A telephone line or telephone circuit is a single-user circuit on a telephone communication system. This is the physical wire or other signaling medium connecting the user's telephone apparatus to the telecommunications network, also implies a single telephone number for billing purposes reserved for that user. Telephone lines are used to deliver landline telephone service and Digital subscriber line phone cable service to the premises. Telephone overhead lines are connected to the public switched telephone network. In 1878, the Bell Telephone Company began to use two-wire circuits from each user's telephone to end offices which performed any necessary electrical switching to allow voice signals to be transmitted to more distant telephones; these wires were copper, although aluminium has been used, were carried in balanced pairs of open wire, separated by about 25 cm on poles above the ground, as twisted pair cables. Modern lines may run underground, may carry analog or digital signals to the exchange, or may have a device that converts the analog signal to digital for transmission on a carrier system.
The customer end of that wire pair is connected to a data access arrangement. In most cases, two copper wires for each telephone line run from a home or other small building to a local telephone exchange. There is a central junction box for the building where the wires that go to telephone jacks throughout the building and wires that go to the exchange meet and can be connected in different configurations depending upon the subscribed telephone service; the wires between the junction box and the exchange are known as the local loop, the network of wires going to an exchange, the access network. The vast majority of houses in the U. S. are wired with 6-position modular jacks with four conductors wired to the house's junction box with copper wires. Those wires may be connected back to two telephone overhead lines at the local telephone exchange, thus making those jacks RJ14 jacks. More only two of the wires are connected to the exchange as one telephone line, the others are unconnected. In that case, the jacks in the house are RJ11.
Older houses have 4-conductor telephone station cable in the walls color coded with Bell System colors: red, yellow, black as 2-pairs of 22 AWG solid copper. Inside the walls of the house—between the house's outside junction box and the interior wall jacks—the most common telephone cable in new houses is Category 5 cable—4 pairs of 24 AWG solid copper. Inside large buildings, in the outdoor cables that run to the telephone company POP, many telephone lines are bundled together in a single cable using the 25-pair color code
A test point is a location within an electronic circuit, used to either monitor the state of the circuitry or to inject test signals. Test points have two primary uses: During manufacturing they are used to verify that a newly assembled device is working correctly. Any equipment that fails this testing is either discarded or sent to a rework station to attempt to repair the manufacturing defects. After sale of the device to a customer, test points may be used at a time to repair the device if it malfunctions, or if the device needs to be re-calibrated after having components replaced. Test points can be labelled and may include pins for attachment of alligator clips or may have complete connectors for test clips. Modern miniature surface-mount electronics simply have a row of unlabelled, tinned solder pads; the device is placed into a test fixture that holds the device securely, a special surface-contact connector plate is pressed down onto the solder pads to connect them all as a group
Soldering is a process in which two or more items are joined together by melting and putting a filler metal into the joint, the filler metal having a lower melting point than the adjoining metal. Unlike welding, soldering does not involve melting the work pieces. In brazing, the filler metal melts at a higher temperature. In the past, nearly all solders contained lead, but environmental and health concerns have dictated use of lead-free alloys for electronics and plumbing purposes. There is evidence. Soldering and brazing are thought to have originated early in the history of metal-working before 4000 BC. Sumerian swords from c. 3000 BC were assembled using hard soldering. Soldering was used to make jewelry items, cooking ware and tools, as well as other uses such as in assembling stained glass. Soldering is used in plumbing and metalwork from flashing to jewelry and musical instruments. Soldering provides reasonably permanent but reversible connections between copper pipes in plumbing systems as well as joints in sheet metal objects such as food cans, roof flashing, rain gutters and automobile radiators.
Jewelry components, machine tools and some refrigeration and plumbing components are assembled and repaired by the higher temperature silver soldering process. Small mechanical parts are soldered or brazed as well. Soldering is used to join lead came and copper foil in stained glass work. Electronic soldering connects electrical wiring and electronic components to printed circuit boards by utilizing a metallic alloy substance called solder; this special alloy is melted by using a soldering iron, a wave bath, or a specialized oven, as it joins conductors to PCBs, wires. Musical instruments brass and woodwind instruments, use a combination of soldering and brazing in their assembly. Brass bodies are a soldered together, while keywork and braces are most brazed. Soldering filler materials are available in many different alloys for differing applications. In electronics assembly, the eutectic alloy of 63% tin and 37% lead has been the alloy of choice. Other alloys are used for plumbing, mechanical assembly, other applications.
