A computer network is a digital telecommunications network which allows nodes to share resources. In computer networks, computing devices exchange data with each other using connections between nodes; these data links are established over cable media such as wires or optic cables, or wireless media such as Wi-Fi. Network computer devices that originate and terminate the data are called network nodes. Nodes are identified by network addresses, can include hosts such as personal computers and servers, as well as networking hardware such as routers and switches. Two such devices can be said to be networked together when one device is able to exchange information with the other device, whether or not they have a direct connection to each other. In most cases, application-specific communications protocols are layered over other more general communications protocols; this formidable collection of information technology requires skilled network management to keep it all running reliably. Computer networks support an enormous number of applications and services such as access to the World Wide Web, digital video, digital audio, shared use of application and storage servers and fax machines, use of email and instant messaging applications as well as many others.
Computer networks differ in the transmission medium used to carry their signals, communications protocols to organize network traffic, the network's size, traffic control mechanism and organizational intent. The best-known computer network is the Internet; the chronology of significant computer-network developments includes: In the late 1950s, early networks of computers included the U. S. military radar system Semi-Automatic Ground Environment. In 1959, Anatolii Ivanovich Kitov proposed to the Central Committee of the Communist Party of the Soviet Union a detailed plan for the re-organisation of the control of the Soviet armed forces and of the Soviet economy on the basis of a network of computing centres, the OGAS. In 1960, the commercial airline reservation system semi-automatic business research environment went online with two connected mainframes. In 1963, J. C. R. Licklider sent a memorandum to office colleagues discussing the concept of the "Intergalactic Computer Network", a computer network intended to allow general communications among computer users.
In 1964, researchers at Dartmouth College developed the Dartmouth Time Sharing System for distributed users of large computer systems. The same year, at Massachusetts Institute of Technology, a research group supported by General Electric and Bell Labs used a computer to route and manage telephone connections. Throughout the 1960s, Paul Baran and Donald Davies independently developed the concept of packet switching to transfer information between computers over a network. Davies pioneered the implementation of the concept with the NPL network, a local area network at the National Physical Laboratory using a line speed of 768 kbit/s. In 1965, Western Electric introduced the first used telephone switch that implemented true computer control. In 1966, Thomas Marill and Lawrence G. Roberts published a paper on an experimental wide area network for computer time sharing. In 1969, the first four nodes of the ARPANET were connected using 50 kbit/s circuits between the University of California at Los Angeles, the Stanford Research Institute, the University of California at Santa Barbara, the University of Utah.
Leonard Kleinrock carried out theoretical work to model the performance of packet-switched networks, which underpinned the development of the ARPANET. His theoretical work on hierarchical routing in the late 1970s with student Farouk Kamoun remains critical to the operation of the Internet today. In 1972, commercial services using X.25 were deployed, used as an underlying infrastructure for expanding TCP/IP networks. In 1973, the French CYCLADES network was the first to make the hosts responsible for the reliable delivery of data, rather than this being a centralized service of the network itself. In 1973, Robert Metcalfe wrote a formal memo at Xerox PARC describing Ethernet, a networking system, based on the Aloha network, developed in the 1960s by Norman Abramson and colleagues at the University of Hawaii. In July 1976, Robert Metcalfe and David Boggs published their paper "Ethernet: Distributed Packet Switching for Local Computer Networks" and collaborated on several patents received in 1977 and 1978.
In 1979, Robert Metcalfe pursued making Ethernet an open standard. In 1976, John Murphy of Datapoint Corporation created ARCNET, a token-passing network first used to share storage devices. In 1995, the transmission speed capacity for Ethernet increased from 10 Mbit/s to 100 Mbit/s. By 1998, Ethernet supported transmission speeds of a Gigabit. Subsequently, higher speeds of up to 400 Gbit/s were added; the ability of Ethernet to scale is a contributing factor to its continued use. Computer networking may be considered a branch of electrical engineering, electronics engineering, telecommunications, computer science, information technology or computer engineering, since it relies upon the theoretical and practical application of the related disciplines. A computer network facilitates interpersonal communications allowing users to communicate efficiently and via various means: email, instant messaging, online chat, video telephone calls, video conferencing. A network allows sharing of computing resources.
