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Multipath propagation

In radio communication, multipath is the propagation phenomenon that results in radio signals reaching the receiving antenna by two or more paths. Causes of multipath include atmospheric ducting, ionospheric reflection and refraction, and reflection from water bodies and terrestrial objects such as mountains and buildings. When the same signal is received over more than one path, it can create interference and phase shifting of the signal. Destructive interference causes fading; this may cause a radio signal to become too weak in certain areas to be received adequately. For this reason, this effect is also known as multipath interference or multipath distortion.

Where the magnitudes of the signals arriving by the various paths have a distribution known as the Rayleigh distribution, this is known as Rayleigh fading. Where one component (often, but not necessarily, a line of sight component) dominates, a Rician distribution provides a more accurate model, and this is known as Rician fading. Where two components dominate, the behavior is best modeled with the two-wave with diffuse power (TWDP) distribution. All of these descriptions are commonly used and accepted and lead to results. However, they are generic and abstract/hide/approximate the underlying physics.

Interference
Multipath interference is a phenomenon in the physics of waves whereby a wave from a source travels to a detector via two or more paths and the two (or more) components of the wave interfere constructively or destructively. Multipath interference is a common cause of "ghosting" in analog television broadcasts and of fading of radio waves.

The condition necessary is that the components of the wave remain coherent throughout the whole extent of their travel.

The interference will arise owing to the two (or more) components of the wave having, in general, travelled a different length (as measured by optical path length – geometric length and refraction (differing optical speed)), and thus arriving at the detector out of phase with each other.

The signal due to indirect paths interferes with the required signal in amplitude as well as phase which is called multipath fading.

Examples
In analog facsimile and television transmission, multipath causes jitter and ghosting, seen as a faded duplicate image to the right of the main image. Ghosts occur when transmissions bounce off a mountain or other large object, while also arriving at the antenna by a shorter, direct route, with the receiver picking up two signals separated by a delay.

In radar processing, multipath causes ghost targets to appear, deceiving the radar receiver. These ghosts are particularly bothersome since they move and behave like the normal targets (which they echo), and so the receiver has difficulty in isolating the correct target echo. These problems can be minimized by incorporating a ground map of the radar's surroundings and eliminating all echoes which appear to originate below the ground or above a certain height (altitude).

In digital radio communications (such as GSM) multipath can cause errors and affect the quality of communications. The errors are due to intersymbol interference (ISI). Equalizers are often used to correct the ISI. Alternatively, techniques such as orthogonal frequency division modulation and rake receivers may be used.

In a Global Positioning System receiver, multipath effects can cause a stationary receiver's output to indicate as if it were randomly jumping about or creeping. When the unit is moving the jumping or creeping may be hidden, but it still degrades the displayed accuracy of location and speed.

In wired media
Multipath propagation is similar in power line communication and in telephone local loops. In either case, impedance mismatch causes signal reflection.

High-speed power line communication systems usually employ multi-carrier modulations (such as OFDM or wavelet OFDM) to avoid the intersymbol interference that multipath propagation would cause. The ITU-T G.hn standard provides a way to create a high-speed (up to 1 gigabit per second) local area network using existing home wiring (power lines, phone lines, and coaxial cables). G.hn uses OFDM with a cyclic prefix to avoid ISI. Because multipath propagation behaves differently in each kind of wire, G.hn uses different OFDM parameters (OFDM symbol duration, guard interval duration) for each media.

DSL modems also use orthogonal frequency-division multiplexing to communicate with their DSLAM despite multipath. In this case the reflections may be caused by mixed wire gauges, but those from bridge taps are usually more intense and complex. Where OFDM training is unsatisfactory, bridge taps may be removed.

Mathematical modeling
The mathematical model of the multipath can be presented using the method of the impulse response used for studying linear systems.

Suppose you want to transmit a single, ideal Dirac pulse of electromagnetic power at time 0, i.e.

 * $$x(t)=\delta(t)$$

At the receiver, due to the presence of the multiple electromagnetic paths, more than one pulse will be received, and each one of them will arrive at different times. In fact, since the electromagnetic signals travel at the speed of light, and since every path has a geometrical length possibly different from that of the other ones, there are different air travelling times (consider that, in free space, the light takes 3 μs to cross a 1 km span). Thus, the received signal will be expressed by

 * $$y(t)=h(t)=\sum_{n=0}^{N-1}{\rho_n e^{j\phi_n} \delta(t-\tau_n)}$$

where $$N$$ is the number of received impulses (equivalent to the number of electromagnetic paths, and possibly very large), $$\tau_n$$ is the time delay of the generic $$n^{th}$$ impulse, and $$\rho_n e^{j\phi_n}$$ represent the complex amplitude (i.e., magnitude and phase) of the generic received pulse. As a consequence, $$y(t)$$ also represents the impulse response function $$h(t)$$ of the equivalent multipath model.

More in general, in presence of time variation of the geometrical reflection conditions, this impulse response is time varying, and as such we have

 * $$\tau_n=\tau_n(t)$$

 * $$\rho_n=\rho_n(t)$$

 * $$\phi_n=\phi_n(t)$$

Very often, just one parameter is used to denote the severity of multipath conditions: it is called the multipath time, $$T_M$$, and it is defined as the time delay existing between the first and the last received impulses

 * $$T_M=\tau_{N-1}-\tau_0$$

In practical conditions and measurement, the multipath time is computed by considering as last impulse the first one which allows receiving a determined amount of the total transmitted power (scaled by the atmospheric and propagation losses), e.g. 99%.

Keeping our aim at linear, time invariant systems, we can also characterize the multipath phenomenon by the channel transfer function $$H(f)$$, which is defined as the continuous time Fourier transform of the impulse response $$h(t)$$

 * $$H(f)=\mathfrak{F}(h(t))=\int_{-\infty}^{+\infty}{h(t)e^{-j 2\pi f t} d t}=\sum_{n=0}^{N-1}{\rho_n e^{j\phi_n} e^{-j2 \pi f \tau_n}}$$

where the last right-hand term of the previous equation is easily obtained by remembering that the Fourier transform of a Dirac pulse is a complex exponential function, an eigenfunction of every linear system.

The obtained channel transfer characteristic has a typical appearance of a sequence of peaks and valleys (also called notches); it can be shown that, on average, the distance (in Hz) between two consecutive valleys (or two consecutive peaks), is roughly inversely proportional to the multipath time. The so-called coherence bandwidth is thus defined as

 * $$B_C \approx \frac{1}{T_M}$$

For example, with a multipath time of 3 μs (corresponding to a 1 km of added on-air travel for the last received impulse), there is a coherence bandwidth of about 330 kHz.

title: Multipath propagation


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Multiple homing

 * In telecommunication, the term multiple homing has the following meanings:
 * telephone systems, the connection of a terminal facility so that it can be served by one or several switching centers. Multiple homing may use a single directory number.
 * In telephone systems, the connection of a terminal facility to more than one switching center by separate access lines. Separate directory numbers are applicable to each switching center accessed.

 * In military, such as Missiles and loitering munitions, it is ability of a single weapon system or projectile to select, focus and simultaneously engage multiple targets.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

title: Multiple homing


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Multiplex baseband

In telecommunication, the term multiplex baseband has the following meanings:
 * 1) In frequency-division multiplexing, the frequency band occupied by the aggregate of the signals in the line interconnecting the multiplexing and radio or line equipment.
 * 2) In frequency division multiplexed carrier systems, at the input to any stage of frequency translation, the frequency band occupied.

For example, the output of a group multiplexer consists of a band of frequencies from 60 kHz to 108 kHz. This is the group-level baseband that results from combining 12 voice-frequency input channels, having a bandwidth of 4 kHz each, including guard bands. In turn, 5 groups are multiplexed into a super group having a baseband of 312 kHz to 552 kHz. This baseband, however, does not represent a group-level baseband. Ten super groups are in turn multiplexed into one master group, the output of which is a baseband that may be used to modulate a microwave-frequency carrier.

title: Multiplex baseband


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Multiplexing

In telecommunications and computer networking, multiplexing (sometimes contracted to muxing) is a method by which multiple analog or digital signals are combined into one signal over a shared medium. The aim is to share a scarce resource a physical transmission medium. For example, in telecommunications, several telephone calls may be carried using one wire. Multiplexing originated in telegraphy in the 1870s, and is now widely applied in communications. In telephony, George Owen Squier is credited with the development of telephone carrier multiplexing in 1910.

The multiplexed signal is transmitted over a communication channel such as a cable. The multiplexing divides the capacity of the communication channel into several logical channels, one for each message signal or data stream to be transferred. A reverse process, known as demultiplexing, extracts the original channels on the receiver end.

A device that performs the multiplexing is called a multiplexer (MUX), and a device that performs the reverse process is called a demultiplexer (DEMUX or DMX).

Inverse multiplexing (IMUX) has the opposite aim as multiplexing, namely to break one data stream into several streams, transfer them simultaneously over several communication channels, and recreate the original data stream.

In computing, I/O multiplexing can also be used to refer to the concept of processing multiple input/output events from a single event loop, with system calls like poll and select (Unix).

