T-carrier
Encyclopedia
Telecommunication
Telecommunication is the transmission of information over significant distances to communicate. In earlier times, telecommunications involved the use of visual signals, such as beacons, smoke signals, semaphore telegraphs, signal flags, and optical heliographs, or audio messages via coded...
s, T-carrier, sometimes abbreviated as T-CXR, is the generic designator for any of several digitally multiplexed
Multiplexing
The multiplexed signal is transmitted over a communication channel, which may be a physical transmission medium. The multiplexing divides the capacity of the low-level communication channel into several higher-level logical channels, one for each message signal or data stream to be transferred...
telecommunications carrier system
Carrier system
In telecommunication, a carrier system is a multichannel telecommunications system in which a number of individual channels are multiplexed for transmission...
s originally developed by Bell Labs
Bell Labs
Bell Laboratories is the research and development subsidiary of the French-owned Alcatel-Lucent and previously of the American Telephone & Telegraph Company , half-owned through its Western Electric manufacturing subsidiary.Bell Laboratories operates its...
and used in North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
, Japan
Communications in Japan
-Telephone:Telephones and ISDN - main lines in use: 52.3981 million IP phone lines in use: 16.766 million Mobile and PHS lines in use: 105.297 million -Mobile phone:...
, and South Korea
Communications in South Korea
Communications services improved dramatically in the 1980s with the assistance of foreign partners and as a result of the development of the electronics industry...
.
The basic unit of the T-carrier system is the DS0
Digital Signal 0
Digital Signal 0 is a basic digital signaling rate of 64 kbit/s, corresponding to the capacity of one voice-frequency-equivalent channel...
, which has a transmission rate of 64 kbit/s
Data rate units
In telecommunications, bit rate or data transfer rate is the average number of bits, characters, or blocks per unit time passing between equipment in a data transmission system. This is typically measured in multiples of the unit bit per second or byte per second.- Avoiding confusion :To be as...
, and is commonly used for one voice circuit.
The E-carrier
E-carrier
In digital telecommunications, where a single physical wire pair can be used to carry many simultaneous voice conversations by time-division multiplexing, worldwide standards have been created and deployed...
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
Bit rate
In telecommunications and computing, bit rate is the number of bits that are conveyed or processed per unit of time....
comparisons.
T1
Existing frequency-division multiplexingFrequency-division multiplexing
Frequency-division multiplexing is a form of signal multiplexing which involves assigning non-overlapping frequency ranges to different signals or to each "user" of a medium.- Telephone :...
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
Bell Labs
Bell Laboratories is the research and development subsidiary of the French-owned Alcatel-Lucent and previously of the American Telephone & Telegraph Company , half-owned through its Western Electric manufacturing subsidiary.Bell Laboratories operates its...
in the late 1950s sought cheaper terminal equipment. Pulse-code modulation
Pulse-code modulation
Pulse-code modulation is a method used to digitally represent sampled analog signals. It is the standard form for digital audio in computers and various Blu-ray, Compact Disc and DVD formats, as well as other uses such as digital telephone systems...
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
Codec
A codec is a device or computer program capable of encoding or decoding a digital data stream or signal. The word codec is a portmanteau of "compressor-decompressor" or, more commonly, "coder-decoder"...
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
Pulse-code modulation
Pulse-code modulation is a method used to digitally represent sampled analog signals. It is the standard form for digital audio in computers and various Blu-ray, Compact Disc and DVD formats, as well as other uses such as digital telephone systems...
, time-division multiplexed
Time-division multiplexing
Time-division multiplexing is a type of digital multiplexing in which two or more bit streams or signals are transferred apparently simultaneously as sub-channels in one communication channel, but are physically taking turns on the channel. The time domain is divided into several recurrent...
speech signals each encoded in 64 kbit/s streams, leaving 8 kbit/s of framing information
Frame synchronization
While receiving a stream of framed data, frame synchronization is the process by which incoming frame alignment signals, i.e., distinctive bit sequences , are identified, i.e., distinguished from data bits, permitting the data bits within the frame to be extracted for decoding or retransmission...
which facilitates the synchronization and demultiplexing at the receiver. The T2 and T3 circuit channels carry multiple T1 channels multiplexed, resulting in transmission rates of 6.312 and 44.736 Mbit/s, respectively. A T3 line comprises 28 T1 lines, each operating at total signaling rate of 1.544 Mbps. It is possible to get a fractional T3 line, meaning a T3 line with some of the 28 lines turned off, resulting in a slower transfer rate but typically at reduced cost.
