STANDARD PRODUCT
PM4351 COMET
DATA SHEET
PMC-1970624
ISSUE 10
COMBINED E1/T1 TRANSCEIVER
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
-3
of finding frame alignment within 1 ms in the presence of 10 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%
-3
probability of being falsely indicated in the presence of a 10 bit error rate. The
block declares loss of frame alignment if 3 consecutive FASs have been received
in error or, additionally, if bit 2 of NFAS frames has been in error for 3
-3
consecutive occasions. In the presence of a random 10 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
Once CRC multiframe alignment is found, the OOCMFV register bit is set to
logic 0, and the E1-FRMR monitors the multiframe alignment signal, indicating
errors occurring in the 6-bit MFAS pattern, errors occurring in the received CRC
and the value of the FEBE bits (bit 1 of frames 13 and 15 of the multiframe).
The E1-FRMR declares loss of CRC multiframe alignment if basic frame
PROPRIETARY AND CONFIDENTIAL
41
PMC [ PMC-SIERRA, INC ]
