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12-channel Carrier System
{{Unreferenced, date= October 2009 In the U.S. telephone network, the 12-channel carrier system was an early frequency-division multiplexing system standard, used to carry multiple telephone calls on a single twisted pair of wires, mostly for short to medium distances. In this system twelve voice channels are multiplexed in a high frequency carrier and passed through a balanced pair trunk line similar to those used for individual voice frequency connections. The original system is obsolete today, but the multiplexing of voice channels in units of 12 or 24 channels in modern digital trunk lines such as T-1 is a legacy of the system. History The twelve channel scheme was first devised in the early 1930s to provide a line spectrum covering 60 to 108 kHz for the Type J Carrier Telephone System, an equivalent four wire (on two wire facilities) open wire carrier that was used almost exclusively for interstate long haul toll telephony. This became the basic building block, the "cha ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Telephone Network
A telephone network is a telecommunications network that connects telephones, which allows telephone calls between two or more parties, as well as newer features such as fax and internet. The idea was revolutionized in the 1920s, as more and more people purchased telephones and used them to communicate news, ideas, and personal information. During the 1990s, it was further revolutionized by the advent of computers and other sophisticated communication devices, and with the use of dial-up internet. There are a number of different types of telephone network: * A landline network where the telephones must be directly wired into a single telephone exchange. This is known as the public switched telephone network or PSTN. * A wireless network where the telephones are mobile and can move around anywhere within the coverage area. * A private network where a closed group of telephones are connected primarily to each other and use a gateway to reach the outside world. This is usually us ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Bell System
The Bell System was a system of telecommunication companies, led by the Bell Telephone Company and later by the American Telephone and Telegraph Company (AT&T), that dominated the telephone services industry in North America for over one hundred 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 the time of the breakup of the Bell System in the early 1980s, it had assets of $150 billion (equivalent to $ billion in ) and employed over one million people. Ever since 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, until in 1974 the Antitrust Division of the U.S. Department of Justice brought a lawsuit against Be ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Hush-A-Phone V
''Hush-A-Phone v. United States'', 238 F.2d 266 (D.C. Cir. 1956) was a seminal ruling in United States telecommunications decided by the D.C. Circuit Court of Appeals. Hush-A-Phone Corporation marketed a small, cup-like device which mounted on the speaking party's microphone, reducing the risk of conversations being overheard and increasing sound fidelity for the listening party. At the time, AT&T had a near-monopoly on America's phone system, even controlling the equipment attached to its network. In this era, Americans had to lease equipment from " Ma Bell" or use approved devices. At this time Hush-A-Phone had been around for 20 years without any issues. However, when an AT&T lawyer saw one in a store window, the company decided to sue on the grounds that anything attached to a phone could damage their network. AT&T, citing the Communications Act of 1934, which stated in part that the company had the right to make changes and dictate "the classifications, practices, and regulat ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
General Telephone
GTE Corporation, formerly General Telephone & Electronics Corporation (1955–1982), was the largest independent telephone company in the United States during the days of the Bell System. The company operated from 1926, with roots tracing further back than that, until 2000, when it was acquired by Bell Atlantic; the combined company took the name Verizon. The Wisconsin-based Associated Telephone Utilities Company was founded in 1926; it went bankrupt in 1933 during the Great Depression, and was reorganized as General Telephone in 1934. In 1991, it acquired the third largest independent, Continental Telephone (ConTel). It owned Automatic Electric, a telephone equipment supplier similar in many ways to Western Electric, and Sylvania Lighting, the only non-communications-oriented company under GTE ownership. GTE provided local telephone service to many areas of the U.S. through operating companies, much as American Telephone & Telegraph provided local telephone service throug ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Four Wire Circuit
In telecommunication, a four-wire circuit is a two-way circuit using two paths so arranged that the respective signals are transmitted in one direction only by one path and in the other direction by the other path. The four-wire circuit gets its name from the fact that is uses four conductors to create two complete electrical circuits, one for each direction. The two separate circuits (channels) allow full- duplex operation with low crosstalk. In telephony a four-wire circuit was historically used to transport and switch baseband audio signals in the phone company telephone exchange before the advent of digital modulation and the electronic switching system eliminated baseband audio from the telco plant except for the local loop. The local loop is a two-wire circuit for one reason only: to save copper. Using half the number of copper wire conductors per circuit means that the infrastructure cost for wiring each circuit is halved. Although a lower quality circuit, the local loop ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Frequency Frogging
In telecommunication, the term frequency frogging has the following meanings: # The interchanging of the frequencies of carrier channels to accomplish specific purposes, such as to prevent feedback and oscillation, to reduce crosstalk, and to correct for a high frequency response slope in the transmission line. # In microwave radio relay systems, the alternate use of two frequencies at repeater sites to prevent feedback and oscillation. ''Note:'' Frequency frogging is accomplished by having modulators, which are integrated into specially designed repeaters, translate a low-frequency group to a high-frequency group, and vice versa. A frequency channel will appear in the low group for one repeater section and will then be translated to the high group for the next section because of frequency frogging. This results in nearly constant attenuation with frequency over two successive repeater sections, and eliminates the need for large slope equalization and adjustments. Singing ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Wire Gauge
