Cabletron Systems manual BRIM-A6 User’s Guide B-3

The ATM Cell

Header

Data

Generic Flow Control (GFC)			Virtual Path Identifier (VPI)
VPI				VCI/VPI
Virtual Channel Identifier (VCI)
VCI		Payload Type			Reserved
	Header Check Sequence

Figure B-2 ATM Header Field Breakdown

The Header field is made up of several sub-fields which provide management/control information relating to a cell. Brief descriptions of each header field are as follows:

•Generic Flow Control (GFC) – provides the ability to control the traffic flow for different grades of service.

•Virtual Path Identifier (VPI) – identifies the virtual path to which the cell belongs.

•Virtual Channel Identifier (VCI) – identifies the virtual channel to which the cell belongs.

•Payload Type Indicator (PTI) – describes whether the information is user information, network information, or management information.

•Header Check Sequence (HCS) – detects bit errors; corrects single bit errors.

This method of fixed-cell length has many advantages over conventional packet transfer technologies; one major advantage is time. Since ATM cells are fixed in length, and all of the header data is the same, processing them is dramatically simplified. This eliminates the need for buffers and processors that tend to slow down transmission. By contrast, other LAN packets vary in length, making it necessary to check each incoming packet to ensure that it is complete and contains no errors.

BRIM-A6 User’s Guide Page B-3