Information Technology Reference
In-Depth Information
NNI Cell Header
Figure 5-17
Bits
8
7
6
5
4
3
2
1
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 format of an NNI cell is slightly different than an UNI cell. In an NNI cell, the GFC is
replaced with 4 additional VPI bits for a total of 12 VPI bits. Using 12 bits, a NNI cell can
identify 4096 unique VPs. The ATM Cell Switching section of this chapter describes the
difference between VC and VP switching.
ATM Cell Switching
The header of each cell contains addressing information consisting of a VPI and a VCI. The
VPI/VCI address pair is locally significant. For each interface in a switch, the VPI/VCI
uniquely identifies the cell as belonging to a particular connection.
When a VC connection is established across a network, each switch involved in the connection
creates an entry in its switching table that identifies the incoming VPI/VCI and interface number
for the connection. The switching table entry also identifies the outgoing VPI/VCI and interface
number for the connection.
When a cell arrives on a particular interface, the switch examines the VPI, VCI, and interface
number. The switch then changes the VPI/VCI pair in the header of the cell and switches the
cell out the appropriate interface according to the entries in its switching table. Figure 5-18
shows two cells being switched across two VC connections in an ATM network.
Another form of switching is VP switching. When a VP connection is established across a
network, the entry in a switch's switching table identifies only the incoming and outgoing VPI
and interface number for the connection.
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