Information about T Carrier

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 and is used at most places in the world 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 bandwidth comparisons.

T1

Main article: Digital Signal 1

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 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 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 6600 feet 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 8000 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 4000 Hz, the required digital sampling rate is 8000 Hz (see Nyquist rate). 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. Because each frame of a T1 is 193 bits in length (24 channels X 8 bits per channel + 1 framing 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)

(Note that communications rates are often quoted in literal kilo- and megabits, rather than the kibibits and mebibits often used to describe computer parts)

Initially, T1 used Alternate Mark Inversion (AMI) to reduce bandwidth and eliminate the DC component of the signal. Later B8ZS became common practice. For AMI, each pulse had the opposite polarity of the previous one, 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.

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). The single bit per frame was chosen, not because a single bit saves bandwidth (by a trivial amount 1.544 vs 1.6 Mbit/s), but from a comment from AT&T Marketing. They claim 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. And sure enough, those 8 bits these days are often sold to customers, resulting in losses on OA&M functions.

Higher T

In the late 1960s and early 1970s Bell Labs developed higher rate systems. T-1C with a more sophisticated modulation scheme carried 3 Mbit/s, on those balanced pair cables that could support it. T-2 carried 6.312 Mbit/s, requiring a special low-capacitance cable. This was standard for Picturephone. T-4 and T-5 used coaxial cables, similar to the old L carriers used by AT&T Long Lines. TD microwave radio systems were also fitted with high rate modems to allow them to carry a DS1 signal in a portion of their FM spectrum that had too poor quality for voice service. Later they carried DS3 and DS4 signals. None of these were widespread, because optical fiber and SONET overtook them.

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 (1 for send and 1 for receive) with BNC connectors on the ends.

Bit robbing

Main article: Robbed bit signaling

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 (Disambiguation). 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.

Carrier Pricing

Carriers price DS1 lines in many different ways. However, most boil down to two simple components; local loop (the cost the local incumbent charges to transport the signal from the end user's central office, otherwise known as a CO, to the point of presence, otherwise known as a POP, of the carrier) and the port (the cost to access the telephone network or the Internet through the carrier's network). Typically, the port price is based upon access speed and yearly commitment level while the loop is based on geography. The further the CO and POP, the more the loop cost.

The loop price has several components built into it, including the mileage calculation (performed in V/H coordinates, not standard GPS coordinates) and the telco piece. Each local Bell operating company - namely Verizon, AT&T, and Qwest - charge T-carriers different price per mile rates. Therefore, the price calculation has two distance steps: geomapping and the determination of local price arrangements.

For voice DS1 lines, the calculation is mostly the same, except that the port (required for Internet access) is replaced by LDU (otherwise known as Long Distance Usage). Once the price of the loop is determined, only voice-related charges are added to the total. In short, the total price = loop + LDU x minutes used.

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 and E-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.)	139.264 Mbit/s (2048 Ch.) (E4)
Fifth level	400.352 Mbit/s (DS5) (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.

Note 3: A DS3 is delivered native on a copper trunk. DS3 may be converted to a fiber run when needing longer distances between termination points. When a DS3 is delivered over fiber it is still an analog type trunk connection at the termination points. When delivering data over an OC3 or greater SONET is used. SONET is much faster and has a much lower bit error rate.

See also

• Digital Signal 0 (DS0)
• Digital Signal 1 (DS1)
• DS1 Encoding schemes: B8ZS, HDB3, AMI
• Optical Carrier (OC-n)
• Time-division multiplexing
• Multiplexing
• Plesiochronous Digital Hierarchy
• List of device bandwidths
• Modified AMI code
• E-carrier
• STM-1

References

• ANSI T1.403-1999 - Network and Customer Installation Interface
• Federal Standard 1037C
• T1: A Survival Guide Chapter 5
• Cisco T1 card documentation
• T1 architecture

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