T1: A Survival Guide

1951-1970:The Birth of T-carrier

It may not have been obvious at the time, but the telephone network was on the cusp of a fundamental change driven by several factors. Early long-distance connections, especially those across an ocean, were full of static and could fade quite abruptly. Development of appropriate cabling would eliminate the problems caused by the use of radio waves, but noise and distortion of the analog signal across such great distances posed a large problem. Postwar prosperity in the U.S. dramatically increased the demand for telephone service, bringing with it millions of new telephone users and requiring ever more operators to connect calls. Better means of providing service had to be found. Fortunately, two new technologies came to the rescue.

The Transistor and Computerization

The transistor, invented in 1947 at Bell Labs, made it possible to construct sophisticated electronics. Today’s semiconductor and computer industries owe their existences to the transistor. Transistors also made AT&T’s “Electronic Switching System” possible. In the early 1960s, AT&T built and field-tested specialized, programmable devices for controlling the telephone network. These devices, of course, were computers, but the terms of the 1956 consent decree prohibited AT&T from manufacturing computers. AT&T followed the time-honored tradition of redefining terms in the argument. AT&T deployed the first such device, the No. 1 ESS, in New Jersey in 1962 following a field trial.^[3] Electronic switching made it possible to run the telephone network without having to dramatically increase the number of operators because the previous mundane tasks of taking numbers and connecting calls could be relegated to the phone switches. Electronic switching also made it feasible to build a telephone network with enough call-routing capability to eliminate operator intervention in connecting long-distance calls; by 1965, 90% of telephones in the U.S. were directly dialing long-distance numbers.

Digitalization

In parallel with computerization, Bell Labs researchers were investigating digital voice transmission. Previous efforts had begun before World War II and resumed in earnest in the 1950s. Prior to digital telephone networks, the trunk lines running between switching offices transmitted analog phone calls using frequency division multiplexing (FDM). FDM divides up the allocated resources into frequency bands and transmits each conversation continuously in its assigned frequency band, as shown in Figure 1-1.

Figure 1-1. FDM versus TDM

Digital networks, by contrast, use a technique called time division multiplexing (TDM). FDM is a natural fit for analog signals, and TDM has a similar relationship to digital signals. In practice, nothing prohibits the use of FDM gear for digital networks or TDM gear for analog networks—those combinations are just more expensive than the natural fits. TDM divides the resources into time slots and transmits the time slots in turn, using the full capacity of the channel. Increased capacity was a major motivation for migrating the phone system to digital transmission. Each analog trunk could handle 12 calls, but digitized trunks, which used the same transmission facilities (wires), could handle 24 calls. Adding trunk lines took time and money—and lots of both. To add a trunk line, AT&T needed to evaluate potential routes, survey the chosen route, and acquire the right of way.

For overworked planners coping with mushrooming demand for telephone services, digitized trunks were perfect. In the early 1960s, AT&T began moving the U.S. phone network to digital carrier systems. In this context, carrier is the term telephone companies use to refer to trunk systems that can accommodate multiple calls. The U.S. digital hierarchy became known as T-carrier when Bell Labs engineers used the letter T for the digital carrier systems they were developing. There is no official significance to the letter T, though some sources suggest it is short for terrestrial. (Other systems in development in that era were the N- and L-carrier systems.) In the T-carrier hierarchy, the best-known members are T1 and T3.

Synchronous and Asynchronous Communications

One item of note about the T-carrier system is that it is synchronous. In synchronous communications, the data is sent as a stream. In contrast, asynchronous communications, such as dial-up modems, must have a method of informing the remote end when data will arrive and when the data transmission has finished.

Asynchronous communications are easier to implement in hardware because all that is necessary is to add start and stop sequences to the transmission and reception units. Synchronous communications are far more complex, as they transmit data as a continuous stream. For the remote end to pick out the data bits, a clocking signal is required. Furthermore, the two clocks must agree on when to pull a data bit out of the stream. Obtaining accurate clocking signals at high data rates and long distances, a major challenge for the T-carrier system, is explored in later chapters.

Like all technology decisions, choosing synchronous or asynchronous timing involves a trade-off. Synchronous communications have less overhead on the wire and can run at higher speeds, but require the maintenance of a clock signal and the associated electronics to recover the signal and extract the bit stream from the incoming transmission.

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^[3] Many enhanced services that we take for granted today, such as touch-tone dialing, call waiting, and call forwarding, were made possible by the 1ESS. AT&T originally unveiled the ESS-enhanced telephone network in 1962 at the World’s Fair in Seattle.