The T-carrier is a member of the series of carrier systems developed by AT&T Bell Laboratories for digital transmission of multiplexed telephone calls .
85-542: 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
170-529: A T2 was defined as part of AT&T's T-carrier system, which defined five levels, T1 through T5, only the T1 and T3 were commonly in use. 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
255-557: A frame , two alternatives were considered. Assign (a) just one extra bit, or (b) additional eight bits per frame. The 8-bit choice is cleaner, resulting in a 200-bit frame, twenty-five 8-bit channels , of which 24 are traffic and one 8-bit channel available for operations, administration, and maintenance ( OA&M ). 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
340-735: A framing bit (for frame synchronization ) and a maintenance-signaling bit . T1's maximum data transmission rate is 1.544 megabits per second. 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. 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
425-455: 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. 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
510-604: 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
595-459: A T1 is 193 bits in length (24 channels × 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 × 193 = 1,544,000). Initially, T1 used Alternate Mark Inversion (AMI) to reduce frequency bandwidth and eliminate the DC component of the signal. Later B8ZS became common practice. For AMI, each mark pulse had
680-534: 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
765-442: A digital communication channel is the capacity excluding the physical layer protocol overhead, for example time division multiplex (TDM) framing bits , redundant forward error correction (FEC) codes, equalizer training symbols and other channel coding . Error-correcting codes are common especially in wireless communication systems, broadband modem standards and modern copper-based high-speed LANs. The physical layer net bitrate
850-472: A format sometimes abbreviated like "16bit / 44.1kHz". CD-DA is also stereo , using a left and right channel , so the amount of audio data per second is double that of mono, where only a single channel is used. The bit rate of PCM audio data can be calculated with the following formula: For example, the bit rate of a CD-DA recording (44.1 kHz sampling rate, 16 bits per sample and two channels) can be calculated as follows: The cumulative size of
935-614: A length of PCM audio data (excluding a file header or other metadata ) can be calculated using the following formula: The cumulative size in bytes can be found by dividing the file size in bits by the number of bits in a byte, which is eight: Therefore, 80 minutes (4,800 seconds) of CD-DA data requires 846,720,000 bytes of storage: where MiB is mebibytes with binary prefix Mi, meaning 2 = 1,048,576. The MP3 audio format provides lossy data compression . Audio quality improves with increasing bitrate: For technical reasons (hardware/software protocols, overheads, encoding schemes, etc.)
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#17327728364551020-479: A post- World War II reconstruction relationship with state-owned Nippon Telegraph and Telephone (NTT) before the 1956 boundaries were emplaced. Before 1956, the Bell System's reach was truly gargantuan. Even during the period from 1956 to 1984, the Bell System's dominant reach into all forms of communications was pervasive within the United States and influential in telecommunication standardization throughout
1105-530: 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 Inc. , and Qwest ), maintain geographic pricing models, albeit at wholesale prices. For voice DS1 lines,
1190-415: 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 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
1275-404: A result of this vertical monopoly , the Bell System effectively owned most telephone service in the United States by 1940, from local and long-distance service to the telephones. This allowed Bell to prohibit its customers from connecting equipment not made or sold by Bell to the system without paying fees. For example, if a customer desired a style of telephone not leased by the local Bell company,
1360-438: 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
1445-401: A single Bell System trademark. For each regional operating company, its name was placed where "name of associated company" appears in this template version of the trademark. Bell system telephones and related equipment were made by Western Electric , a wholly owned subsidiary of AT&T Co. Member telephone companies paid a fixed fraction of their revenues as a license fee to Bell Labs . As
1530-459: A single copper wire pair up to approximately twelve thousand (12,000) feet (3.5 km), if all 24 gauge cable is used. HDSL-2 does not employ multiple 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 (150 m) from any HDSL transceiver. Both two or four wire HDSL equipment transmits and receives over
1615-399: A switched trunk network connection if the supporting T1 carrier entity is optioned B8ZS (Clear Channel Capable). 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
