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Advance Data Sheet
T7633 Dual T1/E1 3.3 V Short-Haul Terminator
May 1998
74
Lucent Technologies Inc.
Frame Formats
(continued)
CEPT Loss of CRC-4 Multiframe Alignment Recovery Algorithms
(continued)
CRC-4 Multiframe Alignment Search Algorithm with 400 ms Timer
The CRC-4 multiframe alignment with 400 ms timer mode is enabled by setting FRM_PR9 to 0XXX1XX1 (binary).
This receive CRC-4 multiframe reframe mode is the modified CRC-4 multiframe alignment algorithm described in
ITU Rec. 706 Annex B, where it is referred to as CRC-4-to-Non-CRC-4 equipment interworking. A flow diagram of
this algorithm is illustrated in Figure 26 on page 75. When the interworking algorithm is enabled, it supersedes the
100 ms algorithm described on page 72 and in Figure 25 on page 73. This algorithm assumes that a valid basic
frame alignment signal is consistently present but the CRC-4 multiframe alignment cannot be achieved by the end
of the total CRC-4 multiframe alignment search period of 400 ms, if the distant end is a non-CRC-4 equipment. In
this mode, the following consequent actions are taken:
1.
An indication that there is no incoming CRC-4 multiframe alignment signal.
2.
All CRC-4 processing on the receive
2.048 Mbits/s signal is inhibited.
3.
CRC-4 data is transmitted to the distant end with both E bits set to zero.
This algorithm allows the identification of failure of CRC-4 multiframe alignment generation/detection, but with cor-
rect basic framing, when interworking between each piece of equipment having the modified CRC-4 multiframe
alignment algorithm.
As described in ITU Rec. G.706 Section B.2.3:
1.
A 400 ms timer is triggered on the initial recovery of the primary basic frame alignment.
2.
The 400 ms timer reset if and only if:
A. The criteria for loss of basic frame alignment as described in ITU Rec. G.706 Section 4.1.1 is achieved.
B. If 915 out of 1000 errored CRC-4 blocks are detected resulting in a loss of basic frame alignment as
described in ITU Rec. G.706 Section 4.3.2.
C. On-demand reframe is requested.
D. The receive framer is programmed to the non-CRC-4 mode.
3.
The loss of basic frame alignment checking process runs continuously, irrespective of the state of the CRC-4
multiframe alignment process below it.
4.
A new search for frame alignment is initiated if CRC-4 multiframe alignment cannot be achieved in 8 ms, as
described in ITU Rec. G.706 Section 4.2. This new search for basic frame alignment will not reset the 400 ms
timer or invoke consequent actions associated with loss of the primary basic frame alignment. In particular, all
searches for basic frame alignment are carried out in parallel with, and independent of, the primary basic frame
loss checking process. All subsequent searches for CRC-4 multiframe alignment are associated with each
basic framing sequence found during the parallel search.
5.
During the search for CRC-4 multiframe alignment, traffic is allowed through, upon, and to be synchronized to,
the initially determined primary basic frame alignment.
6.
Upon detection of the CRC-4 multiframe before the 400 ms timer elapsing, the basic frame alignment associ-
ated with the CRC-4 multiframe alignment replaces, if necessary, the initially determined basic frame align-
ment.
7.
If CRC-4 multiframe alignment is not found before the 400 ms timer elapses, it is assumed that a condition of
interworking between equipment with and without CRC-4 capability exists and the actions described above are
taken.
8.
If the 2.048 Mbits/s path is reconfigured at any time, then it is assumed that the (new) pair of path terminating
equipment will need to re-establish the complete framing process, and the algorithm is reset.