T-carrier

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Two Network Interface Units, one with a single card, the other with two

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 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.

The E-carrier system, where 'E' stands for European, is incompatible with the T-carrier and is used most places in the world outside of 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 bandwidth comparisons.

1 T1
- 1.1 Historical Note on the 193-bit T1 frame
2 Higher T
3 Digital signal crossconnect
4 Bit robbing
5 Carrier Pricing
6 Notes
7 See also
8 References

[edit] T1

Main article: Digital Signal 1

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. To accommodate loading coils, cable vault manholes were physically 6600 feet apart, and so the optimum 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. The code did not allow an all zero sample, thus avoiding a string of binary zeros which would cause the repeaters to lose bit sync.

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 frequency at which voice communications occurs is at 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).

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 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 bit error 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.

[edit] Historical Note on the 193-bit T1 frame

The 193-bit frame was decided during the early stages of T1 system design back in 1958. 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 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 result in obsolescence of already installed systems.

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.

[edit] 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 MBPS, on those balanced pair cables that could support it. T-2 carried 6.2 MBPS, 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 DS3 and DS4 signals. None of these were widespread, because optical fiber overtook them.

[edit] 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.

[edit] 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. 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.

[edit] 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.

[edit] 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.)	139.264 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.