Token Ring local area network technology is a communications protocol for local area networks. It uses a special three-byte frame called a "token" that travels around a logical "ring" of workstations or servers; this token passing is a channel access method providing fair access for all stations, eliminating the collisions of contention-based access methods. Introduced by IBM in 1984, it was standardized with protocol IEEE 802.5 and was successful in corporate environments, but eclipsed by the versions of Ethernet. A wide range of different local area network technologies were developed in the early 1970s, of which one, the Cambridge Ring had demonstrated the potential of a token passing ring topology, many teams worldwide began working on their own implementations. At the IBM Zurich Research Laboratory Werner Bux and Hans Müller in particular worked on the design and development of IBM's Token Ring technology, while early work at MIT led to the Proteon 10 Mbit/s ProNet-10 Token Ring network in 1981 – the same year that workstation vendor Apollo Computer introduced their proprietary 12 Mbit/s Apollo Token Ring network running over 75-ohm RG-6U coaxial cabling.
Proteon evolved a 16 Mbit/s version that ran on unshielded twisted pair cable. IBM launched their own proprietary Token Ring product on October 15, 1985, it ran at 4 Mbit/s, attachment was possible from IBM PCs, midrange computers and mainframes. It used a convenient star-wired physical topology, ran over shielded twisted-pair cabling, shortly thereafter became the basis for the /IEEE standard 802.5. During this time, IBM argued that Token Ring LANs were superior to Ethernet under load, but these claims were fiercely debated. In 1988 the faster 16 Mbit/s Token Ring was standardized by the 802.5 working group, an increase to 100 Mbit/s was standardized and marketed during the wane of Token Ring's existence. However it was never used, while a 1000 Mbit/s standard was approved in 2001, no products were brought to market and standards activity came to a standstill as Fast Ethernet and Gigabit Ethernet dominated the local area networking market. Ethernet and Token Ring have some notable differences: Token Ring access is more deterministic, compared to Ethernet's contention-based CSMA/CD Ethernet supports a direct cable connection between two network interface cards by the use of a crossover cable or through auto-sensing if supported.
Token Ring does not inherently support this feature and requires additional software and hardware to operate on a direct cable connection setup. Token Ring eliminates collision by the use of a single-use token and early token release to alleviate the down time. Ethernet alleviates collision by carrier sense multiple access and by the use of an intelligent switch. Token Ring network interface cards contain all of the intelligence required for speed autodetection and can drive themselves on many Multistation Access Units that operate without power. Ethernet network interface cards can theoretically operate on a passive hub to a degree, but not as a large LAN and the issue of collisions is still present. Token Ring employs ` access priority'. Unswitched Ethernet does not have provisioning for an access priority system as all nodes have equal contest for traffic. Multiple identical MAC addresses are supported on Token Ring. Switched Ethernet cannot support duplicate MAC addresses without reprimand.
Token Ring was more complex than Ethernet, requiring a specialized processor and licensed MAC/LLC firmware for each interface. By contrast, Ethernet included both the lower licensing cost in the MAC chip; the cost of a token Ring interface using the Texas Instruments TMS380C16 MAC and PHY was three times that of an Ethernet interface using the Intel 82586 MAC and PHY. Both networks used expensive cable, but once Ethernet was standardized for unshielded twisted pair with 10BASE-T and 100BASE-TX, it had a distinct advantage and sales of it increased markedly. More significant when comparing overall system costs was the much-higher cost of router ports and network cards for Token Ring vs Ethernet; the emergence of Ethernet switches may have been the final straw. Stations on a Token Ring LAN are logically organized in a ring topology with data being transmitted sequentially from one ring station to the next with a control token circulating around the ring controlling access. Similar token passing mechanisms are used by ARCNET, token bus, 100VG-AnyLAN and FDDI, they have theoretical advantages over the CSMA/CD of early Ethernet.
