ATM Configuration and Management Manual

Overview of the ATM Subsystem
ATM Configuration and Management Manual—522307-003
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ATM Cells
The UNI connects an end user to the network through an ATM switch or other device,
thus supplying network access to the end user. The NNI connects nodes within a
network to each other and provides network routing.
ATM Cells
ATM cells consist of a 5-octet header and 48-octet data or payload section. The
48-octet payload is a SAR protocol data unit (SAR PDU) to which the header is
attached. Most of the bits in the header are used for path and channel identification.
The header also contains a cell lost priority (CLP) bit and a header error control (HEC)
field. The cells are not byte-oriented, and a header field can extend over byte
boundaries. Within a cell, bit order goes from left to right, and across octets bit order
goes down as octets go up.
.
Figure 2-1. ATM Cell Structure
Bit 8 Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1
Generic Flow Control Virtual Path Identifier
Virtual Path Identifier
Virtual Channel Idenfitier
Virtual Channel Idenfitier
Virtual Channel Idenfitier Payload Type
CLP
Header Error Control
Payload
Payload
Payload
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1
2
3
4
5
6
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53
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VST201.vsd