
XRT86SH221
PRELIMINARY
91
SDH-TO-PDH FRAMER/MAPPER WITH INTEGRATED 21-CHANNEL E1 SH LIU
REV. P1.0.5
A 400ms CRC-4 multiframe alignment search period is applied for interworking detection. The 400 msec timer
is triggered on the initial recovery of basic frame alignment and is not reset until the loss of basic frame
alignment occurs. A re-search for FAS will be initiated if CRC-4 multifame alignment can not be found in 8ms
and will not reset the 400ms timer or invoke the consequent actions associated with loss of FAS. All
subsequent searches for CRC-4 multiframe alignment are associated with each basic FAS found. In order to
maintain no disturbance to traffic during the 400ms CRC-4 multiframe search, traffic should be allowed through
with synchronization to the initially determined primary basic frame alignment sequence. If a CRC-4 multiframe
alinment signal is found befoe the 400ms timer elapses, then the basic FAS associated with the CRC-4
multiframe alignment signal should be the one chosen, i.e. the primary basic FAS should be amended
accordingly if the basic FAS alignment changed. If a CRC-4 multiframe alignment sequence can not be found
in 400ms, it should be concluded that a condition of interworking between equipments with and without a CRC-
4 capability exist, so the traffic should be maintained to the initially determined FAS alignment.
If the path is reconfigured at any time, then it is assumed that the new pair of path will need to re-establish the
complete framing process, i.e. the algorithm is reset.
Consequent actions are taken while a non-CRC-4 remote side is detected:
I
The Framer will provide an indication that there is no incoming CRC-4 multiframe alignment signal
I
The Framer will inhibit further CRC-4 processing
I
The Framer will continue to transmit CRC-4 data to the remote side with both E bits set to zero
In this modified framing mode, the framer always sets the return E bits to zero until the interworking
relationship has been established. If CRC-4-to-CRC-4 interworking is established, then normal CRC-4
processing of erred CRC-4 block data should commence. If CRC-4-to-non-CRC-4 interworking is established,
the E bits should remain at 0.
CAS Synchronization
After the FAS frame alignment is declared, the third step is to find CAS multiframe alignment. Two user-
selectable algroithms are available.
Algorthm 1 monitors the sixteenth timeslot of each frame and declares CAS multiframe alignment when 15
consecutive frames with bits 1-4 of timeslot 16 not containing the alignment pattern are observed to precede a
frame with timeslot 16 containing the correct alignment pattern.
Algorthm 2 monitors the sixteenth timeslot of each frame and declares CAS multiframe alignment when non-
zero bits 1-4 of timeslot 16 are observed to precede a timeslot 16 containing the correct alignment pattern.
Once the CAS multiframe alignment is found, the Out Of CAS MultiFame alignment indication is cleared. The
CAS synchronizer monitors the multiframe alignment signal, indicates errors occurring in the 4-bit alignment
pattern, and indicates the debounced value of the remote signaling multiframe alarm bit (bit 6 of timeslot 16 of
frame 0 of the multiframe).
When synchronization is achieved, the framer monitors multiframe alignment signals for errors. The CAS LOF
indication turns on if frame alignment is lost. The criteria for loss of CRC multiframe alignment is dictated by the
CASC bits in the E1 Framing Control Register.