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

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.

The E-carrier system, where ‘E’ stands for European, is incompatible with the T-carrier (though cross compliant cards exist) and is used in most locations outside of North America, Japan, and Korea. 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 bit rate comparisons.

Existing frequency-division multiplexing carrier systems worked well for connections between distant cities, but required expensive modulators, demodulators and filters for every voice channel. For connections within metropolitan areas, Bell Labs in the late 1950s sought cheaper terminal equipment. Pulse-code modulation allowed sharing a coder and decoder among several voice trunks, so this method was chosen for the T1 system introduced into local use in 1961. In later decades, the cost of digital electronics declined to the point that an individual codec per voice channel became commonplace, but by then the other advantages of digital transmission had become entrenched.

The most common legacy of this system is the line rate speeds. “T1″ now means 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 6.312 and 44.736 Mbit/s, respectively.

Supposedly, the 1.544 Mbit/s rate was chosen because tests done by AT&T Long Lines in Chicago were conducted underground. To accommodate loading coils, cable vault manholes were physically 2000 meter (6,600 ft) apart, and so the optimum bit rate was chosen empirically — the capacity was increased until the failure rate was unacceptable, then reduced to leave a margin. Companding allowed acceptable audio performance with only seven bits per PCM sample in this original T1/D1 system. The later D3 and D4 channel banks had an extended frame format, allowing eight bits per sample, reduced to seven every sixth sample or frame when one bit was “robbed” for signaling the state of the channel. The standard does not allow an all zero sample which would produce a long string of binary zeros and cause the repeaters to lose bit sync. However, when carrying data (Switched 56) there could be long strings of zeroes, so one bit per sample is set to “1″ (jam bit 7) leaving 7 bits x 8,000 frames per second for data.

A more common understanding of how the rate of 1.544 Mbit/s was achieved is as follows. (This explanation glosses over T1 voice communications, and deals mainly with the numbers involved.) Given that the highest voice frequency which the telephone system transmits is 4,000 Hz, the required digital sampling rate is 8,000 Hz (see Nyquist rate). Since each T1 frame contains 1 byte of voice data for each of the 24 channels, that system needs then 8,000 frames per second to maintain those 24 simultaneous voice channels. Because each frame of a T1 is 193 bits in length (24 channels X 8 bits per channel + 1 framing bit = 193 bits), 8,000 frames per second is multiplied by 193 bits to yield a transfer rate of 1.544 Mbit/s (8,000 X 193 = 1,544,000).

Initially, T1 used Alternate Mark Inversion (AMI) to reduce frequency bandwidth and eliminate the DC component of the signal. Later B8ZS became common practice. For AMI, each mark pulse had the opposite polarity of the previous one and each space was at a level of zero, resulting in a three level signal which however only carried binary data. Similar British 23 channel systems at 1.536 Mbaud in the 1970s were equipped with ternary signal repeaters, in anticipation of using a 3B2T or 4B3T code to increase the number of voice channels in future, but in the 1980s the systems were merely replaced with European standard ones. American T-carriers could only work in AMI or B8ZS mode.

The AMI or B8ZS signal allowed a simple error rate measurement. The D bank in the central office could detect a bit with the wrong polarity, or “bipolarity violation” and sound an alarm. Later systems could count the number of violations and reframes and otherwise measure signal quality and allow a more sophisticated alarm indication signal system.

Historical Note on the 193-bit T1 frame

The decision to use a 193-bit frame was made in 1958, during the early stages of T1 system design. To allow for the identification of information bits within a frame, two alternatives were considered. Assign (a) just one extra bit, or (b) additional 8 bits per frame. The 8-bit choice is cleaner, resulting in a 200-bit frame, 25 8-bit channels, of which 24 are traffic and 1 8-bit channel available for operations, administration, and maintenance (OA&M). AT&T chose the single bit per frame not to reduce the required bit rate (1.544 vs 1.6 Mbit/s), but because AT&T Marketing worried that “if 8 bits were chosen for OA&M function, someone would then try to sell this as a voice channel and you wind up with nothing.”

Soon after commercial success of T1 in 1962, the T1 engineering team realized the mistake of having only one bit to serve the increasing demand for housekeeping functions. They petitioned AT&T management to change to 8-bit framing. This was flatly turned down because it would make installed systems obsolete.

Having this hindsight, some ten years later, CEPT chose 8 bits for framing the European E1.

Digital signal crossconnect

DS1 signals are interconnected typically at Central Office locations at a common metallic cross-connect point known as a DSX-1. A DS1 signal at a DSX-1 is measured typically at 6 Volts Peak-to-peak (0dBdsx signal level at 772 kHz Nyquist) at plus or minus 1.2 volts to permit easy interconnection of DS1 equipment NCI Code=04DS9/ /). When a DS1 is transported over metallic outside plant cable, the signal travels over conditioned cable pairs known as a T1 span. A T1 span can have up to -130 Volts of DC power superimposed on the associated four wire cable pairs to line or “Span” power line repeaters, and T1 NIU’s (T1 Smartjacks). T1 span repeaters are typically engineered up to 6,000 feet apart, depending on cable gauge, and at no more than 36 dB of loss before requiring a repeated span. There can be no cable bridge taps across any pairs.

T1 copper spans are being replaced by optical transport systems, but if a copper (Metallic) span is used, the T1 is typically carried over an HDSL encoded copper line. Four wire HDSL does not require as many repeaters as conventional T1 spans. Newer two wire HDSL (HDSL-2) equipment transports a full 1.54400 Mbit/s T1 over a single copper wire pair up to approximately twelve thousand (12,000) feet (3.5 km), if all 24 gauge cable is used. HDSL-2 does not employ repeaters as does conventional four wire HDSL, or newer HDSL-4 systems.

One advantage of HDSL is its ability to operate with a limited number of bridge taps, with no tap being closer than 500 feet from any HDSL transceiver. Both two or four wire HDSL equipment transmits and receives over the same cable wire pair, as compared to conventional T1 service that utilizes individual cable pairs for transit or receive.

DS3 signals are rare except within buildings, where they are used 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 (180m) between repeaters. A customer who orders a DS3 usually receives a SONET circuit run into the building and a multiplexer mounted in a utility box. The DS3 is delivered in its familiar form, two coax cables (1 for send and 1 for receive) with BNC connectors on the ends.

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