---
title: "Switching, circuit vs packet, X.25 layers"
subject: "Data Communication"
chapter: 7
key: "semester_5/data_communication:switching-define-compare-x25-layers"
source: https://www.bctengineer.com/q/semester_5/data_communication/switching-define-compare-x25-layers
site: BCT Engineering
---

# Switching, circuit vs packet, X.25 layers

Data Communication · Chapter 7: Switching

- Asked in: 2071 Shrawan, 7 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 Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/data_communication/switching-define-compare-x25-layers
- Whole chapter: https://www.bctengineer.com/md/semester_5/data_communication/chapter-7

## Question

Define switching. Compare circuit and packet switching. Draw the X.25 layers and data formats.

## Short answer

Switching transfers data through intermediate nodes without direct links. Circuit switching reserves a dedicated path, constant delay, good for voice. Packet switching shares links on demand, uses packets with headers, variable delay, suitable for data. X.25 has three layers: physical, data link (LAPB), and packet (PLP), with virtual circuits.

## Answer

#### Switching
**Switching** is the technique of transferring data from a source to a destination through intermediate nodes (switches) in a network, so that every device does not need a direct link to every other device. Types: circuit switching, message switching and packet switching.

#### Circuit vs packet switching
| Point | Circuit switching | Packet switching |
|---|---|---|
| Path | Dedicated end-to-end path | Shared links, no dedicated path |
| Setup | Required before transfer | Not needed (datagram) |
| Bandwidth | Reserved, wasted when idle | Used on demand |
| Delay | Setup delay, then constant | Variable queuing delay |
| Data unit | Continuous stream | Packets with headers |
| Failure | Call is lost | Packets re-routed |
| Best for | Voice (PSTN) | Data (Internet) |

#### X.25 layers
```text
  DTE                           DCE (network)
 +------------------+         +------------------+
 | 3 Packet (PLP)   |<------->| 3 Packet (PLP)   |
 | 2 Link   (LAPB)  |<------->| 2 Link   (LAPB)  |
 | 1 Physical (X.21)|<------->| 1 Physical (X.21)|
 +------------------+         +------------------+
```

#### Data formats
```text
 LAPB frame:
 | Flag | Address | Control | Info (packet) | FCS | Flag |
 | 8    | 8       | 8       | variable      | 16  | 8    |

 X.25 packet inside the info field:
 | GFI 4 | LCN 12 | Type 8 | User data |
```
GFI = general format identifier, LCN = logical channel number. LAPB gives error-free frame transfer; the packet layer sets up virtual circuits and does flow control and multiplexing.

## Check yourself

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

1. What is switching?
   A. Transferring data via intermediate nodes so every device needn't be linked directly
   B. Linking all devices directly
   C. Encrypting data
   D. Amplifying signals
   Answer: A. Switching moves data through intermediate nodes. (Key point: Definition)

2. Which are the types of switching?
   A. Circuit, message, packet
   B. Analog, digital, hybrid
   C. Serial, parallel, mixed
   D. Static, dynamic, random
   Answer: A. Circuit, message and packet switching. (Key point: Types)

3. Which is best for voice vs data?
   A. Packet for voice; circuit for data
   B. Circuit for voice (PSTN); packet for data (Internet)
   C. Both circuit
   D. Both packet
   Answer: B. Circuit suits voice, packet suits data. (Key point: Circuit vs packet)

4. In X.25, what sits in the LAPB frame's info field?
   A. Voice samples
   B. The X.25 packet (GFI, LCN, type, data)
   C. The flag
   D. The FCS
   Answer: B. The packet, with GFI 4, LCN 12, type 8 bits, is carried in the info field. (Key point: X.25 frame)

## Related in Data Communication

- [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
- [Datagram vs virtual circuit switching](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Adatagram-vs-virtual-circuit) — Data Communication, chapter 7, asked 3×
- [Packet switching](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Apacket-switching-explain) — Data Communication, chapter 7, asked 3×
- [X.25 protocol](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Ax25-protocol) — Data Communication, chapter 7, asked 3×

## 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×
- [Circuit switch vs packet switch](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Acircuit-vs-packet-switch) — Telecommunications, chapter 8
- [Datagram vs virtual circuit switching](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Adatagram-vs-virtual-circuit) — Telecommunications, chapter 8
- [IPv6 migration; IPv4 vs IPv6 packet structure](https://www.bctengineer.com/question.md?key=semester_8%2Finternet_and_intranet%3Aipv6-migration-router-packet) — Internet and Intranet, chapter 2