1700-601: A term that referred generally to all AT&T companies, of which there were five major divisions: In 1913, the federal government challenged the Bell System's growing monopoly over the phone system under AT&T ownership in an anti-trust suit, leading to the Kingsbury Commitment . Under the commitment, AT&T escaped break-up or nationalization in exchange for divesting itself of Western Union and allowing non-competing independent telephone companies to interconnect with its long-distance network. After 1934,
1785-510: 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
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#17327728364551870-447: A variable R ) is the number of bits that are conveyed or processed per unit of time. The bit rate is expressed in the unit bit per second (symbol: bit/s ), often in conjunction with an SI prefix such as kilo (1 kbit/s = 1,000 bit/s), mega (1 Mbit/s = 1,000 kbit/s), giga (1 Gbit/s = 1,000 Mbit/s) or tera (1 Tbit/s = 1,000 Gbit/s). The non-standard abbreviation bps
1955-447: 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 eight bits for framing
2040-453: Is 125 Mbit/s, due to the 4B5B (four bit over five bit) encoding. In this case, the gross bit rate is equal to the symbol rate or pulse rate of 125 megabaud, due to the NRZI line code . In communications technologies without forward error correction and other physical layer protocol overhead, there is no distinction between gross bit rate and physical layer net bit rate. For example,
2125-804: Is not the case for modern modulation systems used in modems and LAN equipment. For most line codes and modulation methods: More specifically, a line code (or baseband transmission scheme) representing the data using pulse-amplitude modulation with 2 N {\displaystyle 2^{N}} different voltage levels, can transfer N {\displaystyle N} bits per pulse. A digital modulation method (or passband transmission scheme) using 2 N {\displaystyle 2^{N}} different symbols, for example 2 N {\displaystyle 2^{N}} amplitudes, phases or frequencies, can transfer N {\displaystyle N} bits per symbol. This results in: An exception from
2210-669: Is often used to replace the standard symbol bit/s, so that, for example, 1 Mbps is used to mean one million bits per second. In most computing and digital communication environments, one byte per second (symbol: B/s ) corresponds to 8 bit/s. When quantifying large or small bit rates, SI prefixes (also known as metric prefixes or decimal prefixes) are used, thus: Binary prefixes are sometimes used for bit rates. The International Standard ( IEC 80000-13 ) specifies different symbols for binary and decimal (SI) prefixes (e.g., 1 KiB /s = 1024 B/s = 8192 bit/s, and 1 MiB /s = 1024 KiB/s). In digital communication systems,
2295-545: 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 by AT&T Long Lines in Chicago were conducted underground. The test site was typical of Bell System outside plant of the time in that, to accommodate loading coils , cable vault manholes were physically 2,000 meters (6,600 feet) apart, which determined
2380-408: Is possible without bit errors for a certain physical analog node-to-node communication link . The channel capacity is proportional to the analog bandwidth in hertz. This proportionality is called Hartley's law . Consequently, the net bit rate is sometimes called digital bandwidth capacity in bit/s. The term throughput , essentially the same thing as digital bandwidth consumption , denotes
2465-437: Is provided by the network equipment or protocols, we have the following relation: for a certain communication path. These are examples of physical layer net bit rates in proposed communication standard interfaces and devices: In digital multimedia, bit rate represents the amount of information, or detail, that is stored per unit of time of a recording. The bitrate depends on several factors: Generally, choices are made about
2550-536: 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 robbed-bit signaling in 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 data calls to exist over
2635-405: Is the datarate measured at a reference point in the interface between the data link layer and physical layer, and may consequently include data link and higher layer overhead. In modems and wireless systems, link adaptation (automatic adaptation of the data rate and the modulation and/or error coding scheme to the signal quality) is often applied. In that context, the term peak bitrate denotes
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2720-552: The American Telephone and Telegraph Company (AT&T), that dominated the telephone services industry in North America for over 100 years from its creation in 1877 until its antitrust breakup in 1983. The system of companies was often colloquially called Ma Bell (as in "Mother Bell"), as it held a vertical monopoly over telecommunication products and services in most areas of the United States and Canada. At
