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In telecommunications, T-carrier is the generic designator for any of several digitally multiplexed telecommunications carrier systems originally developed by Bell Labs and used in North America and Japan.

The E-carrier system, where 'E' stands for European is compatible with the T-carrier and is used just about everywhere else in the world besides North America and Japan. It typically uses the E1 line rate and the E3 line rate. The E2 line rate is less commonly used. See the table below for speed comparisons.

The basic unit of the T-carrier system is the DS0, which has a transmission rate of 64 kbit/ps, and is commonly used for one voice circuit.

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 Mbps 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.

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.

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 30 voice-frequency channels and multiples thereof. See table below.

T-Carrier Systems North American Japanese European (CEPT)
Level zero (Channel data rate) 64 kbit/s (DS0) 64 kbit/s 64 kbit/s
First level 1.544 Mbit/s (DS1) (24 user channels) (T1) 1.544 Mbit/s (24 user channels) 2.048 Mbit/s (30 user channels) (E1)
(Intermediate level, US. hierarchy only) 3.152 Mbit/s (DS1C) (48 Ch.) - -
Second level 6.312 Mbit/s (DS2) (96 Ch.) 6.312 Mbit/s (96 Ch.), or 7.786 Mb/s (120 Ch.) 8.448 Mbit/s (120 Ch.)
Third level 44.736 Mbit/s (DS3) (672 Ch.) (T3) 32.064 Mbit/s (480 Ch.) 34.368 Mbit/s (480 Ch.) (E3)
Fourth level 274.176 Mbit/s (DS4) (4032 Ch.) 97.728 Mbit/s (1440 Ch.) 139.268 Mbit/s (1920 Ch.)
Fifth level 400.352 Mbit/s (5760 Ch.) 565.148 Mbit/s (7680 Ch.) 565.148 Mbit/s (7680 Ch.)

Note 1: The DS designations are used in connection with the North American hierarchy only.

Note 2: There are other data rates in use, e.g., military systems that operate at six and eight times the DS1 rate. At least one manufacturer has a commercial system that operates at 90 Mbit/s, twice the DS3 rate. New systems, which take advantage of the high data rates offered by optical communications links, are also deployed or are under development. Higher data rates are now often achieved by using Synchronous optical networking, SONET or Synchronous digital hierarchy, SDH.

Source: from Federal Standard 1037C and from MIL-STD-188 and FOLDOC