System information

FCD 901 48
Issue R2A, 07.2009
XMP1 Release 5.5 System Description
Synchronization
Aastra Proprietary Information Page 2-11
Loss of sync
When the frame alignment signal has been received incorrectly three times
in a row, the frame alignment process is reinitiated. The evaluation can also
involve bit 2 of the service digits, i.e. if this bit has been received as logic "0"
three consecutive times, the frame is considered to be out of sync.
Frame realignment
If the frame alignment signal has been identified for the first time and bit 2
of the following frame 0, i.e. one frame duration later, is logic 1, and the
frame alignment signal is found again in the next frame, the synchronization
process is terminated successfully.
If during the synchronization process
- the frame alignment signal has been identified for the first time and one
frame length later,
- if binary value 0 has been identified in time slot 0 of bit position 2, or
- if the frame alignment signal has been identified for the first time and
cannot be found again two frame lengths later, a new synchronization
process will be initiated two frame lengths after having identified the frame
alignment signal for the first time.
Frame alignment using the CRC4 procedure
When using the CRC4 procedure, frame alignment is performed as follows:
• Synchronization to the frame alignment signal of the frame
• Synchronization to the frame alignment signal of the CRC4 multiframe.
While the system is in sync, the frame alignment is permanently monitored
and faults detected during the CRC4 signature comparison are counted.
If frame alignment gets lost or if more than 914 CRC4 errors are counted
within one second, a new synchronization process is initiated.
This latter is started by searching for the frame alignment signal.
As soon as it has been found, it is checked whether bit 2 of the service digits
occurring in the next frame is logic "1".
In this case and if the frame alignment signal is identified again one frame
length later, the frame alignment process is terminated successfully.
Now the system searches for the CRC4 multiframe alignment signal.
Sixteen frames (2 ms) are required for transmitting the latter. It is checked
within an interval of 8 ms, whether at least two CRC4 multiframe alignment
signals can be identified in the 2 ms pattern. If not, the synchronization
process will be restarted. This procedure prevents the system from being
synchronized to incorrectly pretended frame alignment signals or service
digits.