Some examples of soft-solder are tin-lead for general purposes, tin-zinc for joining aluminium, lead-silver for strength at higher than room temperature, cadmium-silver for strength at high temperatures, zinc-aluminium for aluminium and corrosion resistance, tin-silver and tin-bismuth for electronics. A eutectic formulation has advantages when applied to soldering: the liquidus and solidus temperatures are the same, so there is no plastic phase, it has the lowest possible melting point. Having the lowest possible melting point minimizes heat stress on electronic components during soldering. And, having no plastic phase allows for quicker wetting as the solder heats up, quicker setup as the solder cools. A non-eutectic formulation must remain still as the temperature drops through the liquidus and solidus temperatures. Any movement during the plastic phase may result in cracks. Common solder formulations based on tin and lead are listed below; the fraction represent percentage of tin first lead, totaling 100%: 63/37: melts at 183 °C 60/40: melts between 183–190 °C 50/50: melts between 183–215 °C For environmental reasons, lead-free solders are becoming more used.
They are suggested anywhere young children may come into contact with, or for outdoor use where rain and other precipitation may wash the lead into the groundwater. Most lead-free solders are not eutectic formulations, melting at around 250 °C, making it more difficult to create reliable joints with them. Other common solders include low-temperature formulations, which are used to join previously-soldered assemblies without unsoldering earlier connections, high-temperature formulations which are used for high-temperature operation or for first assembly of items which must not become unsoldered during subsequent operations. Alloying silver with other metals changes the melting point and wetting characteristics, tensile strength. Of all the brazing alloys, silver solders have the broadest applications. Specialty alloys are available with properties such as higher strength, the ability to solder aluminum, better electrical conductivity, higher corrosion resistance; the purpose of flux is to facilitate the soldering process.
One of the obstacles to a successful solder joint is an impurity at the site of the joint, for example, oil or oxidation. The impurities can be removed by mechanical cleaning or by chemical means, but the elevated temperatures required to melt the filler metal encourages the work piece to re-oxidize; this effect is accelerated as the soldering temperatures increase and can prevent the solder from joining to the workpiece. One of the earliest forms of flux was charcoal, which acts as a reducing agent and helps prevent oxidation during the soldering process; some fluxes go beyond the simple prevention of oxidation and provide some form of chemical cleaning. Many fluxes act as a wetting agent in the soldering process, reducing the surface tension of
A punch-down block is a type of electrical connection used in telephony. It is named because the solid copper wires are "punched down" into short open-ended slots which are a type of insulation-displacement connector; these slots cut crosswise across an insulating plastic bar, contain two sharp metal blades which cut through the wire's insulation as it is punched down. These blades make the electrical contact with the wire as well. A tool called a punch down tool is used to push the wire down and properly into the slot; some will automatically cut any excess wire off. The exact size and shape of the tool blade varies by manufacturer, which can cause problems for those working on existing installations when there is a poorly documented mix of different brands. Punch-down blocks are a quick and easy way to connect wiring, as there is no stripping of insulation and no screws to loosen and tighten. Punch-down blocks are used as patch panels, or as breakout boxes for PBX or other similar key phone systems with a 50-pin RJ21 connector.
They are sometimes used in other audio applications, such as in reconfigurable patch panels. A separate tool known as a spudger is used to remove small stray pieces of cut off wiring stuck within punch-down blocks, it is possible to insert wiring without the proper tool, but this requires great care to avoid damaging the connectors. For example, pushing a screwdriver down the middle of the block is a bad practice as it forces the two blades of the terminal post apart, leading to bad contacts, it is possible to punch-down multiple wires on top of each other in a single post of a punch-down block, but this practice is discouraged because of reliability concerns. If these multiple wires are of different thicknesses, it is more that the thinner wire will develop contact problems. Stranded wire can be used on punch-down blocks though they were designed for solid wire connections. Marginal practices like these are discouraged in large or mission-critical installations, because they can introduce troublesome intermittent connections, as well as more-obvious outright bad connections.
