NEC UPD98413 User Manual page 67

(c) Byte stuffing
The µ PD98413 supports a byte stuffing function. Upon detecting a 7Dh or 7Eh byte in a POS pocket, the byte
stuffing of the µ PD98413 inserts 7Dh (control escape) before that byte, and EXORs the detected 7Dh or 7Eh
byte with 20h. The data resulting from byte stuffing is as shown below.
Original data
Stuffed data
(d) Data scrambling
The µ PD98413 scrambles the entire SONET payload including the POS packets and flag sequences (excepted
POH and Fixed stuff). The polynomial used for scrambling is as follows:
43
g(x) = 1 + x
The scrambling function is optional and it may be disabled by the PSCM bit of the MDPOST register.
(e) SONET overhead byte for POS
In PPP over SONET, the values of the C2 and H4 bytes of the SONET overhead bytes are defined. Set the
transmission C2 and H4 byte registers to the following values:
C2 byte: 16h when scrambling is used
CFh when scrambling is not used
H4 byte: 00h (Default value of register)
CHAPTER 3 FUNCTIONAL OUTLINE
Original data Stuffed data
7D
7E
Figure 3-22. Byte Stuffing
7D
EXORed with 0x20
7D 5D
Inserts 0x7D
PRELIMINARY
7D-5D
7D-5E
7E
EXORed with 0x20
7D 5E
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