Copper cable certification
In copper twisted pair wire networks, copper cable certification is achieved through a thorough series of tests in accordance with Telecommunications Industry Association or International Organization for Standardization standards. These tests are done using a certification-testing tool, which provide fail information. While certification can be performed by the owner of the network, certification is done by datacom contractors, it is this certification. Installers who need to prove to the network owner that the installation has been done and meets TIA or ISO standards need to certify their work. Network owners who want to guarantee that the infrastructure is capable of handling a certain application will use a tester to certify the network infrastructure. In some cases, these testers are used to pinpoint specific problems. Certification tests are vital if there is a discrepancy between the installer and network owner after an installation has been performed; the performance tests and their procedures have been defined in the ANSI/TIA/EIA-568-B.1 standard and the ISO/IEC 11801 standard.
The TIA standard defines performance in categories and the ISO defines classes. These standards define the procedure to certify that an installation meets performance criteria in a given category or class; the significance of each category or class is the limit values of which the Pass/Fail and frequency ranges are measured: Cat 3 and Class C test and define communication with 16 MHz bandwidth, Cat 5e and Class D with 100 MHz bandwidth, Cat 6 and Class E up to 250 MHz, Cat6A and Class EA up to 500 MHz, Cat7 and Class F up to 600 MHZ and Cat 7A and Class FA with a frequency range through 1000 MHz. The standards define that data from each test result must be collected and stored in either print or electronic format for future inspection; the wiremap test is used to identify physical errors of the installation. See TIA/EIA-568-B for wiring diagram via Mudit The Propagation Delay test tests for the time it takes for the signal to be sent from one end and received by the other end; the Delay Skew test is used to find the difference in propagation delay between the fastest and slowest set of wire pairs.
An ideal skew is between 50 nanoseconds over a 100-meter cable. The lower this skew the better; the Cable Length test verifies that the copper cable from the transmitter to receiver does not exceed the maximum recommended distance of 100 meters in a 10BASE-T/100BASE-TX/1000BASE-T network. Insertion loss referred to as attenuation, refers to the loss of signal strength at the far end of a line compared to the signal, introduced into the line; this loss is due to the electrical resistance of the copper cable, the loss of energy through the cable insulation, impedance mismatches introduced at the connectors. Insertion loss is expressed in decibels dB. Insertion loss increases with frequency. For every 3 dB of loss, signal power is reduced by a factor of 2 and signal amplitude is reduced by a factor of 2. Return Loss is the measurement of the amount of signal, reflected back toward the transmitter; the reflection of the signal is caused by the variations of impedance in the connectors and cable and is attributed to a poorly terminated wire.
The greater the variation in impedance, the greater the return loss reading. If 3 pairs of wire pass by a substantial amount, but the 4 pair passes, it is an indication of a bad crimp or bad connection at the RJ45 plug. Return loss is not significant in the loss of a signal, but rather signal jitter. In twisted-pair cabling Near-End Crosstalk is a measure that describes the effect caused by a signal from one wire pair coupling into another wire pair and interfering with the signal therein, it is the difference, expressed in dB, between the amplitude of a transmitted signal and the amplitude of the signal coupled into another cable pair, at the signal-source end of a cable. A higher value is desirable as it indicates that less of the transmitted signal is coupled into the victim wire pair. NEXT is measured 30 meters from the injector/generator. Higher near-end crosstalk values correspond to higher overall circuit performance. Low NEXT values on a UTP LAN used with older signaling standards are detrimental.