Once the contact blades in a punchdown block are "sprung apart" by poor practices, the entire block must be replaced to restore reliable connections. In addition, punch-down blocks are being used to handle larger numbers of faster data signals, requiring greater care and proper procedures to control impedance and crosstalk. A 66 block is used in older analog telephone systems. A 110 block is used in residential telephone and Cat 5 wire systems. A Krone block is a proprietary European alternative. A BIX block is a proprietary block developed by Nortel Networks. Media related to Punch down blocks at Wikimedia Commons
In telecommunications, a repeater is an electronic device that receives a signal and retransmits it. Repeaters are used to extend transmissions so that the signal can cover longer distances or be received on the other side of an obstruction; some types of repeaters broadcast an identical signal, but alter its method of transmission, for example, on another frequency or baud rate. There are several different types of repeaters. A broadcast relay station is a repeater used in broadcast television; when an information-bearing signal passes through a communication channel, it is progressively degraded due to loss of power. For example, when a telephone call passes through a wire telephone line, some of the power in the electric current which represents the audio signal is dissipated as heat in the resistance of the copper wire; the longer the wire is, the more power is lost, the smaller the amplitude of the signal at the far end. So with a long enough wire the call will not be audible at the other end.
The farther from a radio station a receiver is, the weaker the radio signal, the poorer the reception. A repeater is an electronic device in a communication channel that increases the power of a signal and retransmits it, allowing it to travel further. Since it amplifies the signal, it requires a source of electric power; the term "repeater" originated with telegraphy in the 19th century, referred to an electromechanical device used to regenerate telegraph signals. Use of the term has continued in data communications. In computer networking, because repeaters work with the actual physical signal, do not attempt to interpret the data being transmitted, they operate on the physical layer, the first layer of the OSI model; this is used to increase the range of telephone signals in a telephone line. Land line repeaterThey are most used in trunklines that carry long distance calls. In an analog telephone line consisting of a pair of wires, it consists of an amplifier circuit made of transistors which use power from a DC current source to increase the power of the alternating current audio signal on the line.
Since the telephone is a duplex communication system, the wire pair carries two audio signals, one going in each direction. So telephone repeaters have to be bilateral, amplifying the signal in both directions without causing feedback, which complicates their design considerably. Telephone repeaters were the first type of repeater and were some of the first applications of amplification; the development of telephone repeaters between 1900 and 1915 made long distance phone service possible. Now, most telecommunications cables are fiber optic cables. Before the invention of electronic amplifiers, mechanically coupled carbon microphones were used as amplifiers in telephone repeaters. After the turn of the 20th century it was found that negative resistance mercury lamps could amplify, they were used; the invention of audion tube repeaters around 1916 made transcontinental telephony practical. In the 1930s vacuum tube repeaters using hybrid coils became commonplace, allowing the use of thinner wires.
In the 1950s negative impedance gain devices were more popular, a transistorized version called the E6 repeater was the final major type used in the Bell System before the low cost of digital transmission made all voiceband repeaters obsolete. Frequency frogging repeaters were commonplace in frequency-division multiplexing systems from the middle to late 20th century. Submarine cable repeaterThis is a type of telephone repeater used in underwater submarine telecommunications cables; this is used to increase the range of signals in a fiber optic cable. Digital information travels through a fiber optic cable in the form of short pulses of light; the light is made up of particles called photons, which can be scattered in the fiber. An optical communications repeater consists of a phototransistor which converts the light pulses to an electrical signal, an amplifier to increase the power of the signal, an electronic filter which reshapes the pulses, a laser which converts the electrical signal to light again and sends it out the other fiber.
However, optical amplifiers are being developed for repeaters to amplify the light itself without the need of converting it to an electric signal first. This is used to extend the range of coverage of a radio signal; the history of radio relay repeaters began in 1898 from the publication by Johann Mattausch in Austrian Journal Zeitschrift für Electrotechnik. But his proposal "Translator" was not suitable for use; the first relay system with radio repeaters, which functioned, was that invented in 1899 by Emile Guarini-Foresio. A radio repeater consists of a radio receiver connected to a radio transmitter; the received signal is amplified and retransmitted on another frequency, to provide coverage beyond the obstruction. Usage of a duplexer can allow the repeater to use one antenna for both receive and transmit at the same time. Broadcast relay station, rebroadcastor or translator: This is a repeater used to extend the coverage of a radio or television broadcasting station, it consists of a secondary television transmitter.
