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Intelligent Network

The Intelligent Network (IN) is the standard network architecture specified in the ITU-T Q.1200 series recommendations. It is intended for fixed as well as mobile telecom networks. It allows operators to differentiate themselves by providing value-added services in addition to the standard telecom services such as PSTN, ISDN on fixed networks, and GSM services on mobile phones or other mobile devices.

The intelligence is provided by network nodes on the service layer, distinct from the switching layer of the core network, as opposed to solutions based on intelligence in the core switches or equipment. The IN nodes are typically owned by telecommunications service providers such as a telephone company or mobile phone operator.

IN is supported by the Signaling System #7 (SS7) protocol between network switching centers and other network nodes owned by network operators.

Examples of IN services

 * Televoting
 * Call screening
 * Local number portability
 * Toll-free calls/Freephone
 * Prepaid calling
 * Account card calling
 * Virtual private networks (such as family group calling)
 * Centrex service (Virtual PBX)
 * Private-number plans (with numbers remaining unpublished in directories)
 * Universal Personal Telecommunications service (a universal personal telephone number)
 * Mass-calling service
 * Prefix free dialing from cellphones abroad
 * Seamless MMS message access from abroad
 * Reverse charging
 * Home Area Discount
 * Premium Rate calls
 * Call distribution based on various criteria associated with the call
 * Location-based routing
 * Time-based routing
 * Proportional call distribution (such as between two or more call centres or offices)
 * Call queueing
 * Call transfer

History and key concepts
The IN concepts, architecture and protocols were originally developed as standards by the ITU-T which is the standardization committee of the International Telecommunication Union; prior to this a number of telecommunications providers had proprietary implementations. The primary aim of the IN was to enhance the core telephony services offered by traditional telecommunications networks, which usually amounted to making and receiving voice calls, sometimes with call divert. This core would then provide a basis upon which operators could build services in addition to those already present on a standard telephone exchange.

A complete description of the IN emerged in a set of ITU-T standards named Q.1210 to Q.1219, or Capability Set One (CS-1) as they became known. The standards defined a complete architecture including the architectural view, state machines, physical implementation and protocols. They were universally embraced by telecom suppliers and operators, although many variants were derived for use in different parts of the world (see Variants below).

Following the success of CS-1, further enhancements followed in the form of CS-2. Although the standards were completed, they were not as widely implemented as CS-1, partly because of the increasing power of the variants, but also partly because they addressed issues which pushed traditional telephone exchanges to their limits.

The major driver behind the development of the IN was the need for a more flexible way of adding sophisticated services to the existing network. Before the IN was developed, all new features and/or services had to be implemented directly in the core switch systems. This made for long release cycles as the software testing had to be extensive and thorough to prevent the network from failing. With the advent of the IN, most of these services (such as toll-free numbers and geographical number portability) were moved out of the core switch systems and into self-contained nodes, creating a modular and more secure network that allowed the service providers themselves to develop variations and value-added services to their networks without submitting a request to the core switch manufacturer and waiting for the long development process. The initial use of IN technology was for number translation services, e.g. when translating toll-free numbers to regular PSTN numbers; much more complex services have since been built on the IN, such as Custom Local Area Signaling Services (CLASS) and prepaid telephone calls.

SS7 architecture
The main concepts (functional view) surrounding IN services or architecture are connected with SS7 architecture:

 * Service Switching Function (SSF) or Service Switching Point (SSP) is co-located with the telephone exchange, and acts as the trigger point for further services to be invoked during a call. The SSP implements the Basic Call State Machine (BCSM) which is a Finite state machine that represents an abstract view of a call from beginning to end (off hook, dialing, answer, no answer, busy, hang up, etc.).  As each state is traversed, the exchange encounters Detection Points (DPs) at which the SSP may invoke a query to the SCP to wait for further instructions on how to proceed.  This query is usually called a trigger.  Trigger criteria are defined by the operator and might include the subscriber calling number or the dialed number. The SSF is responsible for controlling calls requiring value added services.
 * Service Control Function (SCF) or Service Control Point (SCP) is a separate set of platforms that receive queries from the SSP. The SCP contains service logic which implements the behaviour desired by the operator, i.e., the services.  During service logic processing, additional data required to process the call may be obtained from the SDF.  The logic on the SCP is created using the SCE.
 * Service Data Function (SDF) or Service Data Point (SDP) is a database that contains additional subscriber data, or other data required to process a call. For example, the subscriber's remaining prepaid credit may be stored in the SDF to be queried in real-time during the call.  The SDF may be a separate platform or co-located with the SCP.
 * Service Management Function (SMF) or Service Management Point (SMP) is a platform or cluster of platforms that operators use to monitor and manage the IN services. It contains the management database which stores the services' configuration, collects the statistics and alarms, and stores the Call Data Reports and Event Data Reports.
 * Service Creation Environment (SCE) is the development environment used to create the services present on the SCP. Although the standards permit any type of environment, it is fairly rare to see low level languages like C used.  Instead, proprietary graphical languages are used to enable telecom engineers to create services directly. The languages are usually of the fourth-generation type, and the engineer may use a graphical interface to build or change a service.
 * Specialized Resource Function (SRF) or Intelligent Peripheral (IP) is a node which can connect to both the SSP and the SCP and deliver special resources into the call, mostly related to voice communication, for example to play voice announcements or collect DTMF tones from the user.

Protocols
The core elements described above use standard protocols to communicate with each other. The use of standard protocols allows different manufacturers to concentrate on different parts of the architecture and be confident that they will all work together in any combination.

The interfaces between the SSP and the SCP are SS7 based and have similarities with TCP/IP protocols. The SS7 protocols implement much of the OSI seven-layer model. This means that the IN standards only had to define the application layer, which is called the Intelligent Networks Application Part or INAP. The INAP messages are encoded using ASN.1.

The interface between the SCP and the SDP is defined in the standards to be an X.500 Directory Access Protocol or DAP. A more lightweight interface called LDAP has emerged from the IETF which is considerably simpler to implement, so many SCPs have implemented that instead.

Variants
The core CS-1 specifications were adopted and extended by other standards bodies. European flavours were developed by ETSI, American flavours were developed by ANSI, and Japanese variants also exist. The main reasons for producing variants in each region was to ensure interoperability between equipment manufactured and deployed locally (for example different versions of the underlying SS7 protocols exist between the regions).

New functionality was also added which meant that variants diverged from each other and the main ITU-T standard. The biggest variant was called Customised Applications for Mobile networks Enhanced Logic, or CAMEL for short. This allowed for extensions to be made for the mobile phone environment, and allowed mobile phone operators to offer the same IN services to subscribers while they are roaming as they receive in the home network.

CAMEL has become a major standard in its own right and is currently maintained by 3GPP. The last major release of the standard was CAMEL phase 4. It is the only IN standard currently being actively worked on.

Bellcore (subsequently Telcordia Technologies) developed the Advanced Intelligent Network (AIN) as the variant of Intelligent Network for North America, and performed the standardization of the AIN on behalf of the major US operators. The original goal of AIN was AIN 1.0, which was specified in the early 1990s (AIN Release 1, Bellcore SR-NWT-002247, 1993). AIN 1.0 proved technically infeasible to implement, which led to the definition of simplified AIN 0.1 and AIN 0.2 specifications. In North America, Telcordia SR-3511 (originally known as TA-1129+) and GR-1129-CORE protocols serve to link switches with the IN systems such as Service Control Points (SCPs) or Service Nodes. SR-3511 details a TCP/IP-based protocol which directly connects the SCP and Service Node. GR-1129-CORE provides generic requirements for an ISDN-based protocol which connects the SCP to the Service Node via the SSP.

Future
While activity in development of IN standards has declined in recent years, there are many systems deployed across the world which use this technology. The architecture has proved to be not only stable, but also a continuing source of revenue with new services added all the time. Manufacturers continue to support the equipment and obsolescence is not an issue.

Nevertheless, new technologies and architectures have emerged, especially in the area of VoIP and SIP. More attention is being paid to the use of APIs in preference to protocols like INAP, and new standards have emerged in the form of JAIN and Parlay. From a technical viewpoint, the SCE began to move away from its proprietary graphical origins towards a Java application server environment.

The meaning of "intelligent network" is evolving in time, largely driven by breakthroughs in computation and algorithms. From networks enhanced by more flexible algorithms and more advanced protocols, to networks designed using data-driven models to AI enabled networks.

title: Intelligent Network


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Intensity modulation

In optical communications, intensity modulation (IM) is a form of modulation in which the optical power output of a source is varied in accordance with some characteristic of the modulating signal. The envelope of the modulated optical signal is an analog of the modulating signal in the sense that the instantaneous power of the envelope is an analog of the characteristic of interest in the modulating signal.

