Avaya M-ACCF/SF User Manual page 39

The ATM cell has 48 bytes of payload (information to be carried) and five bytes of
header information, making the cell 53 bytes in length.
The cell header contains the information used by the network to forward each cell to
its destination. The ATM cell structure is shown in Figure 1.15.
Figure 1.15 ATM Cell Structure
GFC
Key
GFC- Generic FlowControl (on UNI only)
VPI - Virtual Path Identifier
VCI - Virtual Channel Identifer
The ATM cell header consists of the following fields:
Generic Flow Control (GFC) — Provides local functions, such as flow control over
the User-to Network Interface (UNI). The value encoded in the GFC is not carried end-
to-end and can be overwritten by the ATM Switch.
Virtual Path Identifier (VPI) and Virtual Channel Identifier (VCI) — The VPI/
VCI values allow the network to associate a cell with a given connection, so that the
cell can be switched to its destination.
Payload Type Identifier (PTI) — The PTI is used to indicate whether the cell
contains user information, or management information. The management
information is used for resource and network congestion management.
Cell Loss Priority (CLP) — The purpose of the Cell Loss Priority (CLP) bit in the
ATM cell is to indicate that cells with this bit set should be discarded before cells
which do not have the CLP bit set. Cells can be discarded based on CLP condition
and according to the network load. When the network overloads, a discard
mechanism, based on the value of the CLP bit in the cell header, may come into
operation.
Header Error Check (HEC) — The HEC field is used for detecting bit errors in the
cell header. It is also used for cell delineation, defining where the cell begins in a
SONET frame.
Avaya M770 M-ACCF/SF ATM Access Modules User's Guide
Cell Header
VPI VCI PTI CLP
Payload
HEC User Data
PTI - P ayload T ype Identifier
CLP- Cell Loss Priority
HEC- Header Error Control
Chapter 1 Overview
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