In
telecommunicationTelecommunication is transmission over a distance for the purpose of communication. In earlier times, this may have involved the use of smoke signals, drums, semaphore, flags or heliograph. In modern times, telecommunication typically involves the use of electronic devices such as the telephone,...
s,
T-carrier, sometimes abbreviated as
T-CXR, is the generic designator for any of several digitally
multiplexedThe 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 systemIn 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 LabsBell Laboratories is the research and development organization of Alcatel-Lucent and previously of the American Telephone & Telegraph Company .Bell Laboratories has had its headquarters at Murray Hill, New Jersey, and it has research and development facilities...
and used in
North AmericaNorth America is the northern continent of the Americas, situated in the Earth's northern hemisphere and in the western hemisphere. It is bordered on the north by the Arctic Ocean, on the east by the North Atlantic Ocean, on the southeast by the Caribbean Sea, and on the west by the North Pacific...
,
Japan-Telephone:Telephones and ISDN - main lines in use: 52.398 million IP phone lines in use: 16.766 million Mobile and PHS lines in use: 105.297 million -Radio and Television:...
, and
KoreaCommunications 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
DS0Digital Signal 0 is a basic digital signalling rate of 64 kbit/s, corresponding to the capacity of one voice-frequency-equivalent channel...
, which has a transmission rate of 64
kbit/sIn 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 units bit per second or byte per second.- Avoiding confusion :- 'k' vs...
, and is commonly used for one voice circuit.
The
E-carrierIn digital telecommunications, where a single physical wire pair can be used to carry many simultaneous voice conversations, 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 rateThe line rate of a communications link is the data rate of its raw bitstream, including all framing bits and other physical layer overhead. For example, the line rate of a T1 data link is 1.544 Mbit/s, of which 1.536 Mbit/s is available for data communications, and the remaining 8000 bit/s is...
and the E3 line rate. The E2 line rate is less commonly used. See the table below for bit rate comparisons.
T1
Existing
frequency-division multiplexingFrequency-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.-Non 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 LabsBell Laboratories is the research and development organization of Alcatel-Lucent and previously of the American Telephone & Telegraph Company .Bell Laboratories has had its headquarters at Murray Hill, New Jersey, and it has research and development facilities...
in the late 1950s sought cheaper terminal equipment.
Pulse-code modulationPulse-code modulation is a digital representation of an analog signal where the magnitude of the signal is sampled regularly at uniform intervals, then quantized to a series of symbols in a numeric code. PCM has been used in digital telephone systems and 1980s-era electronic musical keyboards...
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
codecA codec is a device or computer program capable of encoding and/or decoding a digital data stream or signal. The word codec is a portmanteau of compressor-decompressor' or, more accurately, coder-decoder'.Historically a modem was a contraction of modulator/demodulator and converted...
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 modulatedPulse-code modulation is a digital representation of an analog signal where the magnitude of the signal is sampled regularly at uniform intervals, then quantized to a series of symbols in a numeric code. PCM has been used in digital telephone systems and 1980s-era electronic musical keyboards...
,
time-division multiplexedTime-division multiplexing is a type of digital or analog multiplexing in which two or more signals or bit streams 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...
speech signals each encoded in 64 kbit/s streams, leaving 8 kbit/s of
framing informationWhile 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. 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
ChicagoChicago is the largest city in the U.S. state of Illinois, and with more than 2.8 million people, the 3rd largest city in the United States...
were conducted underground. To accommodate
loading coilIn 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, and so the optimum bit rate was chosen
empiricallyIn philosophy, empiricism is a theory of knowledge which asserts that knowledge arises from sense experience. Empiricism is one of several competing views about how we know "things," part of the branch of philosophy called epistemology, or "the Theory of Knowledge"...
— the capacity was increased until the failure rate was unacceptable, then reduced to leave a margin.
CompandingIn 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 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
HzThe hertz is a unit of frequency. It is defined as the number of complete cycles per second. It is the basic unit of frequency in the International System of Units , and is used worldwide in both general-purpose and scientific contexts...
, the required digital sampling rate is 8,000 Hz (see
Nyquist rateIn signal processing, the Nyquist rate is two times the bandwidth of a bandlimited signal or a bandlimited channel. This term is used to mean two different things under two different circumstances:...
). 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
DCDirect current is the undirectional 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 be 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 signalIn telecommunication, a ternary signal is a signal that can assume, at any given instant, one of three significant conditions, such as power level, phase position, pulse duration, or frequency....
repeaters, in anticipation of using a 3B2T or
4B3T4B3T 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 viz. + , 0 , and − . This means we have 16 input combinations to represent, using 27 output combinations...
