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X.25 network and features

Short answer

X.25 is a three-layer packet-switched standard with error and flow control at link and packet layers using virtual circuits. It offers reliable communication over noisy lines but introduces heavy overhead, with speeds up to 2 Mbps.

Explain X.25 Network with its key features.

X.25 is an ITU-T standard for packet-switched WAN communication between a DTE (user equipment) and a DCE (network access equipment). It defines the three lowest layers: physical (X.21), data link (LAPB, similar to HDLC) and network (packet layer protocol, PLP). It was widely used in the 1980s, for example in banking networks and cash machines (ATMs).

Architecture

 DTE --- DCE ===[ packet switches ]=== DCE --- DTE
  • DTE: terminal or computer. DCE: modem or packet switch. PAD: a Packet Assembler/Disassembler lets simple character terminals use the network.

Key features

  1. Packet switching with virtual circuits: Permanent (PVC) and Switched (SVC), identified by a logical channel number.
  2. Reliable service: error control and flow control at both the link layer (hop by hop) and the packet layer, with acknowledgements and retransmission. This was needed for the noisy analog lines of its time.
  3. Statistical multiplexing: several virtual circuits share a single physical line.
  4. Connection-oriented with call setup and clear.
  5. Low speed: typically 64 kbps up to 2 Mbps.
  6. Congestion and flow control by window-based mechanism.
  7. Heavy overhead, so slower than Frame Relay, which replaced it.

✓ Answer checked by Claude Opus 5.5 (high effort)

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