Excessive near-end crosstalk can be an indication of improper termination. Power Sum NEXT is the sum of NEXT values from 3 wire pairs; the combined effect of NEXT can be detrimental to the signal. The Equal-Level Far-End Crosstalk test measures Far-End Crosstalk. FEXT is similar to NEXT, but happens at the receiver side of the connection. Due to attenuation on the line, the signal causing the crosstalk diminishes as it gets further away from the transmitter; because of this, FEXT is less detrimental to a signal than NEXT, but still important nonetheless. The designation was changed from ELFEXT to ACR-F. Power Sum ELFEXT is the sum of FEXT values from 3 wire pairs as they affect the other wire pair, minus the insertion loss of the channel; the designation was changed from PSELFEXT to PSACR-F. Attenuation-to-Crosstalk ratio (ACR
In telecommunications, structured cabling is building or campus cabling infrastructure that consists of a number of standardized smaller elements called subsystems. Structured cabling is the design and installation of a cabling system that will support multiple hardware uses and be suitable for today’s needs and those of the future. With a installed system and future requirements can be met, hardware, added in the future will be supported Structured cabling design and installation is governed by a set of standards that specify wiring data centers and apartment buildings for data or voice communications using various kinds of cable, most category 5e, category 6, fiber optic cabling and modular connectors; these standards define how to lay the cabling in various topologies in order to meet the needs of the customer using a central patch panel, from where each modular connection can be used as needed. Each outlet is patched into a network switch for network use or into an IP or PBX telephone system patch panel.
Lines patched as data ports into a network switch require simple straight-through patch cables at each end to connect a computer. Voice patches to PBXs in most countries require an adapter at the remote end to translate the configuration on 8P8C modular connectors into the local standard telephone wall socket. No adapter is needed in North America as the 6P2C and 6P4C plugs most used with RJ11 and RJ14 telephone connections are physically and electrically compatible with the larger 8P8C socket. RJ25 and RJ61 connections are physically but not electrically compatible, cannot be used. In the United Kingdom, an adapter must be present at the remote end as the 6-pin BT socket is physically incompatible with 8P8C, it is common to color-code patch panel cables to identify the type of connection, though structured cabling standards do not require it except in the demarcation wall field. Cabling standards require. IP phone systems can run the telephone and the computer on the same wires, eliminating the need for separate phone wiring.
Regardless of copper cable type, the maximum distance is 90 m for the permanent link installation, plus an allowance for a combined 10 m of patch cords at the ends. Cat 5e and Cat 6 can both run power over Ethernet applications up to 90 m. However, due to greater power dissipation in Cat 5e cable and power efficiency are higher when Cat 6A cabling is used to power and connect to PoE devices. Structured cabling consists of six subsystems: Entrance facilities is the point where the telephone company network ends and connects with the on-premises wiring belonging to the customer. Equipment rooms house equipment and wiring consolidation points that serve the users inside the building or campus. Backbone cabling is the inter-building and intra-building cable connections in structured cabling between entrance facilities, equipment rooms and telecommunications closets. Backbone cabling consists of the transmission media and intermediate cross-connects and terminations at these locations; this system is used in data centers.
Horizontal cabling wiring can be standard inside wiring or plenum cabling and connects telecommunications rooms to individual outlets or work areas on the floor through the wireways, conduits or ceiling spaces of each floor. A horizontal cross-connect is where the horizontal cabling connects to a patch panel or punch up block, connected by backbone cabling to the main distribution facility. Telecommunications rooms or telecommunications enclosure connects between the backbone cabling and horizontal cabling. Work-area components connect end-user equipment to outlets of the horizontal cabling system. Network cabling standards are used internationally and are published by ISO/IEC, CENELEC and the Telecommunications Industry Association. Most European countries use CENELEC, International Electrotechnical Commission or International Organization for Standardization standards; the main CENELEC document is EN50173, which introduces contextual links to the full suite of CENELEC documents. ISO/IEC 11801 heads the ISO/IEC documentation.