Users may access and use resources provided by devices on the network, such as printing a document on a shared network printer or use of a shared storage device. A network allows sharing of files, and
Media Gateway Control Protocol
The Media Gateway Control Protocol is a signaling and call control communications protocol used in voice over IP telecommunication systems. It implements the media gateway control protocol architecture for controlling media gateways on Internet Protocol networks connected to the public switched telephone network; the protocol is a successor to the Simple Gateway Control Protocol, developed by Bellcore and Cisco, the Internet Protocol Device Control. The methodology of MGCP reflects the structure of the PSTN with the power of the network residing in a call control center softswitch, analogous to the central office in the telephone network; the endpoints are low-intelligence devices executing control commands from a call agent or media gateway controller in the softswitch and providing result indications in response. The protocol represents a decomposition of other VoIP models, such as H.323 and the Session Initiation Protocol, in which the endpoint devices of a call have higher levels of signaling intelligence.
MGCP is a text-based protocol consisting of responses. It uses the Session Description Protocol for specifying and negotiating the media streams to be transmitted in a call session and the Real-time Transport Protocol for framing the media streams; the media gateway control protocol architecture and its methodologies and programming interfaces are described in RFC 2805. MGCP is a master-slave protocol in which media gateways are controlled by a call control agent or softswitch; this controller is called call agent. With the network protocol it can control each specific port on a media gateway; this provides scalable IP telephony solutions. The distributed system is composed of at least one call agent and one or multiple media gateways, which performs the conversion of media signals between circuit-switched and packet-switched networks, at least one signaling gateway when connected to the PSTN. MGCP presents a call control architecture with limited intelligence at the edge and intelligence at the core controllers.
The MGCP model assumes that call agents synchronize with each other to send coherent commands and responses to the gateways under their control. The call agent uses MGCP to request event notifications, reports and configuration data from the media gateway, as well as to specify connection parameters and activation of signals toward the PSTN telephony interface. A softswitch is used in conjunction with signaling gateways, for access to Signalling System No. 7 functionality, for example. The call agent does not use MGCP to control a signaling gateway. A media gateway may be configured with a list of call agents from which it may accept control commands. In principle, event notifications may be sent to different call agents for each endpoint on the gateway, according to the instructions received from the call agents by setting the NotifiedEntity parameter. In practice, however, it is desirable that all endpoints of a gateway are controlled by the same call agent. In the event of such a failure it is the backup call agent's responsibility to reconfigure the media gateway so that it reports to the backup call agent.
The gateway may be audited to determine the controlling call agent, a query that may be used to resolve any conflicts. In case of multiple call agents, MGCP assumes that they maintain knowledge of device state among themselves; such failover features take into account both planned and unplanned outages. MGCP recognizes three essential elements of communication, the media gateway controller, the media gateway endpoint, connections between these entities. A media gateway may host multiple endpoints and each endpoint should be able to engage in multiple connections. Multiple connections on the endpoints support calling features such as call waiting and three-way calling. MGCP is a text-based protocol using a response model. Commands and responses are encoded in messages that are structured and formatted with the whitespace characters space, horizontal tab, carriage return, linefeed and full stop. Messages are transmitted using the User Datagram Protocol. Media gateways use the port number 2427, call agents use 2727 by default.
The message sequence of command and its response is known as a transaction, identified by the numerical Transaction Identifier exchanged in each transaction. The protocol specification defines nine standard commands that are distinguished by a four-letter command verb: AUEP, AUCX, CRCX, DLCX, EPCF, MDCX, NTFY, RQNT, RSIP. Responses begin with a three-digit numerical response code that identifies the outcome or result of the transaction. Two verbs are used by a call agent to query the state of its associated connections. AUEP: Audit Endpoint AUCX: Audit ConnectionThree verbs are used by a call agent to manage the connection to a media gateway endpoint. CRCX: Create Connection DLCX: Delete Connection; this command may be issued by an endpoint to terminate a connection. MDCX: Modify Connection; this command is used to alter operating characteristics of the connection, e.g. speech encoders, half-duplex/full-duplex state and others. One verb is used by a call agent to request notification of events occurring at the endpoint, to apply signals to the connected PSTN network link, or to a connected telephony endpoint, e.g
Ethernet is a family of computer networking technologies used in local area networks, metropolitan area networks and wide area networks. It was commercially introduced in 1980 and first standardized in 1983 as IEEE 802.3, has since retained a good deal of backward compatibility and been refined to support higher bit rates and longer link distances. Over time, Ethernet has replaced competing wired LAN technologies such as Token Ring, FDDI and ARCNET; the original 10BASE5 Ethernet uses coaxial cable as a shared medium, while the newer Ethernet variants use twisted pair and fiber optic links in conjunction with switches. Over the course of its history, Ethernet data transfer rates have been increased from the original 2.94 megabits per second to the latest 400 gigabits per second. The Ethernet standards comprise several wiring and signaling variants of the OSI physical layer in use with Ethernet. Systems communicating over Ethernet divide a stream of data into shorter pieces called frames; each frame contains source and destination addresses, error-checking data so that damaged frames can be detected and discarded.
