T-carrier
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The T-carrier is a member of the series of
The first version, the Transmission System 1 (T1), was introduced in 1962 in the Bell System, and could transmit up to 24 telephone calls simultaneously over a single transmission line of copper wire. Subsequent specifications carried multiples of the basic T1 (1.544 Mbit/s) data rates, such as T2 (6.312 Mbit/s) with 96 channels, T3 (44.736 Mbit/s) with 672 channels, and others.
Although a T2 was defined as part of AT&T's T-carrier system, which defined five levels, T1 through T5,[1] only the T1 and T3 were commonly in use.[2][1]
Transmission System 1
The T-carrier is a hardware specification for carrying multiple time-division multiplexed (TDM) telecommunications channels over a single four-wire transmission circuit. It was developed by AT&T at Bell Laboratories ca. 1957 and first employed by 1962 for long-haul pulse-code modulation (PCM) digital voice transmission with the D1 channel bank.
The T-carriers are commonly used for
Before the digital T-carrier system,
Outside of the United States, Canada, Japan, and South Korea, the E-carrier system is used. E-carrier is similar transmission system with higher capacity that is not directly compatible with the T-carrier.
Legacy
Existing frequency-division multiplexing carrier systems worked well for connections between distant cities, but required expensive modulators, demodulators and filters for every voice channel. In the late 1950s, Bell Labs sought cheaper terminal equipment for connections within metropolitan areas. 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 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. 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 Mbit/s. It is possible to get a fractional T3 line,[4][5] 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 by
A more detailed understanding of the development of the 1.544 Mbit/s rate and its division into channels is as follows. Given that the telephone system nominal
Initially, T1 used
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.
The decision to use a 193-bit frame was made in 1958. To allow for the identification of information bits within a
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
Having this hindsight, some ten years later, CEPT chose eight bits for framing the European E1, although, as feared, the extra channel is sometimes appropriated for voice or data.
Higher bandwidth carriers
In the 1970s, Bell Labs developed higher rate systems. T1C 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
Digital signal cross-connect
DS1 signals are interconnected typically at Central Office locations at a common metallic cross-connect point known as a DSX-1. 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 supply power to line or "Span" signal repeaters, and T1 NIU's (T1 Smartjacks). T1 span repeaters are typically engineered up to 6,000 feet (1,800 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 taps or Load Coils 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
One advantage of HDSL is its ability to operate with a limited number of bridge taps, with no tap being closer than 500 feet (150 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 multiplexed onto a SONET circuit. This is because a T3 circuit can only go about 600 feet (180 m) 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.[7][8][9][10]
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 and 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, and D).
Fifty-six 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 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, and a digital end office switch.
The use of
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 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
While most carriers utilize a geographic pricing model as described above, some Competitive Local Exchange Carriers (
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.
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.
See also
- Communications in Japan
- Comparison of T-carrier and E-carrier systems
- List of interface bit rates
- Modified AMI code
- Optical Carrier transmission rates
- Plesiochronous digital hierarchy
- STM-1
- Telecommunications in South Korea
References
- ^ a b "T1 T2 T3 Speed Comparisons". 11 January 2020.
- ^ 1999 ad: On the left, in an aisle seat, a man who very much "filled" his airline seat while on the right side of the aisle is a height-challenged man whose shoe toes barely reach the floor "Is there a comfortable place between T1 and T3". Digital Link.
- ^ J.R. Davis, A. K. Reilly, T-Carrier Characterization Program – Overview, Bell System Technical Journal July–August 1981, Vol 60 No 6 Part 1
- ^ "fractional T3". 29 May 2008.
- Network World. Aug 16, 1993. p. 40.
- ^ Ronald C. Prime; Laurence L. Sheets (December 1973), "The 1A Radio System Makes "Data Under Voice" A Reality", Bell Laboratories Record
- ^ a b ANSI T1.403
- ^ ANSI T1.231
- ^ ANSI T1.404
- ^ ANSI T1.510
- ^ The Book On ESF, Verilink Corporation, 1986
- ^ D4 Digital Channel Bank Family, Bell System Technical Journal, November 1982
- ^ Sweeney, Terry (December 25, 2000). "T1 Price Drop Means Good Deals For Smart Shoppers". InformationWeek.com. Retrieved 2008-01-03.
- This article incorporates public domain material from Federal Standard 1037C. General Services Administration. Archived from the original on 2022-01-22.
External links
- ANSI T1.403-1999 - Network and Customer Installation Interface
- Cisco T1 card documentation; search for T1
- T1 architecture