The Telecommunications Industry Association issue the ANSI/TIA-568 standards for telecommunications cabling in commercial premises: ANSI/TIA-568.0-D, Generic Telecommunications Cabling for Customer Premises, 2015 ANSI/TIA-568.1-D, Commercial Building Telecommunications Infrastructure Standard, 2015 ANSI/TIA-568-C.2, Balanced Twisted-Pair Telecommunication Cabling and Components Standard, published 2009 ANSI/TIA-568-C.3, Optical Fiber Cabling Components Standard, published 2008, plus errata issued in October, 2008. TIA-569-B Commercial Building Standard for Telecommunications Pathways and Spaces ANSI/TIA/EIA-606-A-2002, Administration Standard for Commercial Telecommunications Infrastructure. 110 block American National Standards Institute BICSI Registered jack, a set of standards for telecommunications cabling termination Fiber Optics LAN Section
International standard ISO/IEC 11801 Information technology — Generic cabling for customer premises specifies general-purpose telecommunication cabling systems that are suitable for a wide range of applications. It covers both balanced copper cabling and optical fibre cabling; the standard was designed for use within commercial premises that may consist of either a single building or of multiple buildings on a campus. It was optimized for premises that span up to 3 km, up to 1 km2 office space, with between 50 and 50,000 persons, but can be applied for installations outside this range. A major revision was released in November 2017, unifying requirements for commercial and industrial networks; the standard defines several link/channel classes and cabling categories of twisted-pair copper interconnects, which differ in the maximum frequency for which a certain channel performance is required: Class A: link/channel up to 100 kHz using Category 1 cable/connectors Class B: link/channel up to 1 MHz using Category 2 cable/connectors Class C: link/channel up to 16 MHz using Category 3 cable/connectors Class D: link/channel up to 100 MHz using Category 5e cable/connectors Class E: link/channel up to 250 MHz using Category 6 cable/connectors Class EA: link/channel up to 500 MHz using Category 6A cable/connectors Class F: link/channel up to 600 MHz using Category 7 cable/connectors Class FA: link/channel up to 1000 MHz using Category 7A cable/connectors Class I: link/channel up to 2000 MHz using Category 8.1 cable/connectors Class II: link/channel up to 2000 MHz using Category 8.2 cable/connectors The standard link impedance is 100 Ω.
The standard defines several classes of optical fiber interconnect: OM1: Multimode fiber type 62.5 µm core. Class F channel and Category 7 cable are backward compatible with Class D/Category 5e and Class E/Category 6. Class F features stricter specifications for crosstalk and system noise than Class E. To achieve this, shielding was added for the cable as a whole. Unshielded cables rely on the quality of the twists to protect from EMI; this involves a tight twist and controlled design. Cables with individual shielding per pair such as category 7 rely on the shield and therefore have pairs with longer twists; the Category 7 cable standard was ratified in 2002 to allow 10 Gigabit Ethernet over 100 m of copper cabling. The cable contains four twisted copper wire pairs, just like the earlier standards. Category 7 cable can be terminated either with 8P8C compatible GG45 electrical connectors which incorporate the 8P8C standard or with TERA connectors; when combined with GG-45 or TERA connectors, Category 7 cable is rated for transmission frequencies of up to 600 MHz.
However, in 2008 Category 6A was ratified and allows 10 Gbit/s Ethernet while still using the traditional 8P8C connector. Therefore, all manufacturers of active equipment and network cards have chosen to support the 8P8C for their 10 Gigabit Ethernet products on copper and not the GG45, ARJ45, or TERA; these products therefore require a Class EA channel. As of 2017 there is no equipment. Category 7 is not recognized by the TIA/EIA. Class FA channels and Category 7A cables, introduced by ISO 11801 Edition 2 Amendment 2, are defined at frequencies up to 1000 MHz, suitable for multiple applications including CATV; the intent of the Class FA was to support the future 40Gigabit Ethernet: 40Gbase-T. Simulation results have shown that 40 Gigabit Ethernet may be possible at 50 meters and 100 Gigabit Ethernet at 15 meters. In 2007, researchers at Pennsylvania State University predicted that either 32 nm or 22 nm circuits would allow for 100 Gigabit Ethernet at 100 meters. However, in 2016, the IEEE 802.3bq working group ratified the amendment 3 which defines 25Gbase-T and 40gbase-T on Category 8 cabling specified to 2000 MHz.