2805-492: The Baby Bells and do not provide telephone service. Beginning in 1991, the Baby Bells began to consolidate operations or rename their Bell Operating Companies according to the parent company name, such as "Bell Atlantic – Delaware, Inc." or "U S WEST Communications, Inc.", to unify their corporate images. The Bell System service marks, including the circled-bell logo, especially as redesigned by Saul Bass in 1969, and
2890-583: The Federal Communications Commission (FCC) assumed regulation of AT&T. Proliferation of telephone service allowed the company to become the largest corporation in the world until its dismantling by the United States Department of Justice in 1984, at which time the Bell System ceased to exist. Receiving a U.S. patent for the invention of the telephone on March 7, 1876, Alexander Graham Bell formed
2975-571: The U.S. Department of Justice brought a lawsuit against Bell claiming violations of the Sherman Act . In 1982, anticipating that it could not win, AT&T agreed to a Justice Department-mandated consent decree that settled the lawsuit and ordered it to break itself up into seven " Regional Bell Operating Companies " (known as "The Baby Bells"). This ended the existence of the conglomerate in 1984. The Baby Bells became independent companies and several of them are large corporations today. In 1877,
3060-410: The physical layer gross bitrate , raw bitrate , data signaling rate , gross data transfer rate or uncoded transmission rate (sometimes written as a variable R b or f b ) is the total number of physically transferred bits per second over a communication link, including useful data as well as protocol overhead. In case of serial communications , the gross bit rate is related to
3145-507: The post-World War II occupation was considered an administrative adjunct to the North American Bell System. Immediately before the 1984 break-up, the Bell System had the following corporate structure : On January 1, 1984, the former components of the Bell System were structured into the following Regional Bell Operating Companies (RBOCs), which became known as Baby Bells. After 1984, multiple mergers occurred of
3230-416: The source information rate , also known as the entropy rate . The bitrates in this section are approximately the minimum that the average listener in a typical listening or viewing environment, when using the best available compression, would perceive as not significantly worse than the reference standard. Compact Disc Digital Audio (CD-DA) uses 44,100 samples per second, each with a bit depth of 16,
3315-759: The American Bell Telephone Company, named after Alexander Graham Bell , opened the first telephone exchange in New Haven, Connecticut . Within a few years local exchange companies were established in every major city in the United States. Use of the Bell System name initially referred to those early telephone franchises and eventually comprised all telephone companies owned by American Telephone & Telegraph , referred to internally as associated companies , regional holding companies , or later Bell operating companies (BOCs). In 1899, American Telephone & Telegraph (AT&T) acquired
3400-488: The Bell System's Caribbean regional operating companies. The consent decree also forced Bell to make all of its patents royalty-free . This led to substantial increases in innovation, in particular in the electronics and computer sectors. Steven Weber 's The Success of Open Source characterizes the consent decree as important in fostering the open source movement. The Bell System also owned various Caribbean regional operating companies, as well as 54% of Japan's NEC and
3485-597: The Bell Telephone Company in 1877, which in 1885 became AT&T. When Bell's original patent expired 15 years later in 1894, the telephone market opened to competition and 6,000 new telephone companies started while the Bell Telephone company took a significant financial downturn. On April 30, 1907, Theodore Newton Vail returned as President of AT&T. Vail believed in the superiority of one national telephone system and AT&T adopted
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3570-497: The Bell name and the circled-bell trademark until SBC opted for all of its companies to do business under the "SBC" name in 2002. Bell Atlantic used the Bell name and circled-bell trademark until renaming itself Verizon in 2000. Pacific Bell continued operating in California under that name (or the shortened "PacBell" nickname) until SBC purchased it. In Canada, Bell Canada ( divested from AT&T in 1975) continues to use
3655-480: The Bell name. For the decades that Nortel was named Northern Telecom, its research and development arm was Bell Northern Research. Bell Canada and its holding-company parent, Bell Canada Enterprises , still use the Bell name. They used variations of the circled-bell logo until 1977, which until 1976 strongly resembled the 1921 to 1939 Bell System trademark shown above. Bit rate In telecommunications and computing , bit rate ( bitrate or as
3740-452: The D1 channel bank . The T-carriers are commonly used for trunking between switching centers in a telephone network, including to private branch exchange (PBX) interconnect points. It uses the same twisted pair copper wire that analog trunks used, employing one pair for transmitting, and another pair for receiving. Signal repeaters may be used for extended distance requirements. Before
