---
title: "ATM vs frame relay"
subject: "Data Communication"
chapter: 7
key: "semester_5/data_communication:atm-vs-frame-relay"
source: https://www.bctengineer.com/q/semester_5/data_communication/atm-vs-frame-relay
site: BCT Engineering
---

# ATM vs frame relay

Data Communication · Chapter 7: Switching

- Asked in: 2071 Chaitra, 2+3 marks
- Video: [Frame relay and ATM](https://www.youtube.com/watch?v=1CEb_4KmP-I) — Learning Computer Networks at Pallotti
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/data_communication/atm-vs-frame-relay
- Whole chapter: https://www.bctengineer.com/md/semester_5/data_communication/chapter-7

## Question

How does ATM differ from frame relay? What are the advantages and disadvantages of ATM compared to frame relay?

## Short answer

ATM uses fixed 53-byte cells with VPI/VCI identifiers, offering high speeds (155 Mbps+), predictable delay, and QoS classes. Frame relay uses variable-length frames, DLCI, lower speed. ATM advantages: integrated voice/video/data, guaranteed QoS, scalable. Disadvantages: 9.4% cell tax, segmentation overhead, complexity, higher cost.

## Answer

**ATM (Asynchronous Transfer Mode)** is a cell-switching technology that uses fixed-size **53-byte cells** (5-byte header + 48-byte payload) over virtual circuits identified by VPI/VCI. **Frame relay** uses variable-length frames and DLCI.

#### Differences
| Point | ATM | Frame relay |
|---|---|---|
| Data unit | Fixed 53-byte cell | Variable frame (up to ~1600 B) |
| Speed | 155 Mbps, 622 Mbps and more | 56 kbps to 45 Mbps |
| Identifier | VPI/VCI | DLCI (10 bits) |
| QoS | Classes CBR, VBR, ABR, UBR | Only CIR and DE bit |
| Traffic | Voice, video and data | Mainly data |
| Delay | Small and predictable | Variable |
| Layer | Own layers (AAL, ATM) | Data link layer |
| Cost | High | Lower |

#### Advantages of ATM
- Very high speed, easy hardware switching due to fixed cells.
- Small, predictable delay and jitter, good for real-time voice and video.
- Guaranteed quality of service.
- One network for voice, video and data; scalable.

#### Disadvantages of ATM
- **Cell tax:** 5 of 53 bytes (about 9.4 %) are header overhead, and padding of partly filled cells.
- Segmentation and reassembly of packets (AAL) adds complexity and delay.
- Expensive equipment and complex management.
- Less suited to bursty LAN data than frame relay or Ethernet.

## Check yourself

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

1. What data unit does ATM use?
   A. Variable frames up to 1600 B
   B. Fixed 53-byte cells (5 header + 48 payload)
   C. 1500-byte packets
   D. Bits
   Answer: B. ATM uses fixed 53-byte cells. (Key point: Data unit)

2. Which identifiers are used?
   A. ATM: DLCI; FR: VPI/VCI
   B. ATM: VPI/VCI; FR: DLCI
   C. Both DLCI
   D. Both VPI
   Answer: B. ATM uses VPI/VCI; frame relay uses DLCI. (Key point: Identifiers)

3. Which has stronger QoS classes?
   A. Frame relay (CBR, VBR)
   B. ATM (CBR, VBR, ABR, UBR)
   C. Neither
   D. Both identical
   Answer: B. ATM offers service classes; frame relay only CIR and DE. (Key point: QoS)

4. What is the ATM cell tax?
   A. 5 of 53 bytes (about 9.4%) are header overhead
   B. Cost per cell
   C. 48 of 53 bytes
   D. Frame delay
   Answer: A. 5/53 is about 9.4% overhead. (Key point: Cell tax)

## Related in Data Communication

- [Frame relay vs X.25](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Aframe-relay-vs-x25) — Data Communication, chapter 7
- [Short note: Frame relay](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Aframe-relay-short-note) — 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
- [TDM vs statistical TDM and STDM frame](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Atdm-statistical-tdm) — Telecommunications, chapter 3, asked 2×
- [MBI features; open vs closed loop compared](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Ambi-features-open-closed) — Instrumentation II, chapter 1
- [Frame vs semantic net](https://www.bctengineer.com/question.md?key=semester_6%2Fartificial_intelligence%3Aframe-vs-semantic-net) — Artificial Intelligence, chapter 5