Rainbow Electronics DS21458 - Manual (Page 122)

DS21455/DS21458 Quad T1/E1/J1 Transceivers
122 of 270
21. G.706 INTERMEDIATE CRC-4 UPDATING (E1 MODE ONLY)
The DS21455/DS21458 can implement the G.706 CRC-4 recalculation at intermediate path points. When
this mode is enabled, the data stream presented at TSER will already have the FAS/NFAS, CRC
multiframe alignment word, and CRC-4 checksum in time slot 0. The user can modify the Sa bit
positions; this change in data content will be used to modify the CRC-4 checksum. The modification,
however, will not corrupt any error information the original CRC-4 checksum might contain. In this mode
of operation, TSYNC must be configured to multiframe mode. The data at TSER must be aligned to the
TSYNC signal. If TSYNC is an input then the user must assert TSYNC aligned at the beginning of the
multiframe relative to TSER. If TSYNC is an output, the user must multiframe-align the data presented to
TSER.
Figure 21-1. CRC-4 Recalculate Method
TSER
XOR CRC-4
CALCULATOR
EXTRACT
OLD CRC-4
CODE INSERT
NEW CRC-4
CODE
MODIFY
Sa BIT
POSITIONS
NEW Sa BIT
DATA
+
TPOSO/TNEGO