DS2181A
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CCS SIGNALLING
CCS (selected when TCR.5 = 1 and/or when RCR.1 = 1) utilizes all bit positions of timeslot 16 in every
frame for message-oriented signaling data transmission. In CCS mode one can use either timeslot 16 or
any one of the other 30 data channels for message-oriented signaling. The CCS mode has no multiframe
structure and the insertion of CAS multiframe alignment, distant multiframe alarm and/or extra bits into
timeslot 16 is disabled. TSER is the source of timeslot 16 data.
CRC4 CODING
The need for enhanced error monitoring capability and additional protection against emulators of the
frame alignment word has led to the development of a cyclic redundancy check (CRC) procedure. When
enabled via CCR.2 and/or CCR.3, CRC4 coding replaces the international bit positions in frames 0
through 12 and 14 with a CRC4 multiframe alignment pattern and associated checksum words. The
CRC4 multiframe must begin with a frame containing the frame alignment signal (CCR.6 = 0). A rising
edge at TMSYNC establishes the CRC4 multiframe alignment (TMSYNC will also establish outgoing
CAS multiframe alignment if enabled via TCR.5).
Incoming CRC4 multiframe alignment is indicated by RCSYNC. Detected CRC4 checksum errors are re-
ported at output RFER and logged in the CECR.
RECEIVE SYNCHRONIZER
The fixed characteristics of the receive synchronizer may be modified by use of programmable
characteristics resident in the RCR and CCR. Sync criteria must be met before synchronization is
declared. Resync criteria establish error occurrences which will cause an auto-resync event when enabled
(RCR.1 = 0).
The receive synchronizer searches for the frame alignment pattern first. Once identified, the output timing
set associated with the framing pattern (all outputs except RCSYNC and RMSYNC) is updated to that
new alignment. If enabled, the synchronizer then begins CAS and/or CRC4 multiframe search; outputs
RMSYNC and/or RCSYNC are then updated. Output RLOS is held high during the entire resync process,
then transitions low after the last output timing update indicating resync is complete. For more details
about the receive synchronizer, see the separate DS2181A CEPT Transceiver Application Note.
FIXED FRAME SYNC CRITERIA
Valid frame sync is assumed when the correct frame alignment signal is present in frame N and frame N
+ 2 and not present in frame N + 1 (bit 2 of timeslot 0 of Frame N + 1 is also checked for 1). CAS and/or
CRC4 multiframe alignment search is initiated when the frame search is complete if enabled via RCR.5
and/or CCR.2.
FIXED CAS MULTIFRAME SYNC CRITERIA
CAS multiframe sync is declared when the multiframe alignment pattern is properly detected and timeslot
16 of the previous frame contains code other than zeros. If no valid pattern can be found in 12 to 14
milliseconds (no time-out period exists if CCR.1=1 or TEST=1), frame search is restarted.
FIXED CRC4 MULTIFRAME SYNC CRITERIA
CRC4 multiframe sync is declared if at least two valid CRC4 multiframe alignment signals are found
within 12 to 14 milliseconds (8 ms if CCR.1=1 or TEST=1) after frame alignment is completed. If not
found within 12 to 14 milliseconds (8 ms if CCR.1=1 or TEST=1), frame search is restarted. The search
for the multiframe alignment signal is performed in timeslot 0 of frames not containing the frame
alignment signal.