The Class FA therefore does not support 40G Ethernet. As of 2017 there is no equipment. Category 7A is not recognized in TIA/EIA. Category 8 was ratified by the TR43 working group under ANSI/TIA 568-C.2-1. It is defined up 2000 MHz and only for distances from 30 m to 36 m depending on the patch cords used. ISO is expected to ratify the equivalent in 2018 but will have 2 options: Class I channel: minimum cable design U/FTP or F/UTP backward compatible and interoperable with Class EA using 8P8C connectors Class II channel: F/FTP or S/FTP minimum, interoperable with Class FA using TERA o
Telecommunications Industry Association
The Telecommunications Industry Association is accredited by the American National Standards Institute to develop voluntary, consensus-based industry standards for a wide variety of Information and Communication Technologies products, represents nearly 400 companies. TIA's Standards and Technology Department operates twelve engineering committees, which develop guidelines for private radio equipment, cellular towers, data terminals, telephone terminal equipment, accessibility, VoIP devices, structured cabling, data centers, mobile device communications, multimedia multicast, vehicular telematics, healthcare ICT, machine to machine communications, smart utility networks. Overall, more than 500 active participants, communications equipment manufacturers, service providers, government agencies, academic institutions, end-users are engaged in TIA's standards setting process. To ensure that these standards become incorporated globally, TIA is engaged in the International Telecommunication Union, the International Organization for Standardization, the International Electrotechnical Commission.
The Telecommunications Industry Assoc's most adopted standards include: TIA-942 Telecommunications Infrastructure Standard for Data Centers TIA-568-C. TIA-569-B Commercial Building Standards for Telecommunications Pathways and Spaces TIA-607-B TIA-598-C TIA-222-G Structural Standard for Antenna Supporting Structures and Antennas TIA-602-A Data Transmission Systems and Equipment, which standardized the common basic Hayes command set. TIA-102 - Land Mobile Communications for Public Safety TIA encourages engineers who represent the manufacturers and/or users of network equipment technology products and services, to become engaged in TIA's engineering committees, by voting and submitting technical contributions for inclusion in future standards. TIA is a participating standards organization of the ITU-T Global Standards Collaboration initiative; the GSC has created a Machine-to-Machine Standardization Task Force to foster industry collaboration on standards across different vertical markets, such as finance, e-health, connected vehicles, utilities.
TIA supported the E-LABEL Act, a bill that would direct the Federal Communications Commission to allow manufacturers of electronic devices with a screen to display information required by the agency digitally on the screen rather than on a label affixed to the device. Grant Seiffert argued that "by granting device manufacturers the ability to use e-labels, the legislation eases the technical and logistical burdens on manufactures and improves consumer access to important device information." Telecommunications Industry Association Website
Category 6 cable
Category 6 cable referred to as Cat 6, is a standardized twisted pair cable for Ethernet and other network physical layers, backward compatible with the Category 5/5e and Category 3 cable standards. Compared with Cat 5 and Cat 5e, Cat 6 features more stringent specifications for crosstalk and system noise; the cable standard specifies performance of up to 250 MHz compared to 100 MHz for Cat 5 and Cat 5e. Whereas Category 6 cable has a reduced maximum length of 55 meters when used for 10GBASE-T, Category 6A cable is characterized to 500 MHz and has improved alien crosstalk characteristics, allowing 10GBASE-T to be run for the same 100 meter maximum distance as previous Ethernet variants. Cat 6 cable can be identified by the printing on the side of the cable sheath. Cable types, connector types and cabling topologies are defined by TIA/EIA-568. Cat 6 patch cables are terminated in 8P8C modular connectors. Connectors use either T568B pin assignments. If Cat 6 rated patch cables and connectors are not used with Cat 6 wiring, overall performance is degraded and may not meet Cat 6 performance specifications.