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 violationA 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 signalAlarm 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, during the early stages of T1 system design. To allow for the identification of information bits within a
frameDigital 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 of North America, Japan, and South Korea...
, 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 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 his own time slot. This...
, of which 24 are traffic and 1 8-bit channel available for operations, administration, and maintenance (
OA&MOA&M stands for Operations, Administration, and Management . It 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,
CEPTThe 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
E1In digital telecommunications, where a single physical wire pair can be used to carry many simultaneous voice conversations, 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
Picturephone 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-carriere border=1 align="right" cellpadding=5 cellspacing=4 width=300>
| System | Year | Frequency | Coax per cable | Distance between repeaters | Voice circuits per coax |
|---|
| L-1 | 1941 | 3 MHz | 4 | 8 miles | 600 |
...
s used by AT&T Long Lines. TD
microwave radio relaythumb|right|180px|[[Heinrich-Hertz-Turm]] in GermanyMicrowave radio relay is a technology for transmitting digital and analog signals, such as long-distance telephone calls and the relay of television programs to transmitters, between two locations on a line of sight radio path...
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. Later
optical fiberAn optical fiber is a glass or plastic fiber that carries light along its length. Fiber optics is the overlap of applied science and engineering concerned with the design and application of optical fibers...
, typically using
SONETSonet may refer to:* Sonet Records, European record label* Synchronous optical networking...
transmission scheme, overtook them.
Digital signal crossconnect
DS1Digital 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 of 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 plantIn telecommunication, the term outside plant has the following meanings:*In civilian telecommunications, all cables, conduits, ducts, poles, towers, repeaters, repeater huts, and other equipment located between a demarcation point in a switching facility and a demarcation point in another switching...
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 tapBridged 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 1.54400 Mbit/s T1 over a single copper wire pair up to approximately twelve thousand (12,000) feet (3.5 km), if all 24
gaugeAmerican wire gauge , also known as the Brown & Sharpe wire gauge, is a standardized wire gauge system used since 1857 predominantly in the United States 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 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
muxedThe 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
SONETSynchronous optical networking and synchronous digital hierarchy are standardized multiplexing protocols that transfer multiple digital bit streams over optical fiber using lasers or light-emitting diodes . Lower rates can also be transferred via an electrical interface...
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
multiplexerThe 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
coaxCoaxial cable, or coax, is an electrical cable with an inner conductor surrounded by a tubular insulating layer typically of a flexible material with a high dielectric constant, all of which are surrounded by a conductive layer , and finally covered with a thin insulating layer on the outside...
cables (1 for send and 1 for receive) with
BNC connectorThe BNC connector is a very common type of RF connector used for terminating coaxial cable.-Use:The BNC connector is used for RF signal connections, for analog and Serial Digital Interface video signals, amateur radio antenna connections, aviation electronics and many other types of electronic...
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 comprised 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 Kb/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 KB/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 Kb/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 Kb/s DDS. In DDS, bit eight is used to identify
DTEData 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/DSUA 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 ....
, and a digital end office switch.
The incident use of robbed-bit signaling in North America has decreased significantly as a result of Signaling System Seven (SS7) on inter-office dial trunks. With SS7, the full 64 Kb/s DS0 channel is available for use on a connection, and allows 64 Kb/s, and 128 Kb/s
ISDNIntegrated services digital network is a set of communications standards enabling traditional telephone lines to carry voice, digital network services, and video. Prior to ISDN, the phone system was viewed as a way to transport voice, with some special services available for data...
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 loopIn 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 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&TAT&T Inc. is the largest provider of local, long distance telephone services in the United States, and also serves digital subscriber line Internet access. AT&T is the second largest provider of wireless service in the United States, with over 77 million wireless customers, and more than 150...
, and
QwestQwest Communications International, Inc. is a large telecommunications carrier. Qwest provides local service in 14 western U.S...
- 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
EarthLinkEarthLink , is an Internet service provider headquartered in Atlanta, Georgia, USA. It claims 5.4 million members .- Business :...
Business Solutions, 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&TAT&T Inc. is the largest provider of local, long distance telephone services in the United States, and also serves digital subscriber line Internet access. AT&T is the second largest provider of wireless service in the United States, with over 77 million wireless customers, and more than 150...
and
QwestQwest Communications International, Inc. is a large telecommunications carrier. Qwest provides local service in 14 western U.S...
), 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.