---
title: "X.25 protocol"
subject: "Data Communication"
chapter: 7
key: "semester_5/data_communication:x25-protocol"
source: https://www.bctengineer.com/q/semester_5/data_communication/x25-protocol
site: BCT Engineering
---

# X.25 protocol

Data Communication · Chapter 7: Switching

- Asked in: 2078 Bhadra, 4 marks; 2072 Kartik, 5 marks; 2070 Asar, 3 marks (asked 3 times)
- 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 Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/data_communication/x25-protocol
- Whole chapter: https://www.bctengineer.com/md/semester_5/data_communication/chapter-7

## Question

Explain the X.25 protocol (layers, data formats and packet switching operation).

## Short answer

X.25 is an ITU-T packet-switching standard using three layers: physical (X.21), link (LAPB for error-free frame transfer), and packet layer (PLP for virtual circuits). The LAPB frame carries a packet with logical channel number and packet type. Operation: a Call Request sets up a virtual circuit; data packets follow the same path with hop-by-hop error control; Clear Request ends the call.

## Answer

**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
```text
 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
```text
 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).

## Check yourself

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

1. What are the three X.25 layers?
   A. Physical (X.21), link (LAPB), packet (PLP)
   B. Physical, network, transport
   C. Link, ATM, packet
   D. Physical, LAPD, IP
   Answer: A. X.25 uses X.21, LAPB and PLP. (Key point: Layers)

2. What is LAPB's role?
   A. Sets up virtual circuits
   B. Error-free frame transfer with flow control and retransmission
   C. Routes packets
   D. Defines electrical levels
   Answer: B. LAPB, a subset of HDLC, gives error-free frame transfer between DTE and network. (Key point: LAPB)

3. What do GFI and LCN stand for in the packet header?
   A. Group frame index, link channel number
   B. General format identifier, logical channel number
   C. Gateway flag identifier, link circuit number
   D. General frame index, logical circuit name
   Answer: B. GFI = general format identifier; LCN = logical channel number (12 bits). (Key point: Packet header)

4. What is the order of a virtual circuit call?
   A. Data, call request, clear
   B. Call request, call accepted, data, clear request
   C. Clear, data, accept
   D. Call accepted, request, data
   Answer: B. Request, accepted, data, then clear request. (Key point: Call procedure)

## Related in Data Communication

- [Switching, circuit vs packet, X.25 layers](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Aswitching-define-compare-x25-layers) — Data Communication, chapter 7
- [Packet switching](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Apacket-switching-explain) — Data Communication, chapter 7, asked 3×
- [Circuit vs packet switching](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Acircuit-vs-packet-switching) — Data Communication, chapter 7, asked 4×
- [Packet vs message switching](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Apacket-vs-message-switching) — Data Communication, chapter 7

## Related in other subjects

- [Circuit vs packet switching](https://www.bctengineer.com/question.md?key=semester_7%2Fcomputer_networks_and_security%3Acircuit-vs-packet-switching) — Computer Networks and Security, chapter 2, asked 3×