The standard for Category 6A is ANSI/TIA-568-C.1, defined by the Telecommunications Industry Association for enhanced performance standards for twisted pair cable systems. It was defined in 2009. Cat 6A performance is defined for frequencies up to 500 MHz—twice that of Cat 6. Cat 6A has an alien crosstalk specification as compared to Cat 6, which picks up high levels of alien noise at high frequencies; the global cabling standard ISO/IEC 11801 has been extended by the addition of amendment 2. This amendment defines new specifications for Cat 6A components and Class EA permanent links; these new global Cat 6A/Class EA specifications require a new generation of connecting hardware offering far superior performance compared to the existing products that are based on the American TIA standard. The most important point is a performance difference between ISO/IEC and EIA/TIA component specifications for the NEXT transmission parameter. At a frequency of 500 MHz, an ISO/IEC Cat 6A connector performs 3 dB better than a Cat 6A connector that conforms with the EIA/TIA specification.
3 dB equals 50% reduction of near-end crosstalk noise signal power. Confusion therefore arises because of the different naming conventions and performance benchmarks laid down by the International ISO/IEC and American TIA/EIA standards, which in turn are different from the regional European standard, EN 50173-1. In broad terms, the ISO standard for Cat 6A is the highest, followed by the European standard, the American; when used for 10/100/1000BASE-T, the maximum allowed. This consists of 90 meters of solid "horizontal" cabling between the patch panel and the wall jack, plus 5 meters of stranded patch cable between each jack and the attached device. For 10GBASE-T, an unshielded Cat 6 cable should not exceed 55 meters and a Cat 6A cable should not exceed 100 meters. Category 6 and 6A cable must be terminated to meet specifications; the cable must not be bent too tightly. The wire pairs must not be untwisted and the outer jacket must not be stripped back more than 0.5 in. Cable shielding may be required in order to improve a Cat 6 cable's performance in high electromagnetic interference environments.
This shielding reduces the corrupting effect of EMI on the cable's data. Shielding is maintained from one cable end to the other using a drain wire that runs through the cable alongside the twisted pairs; the shield's electrical connection to the chassis on each end is made through the jacks. The requirement for ground connections at both cable ends creates the possibility that a ground loop may result if one of the networked chassis is at different instantaneous electrical potential with respect to its mate; this undesirable situation may compel currents to flow between chassis through the network cable shield, these currents may in turn induce detrimental noise in the signal being carried by the cable. Soon after the ratification of Cat 6, a number of manufacturers began offering cable labeled as "Category 6e", their intent was to suggest their offering was an upgrade to the Category 6 standard—presumably naming it after Category 5e, a standardized enhancement to Category 5 cable. However, no legitimate Category 6e standard exists, Cat 6e is not a recognized standard by the Telecommunications Industry Association.
Category 7 is an ISO standard, but not a TIA standard. Cat 7 is in place as a shielded cable solution with non-traditional connectors that are not backward-compatible with category 3 through 6A. Category 8 is the next UTP cabling offering to be backward compatible. "10 Gb/s Over Copper: Horizontal Cabling Choices". The Siemon Company. 2006-01-10. Retrieved 2015-02-13. Information on cable construction and alien crosstalk mitigation. Schmidt, John. "Determining the Right Media". BICSI News. 28. Archived from the original on 2010-01-04. Information on TIA TSB-155 37m versus IEEE 55m limitations. "What Really Changes With Category 6". The Siemon Company. Retrieved 2013-01-05
Ethernet over twisted pair
Ethernet over twisted pair technologies use twisted-pair cables for the physical layer of an Ethernet computer network. They are a subset of all Ethernet physical layers. Early Ethernet had used various grades of coaxial cable, but in 1984, StarLAN showed the potential of simple unshielded twisted pair; this led to the development of 10BASE-T and its successors 100BASE-TX, 1000BASE-T and 10GBASE-T, supporting speeds of 10, 100 Mbit/s and 1 and 10 Gbit/s respectively. All these standards use 8P8C connectors, the cables from Cat 3 to Cat 8; the first two early designs of twisted pair networking were StarLAN, standardized by the IEEE Standards Association as IEEE 802.3e in 1986, at one megabit per second, LattisNet, developed in January 1987, at 10 megabit per second. Both were developed before the 10BASE-T standard and used different signalling, so they were not directly compatible with it. In 1988 AT&T released StarLAN 10, named for working at 10 Mbit/s; the StarLAN 10 signalling was used as the basis of 10BASE-T, with the addition of link beat to indicate connection status.
