---
title: "Packet switching"
subject: "Data Communication"
chapter: 7
key: "semester_5/data_communication:packet-switching-explain"
source: https://www.bctengineer.com/q/semester_5/data_communication/packet-switching-explain
site: BCT Engineering
---

# Packet switching

Data Communication · Chapter 7: Switching

- Asked in: 2075 Asoj, 4 marks; 2074 Asoj, 5 marks; 2070 Chaitra, 5 marks (asked 3 times)
- Video: [Packet switching | Packet switched network | Switching technology | TechTerms](https://www.youtube.com/watch?v=k8rJFgeuZRw) — TechTerms
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/data_communication/packet-switching-explain
- Whole chapter: https://www.bctengineer.com/md/semester_5/data_communication/chapter-7

## Question

Explain the operation of packet switching.

## Short answer

In packet switching, a message is split into packets with headers (source, destination, sequence). Each packet is independently routed using store-and-forward at routers, then reassembled in order at the destination. Two approaches: datagram (connectionless) and virtual circuit (connection-oriented).

## Answer

**Packet switching** is a switching method in which a message is divided into small units called **packets**, each carrying a header (source, destination, sequence number), and the packets are sent through the network independently and then reassembled at the destination. Links are shared by many users on demand (statistical multiplexing). The nodes use **store-and-forward**: a packet is fully received, checked, queued and then forwarded to the next node.

#### Operation
1. The source splits the message into packets and adds headers.
2. Each packet is given to the first router, which stores it in a buffer and checks it for errors.
3. The router looks at the destination address (or virtual circuit number) and its routing table and forwards the packet on the best link.
4. The process repeats at every router until the packet reaches the destination.
5. The destination reorders packets by sequence number and rebuilds the message.

```text
        +--R1---R2--+
 A -----+           +----- B
        +--R3---R4--+
 Packets P1,P2,P3 can take different routes
 and arrive: P2 P1 P3 -> reordered at B
```

#### Two approaches
- **Datagram:** each packet is routed independently, no setup; packets may arrive out of order (Internet, IP).
- **Virtual circuit:** a logical path is set up first, and all packets follow it in order (X.25, frame relay, ATM).

#### Features
- Efficient link use for bursty data; no wasted idle bandwidth.
- Alternative routes if a link fails.
- Delay is variable because of queuing; packets can be lost when buffers overflow.
- Packet header is overhead.

Delay in one hop = transmission time (L/R) + propagation + queuing.

## Check yourself

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

1. What does store-and-forward mean at a node?
   A. Forwarding bits as they arrive
   B. A packet is fully received, checked, queued and then forwarded
   C. Storing packets forever
   D. Forwarding without checking
   Answer: B. Nodes receive the whole packet first, check it, then forward it. (Key point: Store and forward)

2. How does the destination rebuild the message?
   A. By timing
   B. By reordering packets using sequence numbers
   C. By discarding late packets
   D. By asking the source to resend all
   Answer: B. Sequence numbers in headers allow reordering. (Key point: Reassembly)

3. Which statement is correct?
   A. Datagram needs setup
   B. Virtual circuit packets can take different routes
   C. Datagram packets are routed independently and may arrive out of order
   D. VC packets arrive out of order
   Answer: C. Datagram routes each packet independently; VC follows a set path in order. (Key point: Datagram vs VC)

4. Delay in one hop is the sum of
   A. Transmission + propagation + queuing
   B. Only propagation
   C. Setup + teardown
   D. Transmission only
   Answer: A. L/R transmission plus propagation plus queuing. (Key point: Delay)

## 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
- [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
- [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×

## 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×
- [Three-phase SPWM inverter switching instants](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_electronics%3Aspwm-switching-instants) — Power Electronics, chapter 5, asked 3×
- [Time division switching and space switch](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Atime-division-space-switch) — Telecommunications, chapter 4
- [Power transistor switching and base drive](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_electronics%3Apower-transistor-switching-base-drive) — Power Electronics, chapter 1