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T-carrier is a time-division multiplexed (TDM) digital signal that has been used by the telephone companies for many decades. The idea being that in order to communicate a voice signal, it is not necessary to transport the entire analog signal, the wave is sampled at the relatively slow rate of 8000 samples per second, and then digitized into an 8 bit number.

time-division multiplexed t-carrier

24 of these 8-bit signals (called a DSO) are time-division multiplexed into a 1.544 Mbps t-carrier signal for transmission over telephone lines. At the other end of the line the 24 samples are de-multiplexed and each 8-bit sample decoded to produce an analog level. With proper smoothing of the signal, the original voice signal is re-created.

For voice communication, only 7-bits are used to code the signal, the 8th-bit being used to carry signaling information (e.g. ready, ring, busy tone). For data communications the entire 8-bit signal is used.

Multiple T1 streams can be further multiplexed onto higher carriers. A T2 circuit is 4 T1 circuits multiplexed, resulting in a transmission rate of 6.312 Mbps. A T3 circuit is 18 T1 circuits multiplexed, resulting in a transmission rate of 44.736 Mbps. A T4 circuit is 168 T1 circuits multiplexed, resulting in a transmission rate of 274.176 Mbps.

T1 streams can be multiplexed to provide networks with high-bandwidth, permanent WAN connections between sites. If such a high-bandwidth is not needed, a customer does not need to purchase all 24 of the T1 DS0 circuits. A low cost option is to purchase only the number of circuits required, called "fractional T1 service".

Although T1 was introduced in 1962, it was so efficient that a huge amount of it was installed. And because of its reasonable cost and great flexibility, much of it is still in use today.

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