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Chapter 1 Overview
Avaya M770 M-ACCF/SF ATM Access Modules User’s Guide 17
physical layer technology to share the same higher layer — the ATM layer.
ATM uses fixed length packets called cells. The ATM cell is defined as 48 bytes of
payload and 5 bytes of header information totaling 53 bytes. The header contains
enough information to allow the network to forward each cell to its proper
destination. The cell header also provides the network with the ability to implement
congestion control and traffic management mechanisms.
ATM advantages include the fact that:
• Fixed-length cells offer smaller and more predictable switching delays, because
cell switching is less complex than variable-length packet switching.
• Having all the data in the same cell format also dramatically increases the speed
of transmission, by eliminating the need for protocol recognition and decoding.
A good analogy is containerized shipping, where uniform shape and weight
containers with standardized labelling, ease and speed up processing.
• Cell switching is less complex and more reliable. ATM hardware can be
implemented more efficiently because control structures, buffers, and buffer
management schemes can be designed to known size criteria.
• Cell-relay switches can process cells in parallel, achieving speeds that far exceed
the limitations of packet switch architectures.
• The cell format also allows for multi-protocol transmissions. Since ATM is
protocol transparent, the various protocols can be transferred at the same time.
With ATM, one line can carry phone, fax, video, data and other information
simultaneously. This multiprotocol advantage also offers scalability, greatly
reducing the configuration changes necessary for adding a new traffic type to
your network.
ATM is Service Transparent
ATM allows for the high speed transfer of a wide range of user traffic, including
voice, video and other data.
The cell format means that more than one service (traffic type) can be multiplexed
over the same physical line, see Figure 1.9.
Figure 1.9 Service Processing