X.25 protocol

X.25 is an ITU-T standard (1976) for packet-switched wide area networks. It defines the interface between a DTE (user device) and a DCE (network node) and provides reliable virtual-circuit service. It was widely used for ATMs and credit-card verification.

Layers

 Layer 3  Packet layer (PLP)    packets
 Layer 2  Link layer (LAPB)     frames
 Layer 1  Physical (X.21)       bits

• Physical layer: X.21 (or X.21bis, V.24 / RS-232) electrical and mechanical interface between DTE and DCE.
• Data link layer: LAPB (Link Access Procedure Balanced, subset of HDLC). It gives error-free frame transfer between the DTE and the network node, with flow control and retransmission using CRC and sequence numbers.
• Network (packet) layer: PLP sets up and clears virtual circuits, assigns a logical channel number, and does multiplexing, flow control and error control per virtual circuit.

Data formats

 LAPB frame:
 | Flag | Address | Control | Packet (info) | FCS | Flag |
 | 8    | 8       | 8       | variable      | 16  | 8    |

 Packet (inside frame):
 | GFI (4) | LCN (12) | Packet type (8) | User data |

• Flag = 01111110. GFI = general format identifier, LCN = logical channel number, type identifies call request, call accepted, data, RR, clear, etc.
• Data packets also carry send and receive sequence numbers (P(S), P(R)), and an M bit (more data).

Packet switching operation

1. The source DTE sends a Call Request packet with the destination address.
2. The network routes it and sets up a virtual circuit; the destination replies with Call Accepted.
3. Data packets follow the same path with the LCN identifying the circuit. Each hop does error checking and acknowledgment (store and forward).
4. After data transfer a Clear Request ends the call.

Both SVC (switched) and PVC (permanent) virtual circuits are supported. Standard speed is up to 64 kbps (up to 2 Mbps in later versions).