Using twisted pair cabling, in a star topology, for Ethernet addressed several weaknesses of the previous standards: Twisted pair cables were in use for telephone service and were present in many office buildings, lowering overall cost The centralized star topology in use for telephone service and was a more common approach to cabling than the bus in earlier standards and easier to manage Using point-to-point links was less prone to failure and simplified troubleshooting compared to a shared bus Exchanging cheap repeater hubs for more advanced switching hubs provided a viable upgrade path Mixing different speeds in a single network became possible with the arrival of Fast Ethernet Depending on cable grades, subsequent upgrading to Gigabit Ethernet or faster could be accomplished by replacing the network switchesAlthough 10BASE-T is used as a normal-operation signaling rate today, it is still in wide use with NICs in Wake-on-LAN power-down mode and for special, low-power, low-bandwidth applications.
10BASE-T is still supported on most twisted-pair Ethernet ports with up to Gigabit Ethernet speed. The common names for the standards derive from aspects of the physical media; the leading number refers to the transmission speed in Mbit/s. BASE denotes; the T designates twisted pair cable. Where there are several standards for the same transmission speed, they are distinguished by a letter or digit following the T, such as TX or T4, referring to the encoding method and number of lanes. Most Ethernet cables are wired "straight-through". In some instances the "crossover" form may still be required. Cables for Ethernet may be wired to either the T568A or T568B termination standards at both ends of the cable. Since these standards differ only in that they swap the positions of the two pairs used for transmitting and receiving, a cable with T568A wiring at one end and T568B wiring at the other results in a crossover cable. A 10BASE-T or 100BASE-TX host uses a connector wiring called medium dependent interfaces, transmitting on pins 1 and 2 and receiving on pins 3 and 6 to a network device.
An infrastructure node accordingly uses a connector wiring called MDI-X, transmitting on pins 3 and 6 and receiving on pins 1 and 2. These ports are connected using a straight-through cable so each transmitter talks to the receiver on the other end of the cable. Nodes can have two types of ports: MDI or MDI-X. Hubs and switches have regular ports. Routers and end hosts have uplink ports; when two nodes having the same type of ports need to be connected, a crossover cable may be required for older equipment. Connecting nodes having different type of ports requires straight-through cable, thus connecting an end host to a hub or switch requires a straight-through cable. Some older switches and hubs provided a button to allow a port to act as either a normal or an uplink port, i.e. using MDI-X or MDI pinout respectively. Many modern Ethernet host adapters can automatically detect another computer connected with a straight-through cable and automatically introduce the required crossover, if needed. Most newer switches have auto MDI-X on all ports allowing all connections to be made with straight-through cables.
If both devices being connected support 1000BASE-T according to the standards, they will connect regardless of whether a straight-through or crossover cable is used. A 10BASE-T transmitter sends two differential voltages, +2.5 V or −2.5 V. A 100BASE-TX transmitter sends three differential voltages, +1 V, 0 V, or −1 V. Unlike earlier Ethernet standards using broadband and coaxial cable, such as 10BASE5 and 10BASE2, 10BASE-T does not specify the exact type of wiring to be used, but instead specifies certain characteristics that a cable must meet; this was done in anticipation of using 10BASE-T in existing twisted-pair wiring systems that did not conform to any specified wiring standard. Some of the specified characteristics are attenuation, characteristic impedance, timing jitter, propagation delay, several types of noise and crosstalk. Cable testers are available to check these parameters to determine if a cable can be used with 10BASE-T; these characteristics are expected to be met by 100 meters of 24-gauge unshielded twisted-pair cable.