A Token Ring network can be modeled as a polling system where a single server provides service to queues in a cyclic order. The data transmission process goes as follows: Empty information frames are continuously circulated on the ring; when a computer has a message to send, it seizes the token. The computer will be able to send the frame; the frame is examined by each successive workstation. The workstation that identifies itself to be the destination for the message copies it from the frame and changes the token back to 0; when the frame gets back to the originat
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
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
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
Copper has been used in electrical wiring since the invention of the electromagnet and the telegraph in the 1820s. The invention of the telephone in 1876 created further demand for copper wire as an electrical conductor. Copper is the electrical conductor in many categories of electrical wiring. Copper wire is used in power generation, power transmission, power distribution, telecommunications, electronics circuitry, countless types of electrical equipment. Copper and its alloys are used to make electrical contacts. Electrical wiring in buildings is the most important market for the copper industry. Half of all copper mined is used to manufacture electrical wire and cable conductors. Electrical conductivity is a measure of; this is an essential property in electrical wiring systems. Copper has the highest electrical conductivity rating of all non-precious metals: the electrical resistivity of copper = 16.78 nΩ•m at 20 °C. Specially-pure Oxygen-Free Electronic copper is about 1% more conductive; the theory of metals in their solid state helps to explain the unusually high electrical conductivity of copper.
In a copper atom, the outermost 4s energy zone, or conduction band, is only half filled, so many electrons are able to carry electric current. When an electric field is applied to a copper wire, the conduction of electrons accelerates towards the electropositive end, thereby creating a current; these electrons encounter resistance to their passage by colliding with impurity atoms, lattice ions, imperfections. The average distance travelled between collisions, defined as the "mean free path", is inversely proportional to the resistivity of the metal. What is unique about copper is its long mean free path; this mean free path increases as copper is chilled. Because of its superior conductivity, annealed copper became the international standard to which all other electrical conductors are compared. In 1913, the International Electrotechnical Commission defined the conductivity of commercially pure copper in its International Annealed Copper Standard, as 100% IACS = 58.0 MS/m at 20 °C, decreasing by 0.393%/°C.
Because commercial purity has improved over the last century, copper conductors used in building wire slightly exceed the 100% IACS standard. The main grade of copper used for electrical applications is electrolytic-tough pitch copper; this copper is at least 99.90% pure and has an electrical conductivity of at least 101% IACS. ETP copper contains a small percentage of oxygen. If high conductivity copper needs to be welded or brazed or used in a reducing atmosphere oxygen-free copper may be used. Several electrically conductive metals are less dense than copper, but require larger cross sections to carry the same current and may not be usable when limited space is a major requirement. Aluminium has 61% of the conductivity of copper; the cross sectional area of an aluminium conductor must be 56% larger than copper for the same current carrying capability. The need to increase the thickness of aluminium wire restricts its use in several applications, such as in small motors and automobiles. In some applications such as aerial electric power transmission cables, copper is used.
Silver, a precious metal, is the only metal with a higher electrical conductivity than copper. The electrical conductivity of silver is 106% of that of annealed copper on the IACS scale, the electrical resistivity of silver = 15.9 nΩ•m at 20 °C. The high cost of silver combined with its low tensile strength limits its use to special applications, such as joint plating and sliding contact surfaces, plating for the conductors in high-quality coaxial cables used at frequencies above 30 MHz Tensile strength measures the force required to pull an object such as rope, wire, or a structural beam to the point where it breaks; the tensile strength of a material is the maximum amount of tensile stress it can take before breaking. Copper’s higher tensile strength compared to aluminium is another reason why copper is used extensively in the building industry. Copper’s high strength resists stretching, neck-down, creep and breaks, thereby prevents failures and service interruptions. Copper is much heavier than aluminum for conductors of equal current carrying capacity, so the high tensile strength is offset by its increased weight.
Ductility is a material's ability to deform under tensile stress. This is characterized by the material's ability to be stretched into a wire. Ductility is important in metalworking because materials that crack or break under stress cannot be hammered, rolled, or drawn. Copper has a higher ductility than alternate metal conductors with the exception of silver; because of copper’s high ductility, it is easy to draw down to diameters with close tolerances. The stronger a metal is, the less pliable it is; this is not the case with copper. A unique combination of high strength and high ductility makes copper ideal for wiring systems. At junction boxes and at terminations, for example, copper can be bent and pulled without stretching or breaking. Creep is the gradual deformation of a material from constant expansions and contractions under “load, no-load” conditions; this process has adverse effects on electrical systems: terminations can become loose, causing connections to heat up or create dangerous arcing.