As per the OSI model, Ethernet provides services up including the data link layer. Features such as the 48-bit MAC address and Ethernet frame format have influenced other networking protocols including Wi-Fi wireless networking technology. Ethernet is used in home and industry; the Internet Protocol is carried over Ethernet and so it is considered one of the key technologies that make up the Internet. Ethernet was developed at Xerox PARC between 1973 and 1974, it was inspired by ALOHAnet. The idea was first documented in a memo that Metcalfe wrote on May 22, 1973, where he named it after the luminiferous aether once postulated to exist as an "omnipresent, completely-passive medium for the propagation of electromagnetic waves." In 1975, Xerox filed a patent application listing Metcalfe, David Boggs, Chuck Thacker, Butler Lampson as inventors. In 1976, after the system was deployed at PARC, Metcalfe and Boggs published a seminal paper; that same year, Ron Crane, Bob Garner, Roy Ogus facilitated the upgrade from the original 2.94 Mbit/s protocol to the 10 Mbit/s protocol, released to the market in 1980.
Metcalfe left Xerox in June 1979 to form 3Com. He convinced Digital Equipment Corporation and Xerox to work together to promote Ethernet as a standard; as part of that process Xerox agreed to relinquish their'Ethernet' trademark. The first standard was published on September 1980 as "The Ethernet, A Local Area Network. Data Link Layer and Physical Layer Specifications"; this so-called DIX standard specified 10 Mbit/s Ethernet, with 48-bit destination and source addresses and a global 16-bit Ethertype-type field. Version 2 was published in November, 1982 and defines what has become known as Ethernet II. Formal standardization efforts proceeded at the same time and resulted in the publication of IEEE 802.3 on June 23, 1983. Ethernet competed with Token Ring and other proprietary protocols. Ethernet was able to adapt to market realities and shift to inexpensive thin coaxial cable and ubiquitous twisted pair wiring. By the end of the 1980s, Ethernet was the dominant network technology. In the process, 3Com became a major company.
3Com shipped its first 10 Mbit/s Ethernet 3C100 NIC in March 1981, that year started selling adapters for PDP-11s and VAXes, as well as Multibus-based Intel and Sun Microsystems computers. This was followed by DEC's Unibus to Ethernet adapter, which DEC sold and used internally to build its own corporate network, which reached over 10,000 nodes by 1986, making it one of the largest computer networks in the world at that time. An Ethernet adapter card for the IBM PC was released in 1982, and, by 1985, 3Com had sold 100,000. Parallel port based Ethernet adapters were produced with drivers for DOS and Windows. By the early 1990s, Ethernet became so prevalent that it was a must-have feature for modern computers, Ethernet ports began to appear on some PCs and most workstations; this process was sped up with the introduction of 10BASE-T and its small modular connector, at which point Ethernet ports appeared on low-end motherboards. Since Ethernet technology has evolved to meet new bandwidth and market requirements.
In addition to computers, Ethernet is now used to interconnect appliances and other personal devices. As Industrial Ethernet it is used in industrial applications and is replacing legacy data transmission systems in the world's telecommunications networks. By 2010, the market for Ethernet equipment amounted to over $16 billion per year. In February 1980, the Institute of Electrical and Electronics Engineers started project 802 to standardize local area networks; the "DIX-group" with Gary Robinson, Phil Arst, Bob Printis submitted the so-called "Blue Book" CSMA/CD specification as a candidate for the LAN specification. In addition to CSMA/CD, Token Ring and Token Bus were considered as candidates for a LAN standard. Competing proposals and broad interest in the initiative led to strong disagreement over which technology to standardize. In December 1980, the group was split into three subgroups, standardization proceeded separately for each proposal. Delays in the standards process put at risk the market introduction of the Xerox Star workstation and 3Com's Ethernet LAN products.
