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Advance Data Sheet
T7633 Dual T1/E1 3.3 V Short-Haul Terminator
May 1998
72
Lucent Technologies Inc.
Frame Formats
(continued)
CEPT Loss of CRC-4 Multiframe Alignment Recovery Algorithms
Several optional algorithms exist in the receive framer. These are selected through programming of register
FRM_PR9.
CRC-4 Multiframe Alignment Algorithm with 8 ms Timer
The default algorithm is as described in ITU Rec. G.706 Section 4.2. The recommendation states that if a condition
of assumed frame alignment has been achieved, CRC-4 multiframe alignment is deemed to have occurred if at
least two valid CRC-4 multiframe alignment signals can be located within 8 ms, the time separating two CRC-4
multiframe signals being 2 ms or a multiple of 2 ms. The search for the CRC-4 multiframe alignment signal is made
only in bit 1 of NOT FAS frames. If multiframe alignment cannot be achieved within 8 ms, it is assumed that frame
alignment is due to a spurious frame alignment signal and a new parallel search for basic frame alignment is initi-
ated. The new search for the basic frame alignment is started at the point just after the location of the assumed
spurious frame alignment signal. During this parallel search for basic frame alignment, there is no indication to the
system of a receive loss of frame alignment (RLFA) state. During the parallel search for basic frame alignment and
while in primary basic frame alignment, data will flow through the receive framer to the system interface as defined
by the current primary frame alignment. The receive framer will continuously search for CRC-4 multiframe align-
ment.
CRC-4 Multiframe Alignment Algorithm with 100 ms Timer
The CRC-4 multiframe alignment with 100 ms timer mode is enabled by setting FRM_PR9 to 0XXXX1X1 (binary).
This CRC-4 multiframe reframe mode starts a 100 ms timer upon detection of basic frame alignment. This is a par-
allel timer to the 8 ms timer. If CRC-4 multiframe alignment cannot be achieved within the time limit of 100 ms due
to the CRC-4 procedure not being implemented at the transmitting side, then an indication is given, and actions are
taken equivalent to those specified for loss of basic frame alignment, namely:
1.
Optional automatic transmission of A = 1 to the line if register FRM_PR27 bit 3 is set to 1.
2.
Optional automatic transmission of E = 0 to the line if register FRM_PR28 bit 5 is set to 1.
3.
Optional automatic transmission of AIS to the system if register FRM_PR19 bit 1 is set to 1.