Avaya M-ACCF/SF manual ATM Adaptation Layer AAL, Asynchronous Transfer Mode ATM Layer, ATM Basics

Chapter 1 Overview

ATM Adaptation Layer (AAL)

Ethernet frames can be between 64 and 1514 bytes in length. ATM transmits data in fixed length cells. Each cell contains 48 bytes of user data. The ATM Adaptation Layer (AAL) converts data between the Ethernet and ATM formats.

The AAL has a Segmentation and Reassembly (SAR) sub-layer that does the conversion.

In the sending device the LEC passes the Ethernet frames to the SAR. The SAR converts the user data into fixed length cells, and passes these cells to the ATM Layer for transmission across the ATM network.

In the receiving device, the SAR converts the ATM cells back into the appropriate user data again, and passes this data to the LEC.

As ATM can carry different traffic types (for example, voice, video, and other data), several Adaptation Layer protocols have been defined. These protocols operate simultaneously within the Adaptation Layer, and allow the ATM Layer to support different applications and traffic types.

Note: The M-ACC module uses the AAL5 ATM Adaptation Layer protocol, which is a data-oriented protocol. The ATM access module will only work with other devices using the AAL5 ATM adaptation layer protocol.

Asynchronous Transfer Mode (ATM) Layer

Asynchronous Transfer Mode (ATM) is a connection-oriented transmission protocol that has the following features:

•ATM uses the Signalling Protocol (Q.2931) to dynamically create, maintain and clear ATM connections between end-systems.

•ATM uses fixed length packets known as cells, and each cell identifies the connection to be used.

•ATM is transparent to the multiple services it supports and can carry cells from different applications over the same physical connection.

•ATM has well-defined user and network interfaces.

ATM Basics

Asynchronous Transfer Mode (ATM) technology transfers network traffic, including voice, video, and data, at high speed. Using this connection-oriented networking technology, centered by a switch, you can set up a great number of virtual connections to support multiple applications through the same physical connection. The switching technology enables dedicated bandwidth for each application, overcoming the problems that exist in a shared-media networking technology, like Ethernet, Token Ring, and FDDI. ATM allows different types of

16 Avaya M770 M-ACCF/SF ATM Access Modules User’s Guide