Types
Multiple variable bit rate digital bit streams may be transferred efficiently over a single fixed bandwidth channel by means of statistical multiplexing. This is an asynchronous mode time-domain multiplexing which is a form of time-division multiplexing.

Digital bit streams can be transferred over an analog channel by means of code-division multiplexing techniques such as frequency-hopping spread spectrum (FHSS) and direct-sequence spread spectrum (DSSS).

In wireless communications, multiplexing can also be accomplished through alternating polarization (horizontal/vertical or clockwise/counterclockwise) on each adjacent channel and satellite, or through phased multi-antenna array combined with a multiple-input multiple-output communications (MIMO) scheme.

Space-division multiplexing
In wired communication, space-division multiplexing, also known as space-division multiple access (SDMA) is the use of separate point-to-point electrical conductors for each transmitted channel. Examples include an analogue stereo audio cable, with one pair of wires for the left channel and another for the right channel, and a multi-pair telephone cable, a switched star network such as a telephone access network, a switched Ethernet network, and a mesh network.

In wireless communication, space-division multiplexing is achieved with multiple antenna elements forming a phased array antenna. Examples are multiple-input and multiple-output (MIMO), single-input and multiple-output (SIMO) and multiple-input and single-output (MISO) multiplexing. An IEEE 802.11g wireless router with k antennas makes it in principle possible to communicate with k multiplexed channels, each with a peak bit rate of 54 Mbit/s, thus increasing the total peak bit rate by the factor k. Different antennas would give different multi-path propagation (echo) signatures, making it possible for digital signal processing techniques to separate different signals from each other. These techniques may also be utilized for space diversity (improved robustness to fading) or beamforming (improved selectivity) rather than multiplexing.

Frequency-division multiplexing
Frequency-division multiplexing (FDM) is inherently an analog technology. FDM achieves the combining of several signals into one medium by sending signals in several distinct frequency ranges over a single medium. In FDM the signals are electrical signals. One of the most common applications for FDM is traditional radio and television broadcasting from terrestrial, mobile or satellite stations, or cable television. Only one cable reaches a customer's residential area, but the service provider can send multiple television channels or signals simultaneously over that cable to all subscribers without interference. Receivers must tune to the appropriate frequency (channel) to access the desired signal.

A variant technology, called wavelength-division multiplexing (WDM) is used in optical communications.

Time-division multiplexing
Time-division multiplexing (TDM) is a digital (or in rare cases, analog) technology which uses time, instead of space or frequency, to separate the different data streams. TDM involves sequencing groups of a few bits or bytes from each individual input stream, one after the other, and in such a way that they can be associated with the appropriate receiver. If done sufficiently quickly, the receiving devices will not detect that some of the circuit time was used to serve another logical communication path.

Consider an application requiring four terminals at an airport to reach a central computer. Each terminal communicated at 2400 baud, so rather than acquire four individual circuits to carry such a low-speed transmission, the airline has installed a pair of multiplexers. A pair of 9600 baud modems and one dedicated analog communications circuit from the airport ticket desk back to the airline data center are also installed. Some web proxy servers (e.g. polipo) use TDM in HTTP pipelining of multiple HTTP transactions onto the same TCP/IP connection.

Carrier-sense multiple access and multidrop communication methods are similar to time-division multiplexing in that multiple data streams are separated by time on the same medium, but because the signals have separate origins instead of being combined into a single signal, are best viewed as channel access methods, rather than a form of multiplexing.

TD is a legacy multiplexing technology still providing the backbone of most National fixed line Telephony networks in Europe, providing the 2m/bit voice and signalling ports on Narrow band Telephone exchanges such as the DMS100. Each E1 or 2m/bit TDM port provides either 30 or 31 speech timeslots in the case of CCITT7 signalling systems and 30 voice channels for customer connected Q931, DASS2, DPNSS, V5 and CASS signalling systems.

Polarization-division multiplexing
Polarization-division multiplexing uses the polarization of electromagnetic radiation to separate orthogonal channels. It is in practical use in both radio and optical communications, particularly in 100 Gbit/s per channel fiber optic transmission systems.

Differential Cross-Polarized Wireless Communications is a novel method for polarized antenna transmission utilizing a differential technique.

Orbital angular momentum multiplexing
Orbital angular momentum multiplexing is a relatively new and experimental technique for multiplexing multiple channels of signals carried using electromagnetic radiation over a single path. It can potentially be used in addition to other physical multiplexing methods to greatly expand the transmission capacity of such systems. it is still in its early research phase, with small-scale laboratory demonstrations of bandwidths of up to 2.5 Tbit/s over a single light path. This is a controversial subject in the academic community, with many claiming it is not a new method of multiplexing, but rather a special case of space-division multiplexing.

Code-division multiplexing
Code-division multiplexing (CDM), code-division multiple access (CDMA) or spread spectrum is a class of techniques where several channels simultaneously share the same frequency spectrum, and this spectral bandwidth is much higher than the bit rate or symbol rate. One form is frequency hopping, another is direct sequence spread spectrum. In the latter case, each channel transmits its bits as a coded channel-specific sequence of pulses called chips. Number of chips per bit, or chips per symbol, is the spreading factor. This coded transmission typically is accomplished by transmitting a unique time-dependent series of short pulses, which are placed within chip times within the larger bit time. All channels, each with a different code, can be transmitted on the same fiber or radio channel or other medium, and asynchronously demultiplexed. Advantages over conventional techniques are that variable bandwidth is possible (just as in statistical multiplexing), that the wide bandwidth allows poor signal-to-noise ratio according to Shannon–Hartley theorem, and that multi-path propagation in wireless communication can be combated by rake receivers.

A significant application of CDMA is the Global Positioning System (GPS).

Multiple access method
A multiplexing technique may be further extended into a multiple access method or channel access method, for example, TDM into time-division multiple access (TDMA) and statistical multiplexing into carrier-sense multiple access (CSMA). A multiple access method makes it possible for several transmitters connected to the same physical medium to share its capacity.

Multiplexing is provided by the Physical Layer of the OSI model, while multiple access also involves a media access control protocol, which is part of the Data Link Layer.

The Transport layer in the OSI model, as well as TCP/IP model, provides statistical multiplexing of several application layer data flows to/from the same computer.

Code-division multiplexing (CDM) is a technique in which each channel transmits its bits as a coded channel-specific sequence of pulses. This coded transmission typically is accomplished by transmitting a unique time-dependent series of short pulses, which are placed within chip times within the larger bit time. All channels, each with a different code, can be transmitted on the same fiber and asynchronously demultiplexed. Other widely used multiple access techniques are time-division multiple access (TDMA) and frequency-division multiple access (FDMA). Code-division multiplex techniques are used as an access technology, namely code-division multiple access (CDMA), in Universal Mobile Telecommunications System (UMTS) standard for the third-generation (3G) mobile communication identified by the ITU.

Telegraphy
The earliest communication technology using electrical wires, and therefore sharing an interest in the economies afforded by multiplexing, was the electric telegraph. Early experiments allowed two separate messages to travel in opposite directions simultaneously, first using an electric battery at both ends, then at only one end.

Émile Baudot developed a time-multiplexing system of multiple Hughes machines in the 1870s. In 1874, the quadruplex telegraph developed by Thomas Edison transmitted two messages in each direction simultaneously, for a total of four messages transiting the same wire at the same time. Several researchers were investigating acoustic telegraphy, a frequency-division multiplexing technique, which led to the invention of the telephone.

Telephony
In telephony, a customer's telephone line now typically ends at the remote concentrator box, where it is multiplexed along with other telephone lines for that neighborhood or other similar area. The multiplexed signal is then carried to the central switching office on significantly fewer wires and for much further distances than a customer's line can practically go. This is likewise also true for digital subscriber lines (DSL).

Fiber in the loop (FITL) is a common method of multiplexing, which uses optical fiber as the backbone. It not only connects POTS phone lines with the rest of the PSTN, but also replaces DSL by connecting directly to Ethernet wired into the home. Asynchronous Transfer Mode is often the communications protocol used.

Cable TV has long carried multiplexed television channels, and late in the 20th century began offering the same services as telephone companies. IPTV also depends on multiplexing.

Video processing
In video editing and processing systems, multiplexing refers to the process of interleaving audio and video into one coherent data stream.

In digital video, such a transport stream is normally a feature of a container format which may include metadata and other information, such as subtitles. The audio and video streams may have variable bit rate. Software that produces such a transport stream and/or container is commonly called a multiplexer or muxer. A demuxer is software that extracts or otherwise makes available for separate processing the components of such a stream or container.

Digital broadcasting
In digital television systems, several variable bit-rate data streams are multiplexed together to a fixed bit rate transport stream by means of statistical multiplexing. This makes it possible to transfer several video and audio channels simultaneously over the same frequency channel, together with various services. This may involve several standard-definition television (SDTV) programmes (particularly on DVB-T, DVB-S2, ISDB and ATSC-C), or one HDTV, possibly with a single SDTV companion channel over one 6 to 8 MHz-wide TV channel. The device that accomplishes this is called a statistical multiplexer. In several of these systems, the multiplexing results in an MPEG transport stream. The newer DVB standards DVB-S2 and DVB-T2 has the capacity to carry several HDTV channels in one multiplex.

