Information Technology Reference
In-Depth Information
UNI Cell Header
Figure 5-16
Bits
8
7
6
5
4
3
2
1
GFC
(Generic flow control)
VPI
(Virtual path identifier)
1
VPI
(Virtual path identifier)
VCI
(Virtual channel identifier)
2
Bytes
VCI
(Virtual channel identifier)
3
C
L
P
VCI
(Virtual channel identifier)
PTI
4
5
HEC
(Header error control)
CLP
(Cell loss priority)
PTI
(Payload type identification)
The PTI is made up of three bits. The first bit identifies the cell as user data when set to 0 and
as a management cell when set to 1.
The second bit in a user cell is called the EFCI. It indicates that the cell passed through a
network component that was experiencing congestion, as described in the ATM Traffic
Management section of this chapter.
The third bit in a user cell identifies the last cell that makes up a frame of data. If the third bit
is set to 0, the cell is not the last cell in the frame. If the third bit is set to 1, the cell is the last
cell in the frame. An example of the use of this bit is to determine if the packet can be discarded
because of congestion in the network. A switch might want to discard all but the final cell
belonging to a single frame, as described in the ATM Traffic Management section of this
chapter.
If the cell is a management cell, the second bit identifies a segment management cell when the
bit is set to 0 and an end-to-end management cell when the bit is set to 1. The third bit in a
management cell is reserved for future use.
The cell loss priority (CLP) bit identifies the cell as being high or low priority. If the network
experiences congestion, the cells with the CLP set to 1 are discarded before any cell with the
CLP bit is set to 0. The CLP bit can be set by the end device or as a result of ingress policing,
as described in the ATM Traffic Management section of this chapter.
The HEC field is an 8-bit cyclic redundancy check on the cell header.
The payload of a user cell contains the data being transported. The payload of a management
cell can contain flow control information or alarm indicators.
Figure 5-17 shows the format of an NNI cell.
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