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 :
i)DTE (Data terminal equipment) --- Are end systems which communicate with each other on the network with the help of DCEs. ii)DCE(Data Circuit Terminal Equipment)--- Are communicating devices such as modems, switches which provide interface between DTEs and a PSE. iii)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:
i)It acts as a packet buffer
ii)It assembles the packets
iii)It disassembles the packets.
At the source end PAD converts the source’s packet format into X.25 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. X.25 has 3 layers :
i) X.25 physical layer (X.21),layer1 (Physical layer of OSI) ii) X.25 Frame layer(LAPB),layer2 (Data link layer of OSI) iii) X.25 Packet layer(PLP),layer3 (Network layer of OSI)
X.25 specifies X.21,X.24,X.27 as physical interface standards which correspond to EIA RS-232,RS-423A and RS 422A standards respectively.
The physical layer protocol for X.25 layer defined by ITU-T is X.21(or X.21 bis). The standards supported by X.25 are V.35,RS-232. ITU-T designed X.21 interface to make all...