ATM protocol

Architecture Overview

Asynchronous Transport Mode protocol is designed to transport data with guarantees for QOS in which information is organised into small, fixed-length packets, called cells. Due to this nature ATM switches can perform cell switching in hardware improving speed and efficiency.

ATM functionality corresponds to the physical and part of the data-link layers of the OSI model. ATM supported functionality is often described as a three-dimensional reference model that is composed of layers and planes.

The ATM reference model is composed of the following ATM layers:

Physical Layer is responsible for synchronising transmission and reception by sending and receiving a continuous flow of bits with associated timing information. This layer corresponds to the OSI physical layer.

ATM Layer is responsible for the simultaneous sharing of virtual circuits over the physical link and passing cells through the ATM network. To do this, it uses the VPI and VCI information in the header of each ATM cell. Combined with the ATM adaptation layer, the ATM layer corresponds to the data-link layer of the OSI model.

ATM adaptation Layer is responsible for isolating higher layer protocols from the ATM process. The AAL prepares data into payloads.

The ATM reference model is composed of the following planes:

Control plane is responsible for managing signal requests.

User plane is responsible for managing data transfer

Management plane is divided into two sub-planes:

• Layer management control specific layer functions, such as detection of failures and protocol problems.
• Plane management manages and coordinates functions related to the complete system. 

Protocols in the ATM layers provide communications between ATM switches while the protocols in the ATM adaption layer provide end-to-end user communications. Interfaces between users and network equipment are called UNI (user network interface), interfaces between ATM switches are called NNI (network-network-interfaces).

ATM Packets (cells)

ATM cells are 53 bytes in length where the first 5 bytes are used for header information and the last 48 bytes are used for payload information.

The difference between the two types of ATM cells is that the cells at the user-network interface carry a data field for the flow control of data from users. This means that only eight bits are available for virtual path identifiers, rather than 12 bits at the network-node interface.

ATM networks identify virtual connections using a combination of Virtual path identifiers and Virtual channel identifiers, this type of combination provide hierarchy in the numbering of virtual connections, by which a virtual path contains a number of virtual channels.

The number of virtual connections available between a user and network interface is less than between network interfaces, due to the fact that user-network interface cells have less 4 bits available for  virtual connections.

The payload type field is used to identify the type of ATM cells. The Cell Loss Priority (CLP) field is used to provide guidance to the network in the event of congestion. A value of 0 indicates a cell of relatively higher priority, which should not be discarded unless no other alternative is available. The header error control field contains a cyclic redundancy check on the other bytes in the header.