Cabletron Systems BRIM-A6 - Manual (Page 41)

The ATM Cell
BRIM-A6 User’s Guide Page B-3
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.
Header Data
Generic Flow Control (GFC) Virtual Path Identifier (VPI)
VPI VCI/VPI
Virtual Channel Identifier (VCI)
VCI Payload Type Reserved
Header Check Sequence
book Page 3 Thursday, April 18, 1996 2:18 PM