AT&T Long Lines – A Forgotten System (2018)
personal.garrettfuller.org
personal.garrettfuller.org
1. They are a milestone of microwave RF transmission coming to maturity, and being widely and economically used. This includes such aspects as analog transceiver front-end design, antenna design, reliability know-how etc.
2. Long Lines managed to relay analog voice calls over dozens of hops with enough quality to allow low-distortion human and early fax/modem communication over thousand of miles overall. This is an amazing achievement for the American society and economy at large.
Just to highlight how far we've come in terms of data capacity, a single TH-2 Long Line link could carry 900 calls. STM-1, the slowest standardized carrier grade payload framing currently in use (predominantly over fiber), carries 63 VC-12 containers, each with an E1, for a total capacity of 1890 calls.
Of course, the above is not an apples to oranges comparison as they use different transmission media. I was not able to locate information about the bandwidth of specific Long Line link types to calculate RF spectral efficiency, but the ballpark figure is an improvement of ~100x when compared to modern digital microwave links of similar bandwidth (accounting for both modulation and digital voice compression).
The Redwood City site is interesting in its own way, as I believe the building below contained the Redwood City tandem at the time prior to its elimination in favor of a single, very large 4ESS tandem at Oakland. It was not very typical for tandem switches to be colocated with large microwave sites, usually being end offices on the microwave network with only two antennas (tandem offices are typically in large cities so AT&T often used a "city and junction" pattern where the tandem office, named for the city, was an end office on a short leg from a large microwave site on the outskirts of town named city junction). The exceptions tended to be in highly urbanized areas where space was difficult to obtain, or a few odd cases like Mojave where an end office grew over time into a gateway site.
In the satellite photo you can see enormous numbers of portable generators waiting for The Big One.
It was presumably out in the middle of nowhere to survive a nuclear attack on San Francisco and Sacramento.
You can see one in the link below, the tower on the left. The one on the right was (when I used to maintain kit up on this site) run by Scottish Power as part of their communications network, with tower space leased to other customers. Just visible at the top of the left tower are two 156MHz folded dipoles used for VHF marine communications in the Clyde estuary, which went to two old Philips VHF transceivers in the wooden shed you can see in the enclosure behind the fence. That was replaced by a metal shed about ten years ago when I went to site one day and found that a combination of rotten wood and the depredations of rabbits along the bottom of the timbers and a particularly severe gale had made it "sink" by about 18"!
Some of the sites on the west coast were quite interesting, with Mallaig having UHF and VHF links stretching 30 miles or so to remote islands with no real line-of-site. The VHF link to Canna just south of Skye carried a single call on around 80MHz (I think it was 84MHz/76MHz split, can't remember) which fed a tiny ten-line telephone exchange. There was a six-channel link to Rhum, a little to the east, which was a much larger island and had a more conventional exchange.
All gone now, and done using IP over 6 or 24GHz microwave links to little 1U servers in the exchanges.
The service building at the base of the tower was unlocked and empty. With a couple hours of cleaning and mopping, it would have made a fabulous spot for an all-night rave. Alas, the pandemic put all such plans on hold; in the meantime, the site was demolished. There's nothing there now but a gravel patch.
I'm glad to have had the chance to appreciate this bit of infrastructure up close before it vanished.
AT&T "The Far Sound", 1961 [2] From tubes to ... magnetic bubbles? This one shows the microwave network.
Even though an AT&T promo, this is a great documentary. It's an excellent exposé of the forerunner to AT&T's post-War Long Lines—the cable and wireless systems—which was pretty much state of the art before the Long Lines microwave system superseded it.
The doco is particularly useful for historians of tech/communications because it provides an excellent overview of how the US's telephone comms worked by the outbreak of WWII—from the women who ran the network to images of line, terminal and transmitting equipment. It's and excellent snapshot in time.
