T1, Where Does the "T" Come From?
Some Bell Labs History from Dr. John Pan

Abstract: After reading our white paper, a number of customers asked us "where'd the "T" come from in T-1, T-3, etc.? There are many stories floating around (such as T=Time, T= Transmission, T=Terrestrial, Just the next letter in sequence, etc.). We thought the best place to get the true story would be from someone who helped develop this technology. So, we asked our friend Dr John Pan, who worked for Bell Labs during the time T carrier was being developed. Dr Pan is now Vice President of Loop Telecommunications International, our strategic partner in the T-1 DACS, and DSU/access device field since 1997.

Here's what he had to say...


The "T" in T1

The story of the "T" in T1 has its roots way back in 1917, when AT&T deployed the first carrier system, called the "A" system. A total of 7 A-systems, providing four voice channels over an open wire pair, were ever deployed. Then came successive analog frequency division multiplex systems named B, C, D, and so forth. Few of these carrier systems ever saw commercial service. AT&T, being a monopoly, could well afford many dogs. A notable success is the "L" system, providing 600 (L1) and later 1800 (L3) voice channels over a pair of coaxial cables, in long haul service from 1944 to 1984, until breakup of the Bell System forced AT&T to migrate to optical fiber. The last of the analog carrier system is the "N" system and its variants, providing 12 voice channels for intracity short haul. Along with the even more forgettable "O", "P", and "U" systems, the emergence of "T" killed them all.

In 1957, when digital systems were first proposed and developed, the boss decided to skip Q, R, S, and to use T, for Time Division. The idea was this will be the world's first time division system. Interestingly, except for "U", another system that never made it, this naming system ended.

Vaiants of T1, called T1C, T2, T3, and T4, all died. They are survived by signals that would have been carried on all these systems, called DS1, DS2, DS3, and DS4.

Among the successor to T1 vying for success at Bell Labs, digital coaxial cables, digital microwave, satellite, circular waveguide, optical mirror, and optical fiber, none achieved commercial success save fiber.


You can also check out our All You Wanted to Know About T1 But Were Afraid to Ask tutorial, which is a complete explanation of T1 in all its gory details from basics to signaling (written for our distributors and rather deep). We have a number of T1 products including DSU/CSUs, Mini-DACs, T-Span repeaters/extender, channel banks, and Intergrated Access Units. More information is available here.

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