X.25 Packet Switching Protocol Explained | Generated by AI
Question: What is X.25?
Answer:
X.25 is an ITU-T standard protocol suite (originally developed by CCITT) for packet-switched data communication networks. It was the dominant wide-area networking protocol from the late 1970s through the 1990s and is still used in some niche applications today.
X.25 defines a complete three-layer protocol stack for reliable, connection-oriented packet switching over public data networks (PDNs):
- Physical layer
- Based on X.21, X.21bis (V.24/RS-232 compatible), or other interfaces
- Specifies electrical and mechanical characteristics for DTE–DCE connection
- Link layer (Layer 2)
- LAPB (Link Access Procedure Balanced)
- A bit-oriented HDLC-derived protocol
- Provides reliable, error-free, sequenced frame delivery between DTE and DCE using numbering, acknowledgments, retransmissions, and error checking
- Packet layer (Layer 3)
- The core of X.25
- Provides virtual circuit service (both switched virtual circuits – SVCs – and permanent virtual circuits – PVCs)
- Main functions:
- Call setup / call clearing
- Data transfer with sequencing and flow control
- Multiplexing of multiple logical channels over a single physical link
- End-to-end error recovery and flow control (windowing)
- Packet sizes typically 128, 256, 512, or 1024 bytes (negotiable)
- Uses modulo-8 or modulo-128 sequence numbering
Key characteristics of X.25:
- Connection-oriented — requires call setup before data transfer
- Reliable — guarantees in-order, error-free delivery (both link layer and packet layer contribute)
- Slow — significant overhead from acknowledgments and retransmissions at every hop
- High reliability on poor lines — designed for noisy analog lines of the 1970s–1980s
- Low throughput — typical speeds were 9.6 kbit/s to 64 kbit/s (rarely higher)
- Expensive — charged per packet and per call duration in most public networks
Main related ITU-T recommendations (X-series):
- X.25 — Interface between DTE and DCE for terminals operating in the packet mode
- X.3 — PAD parameters
- X.28 — DTE–PAD interface (asynchronous terminal to PAD)
- X.29 — PAD–host communication protocol
- X.121 — International numbering plan for public data networks
- X.75 — Internetworking between packet-switched networks
Historical context and decline:
- Dominant in the 1980s–early 1990s for financial transactions (ATMs, credit card terminals), airline reservation systems, and early internet access in many countries
- Largely replaced in the mid-1990s onward by:
- Frame Relay (less overhead, higher speed)
- ATM
- IP / MPLS / Internet-based VPNs
- Still exists in some legacy systems (e.g., certain banking networks, SCADA, air traffic control backups, and some countries’ public X.25 networks)
In summary:
X.25 was the first widely deployed international standard for packet switching, providing reliable virtual circuits over inherently unreliable lines, but its heavy protocol overhead made it inefficient for modern high-speed networks.
References:
- None (standard ITU-T X.25 recommendation and historical networking knowledge)