Few aspects of commercial telecommunications have quite the allure of the T-carrier. Well, to me, at least, but then I have very specific interests.
T-carrier has this odd, enduring influence on discussion of internet connections. I remember that for years, some video game installers (perhaps those using Gamespy?) used to ask what kind of internet service you had, with T1 as the "highest" option. The Steam Hardware Survey included T1 among the options for a long time. This was odd, in a lot of ways. It set T1 as sort of the "gold standard" in the minds of gamers, but residential internet service over T1 would have been very rare. Besides, even by the standards of the 2000s T1 service was actually pretty slow.
Still, T1 involved a healthy life as an important "common denominator" in internet connectivity. As a regulated telephone service, it was expensive, but available pretty much anywhere. It also provided a very high standard for reliability and latency, beyond many of the last-mile media we use today.
Telephone Carriers
We think of telephone calls as being carried over a pair of wires. In the early days of the telephone system, it wasn't all that far off to imagine a phone call as a single long circuit of two wires that extended all the way from your telephone to the phone you had called. This was the most naive and straightforward version of circuit switching: connections were established by creating a circuit.
The era of this extremely literal form of circuit switching did not last as long as you might think. First, we have to remember that two-wire telephone circuits don't actually work that well. Low install cost and convenience means that they are the norm between a telephone exchange and its local callers, but for long-distance carriage over the phone network, you get far better results by splitting the "talk" and "listen" sides into two separate pairs. This is called a four-wire telephone circuit, and while you will rarely see four-wire service at a customer's premises, almost all connectivity between telephone exchanges (and even in the internals of the telephone exchange itself) has been four-wire since the dawn of long-distance service.
Four-wire circuits only exacerbated an obvious scaling problem: in the long distance network, you have connections called a toll leads between two exchanges. In a very simple case, two towns might have a toll lead between them. For simple four-wire telephone lines, that toll lead needs four wires for each channel. If it has four wires, only one phone call can take place between the towns at a time. If it has eight wires, two telephone calls. This got very expensive very fast, considering that even heavily-built four-crossarm open wire routes might only have a capacity for eight simultaneous calls.
For obvious reasons, research in the telephone industry focused heavily on ways to combine more calls onto fewer wires. Some simple electrical techniques could be used, like phantoms that combined two underlying pairs into a single additional "phantom" pair for a 50% increase in capacity. You could extend this technique to create more channels, with a noticeable loss in quality.
By the 1920s, the Bell System relied on a technique that we would later call frequency division multiplexing (FDM). By modulating a phone call onto a higher-frequency carrier, you can put it over the same wires as other phone calls modulated onto different frequencies. The devices that actually did this combined multiple channels onto a single line, so they were known as channel banks. The actual formats they used over the wire, since they originally consisted of modulation onto a carrier, came to be known themselves as carriers. AT&T identified the carriers they developed with simple sequential letters. In the 1940s, the state of the art was up to J- and K-carrier, which allowed 16 channels on a four-wire circuit (over open wire and multipair cable, respectively). A four-crossarm open-wire circuit, with sixteen pairs, could support 256 unidirectional circuits for 128 channels---or simultaneous phone calls.
FDM carriers reached their apex with the coaxial-cable based L-carrier and and microwave radio TH and TD carriers , which combined channels into groups, groups into supergroups, and supergroups into mastergroups for total capacities that reached into thousands of channels. Such huge FDM groups required very large bandwidths, though, which could not be achieved over copper pairs.
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