TELE-X - a Satellite System for TV and Data Communication ...

42
Fig. 8
The frame structure for multiplexer ZAH 45/140-1
FAW Frame alignment word (111110100000)
ABC Bits belonging to the tributaries A. B, C
C,. Control bits tor pulse stuffing
I Alarm bit
F Bits for optional use
This should be compared with the
printed board assemblies with discrete
components, where each printed board
was only able to house the equipment
for a single tributary.
ZAH 45/140-1
The digital multiplexer ZAH45/140-1
converts three 45 Mbit/s (DS3) signals to
a 140 Mbit/s (DS3E) signal corresponding
to level 4 in the European hierarchy,
and vice versa. No definite standard has
been established for the frame structure
of this multiplexer. The chosen frame is
identical to that used by AT&T The
frame structure is shown in fig. 8. The
interfaces meet the requirements of
CCITT Recommendation G.703 and
AT&T Technical Advisory No. 34.
The function of the multiplexer is illustrated
by the block diagram of fig.9.
The function issimilarto that of ZAH 1.5/
45-1.
Emitter-coupled logic (ECL) has been
used to obtain sufficient reliability at the
high bit rates (^45 Mbit/s). The ECL circuits
have excellent transmission
characteristics at high bit rates. Unfortunately
they have a high power consumption,
so that ECL is only used in the
transmission path, while the control information
is processed in TTL circuits,
containing low-power Schottky circuits.
Moreover the data flow is processed in
parallel form, which means that the
highest bit rate in the magazine is
45 Mbit/s. This design has been op-
Fig. 9
Block diagram of multiplexer ZAH 45/140-1