With such business implications in mind, David Liddle an
Wi-Fi is technology for radio wireless local area networking of devices based on the IEEE 802.11 standards. Wi‑Fi is a trademark of the Wi-Fi Alliance, which restricts the use of the term Wi-Fi Certified to products that complete after many years of testing the 802.11 committee interoperability certification testing. Devices that can use Wi-Fi technologies include, among others and laptops, video game consoles and tablets, smart TVs, digital audio players, digital cameras and drones. Wi-Fi compatible devices can connect to the Internet via a wireless access point; such an access point has a range of about 20 meters indoors and a greater range outdoors. Hotspot coverage can be as small as a single room with walls that block radio waves, or as large as many square kilometres achieved by using multiple overlapping access points. Different versions of Wi-Fi exist, with radio bands and speeds. Wi-Fi most uses the 2.4 gigahertz UHF and 5 gigahertz SHF ISM radio bands. Each channel can be time-shared by multiple networks.
These wavelengths work best for line-of-sight. Many common materials absorb or reflect them, which further restricts range, but can tend to help minimise interference between different networks in crowded environments. At close range, some versions of Wi-Fi, running on suitable hardware, can achieve speeds of over 1 Gbit/s. Anyone within range with a wireless network interface controller can attempt to access a network. Wi-Fi Protected Access is a family of technologies created to protect information moving across Wi-Fi networks and includes solutions for personal and enterprise networks. Security features of WPA have included stronger protections and new security practices as the security landscape has changed over time. In 1971, ALOHAnet connected the Hawaiian Islands with a UHF wireless packet network. ALOHAnet and the ALOHA protocol were early forerunners to Ethernet, the IEEE 802.11 protocols, respectively. A 1985 ruling by the U. S. Federal Communications Commission released the ISM band for unlicensed use.
These frequency bands are the same ones used by equipment such as microwave ovens and are subject to interference. In 1991, NCR Corporation with AT&T Corporation invented the precursor to 802.11, intended for use in cashier systems, under the name WaveLAN. The Australian radio-astronomer Dr John O'Sullivan with his colleagues Terence Percival, Graham Daniels, Diet Ostry, John Deane developed a key patent used in Wi-Fi as a by-product of a Commonwealth Scientific and Industrial Research Organisation research project, "a failed experiment to detect exploding mini black holes the size of an atomic particle". Dr O'Sullivan and his colleagues are credited with inventing Wi-Fi. In 1992 and 1996, CSIRO obtained patents for a method used in Wi-Fi to "unsmear" the signal; the first version of the 802.11 protocol was released in 1997, provided up to 2 Mbit/s link speeds. This was updated in 1999 with 802.11b to permit 11 Mbit/s link speeds, this proved to be popular. In 1999, the Wi-Fi Alliance formed as a trade association to hold the Wi-Fi trademark under which most products are sold.
Wi-Fi uses a large number of patents held by many different organizations. In April 2009, 14 technology companies agreed to pay CSIRO $1 billion for infringements on CSIRO patents; this led to Australia labeling Wi-Fi as an Australian invention, though this has been the subject of some controversy. CSIRO won a further $220 million settlement for Wi-Fi patent-infringements in 2012 with global firms in the United States required to pay the CSIRO licensing rights estimated to be worth an additional $1 billion in royalties. In 2016, the wireless local area network Test Bed was chosen as Australia's contribution to the exhibition A History of the World in 100 Objects held in the National Museum of Australia; the name Wi-Fi, commercially used at least as early as August 1999, was coined by the brand-consulting firm Interbrand. The Wi-Fi Alliance had hired Interbrand to create a name, "a little catchier than'IEEE 802.11b Direct Sequence'." Phil Belanger, a founding member of the Wi-Fi Alliance who presided over the selection of the name "Wi-Fi", has stated that Interbrand invented Wi-Fi as a pun on the word hi-fi, a term for high-quality audio technology.
Interbrand created the Wi-Fi logo. The yin-yang Wi-Fi logo indicates the certification of a product for interoperability; the Wi-Fi Alliance used the advertising slogan "The Standard for Wireless Fidelity" for a short time after the brand name was created. While inspired by the term hi-fi, the name was never "Wireless Fidelity"; the Wi-Fi Alliance was called the "Wireless Fidelity Alliance Inc" in some publications. Non-Wi-Fi technologies intended for fixed points, such as Motorola Canopy, are described as fixed wireless. Alternative wireless technologies include mobile phone standards, such as 2G, 3G, 4G, LTE; the name is sometimes written as WiFi, Wifi, or wifi, but these are not approved by the Wi-Fi Alliance. IEEE is a separate, but related organization and their website has stated "WiFi is a short name for Wireless Fidelity". To connect to a Wi-Fi LAN, a computer has to be equipped with a wireless network interface controller; the combination of computer and interface controllers is called a station.
