Data Communications & Networks

Asynchronous
Transfer Mode

Mastering the high-speed switching technology that unified voice, video, and data transmission. The ultimate guide for undergraduate ECE students.

Start Learning View Cell Structure

What is ATM?

1 Cell-Based Switching

Unlike Ethernet (variable packets) or TDM (fixed time slots), ATM uses fixed-size 53-byte cells (48 payload + 5 header). This allows for predictable hardware processing at high speeds (150 Mbps to Gbps).

2 Connection-Oriented

A Virtual Circuit (VC) must be established before data is sent. This allows for Quality of Service (QoS) guarantees, making it ideal for real-time applications like voice and video.

3 Asynchronous Multiplexing

Cells are allocated on demand. Unlike Synchronous TDM (which wastes bandwidth if a slot is empty), ATM sends cells only when there is data, maximizing link utilization.

Conceptual Model
📞
VOICE
🌐
DATA
🎬
VIDEO
ATM Multiplexer

Fixed Cell Stream

The 53-Byte Cell

Click on the cell components to analyze the structure

GFC (4)
VPI (8)
VCI (16)
PT (3)
CLP (1)
HEC (8)
PAYLOAD (48 Bytes)
Byte 0 Byte 4 Byte 52

Select a Component

Click on the Header (Orange/Blue) or Payload (Green) above to see detailed field descriptions.

Cell Metrics

Total Size 53 Bytes
Header: 5 Bytes (9.4%) Payload: 48 Bytes (90.6%)

Why 53 Bytes?

A compromise between the US (64 bytes) and Europe (32 bytes) proposals. 48 was chosen because it accommodates a full DS0 (64 kbps) voice sample with some overhead.

ATM Reference Model

The three-layer stack

3

AAL Layer

ATM Adaptation Layer

  • Segments upper-layer data into 48-byte payloads.
  • Classes: AAL1 (CBR), AAL5 (Data/IP).
  • Reassembles cells at destination.
2

ATM Layer

Cell Processing

  • Adds 5-byte header.
  • Multiplexes cells from different sources.
  • Functions: Switching, Header Translation.
1

Physical Layer

Transmission

  • Medium dependent (Optical Fiber, Copper).
  • Bit timing & Line coding.
  • TC Sublayer: Cell delineation.

Traffic Management & QoS

ATM's killer feature was its ability to guarantee performance. Before sending data, a contract is negotiated between the user and the network.

1

Traffic Contract

Defines Peak Cell Rate (PCR), Sustainable Cell Rate (SCR), and Max Burst Size (MBS).

2

Usage Parameter Control (UPC)

The network polices the connection using the "Leaky Bucket" algorithm. Violating cells are dropped or tagged (CLP=1).

3

Service Categories

CBR (Constant Bit Rate), VBR (Variable), ABR (Available), UBR (Unspecified).

Leaky Bucket Visualization

State: CONFORMING
Rate: 50 cells/sec
Bucket Level: 40%

ATM Segmentation & Reassembly Simulator

Visualize how packets are chopped into cells and reassembled

10B 100 Bytes 200B
Cells Needed: -
Efficiency: -
Overhead: -
Original Packet
[ 100 Bytes Data ]
Cell Stream
Reassembled Packet
[ 100 Bytes Data ]
Checksum Verified. Data Intact.