In digital radio, a multiplex (also known as an ensemble) is a number of radio stations that are grouped together. A multiplex is a stream of digital information that includes audio and other data.

On communications satellites which carry broadcast television networks and radio networks, this is known as multiple channel per carrier or MCPC. Where multiplexing is not practical (such as where there are different sources using a single transponder), single channel per carrier mode is used.

Analog broadcasting
In FM broadcasting and other analog radio media, multiplexing is a term commonly given to the process of adding subcarriers to the audio signal before it enters the transmitter, where modulation occurs. (In fact, the stereo multiplex signal can be generated using time-division multiplexing, by switching between the two (left channel and right channel) input signals at an ultrasonic rate (the subcarrier), and then filtering out the higher harmonics.) Multiplexing in this sense is sometimes known as MPX, which in turn is also an old term for stereophonic FM, seen on stereo systems since the 1960s.

Other meanings
In spectroscopy the term is used to indicate that the experiment is performed with a mixture of frequencies at once and their respective response unravelled afterwards using the Fourier transform principle.

In computer programming, it may refer to using a single in-memory resource (such as a file handle) to handle multiple external resources (such as on-disk files).

Some electrical multiplexing techniques do not require a physical "multiplexer" device, they refer to a "keyboard matrix" or "Charlieplexing" design style:
 * Multiplexing may refer to the design of a multiplexed display (non-multiplexed displays are immune to break up).
 * Multiplexing may refer to the design of a "switch matrix" (non-multiplexed buttons are immune to "phantom keys" and also immune to "phantom key blocking").

In high-throughput DNA sequencing, the term is used to indicate that some artificial sequences (often called barcodes or indexes) have been added to link given sequence reads to a given sample, and thus allow for the sequencing of multiple samples in the same reaction.

In sociolinguistics, multiplexity is used to describe the number of distinct connections between individuals who are part of a social network. A multiplex network is one in which members share a number of ties stemming from more than one social context, such as workmates, neighbors, or relatives.

title: Multiplexing


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Narrative traffic

Narrative traffic is data communications consisting of plain or encrypted messages written in a natural language and transmitted in accordance with standard formats and procedures.

Examples of narrative traffic include:
 * Messages that are placed on paper tape and transmitted via a teletypewriter (TTY), and on reception, are converted back to a printed page on another teletypewriter or teleprinter
 * Messages printed on a sheet of paper, transmitted via optical character recognition (OCR) equipment, and on reception, converted back to a printed page on a printer.

title: Narrative traffic


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Narrowband modem

In telecommunication, a narrowband modem is a modem whose modulated output signal has an essential frequency spectrum that is limited to that which can be wholly contained within, and faithfully transmitted through, a voice channel with a nominal 4 kHz bandwidth.

Note: High frequency (HF) modems are limited to operation over a voice channel with a nominal 3 kHz bandwidth.

title: Narrowband modem


text:
National Communications System

The National Communications System (NCS) was an office within the United States Department of Homeland Security charged with enabling national security and emergency preparedness communications (NS/EP telecommunications) using the national telecommunications system. The NCS was disbanded by Executive Order 13618 on July 6, 2012.

Background and history
The genesis of the NCS began in 1962 after the Cuban Missile Crisis when communications problems among the United States, the Union of Soviet Socialist Republics, the North Atlantic Treaty Organization, and foreign heads of state threatened to complicate the crisis further. After the crisis, President John F. Kennedy ordered an investigation of national security communications, and the National Security Council (NSC) formed an interdepartmental committee to examine the communications networks and institute changes. This interdepartmental committee recommended the formation of a single unified communications system to serve the President, Department of Defense, diplomatic and intelligence activities, and civilian leaders. Consequently, in order to provide better communications support to critical government functions during emergencies, President Kennedy established the National Communications System by a Presidential Memorandum on August 21, 1963. The NCS mandate included linking, improving, and extending the communications facilities and components of various Federal agencies, focusing on interconnectivity and survivability.

On April 3, 1984, President Ronald Reagan signed Executive Order 12472 which broadened the NCS' national security and emergency preparedness (NS/EP) capabilities and superseded President Kennedy's original 1963 memorandum. The NCS expanded from its original six members to an interagency group of 23 federal departments and agencies, and began coordinating and planning NS/EP telecommunications to support crises and disasters.

With the addition of the Office of the Director of National Intelligence (ODNI) on September 30, 2007, the NCS membership stood at 24 members.

Each NCS member organization was represented on the NCS through the Committee of Principals (COP) – and its subordinate Council of Representatives (COR). The COP, formed as a result of Executive Order 12472, provided advice and recommendations to the NCS and the National Security Council through the President's Critical Infrastructure Protection Board on NS/EP telecommunications and its ties to other critical infrastructures. The NCS also participated in joint industry-Government planning through its work with the President's National Security Telecommunications Advisory Committee (NSTAC), with the NCS's National Coordinating Center for Telecommunications (NCC) and the NCC's subordinate Information Sharing and Analysis Center (ISAC).

After nearly forty years with the Secretary of Defense serving as its Executive Agent, President George W. Bush transferred the National Communications System to the Department of Homeland Security (DHS). The NCS was one of 22 federal agencies transferred to the department on March 1, 2003, in accordance with Executive Order 13286. A revised Executive Order 12472 reflects the changes of E.O. 13286. On November 15, 2005, the NCS became part of the department's Directorate for Preparedness after nearly two years under the Information Analysis and Infrastructure Protection Directorate. In March 2007 the NCS became an entity of the National Protection and Programs Directorate. The DHS Under Secretary for National Protection and Programs Directorate served as the NCS Manager.

On July 6, 2012, President Barack Obama signed Executive Order 13618, which replaced Executive Order 12472, thus eliminating the NCS as a separate organization; it was merged into the Office of Emergency Communications (OEC) of DHS' National Preparedness and Programs Directorate (NPPD) which had been created in 2007. A ceremony to retire the colors of the NCS and to celebrate the legacy of the organization was held on August 30, 2012 in Arlington, VA. Upon establishment of the Cybersecurity and Infrastructure Security Agency (CISA) the OEC was renamed the Emergency Communications Division (ECD).

The President's National Security Telecommunications Advisory Committee helps strengthen United States national security, enhancing cybersecurity, maintaining the global communications infrastructure, assuring communications for disaster response, and addressing critical infrastructure interdependencies and dependencies. On September 29, 2017, President Donald Trump renewed several committees including the President's National Security Telecommunications Advisory Committee.

Services
In fulfillment of their mission to enable emergency communications, the NCS has created a number of different services.
 * NS/EP Priority Telecommunications
 * Government Emergency Telecommunications Service (GETS) - provides emergency access and priority processing in the local and long distance segments of the public switched wireline network. Used in an emergency or crisis situation during which the probability of completing a call over normal or other alternate telecommunication means has significantly decreased.
 * Telecommunications Service Priority (TSP) - provides service vendors with a Federal Communications Commission (FCC) mandate for prioritizing service requests by identifying those services critical to NS/EP. A telecommunications service with a TSP assignment is assured of receiving full attention by the service vendor before a non-TSP service.
 * Wireless Priority Service (WPS) - provides priority cellular network access. The WPS was approved by the FCC for NS/EP requirements on a call-by-call priority basis. The NCS executes the program on behalf of the Executive Office of the President. Only individuals in NS/EP key leadership positions are authorized use of WPS.
 * National Coordinating Center (NCC) for Telecommunications
 * Alerting and Coordination Network (ACN) - The Alerting and Coordination Network (ACN) provides a stable emergency voice communications network connecting telecommunications service providers’ Emergency Operations Centers (EOCs) and Network Operations Centers (NOCs) to support national security and emergency preparedness (NS/EP) telecommunications network restoration coordination, transmission of telecommunications requirements and priorities, and incident reporting when the Public Switched Network (PSN) is inoperable, stressed or congested. It is engineered to provide a reliable and survivable network capability, and, as such, has no logical dependency on the PSN. As a result, if the PSN suffers disruptions, the ACN will not be affected.
 * Shared Resources High Frequency Radio Program (SHARES) - The SHARES HF Radio Program brings together the assets of over 1,000 HF radio stations worldwide to voluntarily pass emergency messages when normal communications are destroyed or unavailable. SHARES uses common radio operating and message formatting procedures and more than 250 designated frequencies. Participation in SHARES is open to all Federal departments and agencies and their designated affiliates on a voluntary basis. More than 90 Federal, state, and industry organizations currently contribute resources throughout the United States and in 26 countries and U.S. possessions
 * Telecom ISAC - In January 2000, the National Coordinator for Security, Infrastructure Protection, and Counterterrorism designated the NCC-ISAC as the ISAC for telecommunications. On March 1, 2000, the NCC-ISAC commenced operations. The initial NCC-ISAC membership is based on NCC membership, which is evolving to reflect a broader base of technologies comprising the telecommunications infrastructure. NCC-ISAC will support the mission assigned by Executive Order 12472 and the national critical infrastructure protection goals of government and industry. The NCC-ISAC will facilitate voluntary collaboration and information sharing among its participants gathering information on vulnerabilities, threats, intrusions, and anomalies from telecommunications industry, government, and other sources. The NCC-ISAC will analyze the data with the goal of averting or mitigating impact upon the telecommunications infrastructure. Additionally, data will be used to establish baseline statistics and patterns and maintained to provide a library of historical data. Results will be sanitized and disseminated in accordance with sharing agreements established for that purpose by the NCC-ISAC participants.
 * Emergency Response Training (ERT)
 * Planning, Training, and Exercise Support (PTE) - Our mission is to ensure NCS readiness, enhance partnerships within government and industry, coordinate telecommunications operational planning among NCS elements, develop emergency response requirements, and to provide skilled civilians and reservists during crises and emergencies.
 * Individual Mobilization Augmentee (IMA) - The National Communications System (NCS) Augmentee Program was established in 1988 to provide a cadre of skilled civilian and military reservists to enhance the efforts of the Office of the Manager, NCS (OMNCS), the National Coordinating Center for Telecommunications (NCC), and NCS Regional Managers (RMs) during national crises and emergencies. The NCS Augmentee Program consists of two components: the civilian members of the National Defense Executive Reserve (NDER) and the U.S. Army reservists participating in the Individual Mobilization Augmentee (IMA) Program.