The recovery of the modulating signal is typically achieved by direct detection, not heterodyning. However, optical heterodyne detection is possible and has been actively studied since 1979. Bell Laboratories had a working, but impractical, system in 1969. Heterodyne and homodyne systems are of interest because they are expected to produce an increase in sensitivity of up to 20 dB allowing longer hops between islands for instance. Such systems also have the important advantage of very narrow channel spacing in optical frequency-division multiplexing (OFDM) systems. OFDM is a step beyond wavelength-division multiplexing (WDM). Normal WDM using direct detection does not achieve anything like the close channel spacing of radio frequency FDM.

Intensity modulation with direct detection
Intensity Modulation / Direct Detection (IM/DD) is a scheme is simple and cost-effective in fiber optic communication, making it a suitable for various optical communication applications. It involves modulating the optical power of the carrier signal to represent the transmitted data. This modulation can be achieved using techniques, such as on-off keying (OOK). The intensity-modulated optical signal is generated by modulating the amplitude or the current of the light source, typically a laser diode with one or two cavity designs such as Fabry-Perot or distributed feedback (DFB).

At the receiver end, direct detection (DD) is used to recover the modulated signal. The modulated optical signal is detected by a photodetector (most commonly PIN or APD photodiode), which converts the optical power variations into corresponding electrical current or voltage variations. The output of the photodetector is then processed and decoded to retrieve the original information.

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

title: Intensity modulation


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Intercept

Intercept may refer to:
 * X-intercept, the point where a line crosses the x-axis
 * Y-intercept, the point where a line crosses the y-axis
 * Interception, a play in various forms of football
 * The Mona Intercept, a 1980 thriller novel by Donald Hamilton
 * Operation Intercept, an anti-drug measure announced by President Nixon
 * Telephone tapping, the monitoring of telephone and Internet conversations by a third party
 * Tax refund intercept
 * Samsung Intercept (SPH-M910), an Android smartphone
 * Visual Intercept, a Microsoft Windows-based software defect tracking system
 * Intermodulation Intercept Point, a measure of an electrical device's linearity
 * Intercept message, a telephone recording informing the caller that the call cannot be completed
 * The Intercept, an online news publication edited by Glenn Greenwald, Laura Poitras, and Jeremy Scahill

title: Intercept


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Interchangeability

Interchangeability can refer to:
 * Interchangeable parts, the ability to select components for assembly at random and fit them together within proper tolerances
 * Interchangeability (computer science), the ability that an object can be replaced by another object without affecting code using the object
 * Interchangeable random variables in mathematics

title: Interchangeability


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Interchange circuit

In telecommunication, an interchange circuit is a circuit that facilitates the exchange of data and signaling information between data terminal equipment (DTE) and data circuit-terminating equipment (DCE).

An interchange circuit can carry many types of signals and provide many types of service features, such as control signals, timing signals, and common return functions.

title: Interchange circuit


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Intercharacter interval

In telecommunications, the intercharacter interval is the time interval between the end of the stop signal of one character and the beginning of the start signal of the next character of an asynchronous transmission.

The intercharacter interval may be of any duration. The signal sense of the intercharacter interval is always the same as the sense of the stop element, i.e., "1" or "mark."

title: Intercharacter interval


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Interconnect facility

Interconnect facility: In a communications network, one or more communications links that (a) are used to provide local area communications service among several locations and (b) collectively form a node in the network.

An interconnect facility may include network control and administrative circuits as well as the primary traffic circuits.

An interconnect facility may use any medium available and may be redundant.

title: Interconnect facility


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Interface

Interface or interfacing may refer to:

Academic journals

 * Interface (journal), by the Electrochemical Society
 * Interface, Journal of Applied Linguistics, now merged with ITL International Journal of Applied Linguistics
 * Interface: A Journal for and About Social Movements
 * Interface, shortened name for the Journal of the Royal Society Interface, covering the interface between life sciences and physical sciences
 * Interfaces (journal), now INFORMS Journal on Applied Analytics

Arts and entertainment

 * Interface (album), by Dominion, 1996
 * Interface (band), an American music group
 * Interface (film), a 1984 American film
 * Interface (novel), by Stephen Bury (a pseudonym), 1994
 * "Interface" (Star Trek: The Next Generation), an episode of the TV series
 * Interface series, a science fiction horror story in short installments on Reddit

Computing and electronics

 * Interface (computing), a shared boundary between system components
 * Interface (Java)
 * Interface (object-oriented programming)
 * Application binary interface, between two binary program modules
 * Application programming interface, between a client and a server
 * Network interface, between two pieces of equipment or protocol layers in a computer network
 * User interface, between humans and machines
 * Graphical user interface
 * Audio and video interfaces and connectors

Other uses in science and social science

 * Interface (communication studies), in the work environment
 * Interface (matter), in the physical sciences
 * Biointerface, in inorganic/organic material
 * Business interoperability interface, between organizational systems
 * Social interface, a concept in social science

Other uses

 * Interface, Inc., a flooring manufacturer
 * Interface:2010, or Interface, an international-standard-setting body in marketing
 * Interfacing, a textile used on the unseen side of fabrics for rigidity

title: Interface


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Interface functionality

In telephony, interface functionality is the characteristic of interfaces that allows operators to support transmission, switching, and signaling functions identical to those used in the enhanced services provided by the carrier.

As part of its comparably efficient interconnection (CEI) offering, the carrier must make available standardized telephone networking hardware and software interfaces that are able to support transmission, switching, and signaling functions identical to those used in the enhanced services provided by the carrier.

title: Interface functionality


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Interface standard

In telecommunications, an interface standard is a standard that describes one or more functional characteristics (such as code conversion, line assignments, or protocol compliance) or physical characteristics (such as electrical, mechanical, or optical characteristics) necessary to allow the exchange of information between two or more (usually different) systems or pieces of equipment. Communications protocols are an example.

An interface standard may include operational characteristics and acceptable levels of performance.

In the military community, interface standards permit command and control functions to be performed using communication and computer systems.

Telephones
There are many interface standards between analog telephone central office equipment and customer-premises equipment. Single voice paths generally include analog audio connections, either a two-wire circuit or four-wire circuit plus signaling paths to indicate call progress and status information, such as ringing, answer supervision, etc. Some of the known interface types are:
 * FXO foreign exchange office
 * FXS foreign exchange station
 * DPO dial pulse originate
 * DPT dial pulse terminate
 * E and M signaling
 * KS Kewlstart
 * Loop start
 * Ground start

title: Interface standard


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Interference filter

An interference filter, dichroic filter, or thin-film filter is an optical filter that reflects some wavelengths (colors) of light and transmits others, with almost no absorption for all wavelengths of interest. An interference filter may be high-pass, low-pass, bandpass, or band-rejection. They are used in scientific applications, as well as in architectural and theatrical lighting.

An interference filter consists of multiple thin layers of dielectric material having different refractive indices. There may also be metallic layers. Interference filters are wavelength-selective by virtue of the interference effects that take place between the incident and reflected waves at the thin-film boundaries. The principle of operation is similar to a Fabry-Perot etalon.

Dichroic mirrors and dichroic reflectors are the same type of device, but are characterized by the colors of light that they reflect, rather than the colors they pass. Dielectric mirrors operate on the same principle, but focus exclusively on reflection.

Theory
Dichroic filters use the principle of thin-film interference, and produce colors in the same way as oil films on water. When light strikes an oil film at an angle, some of the light is reflected from the top surface of the oil, and some is reflected from the bottom surface where it is in contact with the water. Because the light reflecting from the bottom travels a slightly longer path, some light wavelengths are reinforced by this delay, while others tend to be canceled, producing the colors seen. The color transmitted by the filter exhibits a blue shift with increasing angle of incidence, see Dielectric mirror.

In a dichroic mirror or filter, instead of using an oil film to produce the interference, alternating layers of optical coatings with different refractive indices are built up upon a glass substrate. The interfaces between the layers of different refractive index produce phased reflections, selectively reinforcing certain wavelengths of light and interfering with other wavelengths. The layers are usually added by vacuum deposition. By controlling the thickness and number of the layers, the frequency of the passband of the filter can be tuned and made as wide or narrow as desired. Because unwanted wavelengths are reflected rather than absorbed, dichroic filters do not absorb this unwanted energy during operation and so do not become nearly as hot as the equivalent conventional filter (which attempts to absorb all energy except for that in the passband). (See Fabry–Pérot interferometer for a mathematical description of the effect.)

Where white light is being deliberately separated into various color bands (for example, within a color video projector or color television camera), the similar dichroic prism is used instead. For cameras, however, it is now more common to have an absorption filter array to filter individual pixels on a single CCD array.

