
Advance Data Sheet
May 1998
T7633 Dual T1/E1 3.3 V Short-Haul Terminator
69
Lucent Technologies Inc.
Frame Formats
(continued)
CEPT Loss of Basic Frame Alignment (LFA)
Frame alignment is assumed to be lost when:
1.
As described in ITU Rec. G.706 Section 4.1.1, three consecutive incorrect frame alignment signals have been
received.
So as to limit the effect of spurious frame alignment signals, when bit 2 in time slot 0 in NOT FAS frames have
been received with an error on three consecutive occasions.
Optionally, as described in ITU Rec. G.706 Section 4.3.2, by exceeding a count of >914 errored CRC-4 blocks
out of 1000, with the understanding that a count of
≥
915 errored CRC blocks indicates false frame alignment.
On demand via the control registers.
2.
3.
4.
In the LFA state:
1.
2.
3.
4.
5.
6.
No additional FAS or NOT FAS errors are processed.
The received remote frame alarm (received A bit) is deactivated.
All NOT-FAS bit (Si bit, A bit, and Sa4 to Sa8 bits) processing is halted.
Receive Sa6 status bits are set to 0.
Receive Sa6 code monitoring and counting is halted.
All receive Sa stack data updates are halted. The receive Sa stack ready, register FRM_SR4 bit 6 and bit 7, is
set to 0. If enabled, the receive Sa stack interrupt bit is set to 0.
Receive data link (RFDL) is set to 1 and RFDCLK maintains previous alignment.
Optionally, the remote alarm indication (A = 1) may be automatically transmitted to the line if register
FRM_PR27 bit 0 is set to 1.
Optionally, the alarm indication signal (AIS) may be automatically transmitted to the system if register
FRM_PR19 bit 0 is set to 1.
10. If CRC-4 is enabled, loss of CRC-4 multiframe alignment is forced.
11. If CRC-4 is enabled, the monitoring and processing of CRC-4 checksum errors is halted.
12. If CRC-4 is enabled, all monitoring and processing of received E-bit information is halted.
13. If CRC-4 is enabled, the receive continuous E-bit alarm is deactivated.
14. If CRC-4 is enabled, optionally, E bit = 0 is transmitted to the line for the duration of loss of CRC-4 multiframe
alignment if register FRM_PR28 bit 4 is set to 1.
15. If time slot 16 signaling is enabled, loss of the signaling multiframe alignment is forced.
16. If time slot 16 signaling is enabled, updating of the signaling data is halted.
7.
8.
9.
CEPT Loss of Frame Alignment Recovery Algorithm
The receive framer begins the search for basic frame alignment one bit position beyond the position where the LFA
state was detected. As defined in ITU Rec. G.706.4.1.2, frame alignment will be assumed to have been recovered
when the following sequence is detected:
1.
2.
For the first time, the presence of the correct frame alignment signal in frame n.
The absence of the frame alignment signal in the following frame detected by verifying that bit 2 of the basic
frame is a 1 in frame n + 1.
For the second time, the presence of the correct frame alignment in the next frame, n + 2.
3.
Failure to meet 2 or 3 above will initiate a new basic frame search in frame n + 2.