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The most common legacy of this whole system is the line rate designations. A "T1" now seems to mean 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 synchronisation and demultiplexing at the receiver. T2 and T3 circuit channels carry multiple T1 channels multiplexed, resulting in transmission rates of up to 44.736 Mbit/s.

Supposedly, the 1.544 Mbit/s rate was chosen because tests done by AT&T Long Lines in Chicago were conducted underground, and cable vault manholes were physically 6600 feet apart, and so the optimum rate was chosen empirically-- the speed was increased until the failure rate was unacceptable, then reduced.

DS-1 signals are frequently used to connect equipment within a facility. In this case, a low-level signal (6 volts peak-to-peak differential) called the DSX-1 is used. DSX refers to a digital signal crossconnect, and it is essentially a patch panel allowing easy interconnection. When a DS-1 leaves the building, it becomes a T-1 and is referred to as a span. The signal is boosted to a higher level and superimposed on a DC voltage, enabling repeaters in the field to be powered from the span itself. Repeaters are placed every few thousand feet, to clean up and strengthen the signal.

DS-3 signals are almost exclusively used within buildings, for interconnections and as an intermediate step before being muxed onto a SONET circuit. This is because a T-3 circuit can only go about 600 feet between repeaters. When a customer orders a DS-3, they usually get a (much faster) SONET circuit run into the building and a multiplexer mounted in a big cabinet. The DS-3 is delivered in its familiar form, two coax cables with BNC connectors on the ends.

The T-carrier system traditionally uses in-band signalling or bit robbing, resulting in lower transmission rates than the E-carrier system. This resulted in many US ISDN installations only having an effective data rate of 56 kbit/s over a nominal 64 kbit/s channel. See also A&B. This depends on the framing format used, and almost all systems are now capable of transmitting a "clear" 64kbit/s channel, despite the failure of providers to sell such services.

Note 1: The designators for T-carrier in the North American digital hierarchy correspond to the designators for the digital signal (DS) level hierarchy.

Note 2: T-carrier systems were originally designed to transmit digitized voice signals. Current applications also include digital data transmission.

Note 3: Historically, if an "F" precedes the "T", optical fiber cables are utilised at the same rates.

Note 4: The North American and Japanese hierarchies are based on multiplexing 24 voice-frequency channels and multiples thereof, whereas the European hierarchy is based on multiplexing 32 voice-frequency channels and multiples thereof. See table below.