Applications
Dichroic filters can filter light from a white light source to produce light that is perceived by humans to be highly saturated in color. Such filters are popular in architectural and theatrical applications.

Dichroic reflectors known as cold mirrors are commonly used behind a light source to reflect visible light forward while allowing the invisible infrared light to pass out of the rear of the fixture. Such an arrangement allows intense illumination with less heating of the illuminated object. Many quartz-halogen lamps have an integrated dichroic reflector for this purpose, being originally designed for use in slide projectors to avoid melting the slides, but now widely used for interior home and commercial lighting. This improves whiteness by removing excess red; however, it poses a serious fire hazard if used in recessed or enclosed luminaires by allowing infrared radiation into those luminaires. For these applications non-cool-beam (ALU or Silverback) lamps must be used. Recessed or enclosed luminaires that are unsuitable for use with dichroic reflector lights can be identified by the IEC 60598 No Cool Beam symbol.

In fluorescence microscopy, dichroic filters are used as beam splitters to direct illumination of an excitation frequency toward the sample and then at an analyzer to reject that same excitation frequency but pass a particular emission frequency.

Some LCD projectors use dichroic filters instead of prisms to split the white light from the lamp into the three colours before passing it through the three LCD units.

Older DLP projectors typically transmit a white light source through a color wheel which uses dichroic filters to rapidly switch colors sent through the (monochrome) Digital micromirror device. Newer projectors may use laser or LED light sources to directly emit the desired light wavelengths.

They are used as laser harmonic separators. They separate the various harmonic components of frequency doubled laser systems by selective spectral reflection and transmission.

Dichroic filters are also used to create gobos for high-power lighting products. Pictures are made by overlapping up to four colored dichroic filters.

Photographic enlarger color heads use dichroic filters to adjust the color balance in the print.

Artistic glass jewelry is occasionally fabricated to behave as a dichroic filter. Because the wavelength of light selected by the filter varies with the angle of incidence of the light, such jewelry often has an iridescent effect, changing color as the (for example) earrings swing. Another interesting application of dichroic filters is spatial filtering.

With a technique licensed from Infitec, Dolby Labs uses dichroic filters for screening 3D movies. The left lens of the Dolby 3D glasses transmits specific narrow bands of red, green and blue frequencies, while the right lens transmits a different set of red, green and blue frequencies. The projector uses matching filters to display the images meant for the left and right eyes.

Long-pass dichroic filters applied to ordinary lighting can prevent it from attracting insects. In some cases, such filters can prevent attraction of other wildlife, reducing adverse environmental impact.

Advantages
Dichroic filters have a much longer life than conventional filters; the color is intrinsic in the construction of the hard microscopic layers and cannot "bleach out" over the lifetime of the filter (unlike for example, gel filters). They can be fabricated to pass any passband frequency and block a selected amount of the stopband frequencies. Because light in the stopband is reflected rather than absorbed, there is much less heating of the dichroic filter than with conventional filters. Dichroics are capable of achieving extremely high laser damage thresholds, and are used for all the mirrors on the world's most powerful laser, the National Ignition Facility.

Additional sources

 * M. Bass, Handbook of Optics (2nd ed.) pp. 42.89-42.90 (1995)
 * M. Bass, Handbook of Optics (2nd ed.) pp. 42.89-42.90 (1995)

title: Interference filter


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Intermediate distribution frame

An intermediate distribution frame (IDF) is a distribution frame in a central office or customer premises, which cross connects the user cable media to individual user line circuits and may serve as a distribution point for multipair cables from the main distribution frame (MDF) or combined distribution frame (CDF) to individual cables connected to equipment in areas remote from these frames.

IDFs are used for telephone exchange central office, customer-premises equipment, wide area network (WAN), and local area network (LAN) environments, among others.

In central office environments the IDF may contain circuit termination equipment from various auxiliary components. In WAN and LAN environments IDFs can hold devices of different types including backup systems (hard drives or other media as self-contained, or as RAIDs, CD-ROMs, etc.), networking (switches, hubs, routers), and connections (fiber optics, coaxial, category cables) and so on.

title: Intermediate distribution frame


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Intermediate-field region

In antenna theory, intermediate-field region (also known as intermediate field, intermediate zone or transition zone) refers to the transition region lying between the near-field region and the far-field region in which the field strength of an electromagnetic wave is dependent upon the inverse distance, inverse square of the distance, and the inverse cube of the distance from the antenna. For an antenna that is small compared to the wavelength in question, the intermediate-field region is considered to exist at all distances between 0.1 wavelength and 1.0 wavelength from the antenna.

title: Intermediate-field region


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IP address spoofing

In computer networking, IP address spoofing or IP spoofing is the creation of Internet Protocol (IP) packets with a false source IP address, for the purpose of impersonating another computing system.

Background
The basic protocol for sending data over the Internet network and many other computer networks is the Internet Protocol (IP). The protocol specifies that each IP packet must have a header which contains (among other things) the IP address of the sender of the packet. The source IP address is normally the address that the packet was sent from, but the sender's address in the header can be altered, so that to the recipient it appears that the packet came from another source.

The protocol requires the receiving computer to send back a response to the source IP address therefore spoofing is mainly used when the sender can anticipate the network response or does not care about the response.

The source IP address provides only limited information about the sender. It may provide general information on the region, city and town when on the packet was sent. It does not provide information on the identity of the sender or the computer being used.

Applications
IP address spoofing involving the use of a trusted IP address can be used by network intruders to overcome network security measures, such as authentication based on IP addresses. This type of attack is most effective where trust relationships exist between machines. For example, it is common on some corporate networks to have internal systems trust each other, so that users can log in without a username or password provided they are connecting from another machine on the internal network – which would require them already being logged in. By spoofing a connection from a trusted machine, an attacker on the same network may be able to access the target machine without authentication.

IP address spoofing is most frequently used in denial-of-service attacks, where the objective is to flood the target with an overwhelming volume of traffic, and the attacker does not care about receiving responses to the attack packets. Packets with spoofed IP addresses are more difficult to filter since each spoofed packet appears to come from a different address, and they hide the true source of the attack. Denial of service attacks that use spoofing typically randomly choose addresses from the entire IP address space, though more sophisticated spoofing mechanisms might avoid non-routable addresses or unused portions of the IP address space. The proliferation of large botnets makes spoofing less important in denial of service attacks, but attackers typically have spoofing available as a tool, if they want to use it, so defenses against denial-of-service attacks that rely on the validity of the source IP address in attack packets might have trouble with spoofed packets.

In DDoS attacks, the attacker may decide to spoof the IP source address to randomly generated addresses, so the victim machine cannot distinguish between the spoofed packets and legitimate packets. The replies would then be sent to random addresses that do not end up anywhere in particular. Such packages-to-nowhere are called the backscatter, and there are network telescopes monitoring backscatter to measure the statistical intensity of DDoS attacks on the internet over time.

Legitimate uses
The use of packets with a false source IP address is not always evidence of malicious intent. For example, in performance testing of websites, hundreds or even thousands of "vusers" (virtual users) may be created, each executing a test script against the website under test, in order to simulate what will happen when the system goes "live" and a large number of users log in simultaneously.

Since each user will normally have its own IP address, commercial testing products (such as HP LoadRunner, WebLOAD, and others) can use IP spoofing, allowing each user its own "return address" as well.

IP spoofing is also used in some server-side load balancing. It lets the load balancer spray incoming traffic, but not need to be in the return path from the servers to the client. This saves a networking hop through switches and the load balancer as well as outbound message processing load on the load balancer. Output usually has more packets and bytes, so the savings are significant.

Services vulnerable to IP spoofing
Configuration and services that are vulnerable to IP spoofing:

 * RPC (Remote procedure call services)
 * Any service that uses IP address authentication

 * The R services suite (rlogin, rsh, etc.)

Defense against spoofing attacks
Packet filtering is one defense against IP spoofing attacks. The gateway to a network usually performs ingress filtering, which is blocking of packets from outside the network with a source address inside the network. This prevents an outside attacker spoofing the address of an internal machine. Ideally, the gateway would also perform egress filtering on outgoing packets, which is blocking of packets from inside the network with a source address that is not inside. This prevents an attacker within the network performing filtering from launching IP spoofing attacks against external machines. An intrusion detection system (IDS) is a common use of packet filtering, which has been used to secure the environments for sharing data over network and host-based IDS approaches.

It is also recommended to design network protocols and services so that they do not rely on the source IP address for authentication.

Upper layers
Some upper layer protocols have their own defense against IP spoofing attacks. For example, Transmission Control Protocol (TCP) uses sequence numbers negotiated with the remote machine to ensure that arriving packets are part of an established connection. Since the attacker normally cannot see any reply packets, the sequence number must be guessed in order to hijack the connection. The poor implementation in many older operating systems and network devices, however, means that TCP sequence numbers can be predicted.

