PM6341 E1XC
DATA SHEET
PMC-910419
ISSUE 8
E1 FRAMER/TRANSCEIVER
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 or 4 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 Frame Find Block can be forced to initiate a frame search at any time when
any of the following conditions are met:
• the software re-frame bit in the 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.
9.4.2 CRC Frame Find
Once the basic frame alignment has been found, the CRC Frame Find Block
searches for CRC multiframe alignment by observing whether the International
bits (bit 1 of timeslot 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 block sets the OOCMF indication
low, and monitors the multiframe alignment signal, indicating errors occurring in
the 6-bit pattern, and indicating the value of the FEBE bits (bit 1 of frames 13
and 15 of the multiframe).The block declares loss of CRC multiframe alignment if
four consecutive CRC multiframe alignment signals have been received in error,
or if frame alignment has been lost.
The CRC Frame Find Block will force the Frame Find Block to initiate a basic
frame search when CRC multiframe alignment has not been found for 8 ms.
PROPRIETARY AND CONFIDENTIAL TO PMC-SIERRA, INC., AND FOR ITS CUSTOMERS’ INTERNAL USE
47
PMC [ PMC-SIERRA, INC ]
