Avaya Configuring ATM Services Manual de usuario Pagina 28

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Configuring ATM Services
1-2
Cell Header
After dividing the data into 48-byte segments for transmission, the end device
(that is, the ATM 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-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. Refer to the “Data
Transmission” section, later in this chapter, for additional information on
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
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