Other definitions
The term spoofing is also sometimes used to refer to header forgery, the insertion of false or misleading information in e-mail or netnews headers. Falsified headers are used to mislead the recipient, or network applications, as to the origin of a message. This is a common technique of spammers and sporgers, who wish to conceal the origin of their messages to avoid being tracked.

title: IP address spoofing


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Interoperability

Interoperability is a characteristic of a product or system to work with other products or systems. While the term was initially defined for information technology or systems engineering services to allow for information exchange, a broader definition takes into account social, political, and organizational factors that impact system-to-system performance.

Types of interoperability include syntactic interoperability, where two systems can communicate with each other, and cross-domain interoperability, where multiple organizations work together and exchange information.

Types
If two or more systems use common data formats and communication protocols then they are capable of communicating with each other and they exhibit syntactic interoperability. XML and SQL are examples of common data formats and protocols. Low-level data formats also contribute to syntactic interoperability, ensuring that alphabetical characters are stored in the same ASCII or a Unicode format in all the communicating systems.

Beyond the ability of two or more computer systems to exchange information, semantic interoperability is the ability to automatically interpret the information exchanged meaningfully and accurately in order to produce useful results as defined by the end users of both systems. To achieve semantic interoperability, both sides must refer to a common information exchange reference model. The content of the information exchange requests are unambiguously defined: what is sent is the same as what is understood.

Cross-domain interoperability involves multiple social, organizational, political, legal entities working together for a common interest or information exchange.

Interoperability and open standards
Interoperability implies exchanges between a range of products, or similar products from several different vendors, or even between past and future revisions of the same product. Interoperability may be developed post-facto, as a special measure between two products, while excluding the rest, by using open standards. When a vendor is forced to adapt its system to a dominant system that is not based on open standards, it is compatibility, not interoperability.

Open standards
Open standards rely on a broadly consultative and inclusive group including representatives from vendors, academics and others holding a stake in the development that discusses and debate the technical and economic merits, demerits and feasibility of a proposed common protocol. After the doubts and reservations of all members are addressed, the resulting common document is endorsed as a common standard. This document may be subsequently released to the public, and henceforth becomes an open standard. It is usually published and is available freely or at a nominal cost to any and all comers, with no further encumbrances. Various vendors and individuals (even those who were not part of the original group) can use the standards document to make products that implement the common protocol defined in the standard and are thus interoperable by design, with no specific liability or advantage for customers for choosing one product over another on the basis of standardized features. The vendors' products compete on the quality of their implementation, user interface, ease of use, performance, price, and a host of other factors, while keeping the customer's data intact and transferable even if he chooses to switch to another competing product for business reasons.

Post facto interoperability
Post facto interoperability may be the result of the absolute market dominance of a particular product in contravention of any applicable standards, or if any effective standards were not present at the time of that product's introduction. The vendor behind that product can then choose to ignore any forthcoming standards and not co-operate in any standardization process at all, using its near-monopoly to insist that its product sets the de facto standard by its very market dominance. This is not a problem if the product's implementation is open and minimally encumbered, but it may well be both closed and heavily encumbered (e.g. by patent claims). Because of the network effect, achieving interoperability with such a product is both critical for any other vendor if it wishes to remain relevant in the market, and difficult to accomplish because of lack of cooperation on equal terms with the original vendor, who may well see the new vendor as a potential competitor and threat. The newer implementations often rely on clean-room reverse engineering in the absence of technical data to achieve interoperability. The original vendors may provide such technical data to others, often in the name of encouraging competition, but such data is invariably encumbered, and may be of limited use. Availability of such data is not equivalent to an open standard, because:
 * 1) The data is provided by the original vendor on a discretionary basis, and the vendor has every interest in blocking the effective implementation of competing solutions, and may subtly alter or change its product, often in newer revisions, so that competitors' implementations are almost, but not quite completely interoperable, leading customers to consider them unreliable or of lower quality. These changes may not be passed on to other vendors at all, or passed on after a strategic delay, maintaining the market dominance of the original vendor.
 * 2) The data itself may be encumbered, e.g. by patents or pricing, leading to a dependence of all competing solutions on the original vendor, and possibly leading a revenue stream from the competitors' customers back to the original vendor. This revenue stream is the result of the original product's market dominance and not a result of any innate superiority.
 * 3) Even when the original vendor is genuinely interested in promoting a healthy competition (so that he may also benefit from the resulting innovative market), post-facto interoperability may often be undesirable as many defects or quirks can be directly traced back to the original implementation's technical limitations. Although in an open process, anyone may identify and correct such limitations, and the resulting cleaner specification may be used by all vendors, this is more difficult post-facto, as customers already have valuable information and processes encoded in the faulty but dominant product, and other vendors are forced to replicate those faults and quirks for the sake of preserving interoperability even if they could design better solutions. Alternatively, it can be argued that even open processes are subject to the weight of past implementations and imperfect past designs and that the power of the dominant vendor to unilaterally correct or improve the system and impose the changes to all users facilitates innovation.
 * 4) Lack of an open standard can also become problematic for the customers, as in the case of the original vendor's inability to fix a certain problem that is an artifact of technical limitations in the original product. The customer wants that fault fixed, but the vendor has to maintain that faulty state, even across newer revisions of the same product, because that behavior is a de facto standard and many more customers would have to pay the price of any interoperability issues caused by fixing the original problem and introducing new behavior.

eGovernment
Speaking from an e-government perspective, interoperability refers to the collaboration ability of cross-border services for citizens, businesses and public administrations. Exchanging data can be a challenge due to language barriers, different specifications of formats, varieties of categorizations and other hindrances.

If data is interpreted differently, collaboration is limited, takes longer and is inefficient. For instance, if a citizen of country A wants to purchase land in country B, the person will be asked to submit the proper address data. Address data in both countries include full name details, street name and number as well as a postal code. The order of the address details might vary. In the same language, it is not an obstacle to order the provided address data; but across language barriers, it becomes difficult. If the language uses a different writing system it is almost impossible if no translation tools are available.

Flood risk management
Interoperability is used by researchers in the context of urban flood risk management. Cities and urban areas worldwide are expanding, which creates complex spaces with many interactions between the environment, infrastructure and people. To address this complexity and manage water in urban areas appropriately, a system of systems approach to water and flood control is necessary. In this context, interoperability is important to facilitate system-of-systems thinking, and is defined as: "the ability of any water management system to redirect water and make use of other system(s) to maintain or enhance its performance function during water exceedance events." By assessing the complex properties of urban infrastructure systems, particularly the interoperability between the drainage systems and other urban systems (e.g. infrastructure such as transport), it could be possible to expand the capacity of the overall system to manage flood water towards achieving improved urban flood resilience.

Military forces
Force interoperability is defined in NATO as the ability of the forces of two or more nations to train, exercise and operate effectively together in the execution of assigned missions and tasks. Additionally NATO defines interoperability more generally as the ability to act together coherently, effectively and efficiently to achieve Allied tactical, operational and strategic objectives.

At the strategic level, interoperability is an enabler for coalition building. It facilitates meaningful contributions by coalition partners. At this level, interoperability issues center on harmonizing world views, strategies, doctrines, and force structures. Interoperability is an element of coalition willingness to work together over the long term to achieve and maintain shared interests against common threats. Interoperability at the operational and tactical levels is where strategic interoperability and technological interoperability come together to help allies shape the environment, manage crises, and win wars. The benefits of interoperability at the operational and tactical levels generally derive from the interchangeability of force elements and units. Technological interoperability reflects the interfaces between organizations and systems. It focuses on communications and computers but also involves the technical capabilities of systems and the resulting mission compatibility between the systems and data of coalition partners. At the technological level, the benefits of interoperability come primarily from their impacts at the operational and tactical levels in terms of enhancing flexibility.

Public safety
Because first responders need to be able to communicate during wide-scale emergencies, interoperability is an important issue for law enforcement, fire fighting, emergency medical services, and other public health and safety departments. It has been a major area of investment and research over the last 12 years. Widely disparate and incompatible hardware impedes the exchange of information between agencies. Agencies' information systems such as computer-aided dispatch systems and records management systems functioned largely in isolation, in so-called information islands. Agencies tried to bridge this isolation with inefficient, stop-gap methods while large agencies began implementing limited interoperable systems. These approaches were inadequate and, in the US, the lack of interoperability in the public safety realm become evident during the 9/11 attacks on the Pentagon and World Trade Center structures. Further evidence of a lack of interoperability surfaced when agencies tackled the aftermath of Hurricane Katrina.

In contrast to the overall national picture, some states, including Utah, have already made great strides forward. The Utah Highway Patrol and other departments in Utah have created a statewide data sharing network.

