ATM General Introduction

The Asynchronous Transfer Mode (ATM) was born out of standardisation efforts for Broadband ISDN which began in the CCITT in the mid 1980s. It was originally intimately bound up with the emerging Synchronous Digital Hierarchy (SDH) standards, and was conceived as a way in which arbitrary-bandwidth communication channels could be provided within a multiplexing hierarchy consisting of a defined set of fixed-bandwidth channels.

The basic principles of ATM as put forward by CCITT in Recommendation I.150 are:

• ATM is considered as a specific packet oriented transfer mode based on fixed length cells. Each cell consists of an information field and a header, which is mainly used to determine the virtual channel and to perform the appropriate routing. Cell sequence integrity is preserved per virtual channel.

• ATM is connection-oriented. The header values are assigned to each section of a connection for the complete duration of the connection. Signaling and user information are carried on separate virtual channels.

• The information field of ATM cells is carried transparently through the network. No processing like error control is performed on it inside the network.

• All services (voice, video, data, ) can be transported via ATM, including connectionless services. To accommodate various services an adaptation function is provided to fit information of all services into ATM cells and to provide service specific functions (e.g. clock recovery, cell loss recovery, ...).