title: National Communications System


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National Information Infrastructure

The National Information Infrastructure (NII) was the product of the High Performance Computing Act of 1991. It was a telecommunications policy buzzword, which was popularized during the Clinton Administration under the leadership of Vice-President Al Gore.

It proposed to build communications networks, interactive services, interoperable computer hardware and software, computers, databases, and consumer electronics in order to put vast amounts of information available to both public and private sectors. NII was to have included more than just the physical facilities (more than the cameras, scanners, keyboards, telephones, fax machines, computers, switches, compact disks, video and audio tape, cable, wire, satellites, optical fiber transmission lines, microwave nets, switches, televisions, monitors, and printers) used to transmit, store, process, and display voice, data, and images; it was also to encompass a wide range of interactive functions, user-tailored services, and multimedia databases that were interconnected in a technology-neutral manner that will favor no one industry over any other.

title: National Information Infrastructure


text:
Neper

The neper (symbol: Np) is a logarithmic unit for ratios of measurements of physical field and power quantities, such as gain and loss of electronic signals. The unit's name is derived from the name of John Napier, the inventor of logarithms. As is the case for the decibel and bel, the neper is a unit defined in the international standard ISO 80000. It is not part of the International System of Units (SI), but is accepted for use alongside the SI.

Definition
Like the decibel, the neper is a unit in a logarithmic scale. While the bel uses the decadic (base-10) logarithm to compute ratios, the neper uses the natural logarithm, based on Euler's number (e ≈ 2.71828). The level of a ratio of two signal amplitudes or root-power quantities, with the unit neper, is given by

L = \ln\frac{x_1}{x_2}\mathrm{~Np}, $$

where $$x_1$$ and $$x_2$$ are the signal amplitudes, and $ln$ is the natural logarithm. The level of a ratio of two power quantities, with the unit neper, is given by

L = \frac{1}{2} \ln\frac{p_1}{p_2}\mathrm{~Np}, $$

where $$p_1$$ and $$p_2$$ are the signal powers.

In the International System of Quantities, the neper is defined as 1 Np = 1.

Units
The neper is defined in terms of ratios of field quantities — also called root-power quantities — (for example, voltage or current amplitudes in electrical circuits, or pressure in acoustics), whereas the decibel was originally defined in terms of power ratios. A power ratio 10 log r dB is equivalent to a field-quantity ratio 20 log r dB, since power in a linear system is proportional to the square (Joule's laws) of the amplitude. Hence the decibel and the neper have a fixed ratio to each other:
 * $$ 1\ \text{Np} = 20 \log_{10} e\ \text{dB} \approx \text{8.685889638 dB} $$

and

1\ \mathrm{dB} = \frac{1}{20} \ln(10)\ \mathrm{Np} \approx \text{0.115129255 Np} .$$

The (voltage) level ratio is

\begin{align} L & = 10 \log_{10} \frac{x_1^2}{x_2^2} & \text{dB} \\ & = 10 \log_{10} {\left(\frac{x_1}{x_2}\right)}^2 & \text{dB} \\ & = 20 \log_{10} \frac{x_1}{x_2} & \text{dB} \\ & = \ln \frac{x_1}{x_2} & \text{Np}. \\ \end{align} $$

Like the decibel, the neper is a dimensionless unit. The International Telecommunication Union (ITU) recognizes both units. Only the neper is coherent with the SI.

Applications
The neper is a natural linear unit of relative difference, meaning in nepers (logarithmic units) relative differences add rather than multiply. This property is shared with logarithmic units in other bases, such as the bel.

The derived units decineper (1 dNp = 0.1 neper) and centineper (1 cNp = 0.01 neper) are also used. The centineper for root-power quantities corresponds to a log point or log percentage, see .

title: Neper


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Net gain (telecommunications)

In telecommunications, net gain is the overall gain of a transmission circuit. Net gain is measured by applying a test signal at an appropriate power level at the input port of a circuit and measuring the power delivered at the output port. The net gain in dB is calculated by taking 10 times the common logarithm of the ratio of the output power to the input power.

The net gain expressed in dB may be positive or negative. If the net gain expressed in dB is negative, it is also called the net loss. If the net gain is expressed as a ratio, and the ratio is less than unity, a net loss is indicated.

The test signal must be chosen so that its power level is within the usual operating range of the circuit being tested.

title: Net gain (telecommunications)


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Net operation

A radio net is three or more radio stations communicating with each other on a common channel or frequency. A net is essentially a moderated conference call conducted over two-way radio, typically in half-duplex operating conditions. The use of half-duplex operation requires a very particular set of operating procedures to be followed in order to avoid inefficiencies and chaos.

Nets operate either on schedule or continuously (continuous watch). Nets operating on schedule handle traffic only at definite, prearranged times and in accordance with a prearranged schedule of intercommunication. Nets operating continuously are prepared to handle traffic at any time; they maintain operators on duty at all stations in the net at all times. When practicable, messages relating to schedules will be transmitted by a means of signal communication other than radio.

Net operations:
 * allow participants to conduct ordered conferences among participants who usually have common information needs or related functions to perform
 * are characterized by adherence to standard formats and procedures, and
 * are responsive to a common supervisory station, called the "net control station", which permits access to the net and maintains net operational discipline.

Net manager
A net manager is the person who supervises the creation and operation of a net over multiple sessions. This person will specify the format, date, time, participants, and the net control script. The net manager will also choose the Net Control Station for each net, and may occasionally take on that function, especially in smaller organizations.

Net Control Station
Radio nets are like conference calls in that both have a moderator who initiates the group communication, who ensures all participants follow the standard procedures, and who determines and directs when each other station may talk. The moderator in a radio net is called the Net Control Station, formally abbreviated NCS, and has the following duties: The Net Control Station will, for each net, appoint at least one Alternate Net Control Station, formally abbreviated ANCS (abbreviated NC2 in WWII procedures), who has the following duties:
 * Establishes the net and closes the net;
 * Directs Net activities, such as passing traffic, to maintain optimum efficiency;
 * Chooses net frequency, maintains circuit discipline and frequency accuracy;
 * Maintains a net log and records participation in the net and movement of messages; (always knows who is on and off net)
 * Appoints one or more Alternate Net Control Stations (ANCS);
 * Determines whether and when to conduct network continuity checks;
 * Determines when full procedure and full call signs may enhance communications;
 * Subject to Net Manager guidance, directs a net to be directed or free.
 * Assists the NCS to maintain optimum efficiency;
 * Assumes NCS duties in event that the NCS develops station problems;
 * Assumes NCS duties for a portion of the net, as directed or as needed;
 * Serves as a resource for the NCS; echoes transmissions of the NCS if, and only if, directed to do so by the NCS;
 * Maintains a duplicate net log

Structure of the net
Nets can be described as always having a net opening and a net closing, with a roll call normally following the net opening, itself followed by regular net business, which may include announcements, official business, and message passing. Military nets will follow a very abbreviated and opaque version of the structure outlined below, but will still have the critical elements of opening, roll call, late check-ins, and closing.

A net should always operate on the same principle as the inverted pyramid used in journalism—the most important communications always come first, followed by content in ever lower levels of priority. Each net will typically have a main purpose, which varies according to the organization conducting the net, which occurs during the net business phase. For amateur radio nets, it's typically for the purpose of allowing stations to discuss their recent operating activities (stations worked, antennas built, etc.) or to swap equipment. For Military Auxiliary Radio System and National Traffic System nets, net business will involve mainly the passing of formal messages, known as radiograms.
 * 1) Net opening
 * 2) Identification of the NCS
 * 3) Announcement of the regular date, time, and frequency of the net
 * 4) Purpose of the net
 * 5) Roll call
 * 6) A call for stations to check in, oftentimes from a roster of regular stations
 * 7) A call for late check-ins (stations on the roster who did not respond to the first check-in period)
 * 8) A call for guest stations to check in
 * 9) Net business
 * 10) Optional conversion to a free net
 * 11) Net closing

Two modes of net operation

 * Directed Net
 * A net in which no station other than the net control station can communicate with any other station, except for the transmission of urgent messages, without first obtaining the permission of the net control station.
 * Free net
 * A net in which any station may communicate with any other station in the same net without first obtaining permission from the net control station to do so.

