
Understanding ATM, MPOA, ATM Router Redundancy, and OAM
117374-C Rev 00
1-3
Cell Header
After dividing the data into 48-byte segments for transmission, the end device --
that is, the ATM data service unit/channel service unit (DSU/CSU) or native ATM
device -- attaches the required header information (Figure 1-2
).
Figure 1-2. ATM Cell Header
The fields in each ATM cell header provide all the information necessary for
networking. These fields include the following:
• Generic flow control (GFC): The first 4 bits of the cell header contain the
GFC. The GFC controls traffic flow onto the ATM network by controlling the
user-to-network interface (UNI).
• Virtual path identifier (VPI): The next 8 bits of the cell header (that is, the last
half of byte 1 and the first half of byte 2) contain the VPI. The VPI specifies a
virtual path on the physical ATM link. See the next section, “Data
Transmission,” for additional information about virtual paths.
48-byte
information field
5-byte
header
Generic flow control (GFC)
Virtual path identifier (VPI)
Virtual path identifier (VPI) Virtual channel identifier (VCI)
Virtual channel identifier (VCI)
Virtual channel identifier (VCI)
Payload type (PT)
Header error control (HEC)
Cell loss
priority
Bytes
2345678
Bits
1
1
2
3
4
5
ATM0002A
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