Supposedly, the 1.544 Mbit/s rate was chosen because tests done by AT&T Long Lines in Chicago
Chicago
Chicago is the largest city in the US state of Illinois. With nearly 2.7 million residents, it is the most populous city in the Midwestern United States and the third most populous in the US, after New York City and Los Angeles...
were conducted underground. The test site was typical of Bell System outside plant
Outside plant
In telecommunication, the term outside plant has the following meanings:*In civilian telecommunications, outside plant refers to all of the physical cabling and supporting infrastructure , and any associated hardware located between a demarcation point in a switching facility and a demarcation...
of the time in that, to accommodate loading coil
Loading coil
In electronics, a loading coil or load coil is a coil that does not provide coupling to any other circuit, but is inserted in a circuit to increase its inductance. The need was discovered by Oliver Heaviside in studying the disappointing slow speed of the Transatlantic telegraph cable...
s, cable vault manholes were physically 2000 meter (6,600 ft) apart, which determined the repeater spacing. The optimum bit rate
Bit rate
In telecommunications and computing, bit rate is the number of bits that are conveyed or processed per unit of time....
was chosen empirically
Empiricism
Empiricism is a theory of knowledge that asserts that knowledge comes only or primarily via sensory experience. One of several views of epistemology, the study of human knowledge, along with rationalism, idealism and historicism, empiricism emphasizes the role of experience and evidence,...
— the capacity was increased until the failure rate was unacceptable, then reduced to leave a margin. Companding
Companding
In telecommunication, signal processing, and thermodynamics, companding is a method of mitigating the detrimental effects of a channel with limited dynamic range...
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 detailed understanding of how the rate of 1.544 Mbit/s was divided into channels is as follows. (This explanation glosses over T1 voice communications, and deals mainly with the numbers involved.) Given that the telephone system nominal voiceband
Voiceband
In electronics, voiceband means the typical human hearing frequency range that is from 20 Hz to 20 kHz. In telephony, it means the frequency range normally transmitted by a telephone line, generally about 200–3600 Hz. Frequency-division multiplexing in telephony normally uses...
(including guardband
Guard band
-Radio and electronic signalling:In radio, a guard band is an unused part of the radio spectrum between radio bands, for the purpose of preventing interference....
) is 4,000 Hz
Hertz
The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....
, the required digital sampling rate is 8,000 Hz (see Nyquist rate
Nyquist rate
In signal processing, the Nyquist rate, named after Harry Nyquist, is two times the bandwidth of a bandlimited signal or a bandlimited channel...
). 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
Direct current
Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...
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
Ternary signal
In telecommunication, a ternary signal is a signal that can assume, at any given instant, one of three states or significant conditions, such as power level, phase position, pulse duration, or frequency....
repeaters, in anticipation of using a 3B2T or 4B3T
4B3T
4B3T stands for 4 Binary 3 Ternary, a line encoding scheme used for ISDN BRI interface. 4B3T represents four binary bits using three pulses.It uses three states:* + ,* 0 , and* − ....
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
Bipolar violation
A bipolar violation, bipolarity violation, or BPV, is a violation of the bipolar encoding rules where two pulses of the same polarity occur without an intervening pulse of the opposite polarity. This indicates an error in the transmission of the signal.T-carrier and E-carrier signals are...
" 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
Alarm indication signal
Alarm indication signal is a signal transmitted by an intermediate element of a multi-node transport circuit that is part of a concatenated telecommunications system to alert the receiving end of the circuit that a segment of the end-to-end link has failed at a logical or physical level, even if...
system.
Historical note on the 193-bit T1 frame
The decision to use a 193-bit frame was made 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 channelsTime division multiple access
Time division multiple access is a channel access method for shared medium networks. It allows several users to share the same frequency channel by dividing the signal into different time slots. The users transmit in rapid succession, one after the other, each using its own time slot. This...
, of which 24 are traffic and 1 8-bit channel available for operations, administration, and maintenance (OA&M
OA&M
Operations, administration and management or operations, administration and maintenance is a general term used to describe the processes, activities, tools, standards, etc involved with operating, administering, managing and maintaining any system...
). 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
European Conference of Postal and Telecommunications Administrations
The European Conference of Postal and Telecommunications Administrations was established on June 26, 1959, as a coordinating body for European state telecommunications and postal organizations...
chose 8 bits for framing the European E1
E-carrier
In digital telecommunications, where a single physical wire pair can be used to carry many simultaneous voice conversations by time-division multiplexing, worldwide standards have been created and deployed...
.
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 with foam insulation. This was standard for PicturephoneVideophone
A videophone is a telephone with a video screen, and is capable of full duplex video and audio transmissions for communication between people in real-time...
