channel) which is derived from LAPB used for ISDN services. LAPD helps in the communication between a DTE and an ISDN node through D channels.
- Logical link control(LLC) defined by IEEE 802 used for transfer of packets across local area network.
X. 25,layer3(Packet layer): Layer3 creates network data units called packets which holds data as well as control information and X. 5 uses the packet layer protocols to establish a connection, transferring data over connection and to terminate the connection. The packet layer protocols are responsible also to create a virtual circuit and to make negotiation of network services between DTE and DCE .
Where the frame layer is responsible for establishing a link between DTE and DCE, the packet layer is responsible for end-to-end connection between two DTEs. Flow and error control between two DTEs is the main job of X. 25 packet layer, whereas flow and error control between DTE and DCE was one of the functions of frame layer.