Telecom Dictionary - Definitions of terms
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T-carrier - Definition and Use
T-carrier - 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 basic unit of the T-carrier system is the DS0, which has a transmission rate of 64 kbit/s, and is commonly used for one voice circuit.
Note 1: The designators for T-carrier in the North American digital hierarchy correspond to the designators for the digital signal (DS) level hierarchy. See the associated table below. Note 2: T-carrier systems were originally designed to transmit digitized voice signals. Current applications also include digital data transmission. Note 3: If an "F" precedes the "T", a fiber optic cable system is indicated at the same rates. Note 4: The table below lists the designators and rates for current T-Carrier systems. Note 5: 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.
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" 64 kbit/s channel, despite the failure of providers to sell such services.
The E-carrier system, where 'E' stands for European, is incompatible 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.
| 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 (32 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 Mbit/s (120 Ch.) | 8.448 Mbit/s (128 Ch.) (E2) |
| Third level | 44.736 Mbit/s (DS3) (672 Ch.) (T3) | 32.064 Mbit/s (480 Ch.) | 34.368 Mbit/s (512 Ch.) (E3) |
| Fourth level | 274.176 Mbit/s (DS4) (4032 Ch.) | 97.728 Mbit/s (1440 Ch.) | 139.268 Mbit/s (2048 Ch.) (E4) |
| Fifth level | 400.352 Mbit/s (5760 Ch.) | 565.148 Mbit/s (8192 Ch.) | 565.148 Mbit/s (8192 Ch.) (E5) |
Note 1: The DS designations are used in connection with the North American hierarchy only. Technically a DS1 is the data carried on a T1 circuit, and likewise for a DS3 and a T3, but the terms are almost always used interchangeably.
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.
See T1 Line.