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3.2 ATM Function
The µ PD98413 can map ATM cells into a SONET/SDH payload. The mapping of ATM cells into a SONET
payload is shown below.
SO H
Figure 3-15 shows the format of the ATM cell (user network interface). An ATM cell consists of a 5-byte header
and 48-byte payload data. In the case of the user network interface (UNI), the header contains a GFC field (4 bits).
In the case of the network node interface (NNI), however, the corresponding field is defined as a VPI field that is 12
bits long.
User network interface (UNI)
GFC: Generic flow control
VPI: Virtual path identifier
VCI: Virtual channel identifier
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CHAPTER 3 FUNCTIONAL OUTLINE
Figure 3-14. Mapping of ATM cell
ATM cell
POH
Fixed Stuff
Figure 3-15. Structure of ATM Cell
8
7
6
5
4
3
2
1
VPI
GFC
VPI
VCI
Header
PTI
CLP
HEC
Payload data
48 bytes
PRELIMINARY
ATM cell
ATM cell
Payload
Network node interface (NNI)
8
7
6
5
4
3
VPI
VCI
PTI
HEC
Payload data
48 bytes
PTI:
Payload type identifier
CLP: Cell loss priority
HEC: Header error control
ATM cell
2
1
1
2
3
CLP
4
5
6
7
8
9
10
.
.
.
.
.
53
NEC confidential and Proprietary
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