The signal from the main transmitter comes over leased telephone lines or by microwave relay. Microwave relay: This is a specialized point-to-point telecommunications link, consisting of a microwave receiver that receives information over a beam of microwaves from an
Business telephone system
A business telephone system is a multiline telephone system used in business environments, encompassing systems ranging from the small key telephone system to the large private branch exchange. A business telephone system differs from an installation of several telephones with multiple central office lines in that the CO lines used are directly controllable in key telephone systems from multiple telephone stations, that such a system provides additional features related to call handling. Business telephone systems are broadly classified into key telephone systems, private branch exchanges, but many hybrid systems exist. A key telephone system was distinguished from a private branch exchange in that it did not require an operator or attendant at the switchboard to establish connections between the central office trunks and stations, or between stations. Technologically, private branch exchanges share lineage with central office telephone systems, in larger or more complex systems, may rival a central office system in capacity and features.
With a key telephone system, a station user could control the connections directly using line buttons, which indicated the status of lines with built-in lamps. Key telephone systems are defined by arrangements with individual line selection buttons for each available telephone line; the earliest systems were known as wiring plans and consisted of telephone sets, keys and wiring. Key was a Bell System term of art for a customer-controlled switching system such as the line-buttons on the phones associated with such systems; the wiring plans evolved into modular hardware building blocks with a variety of functionality and services in the 1A key telephone system developed in the Bell System in the 1930s. Key systems can be built using three principal architectures: electromechanical shared-control, electronic shared-control, or independent key sets. New installations of key telephone systems have become less common, as hybrid systems and private branch exchanges of comparable size have similar cost and greater functionality.
Before the advent of large-scale integrated circuits, key systems were composed of electromechanical components as were larger telephone switching systems. The systems marketed in North America as the 1A, 6A, 1A1 and the 1A2 Key System are typical examples and sold for many decades; the 1A family of Western Electric Company key telephone units were introduced in the late 1930s and remained in use to the 1950s. 1A equipment required at least two KTUs per line. The telephone instrument used by 1A systems was the WECo 300-series telephone. Introduced in 1953, 1A1 key systems simplified wiring with a single KTU for both line and station termination, increased the features available; as the 1A1 systems became commonplace, requirements for intercom features grew. The original intercom KTUs, WECo Model 207, were wired for a single talk link, that is, a single conversation on the intercom at a time; the WECo 6A dial intercom system provided two talk links and was installed as the dial intercom in a 1A1 or 1A2 key system.
The 6A systems were complex and expensive, never became popular. The advent of 1A2 technology in the 1964 simplified key system maintenance; these continued to be used throughout the 1980s, when the arrival of electronic key systems with their easier installation and greater features signaled the end of electromechanical key systems. Two lesser-known key systems were used at airports for air traffic control communications, the 102 and 302 key systems; these were uniquely designed for communications between the air traffic control tower and radar approach control or ground control approach, included radio line connections. Automatic Electric Company produced a family of key telephone equipment, some of it compatible with Western Electric equipment, but it did not gain the widespread use enjoyed by Western Electric equipment. With the advent of LSI ICs, the same architecture could be implemented much less expensively than was possible using relays. In addition, it was possible to eliminate the many-wire cabling and replace it with much simpler cable similar to that used by non-key systems.
Electronic shared-control systems led to the modern hybrid telephone system, as the features of PBX and key system merged. One of the most recognized such systems is the AT&T Merlin. Additionally, these more modern systems allowed a diverse set of features including: Answering machine functions Automatic call accounting Caller ID Remote supervision of the entire system Selection of signaling sounds Speed dialing Station-specific limitations Features could be added or modified using software, allowing easy customization of these systems; the stations were easier to maintain than the previous electromechanical key systems, as they used efficient LEDs instead of incandescent light bulbs for line status indication. LSI allowed smaller systems to distribute the control into individual telephone sets that don't require any single shared control unit; these systems are used with a few telephone sets and it is more difficult to keep the feature set in synchrony between the various sets. Into the 21st century, the distinction between key systems and PBX systems has become blurred.
