ISO-CMOS
MT8979
125µs
CHANNEL
31
CHANNEL
0
• • •
BIT
7
BIT
6
BIT
5
BIT
4
CHANNEL
30
CHANNEL
31
CHANNEL
0
Most
Significant
Bit (First)
BIT
3
BIT
2
BIT
1
BIT
0
Least
Significant
Bit (Last)
(8/2.048)µs
Figure 7 - ST-BUS Stream Format
the ERR bit can be used to evaluate the bit error
rate of the line according to the CCITT
Recommendation G.732 (see section on Frame
Alignment Error Counter).
Channel 19 contains the Phase Status Word (see
Table 15), which can be used to determine the phase
relationship between the ST-BUS frame pulse (F0i)
and the rising edge of E8Ko. This information could
be used to determine the long term trend of the
received data rate, or to identify the direction of a
slip.
Channel 20 contains the CRC error count (see Table
16). This counter will wrap around once terminal
count is achieved (256 errors). If the maintenance
option is selected (bit 3 of MCW3) the counter is
reset once per second.
Channel 21 contains the Master Status Word 2 (see
Table 17). This byte identifies the status of the CRC
reframe and CRC sync. It also reports the Si bits
received in timeslot 0 of frames 13 and 15 and
the ninth and most significant bit (b
8
) of the 9-bit
Phase Status Word.
selected as the clock source for the PBX) then the
data rate at which the data is being written into the
device on the line side may differ from the rate at
which it is being read out on the ST-BUS side.
When the clocks are
situations can occur:
not
phase-locked,
two
Case #1:
If the data on the line side is being written
in at a rate SLOWER than it is being read out on the
ST-BUS side, the distance between the write pointer
and the read pointer will begin to decrease over time.
When the distance is less than two channels, the
buffer will perform a controlled slip which will move
the read pointers to a new location 34 channels
away from the write pointer. This will result in the
REPETITION of the received frame.
Case #2:
If the data on the line side is being written
in at a rate FASTER than it is being read out on the
ST-BUS side, the distance between the write pointer
and the read pointer will begin to increase over time.
When the distance exceeds 42 channels, the elastic
buffer will perform a controlled slip which will move
the read pointer to a new location ten channels away
from the write pointer. This will result in the LOSS of
the last received frame.
Note that when the device performs a controlled slip,
the ST-BUS address pointer is repositioned so that
there is either a 10 channel or 34 channel delay
between the input CEPT frame and the output
ST-BUS frame. Since the buffer performs a
controlled slip only if the delay exceeds 42 channels
or is less than two channels, there is a minimum
eight channel hysteresis built into the slip
mechanism. The device can, therefore, absorb eight
channels or 32.5µs of jitter in the received signal.
There is no loss of frame synchronization, multiframe
synchronization or any errors in the signalling bits
when the device performs a slip.
Elastic Buffer
The MT8979 has a two frame elastic buffer at the
receiver, which absorbs the jitter and wander in the
received signal. The received data is written into the
elastic buffer with the extracted E2i (2048 kHz) clock
and read out of the buffer on the ST-BUS side with
the system C2i (2048 kHz) clock (e.g., PBX system
clock). Under normal operating conditions, in a
synchronous network, the system C2i clock is
phase-locked to the extracted E2i clock. In this
situation every write operation to the elastic buffer is
followed by a read operation. Therefore, underflow
or overflow of data in the elastic buffer will not occur.
If the system clock is not phase-locked to the
extracted clock (e.g., lower quality link which is not
4-167
MITEL [ MITEL NETWORKS CORPORATION ]