Copper has excellent creep characteristics. For other met
In communication systems, signal processing, electrical engineering, a signal is a function that "conveys information about the behavior or attributes of some phenomenon". In its most common usage, in electronics and telecommunication, this is a time varying voltage, current or electromagnetic wave used to carry information. A signal may be defined as an "observable change in a quantifiable entity". In the physical world, any quantity exhibiting variation in time or variation in space is a signal that might provide information on the status of a physical system, or convey a message between observers, among other possibilities; the IEEE Transactions on Signal Processing states that the term "signal" includes audio, speech, communication, sonar, radar and musical signals. In a effort of redefining a signal, anything, only a function of space, such as an image, is excluded from the category of signals, it is stated that a signal may or may not contain any information. In nature, signals can take the form of any action by one organism able to be perceived by other organisms, ranging from the release of chemicals by plants to alert nearby plants of the same type of a predator, to sounds or motions made by animals to alert other animals of the presence of danger or of food.
Signaling occurs in organisms all the way down to the cellular level, with cell signaling. Signaling theory, in evolutionary biology, proposes that a substantial driver for evolution is the ability for animals to communicate with each other by developing ways of signaling. In human engineering, signals are provided by a sensor, the original form of a signal is converted to another form of energy using a transducer. For example, a microphone converts an acoustic signal to a voltage waveform, a speaker does the reverse; the formal study of the information content of signals is the field of information theory. The information in a signal is accompanied by noise; the term noise means an undesirable random disturbance, but is extended to include unwanted signals conflicting with the desired signal. The prevention of noise is covered in part under the heading of signal integrity; the separation of desired signals from a background is the field of signal recovery, one branch of, estimation theory, a probabilistic approach to suppressing random disturbances.
Engineering disciplines such as electrical engineering have led the way in the design and implementation of systems involving transmission and manipulation of information. In the latter half of the 20th century, electrical engineering itself separated into several disciplines, specialising in the design and analysis of systems that manipulate physical signals. Definitions specific to sub-fields are common. For example, in information theory, a signal is a codified message, that is, the sequence of states in a communication channel that encodes a message. In the context of signal processing, signals are analog and digital representations of analog physical quantities. In terms of their spatial distributions, signals may be categorized as point source signals and distributed source signals. In a communication system, a transmitter encodes a message to create a signal, carried to a receiver by the communications channel. For example, the words "Mary had a little lamb" might be the message spoken into a telephone.
The telephone transmitter converts the sounds into an electrical signal. The signal is transmitted to the receiving telephone by wires. In telephone networks, for example common-channel signaling, refers to phone number and other digital control information rather than the actual voice signal. Signals can be categorized in various ways; the most common distinction is between discrete and continuous spaces that the functions are defined over, for example discrete and continuous time domains. Discrete-time signals are referred to as time series in other fields. Continuous-time signals are referred to as continuous signals. A second important distinction is between continuous-valued. In digital signal processing, a digital signal may be defined as a sequence of discrete values associated with an underlying continuous-valued physical process. In digital electronics, digital signals are the continuous-time waveform signals in a digital system, representing a bit-stream. Another important property of a signal is its information content.
Two main types of signals encountered in practice are digital. The figure shows a digital signal that results from approximating an analog signal by its values at particular time instants. Digital signals are quantized. An analog signal is any continuous signal for which the time varying feature of the signal is a representation of some other time varying quantity, i.e. analogous to another time varying signal. For example, in an analog audio signal, the instantaneous voltage of the signal varies continuously with the pressure of the sound waves, it differs from a digital signal, in which the continuous quantity is a representation of a sequence of discrete values which can only take on one of a finite number of values. The term analog signal refers to electrical signals. An analog signal uses some property of the medium to convey the signal's information. For ex