3825-525: The DOJ had agreed upon in 1956. Before the 1956 break-up, the Bell System included the companies listed below, plus those listed in the pre-1984 section. Northern Electric and the Caribbean regional operating companies were considered part of the Bell System proper before the break-up. Nippon Electric was considered a more distant affiliate of Western Electric, and through its own research and development adapted
3910-715: The Department of Justice in 1913. AT&T committed to sell its $ 30 million in Western Union capital stock, allow competitors to interconnect with its long-distance telephone network, and not acquire other independent companies without permission from the Interstate Commerce Commission . The Bell trademark was used from 1921 through 1969 by both the AT&;T corporation and the regional operating corporations to co-brand themselves under
3995-520: The European E1 , although, as feared, the extra channel is sometimes appropriated for voice or data. 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 Picturephone . T-4 and T-5 used coaxial cables, similar to
4080-511: The Northwestern Bell name. In 1984, each regional Bell operating company was assigned a set list of names it was allowed to use in combination with the Bell marks. By 2022, all these Bell System names had disappeared from the United States business landscape. Cincinnati Bell was the last to use the name, until 2022 when it rebranded to Altafiber , though it still has Cincinnati Bell as its corporate name. Southwestern Bell used both
4165-443: The above factors in order to achieve the desired trade-off between minimizing the bitrate and maximizing the quality of the material when it is played. If lossy data compression is used on audio or visual data, differences from the original signal will be introduced; if the compression is substantial, or lossy data is decompressed and recompressed, this may become noticeable in the form of compression artifacts . Whether these affect
4250-694: The above is some self-synchronizing line codes, for example Manchester coding and return-to-zero (RTZ) coding, where each bit is represented by two pulses (signal states), resulting in: A theoretical upper bound for the symbol rate in baud, symbols/s or pulses/s for a certain spectral bandwidth in hertz is given by the Nyquist law : In practice this upper bound can only be approached for line coding schemes and for so-called vestigial sideband digital modulation. Most other digital carrier-modulated schemes, for example ASK , PSK , QAM and OFDM , can be characterized as double sideband modulation, resulting in
4335-400: The achieved average useful bit rate in a computer network over a logical or physical communication link or through a network node, typically measured at a reference point above the data link layer. This implies that the throughput often excludes data link layer protocol overhead. The throughput is affected by the traffic load from the data source in question, as well as from other sources sharing
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#17327728364554420-547: The assets of its parent, the American Bell Telephone Company. American Bell had created AT&T to provide long-distance calls between New York and Chicago and beyond. AT&T became the parent of American Bell Telephone Company, and thus the head of the Bell System, because regulatory and tax rules were leaner in New York than in Boston, where American Bell was headquartered. Later, the Bell System and its moniker "Ma Bell" became
4505-462: The bit transmission time T b {\displaystyle T_{\text{b}}} as: The gross bit rate is related to the symbol rate or modulation rate, which is expressed in bauds or symbols per second. However, the gross bit rate and the baud value are equal only when there are only two levels per symbol, representing 0 and 1, meaning that each symbol of a data transmission system carries exactly one bit of data; for example, this
4590-508: 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. Bell System The Bell System was a system of telecommunication companies, led by the Bell Telephone Company and later by
4675-501: The connection establishment phase due to adaptive modulation – slower but more robust modulation schemes are chosen in case of poor signal-to-noise ratio . Due to data compression, the actual data transmission rate or throughput (see below) may be higher. The channel capacity , also known as the Shannon capacity, is a theoretical upper bound for the maximum net bitrate, exclusive of forward error correction coding, that
4760-445: The customer was required to purchase the instrument at cost, furnish it to the telephone company for rewiring, pay a service charge, and a monthly lease fee for using it. In 1949, the United States Department of Justice alleged in an antitrust lawsuit that AT&T and the Bell System operating companies were using their near-monopoly in telecommunications to attempt to establish unfair advantage in related technologies. The outcome
