diavox courier 700, digital system telephone for md 110 data

93
Fig. 4
The CMI decoder, which consists of a four-layer
printed wiring board on which are mounted ECL
circuits for the decoding, a controlled crystal
oscillator for the timing recovery and 5 GHz
transistors for the input stage
on coaxial cable is carried out in the line
terminating magazine, fig. 2. Conversion
between CMI code and the internal
binary code is done in the CMI
encoder and decoder. The send side
scrambler makes the signal pattern
more random, mainly, in order to simplify
the timing recovery in the repeaters.
The original signal pattern is restored
in the receive side descrambler
These two units also contain special
supervision circuits which prevent certain
repetitive patterns from ocurring.
The line encoder carries out the recoding
of the signal that is necessary in
order to reduce the symbol rate, and
thus also the influence of the cable attenuation
which the intermediate repeaters
must bridge. The chosen code
is of the 4B3T type, i.e. four incoming
binary symbols are reduced to three
outgoing ternary symbols This gives a
symbol rate on the cable of 3 / 4 x140 ~
105 Mbaud. Since the number of binary
states, 2 4 = 16, is less than the number
of ternary states, 3 3 = 27, the code has a
built-in redundancy. This spare capacity
is used for fault location. It can also
be used for data transmission. The magazine
is therefore designed so that a
data adapter for a 300 baud asynchronous
data channel can be connected.
It would have been possible to use another
code giving greater reduction of
the symbol rate than 4B3T, but such
codes would have rendered the signal
more sensitive to variations in the transmission
quality of the cable.
The line terminating magazine also reduces
the jitter. Within the magazine
the mode of operation is in four parallel
35 Mbit/s flows instead of one serial 140
Mbit/s flow. This means that the line and
CMI encoders tolerate four times more
jitter. Consequently the bandwidth of
the timing recovery circuits can be
made narrow in order to reduce the
jitter on the signal before it is sent out
on the cable or to the D4 interface. All
timing recovery circuits in the magazine
have been designed as phase-locked
circuits with crystal oscillators in order
to obtain good jitter characteristics.
The primary alarms are those recommended
by CCITT. They are compiled
by the alarm unit in the magazine. The
compilation and transmission of alarms
are carried out in the same way as in
other transmission equipment in the
BYB construction practice 2 . In the case
of a loss of signal or high error rate an
alarm indication signal (AIS) is inserted
in the send or receive direction. In the
140 Mbit/s system an AIS is also required
in the send direction, since recoding
is carried out. The AIS in the
send direction, which is made more random
by the scrambler, can be used instead
of an external pattern generator,
for example during commissioning or
when the system serves as a standby.
The alarm "absence of signal in the
send direction", which is obtained
when the equipment is not connected
to the D4 interface, can be inhibited by
means of a switch on the front of the
magazine.
Fig. 3
The line terminating magazine where recoding,
iiMM- -~^..~*:rt« onH eiinamieinn aro rarripri nut