What Is X.25 Protocol?

X.25 was the original packet-switched network.

X.25 is a protocol for packet switching that was developed during the 1970s to connect asynchronous terminals to large host computers. X.25 offers reliable packet delivery and a call connection mechanism. Terminals connect to the network using a packet assembler/disassembler, or PAD.

Call Setup

X.25 requires a call to establish a virtual circuit.

X.25 communications begin with a call setup. A terminal user enters the address of a host and the PAD sends a call setup packet to the network. Network nodes route the setup packet to the destination host and the host returns either a call accept packet or a call clear packet. The call packet defines the virtual circuit to use for data transfer.

Data Transfer

X.25 uses serial links.

After the call is established, data can be sent over the virtual circuits. Circuit numbers are part of the X.25 header, so each node knows where to send each packet. Errors are detected by a frame check sequence at the end of the packet. Any packet that contains an error is discarded and retransmitted over the link until it is received correctly.

Related Protocols

X.3 controls how the terminal traffic was packetized.

The PAD is controlled by a set of parameters defined by the X.3 standard. X.29 defines how the computer can change the parameters and X.28 defines how the terminal user can change these parameters. X.25 address are defined in X.121.

References

• ITU-T: Study Group 17
• Javvin: X.25
• Protocols: X.29
• Cisco: X.3 Pad Parameters.