System information

FCD 901 48
Issue R2A, 07.2009
XMP1 Release 5.5 System Description
Synchronization
Page 2-10 Proprietary Information Aastra
Transmission and evaluation of the CRC4 signature
The calculated CRC4 signature is stored before it is sent to the receive side
via the next multiframe section. The CRC4 signature bits C1 to C4 are
transmitted in bit positions 1 of the frame alignment signals of the frames 0,
2, 4, 6 and 8, 10, 12, 14 respectively.
On the receive side, the CRC4 signature is calculated from the received bit
stream and compared with the CRC4 signature received in the next
multiframe. If these signatures are not identical, at least one bit error has
occurred in the multiframe section concerned.
All CRC4 signature errors detected are permanently counted and added up.
If more than 914 such errors occur in one second, the frame must be
resynchronized.
This signature comparison is no longer performed on loss of sync of the
frame or CRC4 multiframe.
2.2.3 Synchronization
Frame alignment
Frame alignment is performed using the synchronization pattern (bits 2 to 8
of the frame alignment signal, bit 2 of the service digits) or optionally using
the multiframe and/or CRC4 procedure.
The synchronization process starts with the search for the frame alignment
signal (bits 2 to bit 8) in the receive frame. As soon as the frame alignment
signal has been found, it is checked whether bit 2 of the service digits
contained in the next frame is logic 1. If this is the case and if the frame
alignment signal is identified again in the following frame, the frame
synchronization process is terminated.
Synchronization to the multiframe is terminated as soon as the multiframe
alignment signal has been correctly received in frame 0 and no other
multiframe alignment signal is detected in the following 15 frames (1 to 15).
In order to avoid that the absence of the frame alignment signal (continuous
0) pretends a "multiframe in sync" status, one or several frames (1 to 15)
are additionally monitored for the presence of bits with the binary value 1. If
no such bits can be found, the multiframe is out of sync.
Persistence check
In order to avoid bit errors on the transmission link, a persistence check is
performed, i.e. the information of the individual signalling bits (continuous
"0") transmitted in the signalling channel of two consecutive multiframes is
compared. A change of the information value is accepted and passed on to
the signal processing circuit only when both values are identical.
As soon as the signalling converter receives a wrong multiframe alignment
signal, changes of the information status of the individual signalling bits are
not evaluated until the multiframe alignment signal has been received again
correctly, i.e. the multiframe is again in sync.