A service set is the set of all the devices associated with a particular Wi-Fi network. The service set can be local, extended or mesh; each service set has an associated identifier, the 32-byte Service Set Identifier, which identifies the partic
Internet protocol suite
The Internet protocol suite is the conceptual model and set of communications protocols used in the Internet and similar computer networks. It is known as TCP/IP because the foundational protocols in the suite are the Transmission Control Protocol and the Internet Protocol, it is known as the Department of Defense model because the development of the networking method was funded by the United States Department of Defense through DARPA. The Internet protocol suite provides end-to-end data communication specifying how data should be packetized, transmitted and received; this functionality is organized into four abstraction layers, which classify all related protocols according to the scope of networking involved. From lowest to highest, the layers are the link layer, containing communication methods for data that remains within a single network segment; the technical standards underlying the Internet protocol suite and its constituent protocols are maintained by the Internet Engineering Task Force.
The Internet protocol suite predates the OSI model, a more comprehensive reference framework for general networking systems. The Internet protocol suite resulted from research and development conducted by the Defense Advanced Research Projects Agency in the late 1960s. After initiating the pioneering ARPANET in 1969, DARPA started work on a number of other data transmission technologies. In 1972, Robert E. Kahn joined the DARPA Information Processing Technology Office, where he worked on both satellite packet networks and ground-based radio packet networks, recognized the value of being able to communicate across both. In the spring of 1973, Vinton Cerf, who helped develop the existing ARPANET Network Control Program protocol, joined Kahn to work on open-architecture interconnection models with the goal of designing the next protocol generation for the ARPANET. By the summer of 1973, Kahn and Cerf had worked out a fundamental reformulation, in which the differences between local network protocols were hidden by using a common internetwork protocol, instead of the network being responsible for reliability, as in the ARPANET, this function was delegated to the hosts.
Cerf credits Hubert Zimmermann and Louis Pouzin, designer of the CYCLADES network, with important influences on this design. The protocol was implemented as the Transmission Control Program, first published in 1974; the TCP managed both datagram transmissions and routing, but as the protocol grew, other researchers recommended a division of functionality into protocol layers. Advocates included Jonathan Postel of the University of Southern California's Information Sciences Institute, who edited the Request for Comments, the technical and strategic document series that has both documented and catalyzed Internet development. Postel stated, "We are screwing up in our design of Internet protocols by violating the principle of layering." Encapsulation of different mechanisms was intended to create an environment where the upper layers could access only what was needed from the lower layers. A monolithic design would lead to scalability issues; the Transmission Control Program was split into two distinct protocols, the Transmission Control Protocol and the Internet Protocol.
The design of the network included the recognition that it should provide only the functions of efficiently transmitting and routing traffic between end nodes and that all other intelligence should be located at the edge of the network, in the end nodes. This design is known as the end-to-end principle. Using this design, it became possible to connect any network to the ARPANET, irrespective of the local characteristics, thereby solving Kahn's initial internetworking problem. One popular expression is that TCP/IP, the eventual product of Cerf and Kahn's work, can run over "two tin cans and a string." Years as a joke, the IP over Avian Carriers formal protocol specification was created and tested. A computer called, it forwards network packets forth between them. A router was called gateway, but the term was changed to avoid confusion with other types of gateways. From 1973 to 1974, Cerf's networking research group at Stanford worked out details of the idea, resulting in the first TCP specification.
A significant technical influence was the early networking work at Xerox PARC, which produced the PARC Universal Packet protocol suite, much of which existed around that time. DARPA contracted with BBN Technologies, Stanford University, the University College London to develop operational versions of the protocol on different hardware platforms. Four versions were developed: TCP v1, TCP v2, TCP v3 and IP v3, TCP/IP v4; the last protocol is still in use today. In 1975, a two-network TCP/IP communications test was performed between Stanford and University College London. In November 1977, a three-network TCP/IP test was conducted between sites in the US, the UK, Norway. Several other TCP/IP prototypes were developed at multiple research centers between 1978 and 1983. In March 1982, the US Department of Defense declared TCP/IP as the standard for all military computer networking. In the same year, Peter T. Kirstein's research group at University College London adopted the protocol; the migration of the ARPANET to TCP/IP was completed on flag day January 1, 1983, when the new protocols were permanently activated.
