6-6

Theory of Operation

The MT8950 is an encoder/decoder which operates

on low baud rate data (up to 19.2 kbps) to convert it

to the ST-BUS format. The data can subsequently

be transparently switched or transmitted in a manner

identical to PCM encoded voice. In this respect, the

functional characteristics of the device are very

similar to many industry standard voice codecs.

Asynchronous and synchronous data from 0 to 8

kbps and at 9.6 kbps is accepted by the codec

without any restrictions. Asynchronous data at 19.2

kbps should have at least two stop bits for the device

to encode it properly. The data is encoded by the

Codec into an eight bit word which occupies one 64

kbps channel on the ST-BUS. Conversely, it accepts

an encoded 8 bit word from an incoming ST-BUS

stream and regenerates the original digital signal.

Mitel’s ST-BUS is a synchronous time division

multiplexed serial stream with a bit rate of 2048

kbps.

In a telecommunications environment, it is

generally divided into 32 channels made up of 8 bits

each, with an effective bandwidth of 64 kbps per

channel.

These channels may carry data or PCM

encoded voice.

Low Speed Data Format

The Data Codec can accept low speed data in either

Non Return to Zero (NRZ) or Return to Zero (RZ)

format.

The NRZ format requires only one line to

carry the data. This format is suitable for interfacing

the data codec with RS-232 type terminals and

microprocessor peripherals such as UARTS, ACIAs,

etc. All signals have to be converted to TTL voltage

levels before being input to the codec.

The RZ format requires two separate lines to

represent the MARKs and SPACEs in the data as

illustrated in Figure 4. This format is useful when the

data terminal is located some distance from the

codec and the data is to be transmitted over a line as

a three level signal (a positive pulse for the

beginning

of

MARKs,

negative

pulse

for

the

beginning of SPACEs and zero level for no change in

the signal). The three level signal is converted to its

TTL-Compatible binary form as shown in Figure 4

before being applied to the codec. A pulse appears

on one line of the input indicating the beginning of

MARKs. This is followed by a pulse on the second

line indicating the beginning of SPACEs.

If two or

more pulses appear consecutively on the same line

before the second line of the pair receives or

transmits another pulse, then these pulses can be

considered to be violating the normal rule of the RZ

format and are called "Violation Pulses". The data

codec will accept these violations with the restriction

that the time difference between a violation pulse

Table 1. Summary of Data Codec Capabilities.

† Refers to the maximum distortion in the bit period timing of the

regenerated data. (Channel Bandwidth = 64kbps )

where

and an actual data transition be at least 125µs. The

violation pulses can be on the MARK or SPACE line.

In a communications system, these violations can be

used to carry other information when no data is

being transmitted.

Encoding/Decoding Scheme

The

Data

Codec

uses

a

Transition

Encoded

Modulation (TEM) technique to encode low speed

data onto a 56 or 64 kbps equivalent PCM voice

channel. This coding algorithm significantly reduces

data bit distortion. The timing distortion in the

regenerated data is summarized in Table 1. A simple

sampling method for encoding the data would

require a 256 kbps channel to obtain the same low

distortion figures.

If the encoded information is to be transmitted over

digital T1/DS1 trunks, the maximum percentage

distortion in the regenerated data is effectively

doubled.

This is due to the fact that the least

significant bit in specific channels on these trunks is

used to transmit signalling information. Thus the

bandwidth per channel is reduced to 56 kbps.

The encoder stage of the Data Codec

observes

data transitions in discrete timing windows which are

125µs wide. These timing frames are further divided

into 32 timeslots of 3.906µs duration each (see

Figure 3). The position of the first data transition, the

total number of transitions, and, the time period

between the transitions in this 125µs frame is

encoded as an 8 bit word.

The first five bits (b0 to b4) indicate the position of

the first data transition with respect to the 32

timeslots in the window. Bit 7 in the encoded word

represents the absolute value of the data in the 31st

timeslot. Bits 5 and 6 in conjunction with bit 7 are

used to identify the total number of transitions and

the time period between the transitions.

Due to the

fixed bit rate restrictions above 8 kbps, a maximum

Data Rate

Bits/Sec

0 - 8000

9600

19200

Asynchronous

Restrictions

None

None

Minimum

2 Stop Bits

Synchronous

Restrictions

None

None

None

Percentage

±3.2

±3.8

±7.5