---
title: "Frame Relay"
subject: "Computer Networks and Security"
chapter: 2
key: "semester_7/computer_networks_and_security:frame-relay"
source: https://www.bctengineer.com/q/semester_7/computer_networks_and_security/frame-relay
site: BCT Engineering
---

# Frame Relay

Computer Networks and Security · Chapter 2: Physical Layer

- Asked in: 2073 Shrawan, 3 marks
- Video: [What is frame relay? || How it works](https://www.youtube.com/watch?v=dhPPIT9roHs) — Waqas Tech Videos
- 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/frame-relay
- Whole chapter: https://www.bctengineer.com/md/semester_7/computer_networks_and_security/chapter-2

## Question

Define Frame Relay in detail.

## Short answer

Frame Relay is a connection-oriented WAN technology using DLCI-identified virtual circuits with no error/flow control inside the network. It discards errored frames for higher-layer recovery, offering speeds up to 45 Mbps and guaranteed CIR.

## Answer

**Frame Relay** is a high-speed, connection-oriented packet-switching WAN technology that works at the physical and data link layers. It was designed as a faster, simpler successor of X.25 for reliable digital (fibre) lines, and it is standardised by ITU-T and ANSI.

#### Key features
- **Virtual circuits:** data moves over Permanent (PVC) or Switched (SVC) virtual circuits identified by a **DLCI** (Data Link Connection Identifier) in the frame header.
- **Low overhead:** no error correction or flow control inside the network. Frames with errors are simply discarded and higher layers (TCP) recover them. This makes it much faster than X.25.
- **Speeds:** typically 56 kbps up to 45 Mbps (T3/E3).
- **Variable frame length** (up to about 4096 bytes), good for LAN traffic.
- **Statistical multiplexing** of several virtual circuits on one link.
- **Committed Information Rate (CIR):** the guaranteed bandwidth of a circuit; bursts above it may be dropped.
- **Congestion notification** by FECN and BECN bits and the DE (discard eligibility) bit.

#### Frame format
```text
| Flag | Address(DLCI,C/R,FECN,BECN,DE) | Data | FCS | Flag |
```

#### Use
It connects LANs of branch offices to the head office through a service provider, replacing costly leased lines. It was widely used before MPLS and the Internet-based VPNs.

## Check yourself

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

1. Why is Frame Relay faster than X.25?
   A. It adds more error correction
   B. No error correction or flow control inside the network; errored frames are discarded and higher layers recover
   C. It uses analog lines
   D. It has no virtual circuits
   Answer: B. Stripping error and flow control from the network core lowers overhead. (Key point: Low overhead)

2. What does the DLCI identify?
   A. IP address
   B. The virtual circuit in the frame header
   C. A port
   D. A MAC address
   Answer: B. The Data Link Connection Identifier identifies PVC/SVC. (Key point: DLCI)

3. Which statement is right?
   A. CIR is the guaranteed bandwidth; FECN, BECN and DE bits handle congestion
   B. CIR is maximum speed
   C. DE means data encrypted
   D. BECN is a checksum
   Answer: A. CIR is guaranteed bandwidth, and FECN/BECN/DE signal congestion and discard eligibility. (Key point: CIR and congestion)

## Related in other subjects

- [Short note: Frame relay](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Aframe-relay-short-note) — Data Communication, chapter 7
- [ATM vs frame relay](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Aatm-vs-frame-relay) — Data Communication, chapter 7
- [Frame relay vs X.25](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Aframe-relay-vs-x25) — Data Communication, chapter 7