---
title: "Short note: Frame relay"
subject: "Data Communication"
chapter: 7
key: "semester_5/data_communication:frame-relay-short-note"
source: https://www.bctengineer.com/q/semester_5/data_communication/frame-relay-short-note
site: BCT Engineering
---

# Short note: Frame relay

Data Communication · Chapter 7: Switching

- Asked in: 2076 Chaitra, 4 marks
- Video: [5. Intro to Frame Relay](https://www.youtube.com/watch?v=F926ApfBMfQ) — System Engineer
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/data_communication/frame-relay-short-note
- Whole chapter: https://www.bctengineer.com/md/semester_5/data_communication/chapter-7

## Question

Write a short note on frame relay.

## Short answer

Frame relay is a connection-oriented WAN technology operating at the data link layer, using virtual circuits identified by a 10-bit DLCI. It discards errored frames without hop-by-hop error recovery, relying on higher layers. Speeds up to 45 Mbps; uses CIR, FECN/BECN for congestion management.

## Answer

**Frame relay** is a fast, connection-oriented packet-switching WAN technology (ITU-T Q.922 / ANSI T1.618) that works at the physical and data link layers only. It was designed as a simplified successor of X.25 for reliable digital links, with speeds of 56 kbps to 44.7 Mbps (T1/T3).

#### Features
- Uses **virtual circuits**, mostly permanent (PVC), identified by a 10-bit **DLCI** (data link connection identifier).
- No hop-by-hop error correction or flow control: a frame with errors is just discarded and higher layers (TCP) recover. This makes it much faster than X.25.
- Variable-size frames up to about 1600 bytes (up to 8 kB in some cases).
- Bursty traffic is handled with **CIR** (committed information rate); bits above CIR are marked DE (discard eligible).
- Congestion indicated by FECN and BECN bits.

#### Frame format
```text
 | Flag | Address (DLCI,C/R,EA,FECN,BECN,DE) | Info | FCS | Flag |
 |  1   |             2 bytes                | var  |  2  |  1   |
```

#### Uses
Connecting LANs of branch offices (LAN-to-LAN) over WAN, voice over frame relay, and Internet access. It has been largely replaced by MPLS and Ethernet WAN.

## Check yourself

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

1. Frame relay works at which layers?
   A. Physical and data link only
   B. Network and transport
   C. Physical, link and network
   D. Application
   Answer: A. It operates at layers 1 and 2 only. (Key point: Layers)

2. What happens to a frame with errors?
   A. Retransmitted hop by hop
   B. Corrected at each node
   C. Discarded; higher layers recover
   D. Flagged and forwarded
   Answer: C. No hop-by-hop error correction; bad frames are discarded. (Key point: Error handling)

3. What does DLCI identify?
   A. The destination host
   B. A virtual circuit (10 bits)
   C. A frame check
   D. A flow window
   Answer: B. DLCI is the data link connection identifier of the virtual circuit. (Key point: DLCI)

4. What is CIR and DE?
   A. Committed information rate; bits above it are marked discard eligible
   B. Circuit identification route; error flag
   C. Congestion indicator; delay
   D. Cell information rate; data end
   Answer: A. Traffic above CIR is marked DE and dropped first under congestion. (Key point: CIR)

## Related in Data Communication

- [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

## Related in other subjects

- [Frame Relay](https://www.bctengineer.com/question.md?key=semester_7%2Fcomputer_networks_and_security%3Aframe-relay) — Computer Networks and Security, chapter 2