RELEASED
PM4354 COMET-QUAD
DATASHEET
PMC-1990315
ISSUE 6
FOUR CHANNEL COMBINED E1/T1/J1
TRANSCEIVER / FRAMER
PROPRIETARY AND CONFIDENTIAL TO PMC-SIERRA, INC. AND FOR ITS CUSTOMERS’ INTERNAL USE
41
The algorithm finds frame alignment by using the following sequence:
1. Search for the presence of the correct 7-bit FAS (‘0011011’);
2. Check that the FAS is absent in the following frame by verifying that bit 2 of the assumed non-
frame alignment sequence (NFAS) TS 0 byte is a logic 1;
3. Check that the correct 7-bit FAS is present in the assumed TS 0 byte of the next frame.
If either of the conditions in steps 2 or 3 are not met, a new search for frame alignment is initiated
in the bit immediately following the second 7-bit FAS sequence check. This "hold-off" is done to
ensure that new frame alignment searches are done in the next bit position, modulo 512. This
facilitates the discovery of the correct frame alignment, even in the presence of fixed timeslot data
imitating the FAS.
These algorithms provide robust framing operation even in the presence of random bit errors:
framing with algorithm #1 or #2 provides a 99.98% probability of finding frame alignment within
1 ms in the presence of 10-3 bit error rate and no mimic patterns.
Once frame alignment is found, the block sets the OOF indication low, indicates a change of
frame alignment (if it occurred), and monitors the frame alignment signal, indicating errors
occurring in the 7-bit FAS pattern and in bit 2 of NFAS frames, and indicating the debounced value
of the Remote Alarm bit (bit 3 of NFAS frames). Using debounce, the Remote Alarm bit has
<0.00001% probability of being falsely indicated in the presence of a 10-3 bit error rate. The block
declares loss of frame alignment if 3 consecutive FAS's have been received in error or,
additionally, if bit 2 of NFAS frames has been in error for 3 consecutive occasions. In the
presence of a random 10-3 bit error rate the frame loss criteria provides a mean time to falsely
lose frame alignment of >12 minutes.
The E1-FRMR can be forced to initiate a basic frame search at any time when any of the following
conditions are met:
the software re-frame bit in the E1-FRMR Frame Alignment Options register goes to logic 1;
the CRC Frame Find Block is unable to find CRC multiframe alignment; or
the CRC Frame Find Block accumulates excessive CRC evaluation errors (
≥
915 CRC errors
in 1 second) and is enabled to force a re-frame under that condition.
CRC Multiframe Alignment Procedure
The E1-FRMR searches for CRC multiframe alignment by observing whether the International bits
(bit 1 of TS 0) of NFAS frames follow the CRC multiframe alignment pattern. Multiframe
alignment is declared if at least two valid CRC multiframe alignment signals are observed within
8 ms, with the time separating two alignment signals being a multiple of 2 ms