T1 Connection – T1 Carrier

Existing frequency-division multiplexing carrier systems worked well for connections

between distant cities, but required expensive modulators, demodulators and filters for

every voice channel. For connections within metropolitan areas, Bell Labs in the late 1950s

sought cheaper terminal equipment. Pulse-code modulation allowed sharing a coder and

decoder among several voice trunks, so this method was chosen for the T1 system introduced

into local use in 1961. In later decades, the cost of digital electronics declined to the

point that an individual codec per voice channel became commonplace, but by then the other

advantages of digital transmission had become entrenched.

The most common legacy of this system is the line rate speeds. “T1″ now means any data

circuit that runs at the original 1.544 Mbit/s line rate. Originally the T1 format carried

24 pulse-code modulated, time-division multiplexed speech signals each encoded in 64 kbit/s

streams, leaving 8 kbit/s of framing information which facilitates the synchronization and

demultiplexing at the receiver. T2 and T3 circuit channels carry multiple T1 channels

multiplexed, resulting in transmission rates of 6.312 and 44.736 Mbit/s, respectively.

Supposedly, the 1.544 Mbit/s rate was chosen because tests done by AT&T Long Lines in

Chicago were conducted underground. To accommodate loading coils, cable vault manholes were

physically 2000 meter (6,600 ft) apart, and so the optimum bit rate was chosen empirically

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