MT8979 ISO-CMOS
between the frame alignment pattern and the
non-frame alignment pattern as described in Figure
4. Bit 1 of the frame alignment and non-frame
alignment bytes have provisions for additional
protection against false synchronization or enhanced
error monitoring. This is described in more detail in
the following section.
Functional Description
The MT8979 is a CEPT trunk digital link interface
conforming to CCITT Recommendation G.704 for
PCM 30 and I.431 for ISDN. It includes features
such as: insertion and detection of synchronization
patterns, optional cyclical redundancy check and far
end error performance reporting, HDB3 decoding
and optional coding, channel associated or common
channel signalling, programmable digital attenuation
In order to accomplish multiframe synchronization, a
16 frame multiframe is defined by sending four zeros
in the high order quartet of timeslot 16 frame 0, i.e.,
once every 16 frames (see Figure 5). The CEPT
format has four signalling bits, A, B, C and D.
Signalling bits for all 30 information channels are
transmitted in timeslot 16 of frames 1 to 15. These
timeslots are subdivided into two quartets (see Table
6).
and a two frame received elastic buffer.
The
MT8979 can also monitor several conditions on the
CEPT digital trunk, which include, frame and
multiframe synchronization, received all 1’s alarms,
data slips as well as framing and CRC errors, both
near and far end.
The system interface to the MT8979 is a TDM bus
structure that operates at 2048 kbit/s known as the
ST-BUS. This serial stream is divided into 125 µs
frames that are made up of 32 x 8 bit channels.
Cyclic Redundancy Check (CRC)
An optional cyclic redundancy check (CRC) has
been incorporated within CEPT bit stream to provide
additional protection against simulation of the frame
alignment signal, and/or where there is a need for an
enhanced error monitoring capability. The CRC
process treats the binary string of ones and zeros
contained in a submultiframe (with CRC bits set to
binary zero) as a single long binary number. This
The line interface to the MT8979 consists of split
phase unipolar inputs and outputs which are
supplied from/to
respectively.
a
bipolar line receiver/driver,
CEPT Interface
4
string of data is first multiplied by x then divided by
4
the generating polynomial x +x+1. This division
process takes place at both the transmitter and
receiver end of the link. The remainder calculated at
the receiver is compared to the one received with the
data over the link. If they are the same, it is of high
probability that the previous submultiframe was
received error free.
The CEPT frame format consists of 32, 8 bit
timeslots. Of the 32 timeslots in a frame, 30 are
defined as information channels, timeslots 1-15 and
17-31 which correspond to telephone channels 1-30.
An additional voice/data channel may be obtained by
placing the device in common channel signalling
mode. This allows use of timeslot 16 for 64 kbit/s
common channel signalling.
The CRC procedure is based on a 16 frame
multiframe, which is divided into two 8 frame
submultiframes (SMF). The frames which contain
Synchronization is included within the CEPT bit
stream in the form of a bit pattern inserted into
timeslot 0. The contents of timeslot 0 alternate
the frame alignment pattern contain the CRC bits, C
1
to C respectively, in the bit 1 position. The frames
4
2.0 ms
Frame
15
Frame
0
Frame
14
Frame
15
Frame
0
• • • • • • • •
Timeslot
0
Timeslot
Timeslot
30
Timeslot
31
• • • •
1
125 µs
Least
Significant
Bit (Last)
Most
Significant
Bit (First)
Bit
1
Bit
2
Bit
3
Bit
4
Bit
5
Bit
6
Bit
7
Bit
8
(8/2.048) µs
Figure 3 - CEPT Link Frame & Multiframe Format
4-164
MITEL [ MITEL NETWORKS CORPORATION ]
