STANDARD PRODUCT
PM4328 TECT3
DATASHEET
PMC-2011596
ISSUE 1
HIGH DENSITY T1/E1 FRAMER
AND M13 MULTIPLEXER
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.
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:
Sꢀ the software re-frame bit in the E1-FRMR Frame Alignment Options register
goes to logic 1;
Sꢀ the CRC Frame Find Block is unable to find CRC multiframe alignment; or
Sꢀ the CRC Frame Find Block accumulates excessive CRC evaluation errors
(O 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
PROPRIETARY AND CONFIDENTIAL
57
PMC [ PMC-SIERRA, INC ]
