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
The basic principles of ATM as put forward by CCITT in Recommendation
- 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, ...).
Daniel Davids - 9 January 1997 (firstname.lastname@example.org)