The Commonwealth of Virginia is one of the leading states in the United States in improving interoperability. The Interoperability Coordinator leverages a regional structure to better allocate grant funding around the Commonwealth so that all areas have an opportunity to improve communications interoperability. Virginia's strategic plan for communications is updated yearly to include new initiatives for the Commonwealth – all projects and efforts are tied to this plan, which is aligned with the National Emergency Communications Plan, authored by the Department of Homeland Security's Office of Emergency Communications.

The State of Washington seeks to enhance interoperability statewide. The State Interoperability Executive Committee (SIEC), established by the legislature in 2003, works to assist emergency responder agencies (police, fire, sheriff, medical, hazmat, etc.) at all levels of government (city, county, state, tribal, federal) to define interoperability for their local region. Washington recognizes that collaborating on system design and development for wireless radio systems enables emergency responder agencies to efficiently provide additional services, increase interoperability, and reduce long-term costs. This work saves the lives of emergency personnel and the citizens they serve.

The U.S. government is making an effort to overcome the nation's lack of public safety interoperability. The Department of Homeland Security's Office for Interoperability and Compatibility (OIC) is pursuing the SAFECOM and CADIP and Project 25 programs, which are designed to help agencies as they integrate their CAD and other IT systems.

The OIC launched CADIP in August 2007. This project will partner the OIC with agencies in several locations, including Silicon Valley. This program will use case studies to identify the best practices and challenges associated with linking CAD systems across jurisdictional boundaries. These lessons will create the tools and resources public safety agencies can use to build interoperable CAD systems and communicate across local, state, and federal boundaries.

As regulator for interoperability
Governance entities can increase interoperability through their legislative and executive powers. For instance, in 2021 the European Commission, after commissioning two impact assessment studies and a technology analysis study, proposed the implementation of a standardization – for iterations of USB-C – of phone charger products, which may increase interoperability along with convergence and convenience for consumers while decreasing resource needs, redundancy and electronic waste.

Desktop
Desktop interoperability is a subset of software interoperability. In the early days, the focus of interoperability was to integrate web applications with other web applications. Over time, open-system containers were developed to create a virtual desktop environment in which these applications could be registered and then communicate with each other using simple publish–subscribe patterns. Rudimentary UI capabilities were also supported allowing windows to be grouped with other windows. Today, desktop interoperability has evolved into full-service platforms which include container support, basic exchange between web and web, but also native support for other application types and advanced window management. The very latest interop platforms also include application services such as universal search, notifications, user permissions and preferences, 3rd party application connectors and language adapters for in-house applications.

Information search
Search interoperability refers to the ability of two or more information collections to be searched by a single query.

Specifically related to web-based search, the challenge of interoperability stems from the fact designers of web resources typically have little or no need to concern themselves with exchanging information with other web resources. Federated Search technology, which does not place format requirements on the data owner, has emerged as one solution to search interoperability challenges. In addition, standards, such as Open Archives Initiative Protocol for Metadata Harvesting, Resource Description Framework, and SPARQL, have emerged that also help address the issue of search interoperability related to web resources. Such standards also address broader topics of interoperability, such as allowing data mining.

Software

With respect to software, the term interoperability is used to describe the capability of different programs to exchange data via a common set of exchange formats, to read and write the same file formats, and to use the same communication protocols. The lack of interoperability can be a consequence of a lack of attention to standardization during the design of a program. Indeed, interoperability is not taken for granted in the non-standards-based portion of the computing world.

According to ISO/IEC 2382-01, Information Technology Vocabulary, Fundamental Terms, interoperability is defined as follows: "The capability to communicate, execute programs, or transfer data among various functional units in a manner that requires the user to have little or no knowledge of the unique characteristics of those units".

Standards-developing organizations provide open public software specifications to facilitate interoperability; examples include the Oasis-Open organization and buildingSMART (formerly the International Alliance for Interoperability). Another example of a neutral party is the RFC documents from the Internet Engineering Task Force (IETF).

The Open Service for Lifecycle Collaboration community is working on finding a common standard in order that software tools can share and exchange data e.g. bugs, tasks, requirements etc. The final goal is to agree on an open standard for interoperability of open source application lifecycle management tools.

Java is an example of an interoperable programming language that allows for programs to be written once and run anywhere with a Java virtual machine. A program in Java, so long as it does not use system-specific functionality, will maintain interoperability with all systems that have a Java virtual machine available. Applications will maintain compatibility because, while the implementation is different, the underlying language interfaces are the same.

Achieving software
Software interoperability is achieved through five interrelated ways:
 * 1) Product testing
 * Products produced to a common standard, or to a sub-profile thereof, depend on the clarity of the standards, but there may be discrepancies in their implementations that system or unit testing may not uncover. This requires that systems formally be tested in a production scenario – as they will be finally implemented – to ensure they actually will intercommunicate as advertised, i.e. they are interoperable. Interoperable product testing is different from conformance-based product testing as conformance to a standard does not necessarily engender interoperability with another product which is also tested for conformance.
 * 1) Product engineering
 * Implements the common standard, or a sub-profile thereof, as defined by the industry and community partnerships with the specific intention of achieving interoperability with other software implementations also following the same standard or sub-profile thereof.
 * 1) Industry and community partnership
 * Industry and community partnerships, either domestic or international, sponsor standard workgroups with the purpose of defining a common standard that may be used to allow software systems to intercommunicate for a defined purpose. At times an industry or community will sub-profile an existing standard produced by another organization to reduce options and thus make interoperability more achievable for implementations.
 * 1) Common technology and intellectual property
 * The use of a common technology or intellectual property may speed up and reduce the complexity of interoperability by reducing variability between components from different sets of separately developed software products and thus allowing them to intercommunicate more readily. This technique has some of the same technical results as using a common vendor product to produce interoperability. The common technology can come through third-party libraries or open-source developments.
 * 1) Standard implementation
 * Software interoperability requires a common agreement that is normally arrived at via an industrial, national or international standard.

Each of these has an important role in reducing variability in intercommunication software and enhancing a common understanding of the end goal to be achieved.

Market dominance and power
Interoperability tends to be regarded as an issue for experts and its implications for daily living are sometimes underrated. The European Union Microsoft competition case shows how interoperability concerns important questions of power relationships. In 2004, the European Commission found that Microsoft had abused its market power by deliberately restricting interoperability between Windows work group servers and non-Microsoft work group servers. By doing so, Microsoft was able to protect its dominant market position for work group server operating systems, the heart of corporate IT networks. Microsoft was ordered to disclose complete and accurate interface documentation, which could enable rival vendors to compete on an equal footing (the interoperability remedy).

Interoperability has also surfaced in the software patent debate in the European Parliament (June–July 2005). Critics claim that because patents on techniques required for interoperability are kept under RAND (reasonable and non-discriminatory licensing) conditions, customers will have to pay license fees twice: once for the product and, in the appropriate case, once for the patent-protected program the product uses.

Business processes
Interoperability is often more of an organizational issue. Interoperability can have a significant impact on the organizations concerned, raising issues of ownership (do people want to share their data? or are they dealing with information silos?), labor relations (are people prepared to undergo training?) and usability. In this context, a more apt definition is captured in the term business process interoperability.

Interoperability can have important economic consequences; for example, research has estimated the cost of inadequate interoperability in the US capital facilities industry to be $15.8 billion a year. If competitors' products are not interoperable (due to causes such as patents, trade secrets or coordination failures), the result may well be monopoly or market failure. For this reason, it may be prudent for user communities or governments to take steps to encourage interoperability in various situations. At least 30 international bodies and countries have implemented eGovernment-based interoperability framework initiatives called e-GIF while in the US there is the NIEM initiative.

Medical industry
The need for plug-and-play interoperability – the ability to take a medical device out of its box and easily make it work with one's other devices – has attracted great attention from both healthcare providers and industry.

Increasingly, medical devices like incubators and imaging systems feature software that integrates at the point of care and with electronic systems, such as electronic medical records. At the 2016 Regulatory Affairs Professionals Society (RAPS) meeting, experts in the field like Angela N. Johnson with GE Healthcare and Jeff Shuren of the United States Food and Drug Administration provided practical seminars on how companies developing new medical devices, and hospitals installing them, can work more effectively to align interoperable software systems.

Railways
Railways have greater or lesser interoperability depending on conforming to standards of gauge, couplings, brakes, signalling, loading gauge, and structure gauge to mention a few parameters. For passenger rail service, different railway platform height and width clearance standards may also affect interoperability.

North American freight and intercity passenger railroads are highly interoperable, but systems in Europe, Asia, Africa, Central and South America, and Australia are much less so. The parameter most difficult to overcome (at reasonable cost) is incompatibility of gauge, though variable gauge axle systems are increasingly used.