Net-control procedure words
U.S. Army Field Manual ACP 125(G) has the most complete set of procedure words used in radio nets:

Types of radio nets
Maritime mobile nets serve the needs of seagoing vessels.

Civil Air Patrol nets
The Civil Air Patrol defines a different set of nets:

Amateur radio nets
The International Amateur Radio Union defines six different types of nets in its IARU Emergency Telecommunications Guide: Other Amateur radio net types

U.S. Military radio nets
The U.S. Army Field Manual FM 6-02.53, Tactical Radio Operations, defines the following types of radio nets:

Maritime radio nets
When boats or ships are in distress, they will operate a maritime broadcast communications net to communicate among the vessel in distress and all the other vessels, aircraft, and shore stations assisting in the distress response.

title: Net operation


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Network architecture

Network architecture is the design of a computer network. It is a framework for the specification of a network's physical components and their functional organization and configuration, its operational principles and procedures, as well as communication protocols used.

In telecommunication, the specification of a network architecture may also include a detailed description of products and services delivered via a communications network, as well as detailed rate and billing structures under which services are compensated.

The network architecture of the Internet is predominantly expressed by its use of the Internet protocol suite, rather than a specific model for interconnecting networks or nodes in the network, or the usage of specific types of hardware links.

OSI model
The Open Systems Interconnection model (OSI model) defines and codifies the concept of layered network architecture. Abstraction layers are used to subdivide a communications system further into smaller manageable parts. A layer is a collection of similar functions that provide services to the layer above it and receives services from the layer below it. On each layer, an instance provides services to the instances at the layer above and requests services from the layer below.

Distributed computing
In distributed computing, the network architecture often describes the structure and classification of a distributed application architecture, as the participating nodes in a distributed application are often referred to as a network. For example, the applications architecture of the public switched telephone network (PSTN) has been termed the Intelligent Network. There are a number of specific classifications but all lie on a continuum between the dumb network (e.g. the Internet) and the intelligent network (e.g. the PSTN).

A popular example of such usage of the term in distributed applications, as well as permanent virtual circuits, is the organization of nodes in peer-to-peer (P2P) services and networks. P2P networks usually implement overlay networks running over an underlying physical or logical network. These overlay networks may implement certain organizational structures of the nodes according to several distinct models, the network architecture of the system.

title: Network architecture


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Network engineering

Network engineering may refer to:

 * The field concerned with internetworking service requirements for switched telephone networks
 * The field concerned with Computer Networking; the design and management of computer networks
 * The field concerned with Telecommunications Engineering; developing telecommunications network topologies
 * The field concerned with Broadcasting; spreading messages to a dispersed audience electronically.

title: Network engineering


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Network interface

Network interface may refer to:

 * Network interface controller, a computer hardware component that connects a computer to a computer network
 * Network interface device, a device that serves as the demarcation point between a telephone carrier's local loop and the customer's wiring
 * Virtual network interface, an abstract virtualized representation of a computer network interface
 * Loopback interface, a virtual network interface that connects a host to itself

title: Network interface


text:
Network interface device

In telecommunications, a network interface device (NID; also known by several other names) is a device that serves as the demarcation point between the carrier's local loop and the customer's premises wiring. Outdoor telephone NIDs also provide the subscriber with access to the station wiring and serve as a convenient test point for verification of loop integrity and of the subscriber's inside wiring.

Naming
Generically, an NID may also be called a network interface unit (NIU), telephone network interface (TNI), system network interface (SNI), or telephone network box.

Australia's National Broadband Network uses the term network termination device or NTD.

A smartjack is a type of NID with capabilities beyond simple electrical connection, such as diagnostics.

An optical network terminal (ONT) is a type of NID used with fiber-to-the-premises applications.

Wiring termination
The simplest NIDs are essentially just a specialized set of wiring terminals. These will typically take the form of a small, weather-proof box, mounted on the outside of the building. The telephone line from the telephone company will enter the NID and be connected to one side. The customer connects their wiring to the other side. A single NID enclosure may contain termination for a single line or multiple lines.

In its role as the demarcation point (dividing line), the NID separates the telephone company's equipment from the customer's wiring and equipment. The telephone company owns the NID itself, and all wiring up to it. Anything past the NID is the customer's responsibility. To facilitate this, there is typically a test jack inside the NID. Accessing the test jack disconnects the customer premises wiring from the public switched telephone network and allows the customer to plug a "known good" telephone into the jack to isolate trouble. If the telephone works at the test jack, the problem is the customer's wiring, and the customer is responsible for repair. If the telephone does not work, the line is faulty and the telephone company is responsible for repair.

Most NIDs also include "circuit protectors", which are surge protectors for a telephone line. They protect customer wiring, equipment, and personnel from any transient energy on the line, such as from a lightning strike to a telephone pole.

Simple NIDs contain no digital logic; they are "dumb" devices. They have no capabilities beyond wiring termination, circuit protection, and providing a place to connect test equipment.

Smartjack

Several types of NIDs provide more than just a terminal for the connection of wiring. Such NIDs are colloquially called smartjacks or Intelligent Network Interface Devices (INIDs) as an indication of their built-in "intelligence", as opposed to a simple NID, which is just a wiring device. Smartjacks are typically used for more complicated types of telecommunications service, such as T1 lines. Plain old telephone service lines generally cannot be equipped with smartjacks.

Despite the name, most smartjacks are much more than a simple telephone jack. One common form for a smartjack is a printed circuit board with a face plate on one edge, mounted in an enclosure.

A smartjack may provide signal conversion, converting codes and protocols (e.g. framing types) to the type needed by the customer equipment. It may buffer and/or regenerate the signal, to compensate for signal degradation from line transmission, similar to what a repeater does.

Smartjacks also typically provide diagnostic capabilities. A very common capability provided by a smartjack is loopback, such that the signal from the telephone company is transmitted back to the telephone company. This allows the company to test the line from the central telephone exchange, without the need to have test equipment at the customer site. The telephone company usually has the ability to remotely activate loopback, without even needing personnel at the customer site. When looped back, the customer equipment is disconnected from the line.

Additional smartjack diagnostic capabilities include alarm indication signal, which reports trouble at one end of the line to the far end. This helps the telephone company know if trouble is present in the line, the smartjack, or customer equipment. Indicator lights to show configuration, status, and alarms are also common.

Smartjacks typically derive their operating power from the telephone line, rather than relying on premises electrical power, although this is not a universal rule.

Optical network terminals

In fiber-to-the-premises systems, the signal is transmitted to the customer premises using fiber optic technologies. Unlike many conventional telephone technologies, this does not provide power for premises equipment, nor is it suitable for direct connection to customer equipment. An optical network terminal (ONT, an ITU-T term), also known as an optical network unit (ONU, an IEEE term), is used to terminate the fiber optic line, demultiplex the signal into its component parts (voice telephone, television, and Internet access), and provide power to customer telephones. If the device combines all these services into one it is known as an IAD. As the ONT must derive its power from the customer premises electrical supply, many ONTs have the option for a battery backup in order to maintain service in the event of a power outage. These terminals are used in both active optical networks and passive optical networks. An ONT can work in Single Family Unit/SFU mode (modem/bridge) or Home Gateway Unit/HGU mode (router).

Environmental conditions
According to Telcordia GR-49, requirements for telecommunications NIDs vary based on three categories of environmental conditions:

 * 1) Normal conditions: This refers to a normal environment that is expected in most areas of any service provider. Temperatures are expected to be in the range of -20 to 32 °C, and humidity is expected to be less than 90% RH. No unusual contamination is expected.
 * 2) Severe climatic conditions: These cover environments more severe than those of a normal environment (i.e., higher humidity, high lightning activity, exposure to salt-laden atmosphere, and exposure to contaminants). Temperatures are expected to be in the range of -40 to 43 °C, and humidity may exceed 90% RH. Jacks installed in NIDs in such environments are known to become contaminated and develop low insulation resistances and low dielectric breakdown voltages when subjected to high humidity. These problems can cause noisy lines or even service outages.
 * 3) Flooded conditions: These cover areas of a service provider prone to flooding, such as in coastal or flood plain locations. After a flooding incident, temperature is expected to be in the range of 4.5 to 38 °C, and humidity may exceed 90% RH. The requirements are not to determine if the NID will function during a flood, but to review the ability of the NID to function after the flood has subsided.

Service providers must decide which condition best suits their application.

title: Network interface device


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Network management

Network management is the process of administering and managing computer networks. Services provided by this discipline include fault analysis, performance management, provisioning of networks and maintaining quality of service. Network management software is used by network administrators to help perform these functions.