Two notable things are the huge number of women employed to do the manual routing—Strowger switching being essentially limited to towns and crossbar/long distance dialing yet to be deployed—and the methods used to lay underground cables. Women's employment 80 years ago was very different to today and the film illustrates the fact; on the other hand, there's a remarkable resemblance between the drums of cable and cable laying techniques of 1941 and the laying of fiber optic cable of today—those 1941 cable drums could equally have carried fiber.
Some things never change.
Incidentally, I used to work on microwave television links.
https://sometimes-interesting.com/concrete-arrows-and-the-u-...
In 1962 I had a summer job in engineering with Bell of PA (well before the breakup of AT&T) so I have some knowledge about the long lines network.
Interestingly, at that time AT&T owned many of the local operating companies (e.g., Bell of PA which actually owned Diamond State Telephone Company, the operating company that covered Delaware). There were many other operating companies that were not AT&T owned. IT&T was an example of one that owned lots of local operating companies. How they interfaced, both technically and financially, with AT&T is beyond my knowledge.
Long distance phone calls were routed through a hierarchy of several levels of aggregation, ultimately to something like as few as ten (recall is hazy) top-level centers. These had long distance microwave communication from one to the next, where the call would then be routed downward through its hierarchy to the local destination. I don't remember the specific nomenclature for the top-level centers. One is in downtown Wayne. You can see the tower on Google Maps or Google Earth at West Wayne Avenue and South Bellevue Avenue. It used to have horn antennas, but now has various other dishes and antenna structures.
As I recall, the horn antenna can be thought of as being a sector of a full dish. At the frequencies then in use, the full dish would have been huge, probably too big to put up on a tower. However, for a reasonable signal to noise ratio, the directionality you get with a dish would have been important. So they used part of a dish.
I don't know whether in general, they didn't have hazard lights. I'll have to check some night when I'm out, but I'm pretty sure the one in Wayne does have lights. Both the FCC and the FAA have regulations regarding when lights are required. If I remember from my ham radio days, one of the criteria had to do with the height of the antenna and the distance from an airport.
It's odd to think about how a long-distance phone call was limited before then. We take it now for granted that this is possible, even cell coverage works well in most rural areas in the US now.
You would call the operator, tell them who you wanted to talk to, the city they’re in, and their phone number (if you knew it). If the call is close enough, you’d stay on the line while they connected you. Otherwise, you would be instructed to hang up and wait for an operator to call you back and let you know they’d gotten your party on the line (or not). This could take 5 minutes, or a few hours, depending on the distance, complexity of the route, availability of trunks, etc.
By the 1920s you could even call London from the US, but the radio link only had capacity for like 12 calls and it worked better in the evening. There was also a network of ship-to-shore radiotelephone links that operated on HF and VHF.
It would have been entirely possible, but extraordinarily expensive to have a multi-party conference call between someone in London, someone on a ship in the Pacific, a third person riding a train between New York and Washington DC, and a fourth on a mobile phone in a car in Chicago at least as early as 1947.[1]
I’d ballpark the cost of that call at around $100/min in 1947 dollars.
The Wikipedia article on long distance has an excerpt from Dragnet of a call from LA to a rural party line in Utah. It’s worth a listen.
https://en.m.wikipedia.org/wiki/Long-distance_calling
[1] Transatlantic telephone service began in 1927 between NYC and London, VHF mobile service began in 1946 in St. Louis and then Chicago, telephone service on trains began in 1947 (at least as provided by the Bell system, independents and train companies may have gotten there first), ship-to-shore via AT&T High Seas Service was available by the mid 1930s.
Which turns out to be $1337 in 2022 dollars which seems appropriate.
I never knew the tech behind it until now.
https://en.m.wikipedia.org/wiki/33_Thomas_Street
https://theintercept.com/2016/11/16/the-nsas-spy-hub-in-new-...
The systems I'm familiar with were at temporary "forward air control" posts, where a radar system was installed relatively close to the front lines. The portable microwave transmitters were used to relay the radar data and voice calls back to the generals further back from the front lines.
Why coax cables can't be buried to underground?
0: https://long-lines.net/tech-equip/coaxial/4000mi-trench.html