Telecommunications
In telecommunication, the term can be defined as:
 * 1) The ability to provide services to and accept services from other systems, and to use the services exchanged to enable them to operate effectively together. ITU-T provides standards for international telecommunications.
 * 2) The condition achieved among communications-electronics systems or items of communications-electronics equipment when information or services can be exchanged directly and satisfactorily between them or their users. The degree of interoperability should be defined when referring to specific cases.

In two-way radio, interoperability is composed of three dimensions:
 * compatible communications paths (compatible frequencies, equipment and signaling),
 * radio system coverage or adequate signal strength, and;
 * scalable capacity.

Organizations dedicated to interoperability
Many organizations are dedicated to interoperability. All have in common that they want to push the development of the World Wide Web towards the semantic web. Some concentrate on eGovernment, eBusiness or data exchange in general.

Global
Internationally, Network Centric Operations Industry Consortium facilitates global interoperability across borders, language and technical barriers. In the built environment, the International Alliance for Interoperability started in 1994, and was renamed buildingSMART in 2005.

Europe
In Europe, for instance, the European Commission and its IDABC program issue the European Interoperability Framework. IDABC was succeeded by the ISA program. They also initiated the Semantic Interoperability Centre Europe (SEMIC.EU). A European Land Information Service (EULIS) was established in 2006, as a consortium of European National Land Registers. The aim of the service is to establish a single portal through which customers are provided with access to information about individual properties, about land and property registration services, and about the associated legal environment.

The European Interoperability Framework (EIF) considered four kinds of interoperability: 1) legal interoperability, 2) organizational interoperability, 3) semantic interoperability, and 4) technical interoperability. In the European Research Cluster on the Internet of Things (IERC) and IoT Semantic Interoperability Best Practices; four kinds of interoperability are distinguished: 1) syntactical interoperability, 2) technical interoperability, 3) semantic interoperability, and 4) organizational interoperability.

US
In the United States, the General Services Administration Component Organization and Registration Environment (CORE.GOV) initiative provided a collaboration environment for component development, sharing, registration, and reuse in the early 2000s. A related initiative is the ongoing National Information Exchange Model (NIEM) work and component repository. The National Institute of Standards and Technology serves as an agency for measurement standards.

title: Interoperability


text:
Interposition trunk

In telecommunication, the term interposition trunk has the following meanings:

1. A single direct communication channel, e.g., voice-frequency circuit, between two positions of a large switchboard to facilitate the interconnection of other circuits appearing at the respective switchboard positions.

2. Within a technical control facility, a single direct transmission circuit, between positions in a testboard or patch bay, which circuit facilitates testing or patching between the respective positions.

title: Interposition trunk


text:
Intersymbol interference

In telecommunication, intersymbol interference (ISI) is a form of distortion of a signal in which one symbol interferes with subsequent symbols. This is an unwanted phenomenon as the previous symbols have a similar effect as noise, thus making the communication less reliable. The spreading of the pulse beyond its allotted time interval causes it to interfere with neighboring pulses. ISI is usually caused by multipath propagation or the inherent linear or non-linear frequency response of a communication channel causing successive symbols to blur together.

The presence of ISI in the system introduces errors in the decision device at the receiver output. Therefore, in the design of the transmitting and receiving filters, the objective is to minimize the effects of ISI, and thereby deliver the digital data to its destination with the smallest error rate possible.

Ways to alleviate intersymbol interference include adaptive equalization and error correcting codes.

Multipath propagation
One of the causes of intersymbol interference is multipath propagation in which a wireless signal from a transmitter reaches the receiver via multiple paths. The causes of this include reflection (for instance, the signal may bounce off buildings), refraction (such as through the foliage of a tree) and atmospheric effects such as atmospheric ducting and ionospheric reflection. Since the various paths can be of different lengths, this results in the different versions of the signal arriving at the receiver at different times. These delays mean that part or all of a given symbol will be spread into the subsequent symbols, thereby interfering with the correct detection of those symbols. Additionally, the various paths often distort the amplitude and/or phase of the signal, thereby causing further interference with the received signal.

Bandlimited channels
Another cause of intersymbol interference is the transmission of a signal through a bandlimited channel, i.e., one where the frequency response is zero above a certain frequency (the cutoff frequency). Passing a signal through such a channel results in the removal of frequency components above this cutoff frequency. In addition, components of the frequency below the cutoff frequency may also be attenuated by the channel.

This filtering of the transmitted signal affects the shape of the pulse that arrives at the receiver. The effects of filtering a rectangular pulse not only change the shape of the pulse within the first symbol period, but it is also spread out over the subsequent symbol periods. When a message is transmitted through such a channel, the spread pulse of each individual symbol will interfere with following symbols.

Bandlimited channels are present in both wired and wireless communications. The limitation is often imposed by the desire to operate multiple independent signals through the same area/cable; due to this, each system is typically allocated a piece of the total bandwidth available. For wireless systems, they may be allocated a slice of the electromagnetic spectrum to transmit in (for example, FM radio is often broadcast in the 87.5–108 MHz range). This allocation is usually administered by a government agency; in the case of the United States this is the Federal Communications Commission (FCC). In a wired system, such as an optical fiber cable, the allocation will be decided by the owner of the cable.

The bandlimiting can also be due to the physical properties of the medium - for instance, the cable being used in a wired system may have a cutoff frequency above which practically none of the transmitted signal will propagate.

Communication systems that transmit data over bandlimited channels usually implement pulse shaping to avoid interference caused by the bandwidth limitation. If the channel frequency response is flat and the shaping filter has a finite bandwidth, it is possible to communicate with no ISI at all. Often the channel response is not known beforehand, and an adaptive equalizer is used to compensate the frequency response.

Effects on eye patterns
One way to study ISI in a PCM or data transmission system experimentally is to apply the received wave to the vertical deflection plates of an oscilloscope and to apply a sawtooth wave at the transmitted symbol rate R (R = 1/T) to the horizontal deflection plates. The resulting display is called an eye pattern because of its resemblance to the human eye for binary waves. The interior region of the eye pattern is called the eye opening. An eye pattern provides a great deal of information about the performance of the pertinent system.
 * 1) The width of the eye opening defines the time interval over which the received wave can be sampled without error from ISI. It is apparent that the preferred time for sampling is the instant of time at which the eye is open widest.
 * 2) The sensitivity of the system to timing error is determined by the rate of closure of the eye as the sampling time is varied.
 * 3) The height of the eye opening, at a specified sampling time, defines the margin over noise.

An eye pattern, which overlays many samples of a signal, can give a graphical representation of the signal characteristics. The first image above is the eye pattern for a binary phase-shift keying (PSK) system in which a one is represented by an amplitude of −1 and a zero by an amplitude of +1. The current sampling time is at the center of the image and the previous and next sampling times are at the edges of the image. The various transitions from one sampling time to another (such as one-to-zero, one-to-one and so forth) can clearly be seen on the diagram.

The noise margin - the amount of noise required to cause the receiver to get an error - is given by the distance between the signal and the zero amplitude point at the sampling time; in other words, the further from zero at the sampling time the signal is the better. For the signal to be correctly interpreted, it must be sampled somewhere between the two points where the zero-to-one and one-to-zero transitions cross. Again, the further apart these points are the better, as this means the signal will be less sensitive to errors in the timing of the samples at the receiver.

The effects of ISI are shown in the second image which is an eye pattern of the same system when operating over a multipath channel. The effects of receiving delayed and distorted versions of the signal can be seen in the loss of definition of the signal transitions. It also reduces both the noise margin and the window in which the signal can be sampled, which shows that the performance of the system will be worse (i.e. it will have a greater bit error ratio).

Countering ISI
There are several techniques in telecommunication and data storage that try to work around the problem of intersymbol interference.

 * Design systems such that the impulse response is short enough that very little energy from one symbol smears into the next symbol.
 * Separate symbols in time with guard periods.
 * Apply an equalizer at the receiver, that, broadly speaking, attempts to undo the effect of the channel by applying an inverse filter.
 * Apply a sequence detector at the receiver, that attempts to estimate the sequence of transmitted symbols using the Viterbi algorithm.

Intentional intersymbol interference
Coded modulation systems also exist that intentionally build a controlled amount of ISI into the system at the transmitter side, known as faster-than-Nyquist signaling. Such a design trades a computational complexity penalty at the receiver against a Shannon capacity gain of the overall transceiver system.

title: Intersymbol interference


text:
Inverse-square law

In science, an inverse-square law is any scientific law stating that the observed "intensity" of a specified physical quantity is inversely proportional to the square of the distance from the source of that physical quantity. The fundamental cause for this can be understood as geometric dilution corresponding to point-source radiation into three-dimensional space.