Technologies
A small number of accessory methods exist to support network and network device management. Network management allows IT professionals to monitor network components within large network area. Access methods include the SNMP, command-line interface (CLI), custom XML, CMIP, Windows Management Instrumentation (WMI), Transaction Language 1 (TL1), CORBA, NETCONF, and the Java Management Extensions (JMX).

Schemas include the Structure of Management Information (SMI), WBEM, the Common Information Model (CIM Schema), and MTOSI amongst others.

title: Network management


text:
Network operating system

A network operating system (NOS) is a specialized operating system for a network device such as a router, switch or firewall.

Historically operating systems with networking capabilities were described as network operating systems, because they allowed personal computers (PCs) to participate in computer networks and shared file and printer access within a local area network (LAN). This description of operating systems is now largely historical, as common operating systems include a network stack to support a client–server model.

History
Early microcomputer operating systems such as CP/M, MS-DOS and classic Mac OS were designed for one user on one computer. Packet switching networks were developed to share hardware resources, such as a mainframe computer, a printer or a large and expensive hard disk. As local area network technology became available, two general approaches to handle sharing of resources on networks arose.

Historically, a network operating system was an operating system for a computer which implemented network capabilities. Operating systems with a network stack allowed personal computers to participate in a client-server architecture in which a server enables multiple clients to share resources, such as printers. Early examples of client-server operating systems that were shipped with fully integrated network capabilities are Novell NetWare using the Internetwork Packet Exchange (IPX) network protocol and Banyan VINES which used a variant of the Xerox Network Systems (XNS) protocols.

These limited client/server networks were gradually replaced by Peer-to-peer networks, which used networking capabilities to share resources and files located on a variety of computers of all sizes. A peer-to-peer network sets all connected computers equal; they all share the same abilities to use resources available on the network. The most popular peer-to-peer networks as of 2020 are Ethernet, Wi-Fi and the Internet protocol suite. Software that allowed users to interact with these networks, despite a lack of networking support in the underlying manufacturer's operating system, was sometimes called a network operating system. Examples of such add-on software include Phil Karn's KA9Q NOS (adding Internet support to CP/M and MS-DOS), PC/TCP Packet Drivers (adding Ethernet and Internet support to MS-DOS), and LANtastic (for MS-DOS, Microsoft Windows and OS/2), and Windows for Workgroups (adding NetBIOS to Windows). Examples of early operating systems with peer-to-peer networking capabilities built-in include MacOS (using AppleTalk and LocalTalk), and the Berkeley Software Distribution.

Today, distributed computing and groupware applications have become the norm. Computer operating systems include a networking stack as a matter of course. During the 1980s the need to integrate dissimilar computers with network capabilities grew and the number of networked devices grew rapidly. Partly because it allowed for multi-vendor interoperability, and could route packets globally rather than being restricted to a single building, the Internet protocol suite became almost universally adopted in network architectures. Thereafter, computer operating systems and the firmware of network devices tended to support Internet protocols.

Network device operating systems
Network operating systems can be embedded in a router or hardware firewall that operates the functions in the network layer (layer 3). Notable network operating systems include:

Proprietary network operating systems

 * Cisco IOS, a family of network operating systems used on Cisco Systems routers and network switches. (Earlier switches ran the Catalyst operating system, or CatOS)
 * RouterOS by MikroTik
 * ZyNOS, used in network devices made by ZyXEL

FreeBSD, NetBSD, OpenBSD, and Linux-based operating systems

 * Cisco NX-OS, IOS XE, and IOS XR; families of network operating systems used across various Cisco Systems device including the Cisco Nexus and Cisco ASR platforms
 * Junos OS; a network operating system that runs on Juniper Networks platforms
 * Cumulus Linux distribution, which uses the full TCP/IP stack of Linux
 * DD-WRT, a Linux kernel-based firmware for wireless routers and access points as well as low-cost networking device platforms such as the Linksys WRT54G
 * Dell Networking Operating System; DNOS9 is NetBSD based, while OS10 uses the Linux kernel
 * Extensible Operating System runs on switches from Arista and uses an unmodified Linux kernel
 * ExtremeXOS (EXOS), used in network devices made by Extreme Networks
 * FTOS (Force10 Operating System), the firmware family used on Force10 Ethernet switches
 * ONOS, an open source SDN operating system (hosted by Linux Foundation) for communications service providers that is designed for scalability, high performance and high availability.
 * OpenBSD, an open source operating system which includes its own implementations of BGP, RPKI, OSPF, MPLS, VXLAN, and other IETF standardized networking protocols, as well as firewall (PF) and load-balancing functionality.
 * OpenWrt used to route IP packets on embedded devices
 * pfSense, a fork of M0n0wall, which uses PF
 * OPNsense, a fork of pfSense
 * SONiC, a Linux-based network operating system developed by Microsoft
 * VyOS, an open source fork of the Vyatta routing package

title: Network operating system


text:
Network topology

Network topology is the arrangement of the elements (links, nodes, etc.) of a communication network. Network topology can be used to define or describe the arrangement of various types of telecommunication networks, including command and control radio networks, industrial fieldbusses and computer networks.

Network topology is the topological structure of a network and may be depicted physically or logically. It is an application of graph theory wherein communicating devices are modeled as nodes and the connections between the devices are modeled as links or lines between the nodes. Physical topology is the placement of the various components of a network (e.g., device location and cable installation), while logical topology illustrates how data flows within a network. Distances between nodes, physical interconnections, transmission rates, or signal types may differ between two different networks, yet their logical topologies may be identical. A network's physical topology is a particular concern of the physical layer of the OSI model.

Examples of network topologies are found in local area networks (LAN), a common computer network installation. Any given node in the LAN has one or more physical links to other devices in the network; graphically mapping these links results in a geometric shape that can be used to describe the physical topology of the network. A wide variety of physical topologies have been used in LANs, including ring, bus, mesh and star. Conversely, mapping the data flow between the components determines the logical topology of the network. In comparison, Controller Area Networks, common in vehicles, are primarily distributed control system networks of one or more controllers interconnected with sensors and actuators over, invariably, a physical bus topology.

Topologies

Two basic categories of network topologies exist, physical topologies and logical topologies.

The transmission medium layout used to link devices is the physical topology of the network. For conductive or fiber optical mediums, this refers to the layout of cabling, the locations of nodes, and the links between the nodes and the cabling. The physical topology of a network is determined by the capabilities of the network access devices and media, the level of control or fault tolerance desired, and the cost associated with cabling or telecommunication circuits.

In contrast, logical topology is the way that the signals act on the network media, or the way that the data passes through the network from one device to the next without regard to the physical interconnection of the devices. A network's logical topology is not necessarily the same as its physical topology. For example, the original twisted pair Ethernet using repeater hubs was a logical bus topology carried on a physical star topology. Token Ring is a logical ring topology, but is wired as a physical star from the media access unit. Physically, Avionics Full-Duplex Switched Ethernet (AFDX) can be a cascaded star topology of multiple dual redundant Ethernet switches; however, the AFDX virtual links are modeled as time-switched single-transmitter bus connections, thus following the safety model of a single-transmitter bus topology previously used in aircraft. Logical topologies are often closely associated with media access control methods and protocols. Some networks are able to dynamically change their logical topology through configuration changes to their routers and switches.

Links
The transmission media (often referred to in the literature as the physical media) used to link devices to form a computer network include electrical cables (Ethernet, HomePNA, power line communication, G.hn), optical fiber (fiber-optic communication), and radio waves (wireless networking). In the OSI model, these are defined at layers 1 and 2 — the physical layer and the data link layer.

A widely adopted family of transmission media used in local area network (LAN) technology is collectively known as Ethernet. The media and protocol standards that enable communication between networked devices over Ethernet are defined by IEEE 802.3. Ethernet transmits data over both copper and fiber cables. Wireless LAN standards (e.g. those defined by IEEE 802.11) use radio waves, or others use infrared signals as a transmission medium. Power line communication uses a building's power cabling to transmit data.

Wired technologies
The orders of the following wired technologies are, roughly, from slowest to fastest transmission speed.

 * Coaxial cable is widely used for cable television systems, office buildings, and other work-sites for local area networks. The cables consist of copper or aluminum wire surrounded by an insulating layer (typically a flexible material with a high dielectric constant), which itself is surrounded by a conductive layer. The insulation between the conductors helps maintain the characteristic impedance of the cable which can help improve its performance. Transmission speed ranges from 200 million bits per second to more than 500 million bits per second.
 * ITU-T G.hn technology uses existing home wiring (coaxial cable, phone lines and power lines) to create a high-speed (up to 1 Gigabit/s) local area network.
 * Signal traces on printed circuit boards are common for board-level serial communication, particularly between certain types integrated circuits, a common example being SPI.
 * Ribbon cable (untwisted and possibly unshielded) has been a cost-effective media for serial protocols, especially within metallic enclosures or rolled within copper braid or foil, over short distances, or at lower data rates. Several serial network protocols can be deployed without shielded or twisted pair cabling, that is, with "flat" or "ribbon" cable, or a hybrid flat/twisted ribbon cable, should EMC, length, and bandwidth constraints permit: RS-232, RS-422, RS-485, CAN, GPIB, SCSI, etc.
 * Twisted pair wire is the most widely used medium for all telecommunication. Twisted-pair cabling consist of copper wires that are twisted into pairs. Ordinary telephone wires consist of two insulated copper wires twisted into pairs. Computer network cabling (wired Ethernet as defined by IEEE 802.3) consists of 4 pairs of copper cabling that can be utilized for both voice and data transmission. The use of two wires twisted together helps to reduce crosstalk and electromagnetic induction. The transmission speed ranges from 2 million bits per second to 10 billion bits per second. Twisted pair cabling comes in two forms: unshielded twisted pair (UTP) and shielded twisted-pair (STP). Each form comes in several category ratings, designed for use in various scenarios.

