
Configuring ATM Services
2-2 114056 Rev. A
ATM allows the network to operate at a much higher rate than typical
packet-switching systems (for example, X.25), because it provides no error
protection or flow control. Instead, ATM relies on the source and destination
devices to perform error-recovery functions such as retransmission of lost packets.
Cell Headers
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 2-2).
Figure 2-2. 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).
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 (FCI)
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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