. T-4 and T-5 used coaxial cables, similar to the old L-carrier
L-carrier
SystemYearFrequencyCoax per cableDistance between repeatersVoice circuits per coax tubeL-119413 MHz48 miles600L-21942840 kHz416 miles360L-319508 MHz84 miles1,860L-4196717 MHz202 miles3,600L-5197257 MHz221 mile10,800L-5E197566 MHz221 mile13,200...
s used by AT&T Long Lines. TD microwave radio relay 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. During the 80's companies such as RLH Industries, Inc. developed T1 over optical fiber. The industry soon developed and evolved with multiplexed T1 transmission schemes.
Digital signal crossconnect
DS1Digital Signal 1
Digital signal 1 is a T-carrier signaling scheme devised by Bell Labs. DS1 is a widely used standard in telecommunications in North America and Japan to transmit voice and data between devices. E1 is used in place of T1 outside North America, Japan, and South Korea...
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
Outside plant
In telecommunication, the term outside plant has the following meanings:*In civilian telecommunications, outside plant refers to all of the physical cabling and supporting infrastructure , and any associated hardware located between a demarcation point in a switching facility and a demarcation...
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 6000 feet (1,828.8 m) 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 tap
Bridge tap
Bridged tap or bridge tap is a long-used method of cabling for telephone lines. One cable pair will "appear" in several different terminal locations . This allows the telephone company to use or "assign" that pair to any subscriber near those terminal locations. Once that customer disconnects,...
s 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 1.544 Mbit/s T1 over a single copper wire pair up to approximately twelve thousand (12,000) feet (3.5 km), if all 24 gauge
American wire gauge
American wire gauge , also known as the Brown & Sharpe wire gauge, is a standardized wire gauge system used since 1857 predominantly in the United States and Canada for the diameters of round, solid, nonferrous, electrically conducting wire...
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 (152.4 m) 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 transmit or receive.
DS3 signals are rare except within buildings, where they are used for interconnections and as an intermediate step before being muxed
Multiplexing
The multiplexed signal is transmitted over a communication channel, which may be a physical transmission medium. The multiplexing divides the capacity of the low-level communication channel into several higher-level logical channels, one for each message signal or data stream to be transferred...
onto a SONET
Synchronous optical networking
Synchronous Optical Networking and Synchronous Digital Hierarchy are standardized multiplexing protocols that transfer multiple digital bit streams over optical fiber using lasers or highly coherent light from light-emitting diodes . At low transmission rates data can also be transferred via an...
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
Multiplexing
The multiplexed signal is transmitted over a communication channel, which may be a physical transmission medium. The multiplexing divides the capacity of the low-level communication channel into several higher-level logical channels, one for each message signal or data stream to be transferred...
mounted in a utility box. The DS3 is delivered in its familiar form, two coax
Coaxial cable
Coaxial cable, or coax, has an inner conductor surrounded by a flexible, tubular insulating layer, surrounded by a tubular conducting shield. The term coaxial comes from the inner conductor and the outer shield sharing the same geometric axis...
cables (1 for send and 1 for receive) with BNC connector
BNC connector
The BNC connector ' is a common type of RF connector used for coaxial cable. It is used with radio, television, and other radio-frequency electronic equipment, test instruments, video signals, and was once a popular computer network connector. BNC connectors are made to match the characteristic...
s on the ends.
Reference: ANSI T1.403//ANSI T1.231//ANSI T1.404//ANSI T1.510.
Bit robbing
Twelve DS1 frames make up a single T1 Superframe (T1 SF). Each T1 Superframe is composed of two signaling frames. All T1 DS0 channels that employ in-band signaling will have its eighth bit over written, or "robbed" from the full 64 kbit/s DS0 payload, by either a logical ZERO or ONE bit to signify a circuit signaling state or condition. Hence robbed bit signaling will restrict a DS0 channel to a rate of only 56 kbit/s during two of the twelve DS1 frames that make up a T1 SF framed circuit. T1 SF framed circuits yield two independent signaling channels (A&B) T1 ESF framed circuits four signaling frames in a twenty four frame extended frame format that yield four independent signaling channels (A, B,C,& D).NOTE: 56 kbit/s DS0 channels are associated with digital data service (DDS) services typically do not utilize the eighth bit of the DS0 as voice circuits that employ A&B out of band signaling. One exception is Switched 56 kbit/s DDS. In DDS, bit eight is used to identify DTE
Data terminal equipment
Data Terminal Equipment is an end instrument that converts user information into signals or reconverts received signals. These can also be called tail circuits. A DTE device communicates with the data circuit-terminating equipment...
request to send (RTS) condition. With Switched 56 DDS, bit eight is pulsed (alternately set to logical ZERO and ONE) to transmit two state dial pulse signaling information between a SW56 DDS CSU/DSU
CSU/DSU
A CSU/DSU is a digital-interface device used to connect a Data Terminal Equipment device or DTE, such as a router, to a digital circuit, such as a T1 line. The CSU/DSU implements two different functions...