 * An optical fiber is a glass fiber. It carries pulses of light that represent data. Some advantages of optical fibers over metal wires are very low transmission loss and immunity from electrical interference. Optical fibers can simultaneously carry multiple wavelengths of light, which greatly increases the rate that data can be sent, and helps enable data rates of up to trillions of bits per second. Optic fibers can be used for long runs of cable carrying very high data rates, and are used for undersea communications cables to interconnect continents.

Price is a main factor distinguishing wired- and wireless-technology options in a business. Wireless options command a price premium that can make purchasing wired computers, printers and other devices a financial benefit. Before making the decision to purchase hard-wired technology products, a review of the restrictions and limitations of the selections is necessary. Business and employee needs may override any cost considerations.

Wireless technologies

 * Terrestrial microwave – Terrestrial microwave communication uses Earth-based transmitters and receivers resembling satellite dishes. Terrestrial microwaves are in the low gigahertz range, which limits all communications to line-of-sight. Relay stations are spaced approximately 50 km apart.
 * Communications satellites – Satellites communicate via microwave radio waves, which are not deflected by the Earth's atmosphere. The satellites are stationed in space, typically in geostationary orbit 35786 km above the equator. These Earth-orbiting systems are capable of receiving and relaying voice, data, and TV signals.
 * Cellular and PCS systems use several radio communications technologies. The systems divide the region covered into multiple geographic areas. Each area has a low-power transmitter or radio relay antenna device to relay calls from one area to the next area.
 * Radio and spread spectrum technologies – Wireless local area networks use a high-frequency radio technology similar to digital cellular and a low-frequency radio technology. Wireless LANs use spread spectrum technology to enable communication between multiple devices in a limited area. IEEE 802.11 defines a common flavor of open-standards wireless radio-wave technology known as Wi-Fi.
 * Free-space optical communication uses visible or invisible light for communications. In most cases, line-of-sight propagation is used, which limits the physical positioning of communicating devices.

Exotic technologies
There have been various attempts at transporting data over exotic media:

 * IP over Avian Carriers was a humorous April fool's Request for Comments, issued as RFC 1149. It was implemented in real life in 2001.
 * Extending the Internet to interplanetary dimensions via radio waves, the Interplanetary Internet.

Both cases have a large round-trip delay time, which gives slow two-way communication, but does not prevent sending large amounts of information.

Nodes
Network nodes are the points of connection of the transmission medium to transmitters and receivers of the electrical, optical, or radio signals carried in the medium. Nodes may be associated with a computer, but certain types may have only a microcontroller at a node or possibly no programmable device at all. In the simplest of serial arrangements, one RS-232 transmitter can be connected by a pair of wires to one receiver, forming two nodes on one link, or a Point-to-Point topology. Some protocols permit a single node to only either transmit or receive (e.g., ARINC 429). Other protocols have nodes that can both transmit and receive into a single channel (e.g., CAN can have many transceivers connected to a single bus). While the conventional system building blocks of a computer network include network interface controllers (NICs), repeaters, hubs, bridges, switches, routers, modems, gateways, and firewalls, most address network concerns beyond the physical network topology and may be represented as single nodes on a particular physical network topology.

Network interfaces
A network interface controller (NIC) is computer hardware that provides a computer with the ability to access the transmission media, and has the ability to process low-level network information. For example, the NIC may have a connector for accepting a cable, or an aerial for wireless transmission and reception, and the associated circuitry.

The NIC responds to traffic addressed to a network address for either the NIC or the computer as a whole.

In Ethernet networks, each network interface controller has a unique Media Access Control (MAC) address—usually stored in the controller's permanent memory. To avoid address conflicts between network devices, the Institute of Electrical and Electronics Engineers (IEEE) maintains and administers MAC address uniqueness. The size of an Ethernet MAC address is six octets. The three most significant octets are reserved to identify NIC manufacturers. These manufacturers, using only their assigned prefixes, uniquely assign the three least-significant octets of every Ethernet interface they produce.

Repeaters and hubs
A repeater is an electronic device that receives a network signal, cleans it of unnecessary noise and regenerates it. The signal may be reformed or retransmitted at a higher power level, to the other side of an obstruction possibly using a different transmission medium, so that the signal can cover longer distances without degradation. Commercial repeaters have extended RS-232 segments from 15 meters to over a kilometer. In most twisted pair Ethernet configurations, repeaters are required for cable that runs longer than 100 meters. With fiber optics, repeaters can be tens or even hundreds of kilometers apart.

Repeaters work within the physical layer of the OSI model, that is, there is no end-to-end change in the physical protocol across the repeater, or repeater pair, even if a different physical layer may be used between the ends of the repeater, or repeater pair. Repeaters require a small amount of time to regenerate the signal. This can cause a propagation delay that affects network performance and may affect proper function. As a result, many network architectures limit the number of repeaters that can be used in a row, e.g., the Ethernet 5-4-3 rule.

A repeater with multiple ports is known as hub, an Ethernet hub in Ethernet networks, a USB hub in USB networks.
 * USB networks use hubs to form tiered-star topologies.
 * Ethernet hubs and repeaters in LANs have been mostly obsoleted by modern switches.

Bridges
A network bridge connects and filters traffic between two network segments at the data link layer (layer 2) of the OSI model to form a single network. This breaks the network's collision domain but maintains a unified broadcast domain. Network segmentation breaks down a large, congested network into an aggregation of smaller, more efficient networks.

Bridges come in three basic types:
 * Local bridges: Directly connect LANs
 * Remote bridges: Can be used to create a wide area network (WAN) link between LANs. Remote bridges, where the connecting link is slower than the end networks, largely have been replaced with routers.
 * Wireless bridges: Can be used to join LANs or connect remote devices to LANs.

Switches
A network switch is a device that forwards and filters OSI layer 2 datagrams (frames) between ports based on the destination MAC address in each frame. A switch is distinct from a hub in that it only forwards the frames to the physical ports involved in the communication rather than all ports connected. It can be thought of as a multi-port bridge. It learns to associate physical ports to MAC addresses by examining the source addresses of received frames. If an unknown destination is targeted, the switch broadcasts to all ports but the source. Switches normally have numerous ports, facilitating a star topology for devices, and cascading additional switches.

Multi-layer switches are capable of routing based on layer 3 addressing or additional logical levels. The term switch is often used loosely to include devices such as routers and bridges, as well as devices that may distribute traffic based on load or based on application content (e.g., a Web URL identifier).

Routers
A router is an internetworking device that forwards packets between networks by processing the routing information included in the packet or datagram (Internet protocol information from layer 3). The routing information is often processed in conjunction with the routing table (or forwarding table). A router uses its routing table to determine where to forward packets. A destination in a routing table can include a "null" interface, also known as the "black hole" interface because data can go into it, however, no further processing is done for said data, i.e. the packets are dropped.

Modems
Modems (MOdulator-DEModulator) are used to connect network nodes via wire not originally designed for digital network traffic, or for wireless. To do this one or more carrier signals are modulated by the digital signal to produce an analog signal that can be tailored to give the required properties for transmission. Modems are commonly used for telephone lines, using a digital subscriber line technology.

Firewalls
A firewall is a network device for controlling network security and access rules. Firewalls are typically configured to reject access requests from unrecognized sources while allowing actions from recognized ones. The vital role firewalls play in network security grows in parallel with the constant increase in cyber attacks.

Classification
The study of network topology recognizes eight basic topologies: point-to-point, bus, star, ring or circular, mesh, tree, hybrid, or daisy chain.

Point-to-point
The simplest topology with a dedicated link between two endpoints. Easiest to understand, of the variations of point-to-point topology, is a point-to-point communication channel that appears, to the user, to be permanently associated with the two endpoints. A child's tin can telephone is one example of a physical dedicated channel.

Using circuit-switching or packet-switching technologies, a point-to-point circuit can be set up dynamically and dropped when no longer needed. Switched point-to-point topologies are the basic model of conventional telephony.

The value of a permanent point-to-point network is unimpeded communications between the two endpoints. The value of an on-demand point-to-point connection is proportional to the number of potential pairs of subscribers and has been expressed as Metcalfe's Law.

Daisy chain
Daisy chaining is accomplished by connecting each computer in series to the next. If a message is intended for a computer partway down the line, each system bounces it along in sequence until it reaches the destination. A daisy-chained network can take two basic forms: linear and ring.