Radar energy expands during both the signal transmission and the reflected return, so the inverse square for both paths means that the radar will receive energy according to the inverse fourth power of the range.

To prevent dilution of energy while propagating a signal, certain methods can be used such as a waveguide, which acts like a canal does for water, or how a gun barrel restricts hot gas expansion to one dimension in order to prevent loss of energy transfer to a bullet.

Formula
In mathematical notation the inverse square law can be expressed as an intensity (I) varying as a function of distance (d) from some centre. The intensity is proportional (see ∝) to the reciprocal of the square of the distance thus: $$\text{intensity} \ \propto \ \frac{1}{\text{distance}^2} \, $$

It can also be mathematically expressed as : $$\frac{\text{intensity}_1}{\text{intensity}_2} = \frac{\text{distance}_2^2}{\text{distance}_1^2}$$

or as the formulation of a constant quantity: $$\text{intensity}_1 \times \text{distance}_1^2 = \text{intensity}_2 \times \text{distance}_2^2$$

The divergence of a vector field which is the resultant of radial inverse-square law fields with respect to one or more sources is proportional to the strength of the local sources, and hence zero outside sources. Newton's law of universal gravitation follows an inverse-square law, as do the effects of electric, light, sound, and radiation phenomena.

Justification
The inverse-square law generally applies when some force, energy, or other conserved quantity is evenly radiated outward from a point source in three-dimensional space. Since the surface area of a sphere (which is 4πr2) is proportional to the square of the radius, as the emitted radiation gets farther from the source, it is spread out over an area that is increasing in proportion to the square of the distance from the source. Hence, the intensity of radiation passing through any unit area (directly facing the point source) is inversely proportional to the square of the distance from the point source. Gauss's law for gravity is similarly applicable, and can be used with any physical quantity that acts in accordance with the inverse-square relationship.

Gravitation
Gravitation is the attraction between objects that have mass. Newton's law states:

"The gravitational attraction force between two point masses is directly proportional to the product of their masses and inversely proportional to the square of their separation distance. The force is always attractive and acts along the line joining them."

If the distribution of matter in each body is spherically symmetric, then the objects can be treated as point masses without approximation, as shown in the shell theorem. Otherwise, if we want to calculate the attraction between massive bodies, we need to add all the point-point attraction forces vectorially and the net attraction might not be exact inverse square. However, if the separation between the massive bodies is much larger compared to their sizes, then to a good approximation, it is reasonable to treat the masses as a point mass located at the object's center of mass while calculating the gravitational force.

As the law of gravitation, this law was suggested in 1645 by Ismaël Bullialdus. But Bullialdus did not accept Kepler's second and third laws, nor did he appreciate Christiaan Huygens's solution for circular motion (motion in a straight line pulled aside by the central force). Indeed, Bullialdus maintained the sun's force was attractive at aphelion and repulsive at perihelion. Robert Hooke and Giovanni Alfonso Borelli both expounded gravitation in 1666 as an attractive force. Hooke's lecture "On gravity" was at the Royal Society, in London, on 21 March. Borelli's "Theory of the Planets" was published later in 1666. Hooke's 1670 Gresham lecture explained that gravitation applied to "all celestiall bodys" and added the principles that the gravitating power decreases with distance and that in the absence of any such power bodies move in straight lines. By 1679, Hooke thought gravitation had inverse square dependence and communicated this in a letter to Isaac Newton: my supposition is that the attraction always is in duplicate proportion to the distance from the center reciprocall.

Hooke remained bitter about Newton claiming the invention of this principle, even though Newton's 1686 Principia acknowledged that Hooke, along with Wren and Halley, had separately appreciated the inverse square law in the solar system, as well as giving some credit to Bullialdus.

Electrostatics
The force of attraction or repulsion between two electrically charged particles, in addition to being directly proportional to the product of the electric charges, is inversely proportional to the square of the distance between them; this is known as Coulomb's law. The deviation of the exponent from 2 is less than one part in 1015.

$$F=k_\text{e}\frac{q_1 q_2}{r^2}$$

Light and other electromagnetic radiation
The intensity (or illuminance or irradiance) of light or other linear waves radiating from a point source (energy per unit of area perpendicular to the source) is inversely proportional to the square of the distance from the source, so an object (of the same size) twice as far away receives only one-quarter the energy (in the same time period).

More generally, the irradiance, i.e., the intensity (or power per unit area in the direction of propagation), of a spherical wavefront varies inversely with the square of the distance from the source (assuming there are no losses caused by absorption or scattering).

For example, the intensity of radiation from the Sun is 9126 watts per square meter at the distance of Mercury (0.387 AU); but only 1367 watts per square meter at the distance of Earth (1 AU)—an approximate threefold increase in distance results in an approximate ninefold decrease in intensity of radiation.

For non-isotropic radiators such as parabolic antennas, headlights, and lasers, the effective origin is located far behind the beam aperture. If you are close to the origin, you don't have to go far to double the radius, so the signal drops quickly. When you are far from the origin and still have a strong signal, like with a laser, you have to travel very far to double the radius and reduce the signal. This means you have a stronger signal or have antenna gain in the direction of the narrow beam relative to a wide beam in all directions of an isotropic antenna.

In photography and stage lighting, the inverse-square law is used to determine the “fall off” or the difference in illumination on a subject as it moves closer to or further from the light source. For quick approximations, it is enough to remember that doubling the distance reduces illumination to one quarter; or similarly, to halve the illumination increase the distance by a factor of 1.4 (the square root of 2), and to double illumination, reduce the distance to 0.7 (square root of 1/2). When the illuminant is not a point source, the inverse square rule is often still a useful approximation; when the size of the light source is less than one-fifth of the distance to the subject, the calculation error is less than 1%.

The fractional reduction in electromagnetic fluence (Φ) for indirectly ionizing radiation with increasing distance from a point source can be calculated using the inverse-square law. Since emissions from a point source have radial directions, they intercept at a perpendicular incidence. The area of such a shell is 4πr 2 where r is the radial distance from the center. The law is particularly important in diagnostic radiography and radiotherapy treatment planning, though this proportionality does not hold in practical situations unless source dimensions are much smaller than the distance. As stated in Fourier theory of heat “as the point source is magnification by distances, its radiation is dilute proportional to the sin of the angle, of the increasing circumference arc from the point of origin”.

Example
Let P be the total power radiated from a point source (for example, an omnidirectional isotropic radiator). At large distances from the source (compared to the size of the source), this power is distributed over larger and larger spherical surfaces as the distance from the source increases. Since the surface area of a sphere of radius r is A = 4πr2, the intensity I (power per unit area) of radiation at distance r is $$ I = \frac P A = \frac P {4 \pi r^2}. \, $$

The energy or intensity decreases (divided by 4) as the distance r is doubled; if measured in dB would decrease by 6.02 dB per doubling of distance. When referring to measurements of power quantities, a ratio can be expressed as a level in decibels by evaluating ten times the base-10 logarithm of the ratio of the measured quantity to the reference value.

Sound in a gas
In acoustics, the sound pressure of a spherical wavefront radiating from a point source decreases by 50% as the distance r is doubled; measured in dB, the decrease is still 6.02 dB, since dB represents an intensity ratio. The pressure ratio (as opposed to power ratio) is not inverse-square, but is inverse-proportional (inverse distance law):

$$ p \ \propto \ \frac{1}{r} \, $$

The same is true for the component of particle velocity $$ v \,$$ that is in-phase with the instantaneous sound pressure $$p \,$$:

$$ v \ \propto \frac{1}{r} \ \, $$

In the near field is a quadrature component of the particle velocity that is 90° out of phase with the sound pressure and does not contribute to the time-averaged energy or the intensity of the sound. The sound intensity is the product of the RMS sound pressure and the in-phase component of the RMS particle velocity, both of which are inverse-proportional. Accordingly, the intensity follows an inverse-square behaviour:

$$ I \ = \ p v \ \propto \ \frac{1}{r^2}. \, $$

Field theory interpretation
For an irrotational vector field in three-dimensional space, the inverse-square law corresponds to the property that the divergence is zero outside the source. This can be generalized to higher dimensions. Generally, for an irrotational vector field in n-dimensional Euclidean space, the intensity "I" of the vector field falls off with the distance "r" following the inverse (n − 1)th power law

$$I\propto \frac 1 {r^{n-1}},$$

given that the space outside the source is divergence free.

Non-Euclidean implications
The inverse-square law, fundamental in Euclidean spaces, also applies to non-Euclidean geometries, including hyperbolic space. The inherent curvature in these spaces impacts physical laws, underpinning various fields such as cosmology, general relativity, and string theory.

