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

Fig. 3
A sketch of the network
DAMA traffic control station
Signalling channel
Combined signalling and traffic channel
Subscriber connected via fiber optics
area is internatlly connected via an optical
fibre network.
The demand for small and simple stations
has led to the use of the best of
both old and new satellite technologies.
For example, the most modern of highpower
satellites is combined with the
simple but effective SCPC (Single Channel
Per Carrier) transmission method.
With the SCPC method the transponder
signal is frequency divided into a number
of carriers, which form a pool used
by all stations in the network according
to need. Forexample, the distribution of
64 kbit/s and 2 Mbit/s carriers is dynamic,
i.e. it can be adapted to the current
traffic requirement by blocking or releasing
64 kbit/s channels so that the
number available corresponds to the
traffic load.
When a station calls another station a
channel is allocated, and when the
transmission is finished the channel is
returned to the pool.
Each cannel is independent of the others
and is modulated on its own carrier.
This makes for flexible allocation of
channels to the stations, without complicated
signal processing.
All stations in the system (can be thousands)
therefore have full access to
each other when required. This arrangement
is designated DAMA (Demand Assignment
Multiple Access).
Fig. 3 shows a sketch of the network.
SCPC/DAMA can be compared with the
now very popular TDMA (Time Division
Multipel Access) systems. Such a corn-
Comparison between
SCPC/DAMA and TDMA
Full transponder TDMA
With full utilization of the TDMA (Time Division
Multiple Access) method all traffic in the system
passes through the satellite transponder and
the earth stations via one carrier. The carrier is
time divided into short bursts and thus a high
momentary data rate is obtained. Since only the
one carrier is present it is possible to drive the
earth stations as well as the satellite output
stage to saturation, which gives good utilization
of their high-power transmitters.
Another advantage is that with TDMA all subscribers
in the network have access at any instant
to all exchanged information. This
provides unique facilities in a large number of
traffic situations because of the possibility of
varying the length or periodicity of the bursts.
One drawback is that the advantages of TDMA
are closely connected with the fact that at any
moment all earth stations handle and sort all the
traffic in the network. The radio parts in the
earth station must be dimensioned for the full
amount of traffic in the network, and the complexity
of the "sorting equipment" increases
very rapidly with the number of earth stations
and the number of traffic elements per earth
station.
Small-scale TDMA
The designation TDMA is sometimes also used
for rather trivial multiplex-type systems, whose
network structure and capacity allocation are
set during installation and cannot be adjusted
afterwards to suit the traffic requirements.
The relative simplicity is attractive for smallscale
TDMA in which the satellite transponder
signal is divided into several carriers with different
frequencies, like in the SCPC (Single
Channel Per Carrier) method.
Each carrier is time divided, as with full TDMA,
but with a lower data rate per carrier. This demands
less from the individual earth station as
regards data rate and power, but means a loss
of connectivity between the TDMA carriers, and
a limited upper data rate for each station.
The method is economical in systems with a
medium data rate and a fairly small number of
stations in a closed user group.
TDMA or SCPC system ?
TDMA stations with full transponders are complicated
and expensive, as the stations must
handle all traffic in the network regardless of
the needs of the individual station. The method
is economical when the network consists of a
few earth stations, each with a large amount of
traffic. In order to achieve this it is often necessary
to concentrate the traffic. However, it has
been found that just the cost of connecting a
subscriber to an earth station via a cable or
radio relay link can be higher than the cost of a
complete TELE-X SCPC earth station. For reasons
of economy TDMA is thus not suitable for
wide-mesh networks with many subscribers.
The SCPC concept means that each station is
dimensioned for its own traffic (not the total
network traffic), and it is therefore much cheaper
than a TDMA station.
Small-scale TDMA with several carriers in the
same transponder can make full use of the
TELE-X system. This combination may be attractive
for different types of company networks
in which, for example, a 2 Mbit/s channel within
the SCPC system is used for TDMA traffic. The
TELE-X traffic control system DAMA (Demand
Assignment Multiple Access) makes possible
such a combination of pure SCPC and TDMA.
New services, such as data and video communication,
will be used fairly sparsely even in cities.
The possible users of such facilities will be
numbered in tens of thousands, unlike the telephone
subscribers which number millions. This
fact favours TELE-X SCPC, perhaps combined
with small-scale TDMA.