TELECOM Digest OnLine - Sorted: Re: Communications History

Re: Communications History

Paul Coxwell (paulcoxwell@tiscali.co.uk)
Thu, 08 Dec 2005 13:51:33 +0000

> I am a bit of a communications history buff. I've been doing a little
> research about telecom in the days before transoceanic phone service
> before cables and satellites. The only way to bridge the oceans was
> hf radio. It's interesting to note that anyone with a shortwave radio
> could listen to all the international point-to-point phone traffic. I
> am going to assume that a form of independent sideband was used with a
> maximum of two or four circuits going to a specific country. Ssb is
> very easy to receive even with a standard shortwave radio of the day
> as long as it had a bfo to demodulate and recover the signals. I
> would like to know if any form of primitive encryption was used to
> make the circuits a bit more secure. It must have been very easy to
> literally monitor all the international traffic to and from a given
> nation. Anyone who can shed light on this subject would be
> appreciated to help scratch my historical itch.

In the earlier days, quite a number of the HF links were just regular
double-sideband AM with carrier, not even SSB. Even today we still
have ship-to-shore telephone links operated over simple, unencrypted
SSB links in the marine band (2 to 3MHz-ish). I'm on the east coast
of England, and there are still quite a number of calls placed this
way from the off-shore oil rigs in the North Sea.

When satellites started taking over international circuits, there was
no encryption either. They used the same sort of basic carrier
arrangements as were already in use for coax systems, with each
circuit in an SSB channel of 4kHz overall bandwidth (wider than the
telephone speech bandwidth to allow for guard bands, pilot tones,
etc.).

12 channels made up a group, then 5 groups were multiplexed together
to form a supergroup, and so on. The arrangement allowed for easy
routing of circuits at intermediate points without splitting
everything down to individual channels and then recombining. If, say,
one group (12 channels) out of a supergroup was destined for a
particular place,you could just extract that group complete and then
multiplex it on to a different supergroup to send it on its way -- No
need to worry about the individual channel content of the group.

Anyone with a suitable satellite receiver/downconverter could connect
it to a receiver and then just tune across the band, finding an SSB
telephone channel every 4kHz or so. In fact when I worked for BT in
the 1980s that's almost EXACTLY the way we would check a specific
channel off the satellite (or other carrier systems). We had monitor
sets that we could connect to the downconverted satellite baseband.
They had precisely calibrated level meters so we could measure
pilot-tone levels to make adjustments, but they were still, in effect,
just fancy SSB SW receivers.

Of course, in this case you'd be picking up just one direction of the
conversation, as hybrids were used back at the origin to split the
circuit. Finding the "mate" of a channel was easy enough with all the
records in front of you, although even that could take a while to set
up. For someone tuning around without that information, it would be
much, much harder.

Paul

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