
XRT86SH221
PRELIMINARY
89
SDH-TO-PDH FRAMER/MAPPER WITH INTEGRATED 21-CHANNEL E1 SH LIU
REV. P1.0.5
Algorithm 1:
Step 1: Seach for the presence of the correct 7-bit FAS pattern. Go to step 2 if found;
Step 2: Check if the FAS is absent in the following frame by verifying that bit 2 of the assumed timslot 0 byte is
a one. Go back to step 1 if verification failed; Otherwise, go to step 3;
Step 3: Check if the FAS is present in the assumed timeslot 0 byte of the third frame. Go back to step 1 if failed.
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 errored timeslot 0 byte locaiton. If both conditions are met and frame check
sequence is enabled, then an additional check sequence is initiated. The check sequence consists of verifying
correct frame alignment for an additional two frames.
Step 4: Once the frame alignment is found, check if the FAS is absent in the following frame by verifying the bit
2 of timeslot 0 being a one. If verification failed, go back to step 1.
Step 5: Check that the FAS is present of the next frame. If not, go back to step 1.
The second algorithm is similar to the first one, but adds a one frame hold-off in the second step to begin a new
search in the bit immediately following the second (third frame) assumed FAS. This extra frame hold-off is
performed only after the condition in step 2 fails to provide a robust algorithm which allows the framer to
operate correctly in the presence of fixed timeslot imitating the FAS pattern.
Algorithm 2:
Step 1: Seach for the presence of the correct 7-bit FAS pattern. Go to step 2 if found;
Step 2: Check if the FAS is absent in the following frame by verifying that bit 2 of the assumed timslot 0 byte is
a one. Go to step 4 if varification failed; Otherwise, go to step 3;
Step 3: Check if the FAS is present in the assumed timeslot 0 byte of the third frame. Go back to step 1 if failed;
Otherwise start check sequence if enabled.
Step 4: Wait for assumed FAS in next frame, then go back to step 1.
If both conditions are met and frame check sequence is enabled, then an additional check sequence is
initiated. The check sequence consists of verifying correct frame alignment for an additional two frames.
Step 5: Once the frame alignment is found, check if the FAS is absent in the following frame by verifying the bit
2 of timeslot 0 being a one. If verification failed, go back to step 4.
Step 6: Check that the FAS is present of the next frame. If not, go back to step 1.
When synchronization is achieved, the framer monitors alignment signals for errors. A Red Alarm (FASRED)
is generated if frame alignment is lost. The criteria for loss of frame alignment in FAS framing is dictated by an
E1 Framing Control Register (FCR). The MSB of this register is an RSYNC bit which imposes the framer to
restart the resync process even if the frame is currently in sync. This bit will be cleared after the framer
resumes its normal sync monitoring mode. The FAS criteria bits specify the number of consecutive erred FAS
patterns determining the loss of FAS alignment. Note: Loss of FAS alignment forces loss of CAS and loss of
CRC alignment.
It is important to note that the off-line searching is conducted by a shadow synchronizer. The shadow
synchronizer continuously searches for the frame alignment even if the framer is in the in-sync state. Once the
loss of frame is declared and the resync mode is entered, the framer can shift back into monitor mode by
moving to the new alignment at the beginning of the next frame as long as the shadow synchronizer is in-sync.
This feature dramatically reduces the reframe time required.