Brief introduction to Satelite Communications
and the integration PSTN netwokrs.

by hybrid
(hybrid@phunc.com)
(darkcyde.8m.com/hybrid/hybrid.htm)


This is just a very quick file to acomany lowtek's article on satelite
communications. Just did a little research, and found a little more out about
sateleites and the PSTN (Public Switched Telephone Network). The majour
players in the satelite industry are INTELSAT (International
Telecommunications Satelite Organisation), and EUTELSAT (European
Telecommunications Satelite service)- part of the European Space Agency (ESA)
and PASAT (Pan American Satelite Inc) These organisations lease space on
there satelites for other companys such as telcos, private sector operations,
etc, etc. These systems are often interconnected to the PSTN and ISDN nets
on the ground, and implement dishes with diameters ranging from 3.5 to 13m.
An interesting development I discovered is called 'Briefcase satelite
newsgathering (SNG) terminal'.. It is a small, lightweight system developed
by a co-operation between Columbia Broadcasting System (CBS), Teleglobe
Canada, and Skywave Electronics Ltd. The system weighs only 14.5 kg, and is
capable of providing good quality speech, still pictures, plus data packets.
Again, this system is readily being intergrated into our PSTN. The system
operates over INMARSAT-C links of 1.5 to 1.6 GHz to a hub earth station
which operates in the 4 and 6 GHz band. Basically, I would imagine that
satelite systems are connected to the PSTN in the following way:

                                                                  ___
                         _________|______          _______       |   |
        ____            |         |      |        |       |::x:::|   | SAT 1
       | a) |:::::::::::|   PSTN  |      |>-------|_______|::y:::|   |
       |____|           |_________|______|        Base station   |___|
                                  |               A              ////
  (customer A, in                                              ////
   the UK)                                         ____|__   ////  <-- signal
                      PSTN: Intra-lata exchange   |    |  |////
                      carriers, operating over    |    |  |//
                      different switching and     |    |  || Mid-way base
                      signaling systems, such as  |    |  |\\   station.       
                      the CO, 2 4ESS switches,    |____|__|\\\\
                      DMS25O international gates.      |     \\\\  <-- signal  
                                                               \\\\ 
                                                                 \\\\ 
                         _________|______          _______       |   |
        ____            |         |      |        |       |::x:::|   |
       | b) |:::::::::::|   PSTN  |      |>-------|_______|::y:::|   | SAT 2
       |____|           |_________|______|        Base station   |___|
                                  |               B
   (customer B, in                                       x= uplink
    Australia)                                           y= downlink


I will be honest here and say that I don't really know exactly how Earth to
Satelite PSTN networking actually works, but if I was to guess it would be
similar to my above diagram, and involve the following routing proccess over
the PSTN... Say I was customer (A) and I wanted to call customer (B) over in
Austrailia, here is what I believe would happen: I pick up the phone and
begin to dial the international country code for Austrailia.. My local
exchange recognises that conventional routing over the network will not
terminate the call, so then forwards my number query over to a DMS
international exchange unit, where the best route for my call is decided by
call allocation software. Beacuse the distance of the call is not conviniant
for land lines, or sub mersive transmission, I am then forwarded to a special
telco satelite base station, where a channel is selected and researved for my
call. An uplink and downlink channel is then setup betwwen Base station A and
Base station B in Australia. Conventional Signaling protocols in Australia
then setup the call acrross the many switches over there until an incoming
call trunk transmission is detected by customer B's local office, which then
routes a call translation and set's up a channel via it's PSTN back to the
PSTN in the UK... B then picks up the phone and the data link is complete.
Because Australia is the opposite side of the world to the UK, a base
station in the middle would have to rebound the signal, and data transmission
back upto the geo-stationary satelite over Australia. When the call is
terminated, all channels, circuits, switches, contacts, are reset and are
ready for another call. I would imagine there are litteraly 1OO's of geo-
stationary satelites that are used by the telcos for international calling,
note: for those of you who have not been paying attention! - geo-stationary
means the satelite is in syncronous orbit with the earth, and would appear to
be still if seen from the earths surface. The orbit at which these satelites
reside is called 'the clarke belt' named after Authur C Clarkes vision of a
place orbiting the earth that remains syncronous with the earths rotation.
Anyways, this could go on forever, so I'm gonna conclude this short file. For
more info on this subject, just do a web portal search for things like
intelsat, eutelsat, pstn routing, geo-syncronous.. blah blah blah. Hope you
enjoyed...


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