 * A linear topology puts a two-way link between one computer and the next. However, this was expensive in the early days of computing, since each computer (except for the ones at each end) required two receivers and two transmitters.
 * By connecting the computers at each end of the chain, a ring topology can be formed. When a node sends a message, the message is processed by each computer in the ring. An advantage of the ring is that the number of transmitters and receivers can be cut in half. Since a message will eventually loop all of the way around, transmission does not need to go both directions.  Alternatively, the ring can be used to improve fault tolerance. If the ring breaks at a particular link then the transmission can be sent via the reverse path thereby ensuring that all nodes are always connected in the case of a single failure.

Bus

In local area networks using bus topology, each node is connected by interface connectors to a single central cable. This is the 'bus', also referred to as the backbone, or trunk – all data transmission between nodes in the network is transmitted over this common transmission medium and is able to be received by all nodes in the network simultaneously.

A signal containing the address of the intended receiving machine travels from a source machine in both directions to all machines connected to the bus until it finds the intended recipient, which then accepts the data. If the machine address does not match the intended address for the data, the data portion of the signal is ignored. Since the bus topology consists of only one wire it is less expensive to implement than other topologies, but the savings are offset by the higher cost of managing the network. Additionally, since the network is dependent on the single cable, it can be the single point of failure of the network. In this topology data being transferred may be accessed by any node.

Linear bus
In a linear bus network, all of the nodes of the network are connected to a common transmission medium which has just two endpoints. When the electrical signal reaches the end of the bus, the signal is reflected back down the line, causing unwanted interference. To prevent this, the two endpoints of the bus are normally terminated with a device called a terminator.

Distributed bus
In a distributed bus network, all of the nodes of the network are connected to a common transmission medium with more than two endpoints, created by adding branches to the main section of the transmission medium – the physical distributed bus topology functions in exactly the same fashion as the physical linear bus topology because all nodes share a common transmission medium.

Star
In star topology, every peripheral node (computer workstation or any other peripheral) is connected to a central node called a hub or switch. The hub is the server and the peripherals are the clients. The network does not necessarily have to resemble a star to be classified as a star network, but all of the peripheral nodes on the network must be connected to one central hub. All traffic that traverses the network passes through the central hub, which acts as a signal repeater.

The star topology is considered the easiest topology to design and implement. One advantage of the star topology is the simplicity of adding additional nodes. The primary disadvantage of the star topology is that the hub represents a single point of failure. Also, since all peripheral communication must flow through the central hub, the aggregate central bandwidth forms a network bottleneck for large clusters.

Extended star
The extended star network topology extends a physical star topology by one or more repeaters between the central node and the peripheral (or 'spoke') nodes. The repeaters are used to extend the maximum transmission distance of the physical layer, the point-to-point distance between the central node and the peripheral nodes. Repeaters allow greater transmission distance, further than would be possible using just the transmitting power of the central node. The use of repeaters can also overcome limitations from the standard upon which the physical layer is based.

A physical extended star topology in which repeaters are replaced with hubs or switches is a type of hybrid network topology and is referred to as a physical hierarchical star topology, although some texts make no distinction between the two topologies.

A physical hierarchical star topology can also be referred as a tier-star topology. This topology differs from a tree topology in the way star networks are connected together. A tier-star topology uses a central node, while a tree topology uses a central bus and can also be referred as a star-bus network.

Distributed star
A distributed star is a network topology that is composed of individual networks that are based upon the physical star topology connected in a linear fashion – i.e., 'daisy-chained' – with no central or top level connection point (e.g., two or more 'stacked' hubs, along with their associated star connected nodes or 'spokes').

Ring

A ring topology is a daisy chain in a closed loop. Data travels around the ring in one direction. When one node sends data to another, the data passes through each intermediate node on the ring until it reaches its destination. The intermediate nodes repeat (retransmit) the data to keep the signal strong. Every node is a peer; there is no hierarchical relationship of clients and servers. If one node is unable to retransmit data, it severs communication between the nodes before and after it in the bus.

Advantages:
 * When the load on the network increases, its performance is better than bus topology.
 * There is no need of network server to control the connectivity between workstations.

Disadvantages:
 * Aggregate network bandwidth is bottlenecked by the weakest link between two nodes.

Mesh
The value of fully meshed networks is proportional to the exponent of the number of subscribers, assuming that communicating groups of any two endpoints, up to and including all the endpoints, is approximated by Reed's Law.

Fully connected network

In a fully connected network, all nodes are interconnected. (In graph theory this is called a complete graph.) The simplest fully connected network is a two-node network. A fully connected network doesn't need to use packet switching or broadcasting. However, since the number of connections grows quadratically with the number of nodes:

$$c= \frac{n(n-1)}{2}.\,$$

This makes it impractical for large networks. This kind of topology does not trip and affect other nodes in the network.

Partially connected network
In a partially connected network, certain nodes are connected to exactly one other node; but some nodes are connected to two or more other nodes with a point-to-point link. This makes it possible to make use of some of the redundancy of mesh topology that is physically fully connected, without the expense and complexity required for a connection between every node in the network.

Hybrid
Hybrid topology is also known as hybrid network. Hybrid networks combine two or more topologies in such a way that the resulting network does not exhibit one of the standard topologies (e.g., bus, star, ring, etc.). For example, a tree network (or star-bus network) is a hybrid topology in which star networks are interconnected via bus networks. However, a tree network connected to another tree network is still topologically a tree network, not a distinct network type. A hybrid topology is always produced when two different basic network topologies are connected.

A star-ring network consists of two or more ring networks connected using a multistation access unit (MAU) as a centralized hub.

Snowflake topology is a star network of star networks.

Two other hybrid network types are hybrid mesh and hierarchical star.

Centralization
The star topology reduces the probability of a network failure by connecting all of the peripheral nodes (computers, etc.) to a central node. When the physical star topology is applied to a logical bus network such as Ethernet, this central node (traditionally a hub) rebroadcasts all transmissions received from any peripheral node to all peripheral nodes on the network, sometimes including the originating node. All peripheral nodes may thus communicate with all others by transmitting to, and receiving from, the central node only. The failure of a transmission line linking any peripheral node to the central node will result in the isolation of that peripheral node from all others, but the remaining peripheral nodes will be unaffected. However, the disadvantage is that the failure of the central node will cause the failure of all of the peripheral nodes.

If the central node is passive, the originating node must be able to tolerate the reception of an echo of its own transmission, delayed by the two-way round trip transmission time (i.e. to and from the central node) plus any delay generated in the central node. An active star network has an active central node that usually has the means to prevent echo-related problems.

A tree topology (a.k.a. hierarchical topology) can be viewed as a collection of star networks arranged in a hierarchy. This tree structure has individual peripheral nodes (e.g. leaves) which are required to transmit to and receive from one other node only and are not required to act as repeaters or regenerators. Unlike the star network, the functionality of the central node may be distributed.

As in the conventional star network, individual nodes may thus still be isolated from the network by a single-point failure of a transmission path to the node. If a link connecting a leaf fails, that leaf is isolated; if a connection to a non-leaf node fails, an entire section of the network becomes isolated from the rest.

To alleviate the amount of network traffic that comes from broadcasting all signals to all nodes, more advanced central nodes were developed that are able to keep track of the identities of the nodes that are connected to the network. These network switches will "learn" the layout of the network by "listening" on each port during normal data transmission, examining the data packets and recording the address/identifier of each connected node and which port it is connected to in a lookup table held in memory. This lookup table then allows future transmissions to be forwarded to the intended destination only.

Daisy chain topology is a way of connecting network nodes in a linear or ring structure. It is used to transmit messages from one node to the next until they reach the destination node.

A daisy chain network can have two types: linear and ring. A linear daisy chain network is like an electrical series, where the first and last nodes are not connected. A ring daisy chain network is where the first and last nodes are connected, forming a loop.

Decentralization
In a partially connected mesh topology, there are at least two nodes with two or more paths between them to provide redundant paths in case the link providing one of the paths fails. Decentralization is often used to compensate for the single-point-failure disadvantage that is present when using a single device as a central node (e.g., in star and tree networks). A special kind of mesh, limiting the number of hops between two nodes, is a hypercube. The number of arbitrary forks in mesh networks makes them more difficult to design and implement, but their decentralized nature makes them very useful.

This is similar in some ways to a grid network, where a linear or ring topology is used to connect systems in multiple directions. A multidimensional ring has a toroidal topology, for instance.

A fully connected network, complete topology, or full mesh topology is a network topology in which there is a direct link between all pairs of nodes. In a fully connected network with n nodes, there are $$\frac{n(n-1)}{2}\,$$ direct links. Networks designed with this topology are usually very expensive to set up, but provide a high degree of reliability due to the multiple paths for data that are provided by the large number of redundant links between nodes. This topology is mostly seen in military applications.

title: Network topology


text:
Neutral direct-current telegraph system

In telecommunication, a neutral direct-current telegraph system (single-current system, single-current transmission system, single-Morse system) is a telegraph system in which (a) current flows during marking intervals and no current flows during spacing intervals for the transmission of signals over a line, and (b) the direction of current flow (electric polarity) is immaterial.

title: Neutral direct-current telegraph system