T-carrier

From Academic Kids

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 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. 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/s, and is commonly used for one voice circuit.

Contents

T1

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 synchronization 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.

A more common understanding of how the rate of 1.544 Mbit/s was achieved is as follows. (This explanation grazes over T1 voice communications, and deals mainly with the numbers involved.) Given that the highest frequency at which voice communications occurs is at 4000 Hz, one needs, when converting analog voice to digital data, at least double that frequency for the sample rate. This yields the number 8000. Since each T1 frame contains 1 byte of voice data for each of the 24 channels, that system needs then 8000 frames per second to maintain those 24 simultaneous voice channels. Since each frame of a T1 is 193 bits in length (24 channels X 8 bits per channel + 1 control bit = 193 bits), 8000 frames per second is multiplied by 193 bits to yield a transfer rate of 1.544 Mbit/s (8000 X 193 = 1544000).

Digital signal crossconnect

DS1 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 DSX1 is used. DSX refers to a digital signal crossconnect, and it is essentially a patch panel allowing easy interconnection. When a DS1 leaves the building, it becomes a T1 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.

DS3 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 T3 circuit can only go about 600 feet between repeaters. When a customer orders a DS3, they usually get a (much faster) SONET circuit run into the building and a multiplexer mounted in a big cabinet. The DS3 is delivered in its familiar form, two coax cables with BNC connectors on the ends.

Bit robbing

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.

Notes

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 utilized 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.

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 also

References

This article was originally based on material from the Free On-line Dictionary of Computing, which is licensed under the GFDL.
fr:T-carrier
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