---
title: "Short note: Virtual-circuit network"
subject: "Telecommunications"
chapter: 8
key: "semester_8/telecommunications:short-note-virtual-circuit"
source: https://www.bctengineer.com/q/semester_8/telecommunications/short-note-virtual-circuit
site: BCT Engineering
---

# Short note: Virtual-circuit network

Telecommunications · Chapter 8: Data Communication

- Asked in: 2081 Baisakh, 4 marks; 2074 Chaitra, 4 marks (asked 2 times)
- Video: [Datagrams & Virtual Circuits](https://www.youtube.com/watch?v=PVP3xUte5jA) — Spyros Georgios Zygouris
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_8/telecommunications/short-note-virtual-circuit
- Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-8

## Question

Write a short note on virtual-circuit network.

## Short answer

A virtual-circuit network establishes a logical connection before data transfer, so all packets follow the same path and arrive in order. It uses short VC identifiers instead of full addresses, with setup, data transfer, and teardown phases. Resources can be reserved, and VCs may be switched (per call) or permanent (configured).

## Answer

A **virtual-circuit (VC) network** is a packet-switched network in which a **logical connection** is set up between source and destination before data transfer. All packets of the session follow the **same route** and arrive **in order**, although link capacity is still shared with other users (it is "virtual", not a dedicated circuit). Examples: X.25, Frame Relay, ATM, MPLS.

**Three phases**

1. **Setup:** a call-request packet travels through the switches; each switch makes an entry in its table and assigns a **virtual circuit identifier (VCI)** for the incoming and outgoing port.
2. **Data transfer:** each packet carries only the short VCI; switches look it up and forward (label swapping).
3. **Teardown:** a clear packet removes the table entries.

```text
 Switch table:
 In port | In VCI | Out port | Out VCI
    1    |   14   |    3     |   22
 A --[VCI 14]--> S1 --[VCI 22]--> S2 --> B
```

**Features**

- Short header (VCI instead of full address).
- In-order delivery; resources and QoS can be reserved at setup.
- Setup delay; if a switch fails, the VC is lost and must be re-established.

**Types:** switched VC (SVC, set up per call) and permanent VC (PVC, configured by the operator).

## Check yourself

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

1. Why is a virtual circuit called 'virtual'?
   A. Packets take different routes
   B. It uses no switches
   C. It reserves a dedicated physical wire
   D. A logical connection is set up but link capacity is still shared with other users
   Answer: D. Packets follow one logical route in order, but unlike circuit switching the capacity is shared. (Key point: VC network nature)

2. What does each packet carry during the data transfer phase?
   A. A routing table
   B. The full destination address
   C. No header
   D. Only the short VCI, which switches look up and swap
   Answer: D. Switches use table entries (in port, in VCI, out port, out VCI) to forward by label swapping. (Key point: VCI use)

3. Which is a drawback of virtual-circuit networks?
   A. No QoS possible
   B. Packets arrive out of order
   C. Setup delay, and a switch failure loses the VC
   D. Long headers
   Answer: C. Setup takes time and if a switch fails the VC must be re-established. (Key point: Setup and failure)

## Related in Telecommunications

- [Datagram vs virtual circuit switching](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Adatagram-vs-virtual-circuit) — Telecommunications, chapter 8

## Related in other subjects

- [Virtual Private Network with example](https://www.bctengineer.com/question.md?key=semester_7%2Fcomputer_networks_and_security%3Avpn-example) — Computer Networks and Security, chapter 8
- [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×
- [Virtual circuit switching](https://www.bctengineer.com/question.md?key=semester_7%2Fcomputer_networks_and_security%3Avirtual-circuit-switching) — Computer Networks and Security, chapter 2
- [Virtual short circuit and inverting integrator](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Avirtual-short-integrator) — Basic Electronics Engineering, chapter 4