Preliminary Information
MT9075A
Two status bits, RSLIP and RSLPD (page 03H,
address 15H), give indication of a slip occurrence
and direction. RSLIP changes state in the event of a
slip. If RSLPD=0, the slip buffer has overflowed and a
frame was lost; if RSLPD=1, a underflow condition
occurred and a frame was repeated. A maskable
interrupt SLPI (page 01H, address 1BH) is also
provided.
synchronization has been found (i.e., page 03H,
address 10H, bit 7, SYNC is zero).
Once basic frame alignment is acquired the
signalling and CRC-4 multiframe searches will be
initiated. The signalling multiframe algorithm will
align to the first multiframe alignment signal pattern
(MFAS = 0000) it receives in the most significant
nibble of channel 16 (page 3, address 10H, bit 6,
MFSYNC = 0). Signalling multiframing will be lost
when two consecutive multiframes are received in
error.
Figure 9 illustrates the relationship between the read
and write pointers of the receive slip buffer.
Measuring clockwise from the write pointer, if the
read pointer comes within two channels of the write
pointer a frame slip will occur, which will put the read
pointer 34 channels from the write pointer.
Conversely, if the read pointer moves more than 60
channels from the write pointer, a slip will occur,
which will put the read pointer 28 channels from the
write pointer. This provides a worst case hysteresis
of 13 channels peak (26 channels peak-to-peak) or a
wander tolerance of 208 UI.
The CRC-4 multiframe alignment signal is a 001011
bit sequence that appears in PCM 30 bit position one
of the NFAS in frames 1, 3, 5, 7, 9 and 11 (see Table
7). In order to achieve CRC-4 synchronization two
CRC-4 multiframe alignment signals must be
received without error (page 03H, address 10H, bit 5,
CRCSYN = 0) within 8 msec.
The MT9075A 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 bit AUTC (page
01H, address 11H) is set to zero.
Framing Algorithm
The MT9075A contains three distinct framing
algorithms: basic frame alignment, signalling
multiframe alignment and CRC-4 multiframe
alignment. Figure 10 is a state diagram that
illustrates these algorithms and how they interact.
After power-up, the basic frame alignment framer will
search for a frame alignment signal (FAS) in the PCM
30 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
Write
Pointer
Read Pointer
Read Pointer
60 CH
13 CH
2 CH
Wander Tolerance
512 Bit
Elastic
Store
15 CH
47 CH
-13 CH
34 CH
28 CH
Read Pointer
Read Pointer
Figure 9 - Read and Write Pointers in the Slip Buffers
4-149
MITEL [ MITEL NETWORKS CORPORATION ]
