CHAPTER 3 FUNCTIONAL OUTLINE
PRELIMINARY
NEC confidential and Proprietary
71
3.3.2 Reception POS functions
The reception POS processor of the
µ
PD98413 extracts POS packets from the SONET frame payload entered
from the line interface, based on the flag sequence (7Eh). The
µ
PD98413 recognizes a packet boundary by
detecting a flag sequence (7Eh). It then performs HDLC processing on the POS packets and sends them to the
POS interface. The functions, which are executed in the following order, are described in detail below.
Data descrambling
Æ
Æ
Æ
Æ
Byte destuffing
Æ
Æ
Æ
Æ
Address and control check
Æ
Æ
Æ
Æ
Packet size check
Æ
Æ
Æ
Æ
FCS check
(a) Data descrambling
The
µ
PD98413 descrambles the entire SONET payload containing the POS packets and flag sequences. The
polynomial used for descrambling is as follows:
g(x) = 1 + x
43
The descrambling function is optional and it may be disabled by the PSCM bit of the MDPOSR register.
(b) Byte destuffing
The
µ
PD98413 supports the byte destuffing function. Upon detecting a 7Dh (control escape) byte in a POS
packet, the byte destuffing of the
µ
PD98413 deletes that byte, and EXORs the next byte with 20h. The data
resulting from byte destuffing is as shown below.
Stuffed data
Destuffed data
(Original)
7D-5D 7D
7D-5E 7E
Figure 3-25. Byte Destuffing
Remark
If 7D-7Eh bytes are detected in a POS packet, the
µ
PD98413 does not perform byte destuffing. That
packet is processed as an abort packet.
The
µ
PD98413 also performs byte destuffing if the byte following the control escape (7Dh) is not 5Dh, 5Eh,
or 7Eh.
7D
7E
7D 5D
5E
7D
EXORed with 0x20
EXORed with 0x20
Destuffed data
Received data
Removes 0x7D
Содержание NEASCOT-P65
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