, and a digital end office switch.
The use of robbed-bit signaling in North America has decreased significantly as a result of Signaling System No 7 (SS7) on inter-office dial trunks. With SS7, the full 64 kbit/s DS0 channel is available for use on a connection, and allows 64 kbit/s, and 128 kbit/s ISDN
Integrated Services Digital Network
Integrated Services Digital Network is a set of communications standards for simultaneous digital transmission of voice, video, data, and other network services over the traditional circuits of the public switched telephone network...
data calls to exist over a switched trunk network connection if the supporting T1 carrier entity is optioned B8ZS (Clear Channel Capable).
REFERENCES:
ANSI T1.403//The Book On ESF, Verilink Corporation, 1986//D4 Digital Channel Bank Family, Bell System Technical Journal, Nov 1982
Carrier pricing
Carriers price DS1 lines in many different ways. However, most boil down to two simple components; local loopLocal loop
In telephony, the local loop is the physical link or circuit that connects from the demarcation point of the customer premises to the edge of the carrier or telecommunications service provider's network...
(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 farther the CO and POP, the more the loop costs.
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
AT&T
AT&T Inc. is an American multinational telecommunications corporation headquartered in Whitacre Tower, Dallas, Texas, United States. It is the largest provider of mobile telephony and fixed telephony in the United States, and is also a provider of broadband and subscription television services...
, and Qwest
Qwest
Qwest Communications International, Inc. was a large United States telecommunications carrier. Qwest provided local service in 14 western U.S. states: Arizona, Colorado, Idaho, Iowa, Minnesota, Montana, Nebraska, New Mexico, North Dakota, Oregon, South Dakota, Utah, Washington, and Wyoming.On April...
- charge T-carriers different price per mile rates. Therefore, the price calculation has two distance steps: geomapping and the determination of local price arrangements.
While most carriers utilize a geographic pricing model as described above, some Competitive Local Exchange Carriers (CLECs), such as Covad
Covad
Covad Communications Group was an American provider of broadband voice and data communications. By 2006, the company had 530,000 subscribers, and ranked 16th largest ISP in the United States. Covad was acquired by U.S...
, Integra Telecom
Integra Telecom
Integra Telecom is a privately owned, facilities-based, integrated telecommunications carrier and communications equipment provider headquartered in Portland, Oregon, United States. Founded in 1996, the company is one of the largest competitive local exchange carriers in the United States and...
, tw telecom, Windstream, Level 3 Communications
Level 3 Communications
Level 3 Communications is a telecommunications and Internet service provider headquartered in Broomfield, Colorado.It operates a Tier 1 network. The company provides core transport, IP, voice, video and content delivery for most of the medium to large Internet carriers in North America and Europe...
, and XO Communications
XO Communications
XO Communications is a telecommunications company owned by XO Holdings, Inc...
offer national pricing. Under this DS1 pricing model, a provider charges the same price in every geography it services. National pricing is an outgrowth of increased competition in the T-carrier market space and the commoditization of T-carrier products. Providers that have adopted a national pricing strategy may experience widely varying margins as their suppliers, the Bell operating companies (e.g., Verizon, AT&T
AT&T
AT&T Inc. is an American multinational telecommunications corporation headquartered in Whitacre Tower, Dallas, Texas, United States. It is the largest provider of mobile telephony and fixed telephony in the United States, and is also a provider of broadband and subscription television services...
and Qwest
Qwest
Qwest Communications International, Inc. was a large United States telecommunications carrier. Qwest provided local service in 14 western U.S. states: Arizona, Colorado, Idaho, Iowa, Minnesota, Montana, Nebraska, New Mexico, North Dakota, Oregon, South Dakota, Utah, Washington, and Wyoming.On April...
), maintain geographic pricing models, albeit at wholesale prices.
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.
On March 29, 2011 the United States Patent Office issued patent no. 7,916,844 to Telarus
Telarus
Telarus, Inc. is a national sales agency that holds contracts with many commercial telecommunication carriers. Telarus consolidates the sales volume of hundreds of independent sales agents. As a master agent Telarus functions as the top layer in the 2-tier distribution model that is prevelent in...
for software that calculates real-time T-carrier prices.
External links
- ANSIAmerican National Standards InstituteThe American National Standards Institute is a private non-profit organization that oversees the development of voluntary consensus standards for products, services, processes, systems, and personnel in the United States. The organization also coordinates U.S. standards with international...
T1.403-1999 - Network and Customer Installation Interface - T1: A Survival Guide Chapter 5
- Cisco T1 card documentation
- T1 architecture