4845-535: The designs of Western Electric's North American telecommunications equipment for use in Japan, which to this day gives much of Japan's telephone equipment and network a closer resemblance to North American ANSI and iconectiv standards than to European-originated ITU-T standards. Before the 1956 break-up, Northern Electric was focused on manufacturing, without significant telecommunication-equipment research & development of its own. The operation of Japan's NTT during
4930-459: The development of the 1.544 Mbit/s rate and its division into channels is as follows. Given that the telephone system nominal voiceband (including guardband ) is 4,000 Hz , the required digital sampling rate is 8,000 Hz (see Nyquist rate ). 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
5015-433: The digital T-carrier system, carrier wave systems such as 12-channel carrier systems worked by frequency-division multiplexing ; each call was an analog signal . A T1 trunk could transmit 24 telephone calls at a time, because it used a digital carrier signal called Digital Signal 1 (DS-1). DS-1 is a communications protocol for multiplexing the bitstreams of up to 24 telephone calls, along with two special bits :
5100-399: The encoding bit rate is the goodput that is required to avoid playback interruption. The term average bitrate is used in case of variable bitrate multimedia source coding schemes. In this context, the peak bit rate is the maximum number of bits required for any short-term block of compressed data. A theoretical lower bound for the encoding bit rate for lossless data compression is
5185-446: The file size (in bytes) divided by the file transfer time (in seconds) and multiplied by eight. As an example, the goodput or data transfer rate of a V.92 voiceband modem is affected by the modem physical layer and data link layer protocols. It is sometimes higher than the physical layer data rate due to V.44 data compression , and sometimes lower due to bit-errors and automatic repeat request retransmissions. If no data compression
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#17327728364555270-534: The following relation: In case of parallel communication , the gross bit rate is given by where n is the number of parallel channels, M i is the number of symbols or levels of the modulation in the i th channel , and T i is the symbol duration time , expressed in seconds, for the i th channel. The physical layer net bitrate , information rate , useful bit rate , payload rate , net data transfer rate , coded transmission rate , effective data rate or wire speed (informal language) of
5355-405: 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
5440-482: The gross bit rate and net bit rate is affected by the FEC code rate according to the following. The connection speed of a technology that involves forward error correction typically refers to the physical layer net bit rate in accordance with the above definition. For example, the net bitrate (and thus the "connection speed") of an IEEE 802.11a wireless network is the net bit rate of between 6 and 54 Mbit/s, while
5525-472: The gross bit rate is between 12 and 72 Mbit/s inclusive of error-correcting codes. The net bit rate of ISDN2 Basic Rate Interface (2 B-channels + 1 D-channel) of 64+64+16 = 144 kbit/s also refers to the payload data rates, while the D channel signalling rate is 16 kbit/s. The net bit rate of the Ethernet 100BASE-TX physical layer standard is 100 Mbit/s, while the gross bitrate
5610-438: The industrialized world. The 1984 Bell System divestiture brought an end to the affiliation branded as the Bell System. It resulted from another antitrust lawsuit filed by the U.S. Department of Justice in 1974, alleging illegal practices by the Bell System companies to stifle competition in the telecommunications industry. The parties settled the lawsuit on January 8, 1982, superseding the former restrictions that AT&T and
5695-418: 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
5780-477: The marks on rare occasions to maintain their trademark rights, even less now that they have adopted names conceived long after divestiture. Examples include Verizon, which still used the Bell logo on its trucks and payphones until it updated its own logo in 2015, and Qwest, formerly US West , which licenses the Northwestern Bell and Mountain Bell names to Unical Enterprises, who makes telephones under
5865-520: The net as well as gross bit rate of Ethernet 10BASE-T is 10 Mbit/s. Due to the Manchester line code, each bit is represented by two pulses, resulting in a pulse rate of 20 megabaud. The "connection speed" of a V.92 voiceband modem typically refers to the gross bit rate, since there is no additional error-correction code. It can be up to 56,000 bit/s downstream and 48,000 bit/s upstream . A lower bit rate may be chosen during
5950-465: The net bitrate of the fastest and least robust transmission mode, used for example when the distance is very short between sender and transmitter. Some operating systems and network equipment may detect the " connection speed " (informal language) of a network access technology or communication device, implying the current net bit rate. The term line rate in some textbooks is defined as gross bit rate, in others as net bit rate. The relationship between