Early electronic key systems used dedicated handsets which displayed and allowed access to all connected PSTN lines and stations. The modern key system now supports SIP, ISDN, analog handsets (in addition to it
Wire wrap was invented to wire telephone crossbar switches, adapted to construct electronic circuit boards. Electronic components mounted on an insulating board are interconnected by lengths of insulated wire run between their terminals, with the connections made by wrapping several turns of uninsulated sections of the wire around a component lead or a socket pin. Wires can be wrapped by hand or by machine, can be hand-modified afterwards, it was popular for large-scale manufacturing in the 1960s and early 1970s, continues today to be used for short runs and prototypes. The method eliminates the fabrication of a printed circuit board. Wire wrapping is unusual among other prototyping technologies since it allows for complex assemblies to be produced by automated equipment, but easily repaired or modified by hand. Wire wrap construction can produce assemblies which are more reliable than printed circuits: connections are less prone to fail due to vibration or physical stresses on the base board, the lack of solder precludes soldering faults such as corrosion, cold joints and dry joints.
The connections themselves are firmer and have lower electrical resistance due to cold welding of the wire to the terminal post at the corners. Wire wrap was used for assembly of high frequency prototypes and small production runs, including gigahertz microwave circuits and super computers, it is unique among automated prototyping techniques in that wire lengths can be controlled, twisted pairs or magnetically shielded twisted quads can be routed together. Wire wrap construction became popular around 1960 in circuit board manufacturing, use has now declined. Surface-mount technology has made the technique much less useful than in previous decades. Solder-less breadboards and the decreasing cost of professionally made PCBs have nearly eliminated this technology. A made wire-wrap connection for 30 or 28 AWG wire is seven turns of bare wire with half to one and a half turns of insulated wire at the bottom for strain relief; the square hard-gold-plated post thus forms 28 redundant contacts. The silver-plated wire coating cold-welds to the gold.
If corrosion occurs, it occurs on the outside of the wire, not on the gas-tight contact where oxygen cannot penetrate to form oxides. A designed wire-wrap tool applies up to twenty tons of force per square inch on each joint; the electronic parts sometimes plug into sockets. The sockets are attached with cyanoacrylate to thin plates of glass-fiber-reinforced epoxy; the sockets have square posts. The usual posts are 0.025 in square, 1 in high, spaced at 0.1 in intervals. Premium posts are hard-drawn beryllium copper alloy plated with a 0.000025 in of gold to prevent corrosion. Less-expensive posts are bronze with tin plating. 30 gauge silver-plated soft copper wire is insulated with a fluorocarbon that does not emit dangerous gases when heated. The most common insulation is "Kynar"; the 30 AWG Kynar wire is cut into standard lengths one inch of insulation is removed on each end. A "wire wrap tool" has two holes; the wire and 1⁄4 in of insulated wire are placed in a hole near the edge of the tool. The hole in the center of the tool is placed over the post.
The tool is twisted. The result is that 1.5 to 2 turns of insulated wire are wrapped around the post, above that, 7 to 9 turns of bare wire are wrapped around the post. The post has room for three such connections, although only one or two are needed; this permits manual wire-wrapping to be used for repairs. The turn and a half of insulated wire helps prevent wire fatigue. Above the turn of insulated wire, the bare wire wraps around the post; the corners of the post bite in with pressures of tons per square inch. This forces all the gases out of the area between the wire's silver plate and the post's gold or tin corners. Further, with 28 such connections, a reliable connection exists between the wire and the post. Furthermore, the corners of the posts are quite "sharp": they have a quite-small radius of curvature. There are three ways of placing wires on a board. In professionally built wire-wrap boards, long wires are placed first so that shorter wires mechanically secure the long wires. To make an assembly more repairable, wires are applied in layers.
The ends of each wire are always at the same height on the post, so that at most three wires need to be replaced to replace a wire. To make the layers easier to see, they are made with different colors of insulation. In space-rated or airworthy wire-wrap assemblies, the wires are boxed, may be conformally coated with wax to reduce vibration. Epoxy is never used for the coating. Wire-wrap works well with digital circuits with few discrete components, but is less convenient for analog systems with many discrete resistors, capacitors or other components; the sockets are an additional cost compared to directly inserting integrated circuits into a printed circuit board, add size and mass to a system. Multiple strands of wire may introduce cross-talk between circuits, of little consequence for digital circuits but a limitation for analog systems; the interconnected wires can radiate electromagnetic interference and have less predictable impedance than a printed circuit board. Wire-wrap construction cannot provide the ground planes and power distribution planes possible with multilayer printed circuit boards, increasing the possibility of noise.
Wire wrapping comes from the tradition of rope splicing. Early wire wrapping was performed ma