Avaya M-ACCF/SF manual ATM Cell Structure

Chapter 1 Overview

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

	Cell Header				Payload
GFC	VPI VCI	PTI	CLP	HEC	User Data
Key
GFC- Generic FlowControl (on UNI only)				PTI - Payload Type Identifier
VPI - Virtual Path Identifier				CLP- Cell Loss Priority
VCI - Virtual Channel Identifer				HEC- Header Error Control

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 23