6035-529: The old L-carriers 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 1980s companies such as RLH Industries, Inc. developed T1 over optical fiber. The industry soon developed and evolved with multiplexed T1 transmission schemes. DS1 signals are interconnected typically at Central Office locations at
6120-416: The operating companies and between them, so that some components of the former Bell System are now owned by companies independent of the historic Bell System, including foreign telecommunications firms. The structure of the companies today is as follows. The following telephone companies are considered independent of the Baby Bells : The following companies were divested after 1984 from AT&T Corp. or
6205-576: The opposite polarity of the previous one and each space was at a level of zero, resulting in a three level signal which carried only binary data. Similar 1970s British 23 channel systems at 1.536 megabaud were equipped with ternary signal repeaters, in anticipation of using a 3B2T or 4B3T code to increase the number of voice channels in the 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
6290-565: 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
6375-433: The perceived quality, and if so how much, depends on the compression scheme, encoder power, the characteristics of the input data, the listener's perceptions, the listener's familiarity with artifacts, and the listening or viewing environment. The encoding bit rate of a multimedia file is its size in bytes divided by the playback time of the recording (in seconds), multiplied by eight. For real-time streaming multimedia ,
6460-440: The repeater spacing. 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
6545-402: 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
6630-425: The same network resources. See also measuring network throughput . Goodput or data transfer rate refers to the achieved average net bit rate that is delivered to the application layer , exclusive of all protocol overhead, data packets retransmissions, etc. For example, in the case of file transfer, the goodput corresponds to the achieved file transfer rate . The file transfer rate in bit/s can be calculated as
6715-489: The slogan One Policy, One System, Universal Service. This became the company's philosophy for the next 70 years. Under Vail, AT&T began acquiring many of the smaller telephone companies including the Western Union Telegraph Company . In response to the threat of antitrust action from government, AT&T entered into an out-of-court agreement, known as the Kingsbury Commitment with
6800-571: The telco piece. Each local Bell operating company—namely Verizon , AT&T Inc. , 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. While most carriers utilize a geographic pricing model as described above, some Competitive Local Exchange Carriers ( CLECs ), such as TelePacific , Integra Telecom , tw telecom , Windstream , Level 3 Communications , and XO Communications offer national pricing. Under this DS1 pricing model,
6885-470: The time of the breakup of the Bell System in the early 1980s, it had assets of $ 150 billion (equivalent to $ 440 billion in 2023) and employed over one million people. Beginning in the 1910s, American antitrust regulators had been observing and accusing the Bell System of abusing its monopoly power, and had brought legal action multiple times over the decades. In 1974 the Antitrust Division of
6970-414: The various resulting 1984 spinoffs, only BellSouth actively used and promoted the Bell name and logo during its entire history, from the 1984 break-up to its reunion with the new AT&T in 2006. Similarly, cessation of using either the Bell name or logo occurred for many of the other companies more than a decade after the 1984 break-up as part of an acquisition-related rebranding. The others have only used
7055-500: The words Bell System in text, were used before January 1, 1984, when the AT&;T divestiture of its regional operating companies took effect. The word mark Bell , the logo, and other related trademarks, are held by each of the remaining Bell companies, namely AT&T, Verizon , CenturyLink , and Altafiber . International rights to the marks, except for Canada , are held by a joint venture of these companies, Bell IP Holdings . Of
7140-848: Was a 1956 consent decree limiting AT&T to 85% of the United States' national telephone network and certain government contracts, and from continuing to hold interests in Canada and the Caribbean . The Bell System's Canadian operations included the Bell Canada regional operating company and the Northern Electric manufacturing subsidiary of the Bell System's Western Electric equipment manufacturer. Western Electric divested Northern Electric in 1956, but AT&T did not divest itself of Bell Canada until 1975. ITT Inc. , then known as International Telephone & Telegraph Co. , purchased
7225-405: 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 zeros, so one bit per sample is set to "1" (jam bit 7) leaving 7 bits × 8,000 frames per second for data. A more detailed understanding of
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