
Preliminary Information
MT9071
47
5.5
E1 Framing Algorithms
The MT9071 contains three distinct framing algorithms: basic frame alignment, signaling multiframe alignment
and CRC-4 multiframe alignment. To see how these algorithms interact, see Figure 10 - E1 Synchronization
State Diagram.
After power-up, the basic frame alignment framer will search for a frame alignment signal (FAS) in the PCM30
receive bit stream. Once the FAS is detected, the corresponding bit 2 of the non-frame alignment signal (NFAS)
is checked. If bit 2 of the NFAS is zero a new search for basic frame alignment is initiated. If bit 2 of the NFAS
is one and the next FAS is correct, the algorithm declares that basic frame synchronization has been found and
sets status register bit BSYNC =0 (see Table 105 - E1 Synchronization & CRC-4 Remote Status - R Address
Y10).
Once basic frame alignment is acquired the signaling and CRC-4 multiframe searches will be initiated. The
signaling multiframe algorithm will align to the first multiframe alignment signal pattern (MFAS = 0000) it
receives in the most significant nibble of channel 16 and sets status register bit MSYNC = 0 (see Table 105 - E1
Synchronization & CRC-4 Remote Status - R Address Y10).Signaling multiframing will be lost when two
consecutive multiframes are received in error.
The CRC-4 multiframe alignment signal is a 001011 bit sequence that appears in PCM30 bit position one of the
NFAS in frames 1, 3, 5, 7, 9 and 11 (see Table 8 - E1 CRC-4 FAS and NFAS Structure). In order to achieve
CRC-4 synchronization, two consecutive CRC-4 multiframe alignment signals must be received without error,
this is indicated with status register bit CSYNC = 0 (see Table 105 - E1 Synchronization & CRC-4 Remote
Status - R Address Y10).
The MT9071 framing algorithm supports automatic interworking of interfaces with and without CRC-4
processing capabilities. That is, if an interface with CRC-4 capability, achieves valid basic frame alignment, but
does not achieve CRC-4 multiframe alignment by the end of a predefined period, the distant end is considered
to be a non-CRC-4 interface. When the distant end is a non-CRC-4 interface, the near end automatically
suspends receive CRC-4 functions, continues to transmit CRC-4 data to the distant end with its E-bits set to
zero, and provides a status indication. Naturally, if the distant end initially achieves CRC-4 synchronization,
CRC-4 processing will be carried out by both ends. This feature is selected when control register bit AUTC =0
(see Table 79 - E1 Alarms and Framing Control - R/W Address Y00).
Status
CRCIW CSYNC
CRC to Non-CRC Interworking, CRC sync acquired after the 400ms timer, but interworking
operation does not change.
CRC to Non-CRC Interworking, CRC sync not yet acquired.
CRC to CRC Interworking, CRC sync acquired.
CRC to CRC Interworking, CRC sync not yet acquired.
Table 10 - E1 CRC Interworking Status Register Bits
0
0
0
1
1
1
0
1