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ATM General Introduction
The Asynchronous Transfer Mode (ATM) was born out of standardization 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, ...).
- ATM is a connection-oriented, packet-like switching and multiplexing
principle
- Three layers in the B-ISDN protocol reference model
- Physical layer
- various transmission media
- kilobits per second to gigabits per second
- ATM Layer
- short fixed-length cells (53 bytes - 48 bytes payload)
- multiplex logical channels within a physical channel
- fixed length cells
very-high-speed switching hardware
- problems with traffic management
- ATM adaptation layer
- AAL Type 1 --- constant bit rate services
- AAL Type 2 --- variable bit rate services
- AAL Type 3/4 --- connectionless services/data protocols
- AAL Type 5 --- high-speed data protocol
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