However, with high quality cabling, reliable cable runs of 150 meters or longer are o
A telephone, or phone, is a telecommunications device that permits two or more users to conduct a conversation when they are too far apart to be heard directly. A telephone converts sound and most efficiently the human voice, into electronic signals that are transmitted via cables and other communication channels to another telephone which reproduces the sound to the receiving user. In 1876, Scottish emigrant Alexander Graham Bell was the first to be granted a United States patent for a device that produced intelligible replication of the human voice; this instrument was further developed by many others. The telephone was the first device in history that enabled people to talk directly with each other across large distances. Telephones became indispensable to businesses and households and are today some of the most used small appliances; the essential elements of a telephone are a microphone to speak into and an earphone which reproduces the voice in a distant location. In addition, most telephones contain a ringer to announce an incoming telephone call, a dial or keypad to enter a telephone number when initiating a call to another telephone.
The receiver and transmitter are built into a handset, held up to the ear and mouth during conversation. The dial may be located either on a base unit to which the handset is connected; the transmitter converts the sound waves to electrical signals which are sent through a telephone network to the receiving telephone, which converts the signals into audible sound in the receiver or sometimes a loudspeaker. Telephones are duplex devices; the first telephones were directly connected to each other from one customer's office or residence to another customer's location. Being impractical beyond just a few customers, these systems were replaced by manually operated centrally located switchboards; these exchanges were soon connected together forming an automated, worldwide public switched telephone network. For greater mobility, various radio systems were developed for transmission between mobile stations on ships and automobiles in the mid-20th century. Hand-held mobile phones were introduced for personal service starting in 1973.
In decades their analog cellular system evolved into digital networks with greater capability and lower cost. Convergence has given most modern cell phones capabilities far beyond simple voice conversation, they may be able to record spoken messages and receive text messages and display photographs or video, play music or games, surf the Internet, do road navigation or immerse the user in virtual reality. Since 1999, the trend for mobile phones is smartphones that integrate all mobile communication and computing needs. A traditional landline telephone system known as plain old telephone service carries both control and audio signals on the same twisted pair of insulated wires, the telephone line; the control and signaling equipment consists of three components, the ringer, the hookswitch, a dial. The ringer, or beeper, light or other device, alerts the user to incoming calls; the hookswitch signals to the central office that the user has picked up the handset to either answer a call or initiate a call.
A dial, if present, is used by the subscriber to transmit a telephone number to the central office when initiating a call. Until the 1960s dials used exclusively the rotary technology, replaced by dual-tone multi-frequency signaling with pushbutton telephones. A major expense of wire-line telephone service is the outside wire plant. Telephones transmit both the outgoing speech signals on a single pair of wires. A twisted pair line rejects electromagnetic interference and crosstalk better than a single wire or an untwisted pair; the strong outgoing speech signal from the microphone does not overpower the weaker incoming speaker signal with sidetone because a hybrid coil and other components compensate the imbalance. The junction box arrests lightning and adjusts the line's resistance to maximize the signal power for the line length. Telephones have similar adjustments for inside line lengths; the line voltages are negative compared to earth. Negative voltage attracts positive metal ions toward the wires.
The landline telephone contains a switchhook and an alerting device a ringer, that remains connected to the phone line whenever the phone is "on hook", other components which are connected when the phone is "off hook". The off-hook components include a transmitter, a receiver, other circuits for dialing and amplification. A calling party wishing to speak to another party will pick up the telephone's handset, thereby operating a lever which closes the switchhook, which powers the telephone by connecting the transmitter and related audio components to the line; the off-hook circuitry has a low resistance which causes a direct current, which comes down the line from the telephone exchange. The exchange detects this current, attaches a digit receiver circuit to the line, sends a dial tone to indicate readiness. On a modern push-button telephone, the caller presses the number keys to send the telephone number of the called party; the keys control a tone generator circuit. A rotary-dial telephone uses pulse