---
title: "X.25 network and features"
subject: "Computer Networks and Security"
chapter: 2
key: "semester_7/computer_networks_and_security:x25-network"
source: https://www.bctengineer.com/q/semester_7/computer_networks_and_security/x25-network
site: BCT Engineering
---

# X.25 network and features

Computer Networks and Security · Chapter 2: Physical Layer

- Asked in: 2071 Chaitra, 3 marks
- Video: [X.25 | X.25protocol | DTE and DCE | X.25 interface](https://www.youtube.com/watch?v=dT9NE-w55VA) — Easy Engineering By Neha Syed
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/computer_networks_and_security/x25-network
- Whole chapter: https://www.bctengineer.com/md/semester_7/computer_networks_and_security/chapter-2

## Question

Explain X.25 Network with its key 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.

## Answer

**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
```text
 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.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Which three layers does X.25 define?
   A. Physical (X.21), data link (LAPB), network (PLP)
   B. Transport, session, application
   C. Data link, network, transport
   D. Only physical
   Answer: A. X.25 covers the lowest three layers. (Key point: Layers)

2. Why does X.25 have error and flow control at both link and packet layers?
   A. Because it was designed for noisy analog lines
   B. To reduce overhead
   C. Because it was connectionless
   D. Because it uses fibre
   Answer: A. The noisy analog lines of its time needed hop-by-hop and packet-level reliability. (Key point: Reliability)

3. How does X.25 compare with Frame Relay?
   A. Faster
   B. Heavy overhead, slower; replaced by Frame Relay
   C. Same speed
   D. No virtual circuits
   Answer: B. X.25's overhead made it slower than Frame Relay. (Key point: Versus Frame Relay)

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