ATM Network

Asynchronous Transfer Mode 

Asynchronous transfer mode is an International Telecommunication union- Telecommunication section (ITU-T) standard for cell relay. Wherein information for multiple service types such as video, voice or data is conveyed in small fixed size cells. A cell network uses the cell as the basic unit of data exchange. A cell is defined as a small, fixed-size block of information.

 ATM Network Interfaces

An ATM network of a set of ATM switches interconnected by point-to-point ATM links or interfaces. ATM switche support two primary types of interfaces: UNI and NNI.UNI(User Network Interface) connects ATM end systm to an ATM switch. NNI(Network-Network Interface) connects two ATM switches. Connection between two endpoints is accomplished through transmission paths (TPs), virtual paths (YPs), and virtual circuits (YCs). Architecture of an ATM network shown below,

Architecture of an ATM network

Cells

The basic data unit in an ATM network is called a cell. A cell is only 53 bytes long with

5 bytes allocated to the header and 48 bytes carrying the payload, user data may be less than 48 byte.

ATM Cell

ATM Layers

ATM standard consist three layers. They are, the application adaptation layer (AAL), the ATM layer, and the physical layer. The endpoints use all layers but, the switches use only the two bottom layers.

ATM Layers 

Physical Layer

The Physical Layer is responsible for the electrical or optical transmission and receipt along the physical media between two devices. 

ATM Layer

The ATM layer provides routing, traffic management, switching, and multiplexing services. Other functions are,

• Cell header generation/extraction. 
• Multiplexing and de-multiplexing of cells.
• Managing of cell flow and sequencing.
• Handling dropped cells.
• Switch-based routing using virtual paths and virtual circuits.

ATM Adaptation Layer

The main services provided by AAL (ATM Adaptation Layer) are:

Segmentation and reassembly, handling of transmission errors, handling of lost cell conditions, timing and flow control