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When did they stop routing long-distance analog phone calls through satellites? What was the maximum volume at its peak? contains images, sources and descriptions of both

The Intelsat 1 “Early Bird” communications satellite, built by Hughes Aircraft Co.

and

Both were said to be able to carry hundreds of simultaneous telephone conversations.

We can back of the ballpark envelope spherical cow estimate this to be of order a MHz of bandwidth or more.

Question: How were Intelsat 1 “Early Bird” and Telstar 1's "hundreds" of simultaneous telephone conversations multiplexed/demultiplexed?

  • Was it all done on the ground and the satellites were "bent pipes"?
  • Was the multiplexing digital? Or in the frequency domain ("channels") or something else?
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    $\begingroup$ I just realized that there's more about Telstar's inner workings in answer(s) to How were the 100+ antennas around Telstar 1 and 2 configured? $\endgroup$
    – uhoh
    Dec 10, 2021 at 21:30
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    $\begingroup$ Telstar-1 was far from able to handle hundreds of simultaneous phone calls. It transmitted 117 phone calls from Andover station during its whole lifetime! See the link Organic Marble provided. $\endgroup$
    – asdfex
    Dec 11, 2021 at 8:52

3 Answers 3

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It was done on the ground, and frequency-multiplexed.

In this arrangement, 12 individual telephone channels are frequency-multiplexed into the 60 to 108 kc band as shown.

enter image description here

Source: Results of the Telstar System Communication Tests, found in NASA SP-32 Volume II

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    $\begingroup$ The picture shows a test setup with only 12 telephone channels. After testing 600 uplink and 600 downlink channels were used, total bandwidth 4.8 MHz. $\endgroup$
    – Uwe
    Dec 12, 2021 at 16:43
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Was it [the multiplexing] all done on the ground and the satellites were "bent pipes"?

Even nowadays, almost all GEO satellites are "bent pipe", meaning that they re-transmit the uplinked signal "as is", just shifted in frequency. For Early Bird, the signal uplinked at 4GHz (band) was frequency translated to 6GHz (band) then amplified for downlinking back to earth. The 4/6 GHz band is commonly referred to as the "C-band". More recent Intelsat satellites also use other bands, for ex the Ku (10/14 GHz), and the Ka (20/30 GHz) but essentially their common architecture is (still) bent-pipe.

Hence, with a bent-pipe architecture, it is the ground segment that does the multiplexing, whether this multiplexing is in frequency (each one-way communication takes a slice of the bandwidth) or in time (each one-way communication takes a "time slot"). The beauty of the bent-pipe architecture is that it does not make the satellite obsolete to advances in communication technologies. For example, moving from analog to digital, or frequency-multiplexing to time-multiplexing does not require any change in the (already launched) satellites.

Was the multiplexing digital? Or in the frequency domain ("channels") or something else?

The digitalization of the PSTN (Public Switched Telephony Network) started in the 70s, with the first standard of voice digitalization, the ITU G.711. Early Bird being launched in 1965 could not benefit from this, hence the (analogue) voice channels were frequency multiplexed (I talk to you on frequency f1 and listen to you on frequency f2).

Voice digitalization opens the possibility of several communications being "bundled" and transmitted in parallel in a high-speed "multiplex". The US, for example, use (perhaps used?) a Digital Signal-1 (DS1) multiplex to transmit simultaneously 24 voice calls (among other things). Note that this is uncompressed voice at 64kbps (for each direction).

The Intelsat VI series implemented a non bent-pipe technology called SS/TDMA (Satellite-Switched/Time-Division-Multiple-Access), which was used from the 90s to mid 00s, when large investments in undersea optical fibers quickly obsoleted satellite (trunk) telephony.

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  • $\begingroup$ IIRC in the US voice calls were often 56K, not 64K, due to robbed-bit signalling. $\endgroup$
    – jcaron
    Dec 11, 2021 at 23:56
  • $\begingroup$ @jcaron, almost correct, but we have to dig down into the details of this "engineering optimization". For those interested, there is a primer on Wiki $\endgroup$
    – Ng Ph
    Dec 12, 2021 at 14:45
  • $\begingroup$ The last generation of optical fiber cables transmitted 130 wavelengths x 100 Gbit/s per fiber pair = 13 Tbit/s. Several fiber pairs within the same cable are possible. 13 Tbit/s seems impossible for a satellite link that should be possible even with a cloud covered ground station. See en.wikipedia.org/wiki/AEConnect $\endgroup$
    – Uwe
    Dec 12, 2021 at 17:16
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The telephone channels routed over the satellites were frequency multiplexed.

Frequency multiplexing of telephone channels was an established technology used for the transatlantic cables TAT-1 and TAT-2 since 1956. These transatlantic cables used coaxial cables for signal transmission.

TAT-1 was 3600 km long and equipped with 51 amplifiers at every 70 km. Bandwidth per channel plus gap first 4 kHz and later only 3 kHz. There were 36 channels first and 48 channels later.

A 12 channel frequency division multiplex system for land cables of twisted pairs was used since the early 1930s. Repeaters were used about every 10 km. The frequency range 60 to 108 kHz was used, bandwidth 48 kHz, 4 kHz per channel plus gap

So there were more than two decades of experience using frequency division multiplexing for telephone channels transmission.

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  • $\begingroup$ This is really interesting! What was the frequency range used? Was it DC to say 50 kH, or something higher? $\endgroup$
    – uhoh
    Dec 11, 2021 at 0:12
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    $\begingroup$ @uhoh, it looks like the answer is given in the diagram shown in Organic Marble's answer (the 12-channel multiplex occupies frequencies from 60kHz to 108 kHz, KC=kHz in the old doc). $\endgroup$
    – Ng Ph
    Dec 11, 2021 at 9:21
  • $\begingroup$ @NgPh did OrganicMarble's answer talk about undersea cables as this answer does? $\endgroup$
    – uhoh
    Dec 11, 2021 at 10:37
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    $\begingroup$ @NgPh The 12 channel multiplexer was at first used for land cables, then a similar multiplexer for sea cables and at last for satellite connections. From the early 30s to the early 60s, about 3 decades. $\endgroup$
    – Uwe
    Dec 11, 2021 at 11:13
  • $\begingroup$ @uhoh You really don't want any higher frequencies on such cables. TAT-1 needed a repeater every 70 km - later cables capable of 1 MHz had one every 16 km or even more. $\endgroup$
    – asdfex
    Dec 11, 2021 at 14:48

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