Wire gauge is a measurement of wire diameter. This determines the amount of electric current the wire can safely carry, as well as its electrical resistance and weight. Types of wire gauge Wire gauges may be broadly divided into two groups, the empirical and the geometric. The first includes all the older gauge measurements, notably the Birmingham gauge (B.W.G. or Stubs) and the Lancashire. The origin of the B.W.G. is obscure. The numbers of wire were in common use earlier than 1735. It is believed that they originally were based on the series of drawn wires, No. 1 being the original rod, and succeeding numbers corresponding with each draw, so that No. 10, for example, would have passed ten times through the draw plate. But the Birmingham and the Lancashire gauges, the latter being based on an averaging of the dimensions collated from a large number of the former in the possession of Peter Stubs of Warrington (1756-1806), have long held the leading position, and are still retai ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Single Sideband
In radio communications, single-sideband modulation (SSB) or single-sideband suppressed-carrier modulation (SSB-SC) is a type of modulation used to transmit information, such as an audio signal, by radio waves. A refinement of amplitude modulation, it uses transmitter power and bandwidth more efficiently. Amplitude modulation produces an output signal the bandwidth of which is twice the maximum frequency of the original baseband signal. Single-sideband modulation avoids this bandwidth increase, and the power wasted on a carrier, at the cost of increased device complexity and more difficult tuning at the receiver. Basic concept Radio transmitters work by mixing a radio frequency (RF) signal of a specific frequency, the carrier wave, with the audio signal to be broadcast. In AM transmitters this mixing usually takes place in the final RF amplifier (high level modulation). It is less common and much less efficient to do the mixing at low power and then amplify it in a linear ampl ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Frequency Plan
A frequency plan, bandplan, band plan or wavelength plan is a plan for using a particular band of radio frequencies, that are a portion of the electromagnetic spectrum. Each frequency plan defines the frequency range to be included, how channels are to be defined, and what will be carried on those channels. Typical definitions set forth in a frequency plan are: * numbering scheme – which channel numbers or letters (if any) will be assigned * center frequencies – how far apart the carrier wave for each channel will be *bandwidth and/or deviation – how wide each channel will be * spectral mask – how extraneous signals will be attenuated by frequency * modulation – what type will be used or are permissible *content – what types of information are allowed, such as audio or video, analog or digital *licensing – what the procedure will be to obtain a broadcast license The actual authorized frequency bands are defined by the ITU and the local regulating agencies like th ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Sideband
In radio communications, a sideband is a band of frequencies higher than or lower than the carrier frequency, that are the result of the modulation process. The sidebands carry the information transmitted by the radio signal. The sidebands comprise all the spectral components of the modulated signal except the carrier. The signal components above the carrier frequency constitute the upper sideband (USB), and those below the carrier frequency constitute the lower sideband (LSB). All forms of modulation produce sidebands. Sideband creation We can illustrate the creation of sidebands with one trigonometric identity: :\cos(A)\cdot \cos(B) \equiv \tfrac\cos(A+B) + \tfrac\cos(A-B) Adding \cos(A) to both sides: :\cos(A)\cdot +\cos(B)= \tfrac\cos(A+B) + \cos(A) + \tfrac\cos(A-B) Substituting (for instance) A \triangleq 1000\cdot t and B \triangleq 100\cdot t, where t represents time: :\underbrace_\cdot \underbrace_ = \underbrace_ + \underbrace_ + \unde ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Carrier System
A carrier system is a telecommunications system that transmits information, such as the voice signals of a telephone call and the video signals of television, by modulation of one or multiple carrier signals above the principal voice frequency or data rate.Western Electric (1969) ''Fundamentals of Telephone Communication Systems'', p.16.2 Carrier systems typically transmit multiple channels of communication simultaneously over the shared medium using various forms of multiplexing. Prominent multiplexing methods of the carrier signal are time-division multiplexing (TDM) and frequency-division multiplexing (FDM). A cable television system is an example of frequency-division multiplexing. Many television programs are carried simultaneously on the same coaxial cable by sending each at a different frequency. Multiple layers of multiplexing may ultimately be performed upon a given input signal. For example, in the public switched telephone network, many telephone calls are sent over sha ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
L-carrier
The L-carrier system was one of a series of carrier systems developed by AT&T for high-capacity transmission for long-distance communications. Over a period from the late 1930s to the 1970s, the system evolved in six significant phases of development, designated by Bell System engineers as L-1 through L-5, and L-5E. Coaxial cable was the principal transmission medium in all stages, initially lending the system another description i.e. the ''coaxial system''.E.L. Green, ''The Coaxial Cable System'', Bell Laboratories Record 15(9) p274 (May 1937) It was the successor to a series of previous carrier systems, typically identified by capital letters. In the 1960s the system was hardened against the dangers of the Cold War using complete placement of all terminal and repeater equipment in hardened underground vaults. Initial development and testing of the coaxial system took place between 1935 and 1937 on a test bed of a 95-mile two-way coaxial cable between locations in New York Cit ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