In 1985, the Internet Advisory Board held a three-day TCP/
Integrated Services Digital Network
Integrated Services Digital Network is a set of communication standards for simultaneous digital transmission of voice, video and other network services over the traditional circuits of the public switched telephone network. It was first defined in 1988 in the CCITT red book. Prior to ISDN, the telephone system was viewed as a way to transport voice, with some special services available for data; the key feature of ISDN is that it integrates speech and data on the same lines, adding features that were not available in the classic telephone system. The ISDN standards define several kinds of access interfaces, such as Basic Rate Interface, Primary Rate Interface, Narrowband ISDN, Broadband ISDN. ISDN is a circuit-switched telephone network system, which provides access to packet switched networks, designed to allow digital transmission of voice and data over ordinary telephone copper wires, resulting in better voice quality than an analog phone can provide, it offers circuit-switched connections, packet-switched connections, in increments of 64 kilobit/s.
In some countries, ISDN found major market application for Internet access, in which ISDN provides a maximum of 128 kbit/s bandwidth in both upstream and downstream directions. Channel bonding can achieve a greater data rate. ISDN is employed as data-link and physical layers in the context of the OSI model. In common use, ISDN is limited to usage to Q.931 and related protocols, which are a set of signaling protocols establishing and breaking circuit-switched connections, for advanced calling features for the user. They were introduced in 1986. In a videoconference, ISDN provides simultaneous voice and text transmission between individual desktop videoconferencing systems and group videoconferencing systems. Integrated services refers to ISDN's ability to deliver at minimum two simultaneous connections, in any combination of data, voice and fax, over a single line. Multiple devices can be attached to the line, used as needed; that means an ISDN line can take care of what were expected to be most people's complete communications needs at a much higher transmission rate, without forcing the purchase of multiple analog phone lines.
It refers to integrated switching and transmission in that telephone switching and carrier wave transmission are integrated rather than separate as in earlier technology. The entry level interface to ISDN is the Basic Rate Interface, a 128 kbit/s service delivered over a pair of standard telephone copper wires; the 144 kbit/s overall payload rate is divided into two 64 kbit/s bearer channels and one 16 kbit/s signaling channel. This is sometimes referred to as 2B+D; the interface specifies the following network interfaces: The U interface is a two-wire interface between the exchange and a network terminating unit, the demarcation point in non-North American networks. The T interface is a serial interface between a computing device and a terminal adapter, the digital equivalent of a modem; the S interface is a four-wire bus. The R interface defines the point between a non-ISDN device and a terminal adapter which provides translation to and from such a device. BRI-ISDN is popular in Europe but is much less common in North America.
It is common in Japan — where it is known as INS64. The other ISDN access available is the Primary Rate Interface, carried over T-carrier with 24 time slots in North America, over E-carrier with 32 channels in most other countries; each channel provides transmission at a 64 kbit/s data rate. With the E1 carrier, the available channels are divided into 30 bearer channels, one data channel, one timing and alarm channel; this scheme is referred to as 30B+2D. In North America, PRI service is delivered via T1 carriers with only one data channel referred to as 23B+D, a total data rate of 1544 kbit/s. Non-Facility Associated Signalling allows two or more PRI circuits to be controlled by a single D channel, sometimes called 23B+D + n*24B. D-channel backup allows for a second D channel in case the primary fails. NFAS is used on a Digital Signal 3. PRI-ISDN is popular throughout the world for connecting private branch exchanges to the public switched telephone network. Though many network professionals use the term ISDN to refer to the lower-bandwidth BRI circuit, in North America BRI is uncommon whilst PRI circuits serving PBXs are commonplace.
The bearer channel is a standard 64 kbit/s voice channel of 8 bits sampled at 8 kHz with G.711 encoding. B-channels can be used to carry data, since they are nothing more than digital channels; each one of these channels is known as a DS0. Most B channels can carry a 64 kbit/s signal, but some were limited to 56K because they traveled over RBS lines; this has since become less so. X.25 can be carried over the B or D channels of a BRI line, over the B channels of a PRI line. X.25 over the D channel is used at many point-of-sale terminals because it eliminates the modem setup, because it connects to the central system over a B channel, thereby eliminating the need for modems and making much better use of the central system's telephone lines. X.25 was part of an ISDN protocol