Cabletron Systems BRIM-A6 User Manual

Contents

The ATM CellBRIM-A6 User’s GuidePage B-3Figure B-2 ATM Header Field BreakdownThe Header field is made up of several sub-fields which providemanagement/control information relating to a cell. Brief descriptions ofeach header field are as follows:• Generic Flow Control (GFC) – provides the ability to control thetraffic flow for different grades of service.• Virtual Path Identifier (VPI) – identifies the virtual path to which thecell belongs.• Virtual Channel Identifier (VCI) – identifies the virtual channel towhich the cell belongs.• Payload Type Indicator (PTI) – describes whether the information isuser information, network information, or management information.• Header Check Sequence (HCS) – detects bit errors; corrects single biterrors.This method of fixed-cell length has many advantages over conventionalpacket transfer technologies; one major advantage is time. Since ATMcells are fixed in length, and all of the header data is the same, processingthem is dramatically simplified. This eliminates the need for buffers andprocessors that tend to slow down transmission. By contrast, other LANpackets vary in length, making it necessary to check each incomingpacket to ensure that it is complete and contains no errors.Header DataGeneric Flow Control (GFC) Virtual Path Identifier (VPI)VPI VCI/VPIVirtual Channel Identifier (VCI)VCI Payload Type ReservedHeader Check Sequence