John D. Barrow, in his 2020 paper "Non-Euclidean Newtonian Cosmology," elaborates on the behavior of force (F) and potential (Φ) within hyperbolic 3-space (H3). He illustrates that F and Φ obey the formulas F ∝ 1 / R^2 sinh^2(r/R) and Φ ∝ coth(r/R), where R and r represent the curvature radius and the distance from the focal point, respectively.

The concept of the dimensionality of space, first proposed by Immanuel Kant, is an ongoing topic of debate in relation to the inverse-square law. Dimitria Electra Gatzia and Rex D. Ramsier, in their 2021 paper, argue that the inverse-square law pertains more to the symmetry in force distribution than to the dimensionality of space.

Within the realm of non-Euclidean geometries and general relativity, deviations from the inverse-square law might not stem from the law itself but rather from the assumption that the force between bodies depends instantaneously on distance, contradicting special relativity. General relativity instead interprets gravity as a distortion of spacetime, causing freely falling particles to traverse geodesics in this curved spacetime.

History
John Dumbleton of the 14th-century Oxford Calculators, was one of the first to express functional relationships in graphical form. He gave a proof of the mean speed theorem stating that "the latitude of a uniformly difform movement corresponds to the degree of the midpoint" and used this method to study the quantitative decrease in intensity of illumination in his Summa logicæ et philosophiæ naturalis (ca. 1349), stating that it was not linearly proportional to the distance, but was unable to expose the Inverse-square law. In proposition 9 of Book 1 in his book Ad Vitellionem paralipomena, quibus astronomiae pars optica traditur (1604), the astronomer Johannes Kepler argued that the spreading of light from a point source obeys an inverse square law:

In 1645, in his book Astronomia Philolaica ..., the French astronomer Ismaël Bullialdus (1605–1694) refuted Johannes Kepler's suggestion that "gravity" weakens as the inverse of the distance; instead, Bullialdus argued, "gravity" weakens as the inverse square of the distance:

In England, the Anglican bishop Seth Ward (1617–1689) publicized the ideas of Bullialdus in his critique In Ismaelis Bullialdi astronomiae philolaicae fundamenta inquisitio brevis (1653) and publicized the planetary astronomy of Kepler in his book Astronomia geometrica (1656).

In 1663–1664, the English scientist Robert Hooke was writing his book Micrographia (1666) in which he discussed, among other things, the relation between the height of the atmosphere and the barometric pressure at the surface. Since the atmosphere surrounds the Earth, which itself is a sphere, the volume of atmosphere bearing on any unit area of the Earth's surface is a truncated cone (which extends from the Earth's center to the vacuum of space; obviously only the section of the cone from the Earth's surface to space bears on the Earth's surface). Although the volume of a cone is proportional to the cube of its height, Hooke argued that the air's pressure at the Earth's surface is instead proportional to the height of the atmosphere because gravity diminishes with altitude. Although Hooke did not explicitly state so, the relation that he proposed would be true only if gravity decreases as the inverse square of the distance from the Earth's center.

title: Inverse-square law


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Ionospheric sounding

In telecommunication and radio science, an ionospheric sounding is a technique that provides real-time data on high-frequency ionospheric-dependent radio propagation, using a basic system consisting of a synchronized transmitter and receiver.

The time delay between transmission and reception is translated into effective ionospheric layer altitude. Vertical incident sounding uses a collocated transmitter and receiver and involves directing a range of frequencies vertically to the ionosphere and measuring the values of the reflected returned signals to determine the effective ionosphere layer altitude. This technique is also used to determine the critical frequency. Oblique sounders use a transmitter at one end of a given propagation path, and a synchronized receiver, usually with an oscilloscope-type display (ionogram), at the other end. The transmitter emits a stepped- or swept-frequency signal which is displayed or measured at the receiver. The measurement converts time delay to effective altitude of the ionospheric layer. The ionogram display shows the effective altitude of the ionospheric layer as a function of frequency.

title: Ionospheric sounding


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Isochronous timing

A sequence of events is isochronous if the events occur regularly, or at equal time intervals. The term isochronous is used in several technical contexts, but usually refers to the primary subject maintaining a constant period or interval (the reciprocal of frequency), despite variations in other measurable factors in the same system. Isochronous timing is a characteristic of a repeating event whereas synchronous timing refers to the relationship between two or more events.

 * In dynamical systems theory, an oscillator is called isochronous if its frequency is independent of its amplitude.
 * In horology, a mechanical clock or watch is isochronous if it runs at the same rate regardless of changes in its drive force, so that it keeps correct time as its mainspring unwinds or chain length varies. Isochrony is important in timekeeping devices. Simply put, if a power providing device (ie a spring or weight) provides constant torque to the wheel train, it is isochronous (since springs provide weaker power the less pressure is applied it isn't isochronous, but since gravity's force on a mass is constant then it is isochronous).
 * In electrical power generation, isochronous means that the frequency of the electricity generated is constant under varying load; there is zero generator droop. (See Synchronization (alternating current).)
 * In telecommunications, an isochronous signal is one where the time interval separating any two corresponding transitions is equal to the unit interval or to a multiple of the unit interval; but phase is arbitrary and potentially varying.
 * The term is also used in data transmission to describe cases in which corresponding significant instants of two or more sequential signals have a constant phase relationship.
 * Isochronous burst transmission is used when the information-bearer channel rate is higher than the input data signaling rate.
 * In the Universal Serial Bus used in computers, isochronous is one of the four data flow types for USB devices (the others being Control, Interrupt and Bulk). It is commonly used for streaming data types such as video or audio sources. Similarly, the IEEE 1394 interface standard, commonly called Firewire, includes support for isochronous streams of audio and video at known constant rates.
 * In particle accelerators an isochronous cyclotron is a cyclotron where the field strength increases with radius to compensate for relativistic increase in mass with speed.
 * An isochrone is a contour line of equal time, for instance, in geological layers, tree rings or wave fronts. An isochrone map or diagram shows such contours.
 * In linguistics, isochrony is the postulated rhythmic division of time into equal portions by a language.
 * In neurology, isochronic tones are regular beats of a single tone used for brainwave entrainment.

title: Isochronous timing


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Isochronous burst transmission

Isochronous burst transmission is a method of transmission. In a data network where the information-bearer channel rate is higher than the input data signaling rate, transmission is performed by interrupting, at controlled intervals, the data stream being transmitted.

Note 1: Burst transmission in isochronous form enables communication between data terminal equipment (DTE) and data networks that operate at dissimilar data signaling rates, such as when the information-bearer channel rate is higher than the DTE output data signaling rate.

Note 2: The binary digits are transferred at the information-bearer channel rate. The data transfer is interrupted at intervals in order to produce the required average data signaling rate.

Note 3: The interruption is always for an integral number of unit intervals.

Note 4: Isochronous burst transmission has particular application where envelopes are being transferred between data circuit terminating equipment (DCE) and only the bytes contained within the envelopes are being transferred between the DCE and the DTE. Synonyms: burst isochronous (deprecated), interrupted isochronous transmission.

title: Isochronous burst transmission


text:
Isochronous signal

In telecommunication, an isochronous signal is a signal in which the time interval separating any two significant instants is equal to the unit interval or a multiple of the unit interval. Variations in the time intervals are constrained within specified limits.

"Isochronous" is a characteristic of one signal, while "synchronous" indicates a relationship between two or more signals.

title: Isochronous signal


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Jerkiness

Jerkiness (sometimes called strobing or choppy footage) describes the perception of individual still images while watching a motion picture.

Description
Motion pictures are made from still images shown in rapid sequence. Provided there is sufficient continuity between the images and provided the sequence is shown fast enough, the central nervous system interprets the sequence as continuous motion. However, some technologies cannot process or carry data fast enough for sufficiently high frame rates. For example, viewing motion pictures by Internet connection generally necessitates a greatly reduced frame rate, making jerkiness clearly apparent.

In conventional cinematography, the images are filmed and displayed at 24 frames per second, at which speed jerkiness is not normally discernible. Television screens refresh at even higher frequencies. PAL and SÉCAM television (the standards in Europe) refresh at 25 or 50 (HDTV) frames per second. NTSC television displays (the standard in North America) refresh at 29.97 frames per second. Animated cartoon films are typically made at reduced frame rates (accomplished by shooting several film frames of the individual drawings) so as to limit production costs, with the result that jerkiness tends to be apparent, especially on older limited animation features.

Other use in image glossary
Strobing can also refer to cross colour and Moiré patterning. Cross colour refers to when any high frequency luminance content of the picture, close to the TV systems colour sub-carrier frequency, is interpreted by the analogue receiver's decoder as colour information. Moiré patterning is where an interference pattern is produced by fine scene detail beating with the line (or even pixel) structure of the device used to analyse or display the scene.

title: Jerkiness