Reception Pos Functions - NEC UPD98413 User Manual

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
(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:
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g(x) = 1 + x
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.
Received data
Destuffed data
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.
CHAPTER 3 FUNCTIONAL OUTLINE
Byte destuffing Address and control check
Stuffed data
7D-5D
7D-5E
Figure 3-25. Byte Destuffing
Removes 0x7D
7D 5D
EXORed with 0x20
7D
PRELIMINARY
Packet size check
Destuffed data
(Original)
7D
7E
7D 5E
EXORed with 0x20
7E
NEC confidential and Proprietary
FCS check
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