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

Fig. 4
Block diagram for a data and video station, MDVS
MM
CEM
U/C
TWTA
D/C
LNA
Modem shelf
Central electronics unit
Up-converter
Travelling wave tube amplifier
Down-converter
Low noise amplifier
The station is designed to set up connections
automatically This is done
with the aid of an X.21 high-level protocol,
which means that the subscriber
equipment either uses X.21 itself or that
it is equipped with an auxiliary signalling
unit for this protocol.
The incoming data signal is first taken to
the data circuit equipment, DCE, in the
station, fig. 4. The signalling part is processed
by DRP (DAMA Remote Processor).
DRP sends out signalling data
in the form of bursts, which are OQPSK
(Offset Quadrature Phase Shift Keying)
modulated and are then fed to the station
transmitter.
Common channel signalling in accordance
with the ALOHA access method,
based on random access, is used between
the traffic station and the control
station When receiving a request from a
calling station the control station signals
to the other traffic station via its
BSC (Broadcast Signalling Channel)
and then tells the two stations which
pair of channels they are to use. BSC
works continuously with time division
for the stations in the network.
Each station is equipped with a signalling
receiver for BSC. This channel is
BPSK (Binary Phase Shift Keying) modulated,
and the incoming signals are
therefore fed through a BPSK demodulator,
and then a decoder for the error
correction code (FEC) used. The incoming
signals are processed by the DCE,
which sets up the pair of channels indicated
by the control station. This is done
by setting the traffic modem to the allocated
frequencies. The traffic modem
uses OQPSK modulation in combination
with FEC and scrambling. After a
handshake procedure with the other
traffic station the traffic data can be
transmitted. Afterwards the two channels
are disconnected and returned to
the pool of free channels in the control
station.
The station transmitter is equipped with
TWT (Travelling Wave Tube) amplifiers
(except DS, which has a GaAs-FET semiconductor
amplifier). Stations with TWT
are provided with active output power
control, in order to compensate for the
effect of weather on the link to the satellite.
All IF/RF electronic equipment is placed
in a cabinet behind the antenna. The
electronic equipment for indoor installation
consists of between one (DS)
and three (fully extended MDVS) 19"
magazine shelves of type BYB, which
can be placed in an ordinary office environment.
The high-speed stations have more extensive
electronic and microwave
equipment, but they normally have the
same antennas as the low-speed stations.
The control station DVCS (Data/Video
Control Station) is complex, and a detailed
description would be outside the
scope of this article. It consists of a radio
part and a switching part, which is similar
to a switch in an ordinary data network.
It works in accordance with the
SCPC/DAMA method and handles the
switching and supervisory functions in
the data and video network. The control
station does not transmit any traffic, as
this is the task of the traffic stations. The
capacity and facility level of the control
station can be enlarged during operation.
The station can be unmanned and
remotely controlled from, for example,
an AOM101 centre.