Time-division multiple access
Time-division multiple access is a channel access method for shared-medium networks. It allows several users to share the same frequency channel by dividing the signal into different time slots; the users transmit in one after the other, each using its own time slot. This allows multiple stations to share the same transmission medium while using only a part of its channel capacity. TDMA is used in the digital 2G cellular systems such as Global System for Mobile Communications, IS-136, Personal Digital Cellular and iDEN, in the Digital Enhanced Cordless Telecommunications standard for portable phones, it is used extensively in satellite systems, combat-net radio systems, passive optical network networks for upstream traffic from premises to the operator. For usage of Dynamic TDMA packet mode communication, see below. TDMA is a type of time-division multiplexing, with the special point that instead of having one transmitter connected to one receiver, there are multiple transmitters. In the case of the uplink from a mobile phone to a base station this becomes difficult because the mobile phone can move around and vary the timing advance required to make its transmission match the gap in transmission from its peers.
Shares single carrier frequency with multiple users Non-continuous transmission makes handoff simpler Slots can be assigned on demand in dynamic TDMA Less stringent power control than CDMA due to reduced intra cell interference Higher synchronization overhead than CDMA Advanced equalization may be necessary for high data rates if the channel is "frequency selective" and creates Intersymbol interference Cell breathing is more complicated than in CDMA Frequency/slot allocation complexity Pulsating power envelope: interference with other devices Most 2G cellular systems, with the notable exception of IS-95, are based on TDMA. GSM, D-AMPS, PDC, iDEN, PHS are examples of TDMA cellular systems. GSM combines TDMA with Frequency Hopping and wideband transmission to minimize common types of interference. In the GSM system, the synchronization of the mobile phones is achieved by sending timing advance commands from the base station which instructs the mobile phone to transmit earlier and by how much.
This compensates for the propagation delay resulting from the light speed velocity of radio waves. The mobile phone is not allowed to transmit for its entire time slot, but there is a guard interval at the end of each time slot; as the transmission moves into the guard period, the mobile network adjusts the timing advance to synchronize the transmission. Initial synchronization of a phone requires more care. Before a mobile transmits there is no way to know the offset required. For this reason, an entire time slot has to be dedicated to mobiles attempting to contact the network; the mobile attempts to broadcast at the beginning of the time slot. If the mobile is located next to the base station, there will be no time delay and this will succeed. If, the mobile phone is at just less than 35 km from the base station, the time delay will mean the mobile's broadcast arrives at the end of the time slot. In that case, the mobile will be instructed to broadcast its messages starting nearly a whole time slot earlier than would be expected otherwise.
If the mobile is beyond the 35 km cell range in GSM the RACH will arrive in a neighbouring time slot and be ignored. It is this feature, rather than limitations of power, that limits the range of a GSM cell to 35 km when no special extension techniques are used. By changing the synchronization between the uplink and downlink at the base station, this limitation can be overcome. Although most major 3G systems are based upon CDMA, time-division duplexing, packet scheduling and packet oriented multiple access schemes are available in 3G form, combined with CDMA to take advantage of the benefits of both technologies. While the most popular form of the UMTS 3G system uses CDMA and frequency division duplexing instead of TDMA, TDMA is combined with CDMA and time-division duplexing in two standard UMTS UTRA; the ITU-T G.hn standard, which provides high-speed local area networking over existing home wiring is based on a TDMA scheme. In G.hn, a "master" device allocates "Contention-Free Transmission Opportunities" to other "slave" devices in the network.
Only one device can use a CFTXOP at a time. FlexRay protocol, a wired network used for safety-critical communication in modern cars, uses the TDMA method for data transmission control. In radio systems, TDMA is used alongside frequency-division multiple access and frequency division duplex; this is the case in both IS-136 for example. Exceptions to this include the DECT and Personal Handy-phone System micro-cellular systems, UMTS-TDD UMTS variant, China's TD-SCDMA, which use time-division duplexing, where different time slots are allocated for the base station and handsets on the same frequency. A major advantage of TDMA is that the radio part of the mobile only needs to listen and broadcast for its own time slot. For the rest of the time, the mobile can carry out measurements on the network, detecting surrounding transmitters on different frequencies; this allows safe inter frequency handovers, something, difficult in CDMA systems, not supported at all in IS-95 and supported through complex system additions in Universal Mobile Telecommunications System.
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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
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
Telecommunication is the transmission of signs, messages, writings and sounds or information of any nature by wire, optical or other electromagnetic systems. Telecommunication occurs when the exchange of information between communication participants includes the use of technology, it is transmitted either electrically over physical media, such as cables, or via electromagnetic radiation. Such transmission paths are divided into communication channels which afford the advantages of multiplexing. Since the Latin term communicatio is considered the social process of information exchange, the term telecommunications is used in its plural form because it involves many different technologies. Early means of communicating over a distance included visual signals, such as beacons, smoke signals, semaphore telegraphs, signal flags, optical heliographs. Other examples of pre-modern long-distance communication included audio messages such as coded drumbeats, lung-blown horns, loud whistles. 20th- and 21st-century technologies for long-distance communication involve electrical and electromagnetic technologies, such as telegraph and teleprinter, radio, microwave transmission, fiber optics, communications satellites.
A revolution in wireless communication began in the first decade of the 20th century with the pioneering developments in radio communications by Guglielmo Marconi, who won the Nobel Prize in Physics in 1909, other notable pioneering inventors and developers in the field of electrical and electronic telecommunications. These included Charles Wheatstone and Samuel Morse, Alexander Graham Bell, Edwin Armstrong and Lee de Forest, as well as Vladimir K. Zworykin, John Logie Baird and Philo Farnsworth; the word telecommunication is a compound of the Greek prefix tele, meaning distant, far off, or afar, the Latin communicare, meaning to share. Its modern use is adapted from the French, because its written use was recorded in 1904 by the French engineer and novelist Édouard Estaunié. Communication was first used as an English word in the late 14th century, it comes from Old French comunicacion, from Latin communicationem, noun of action from past participle stem of communicare "to share, divide out.
Homing pigeons have been used throughout history by different cultures. Pigeon post had Persian roots, was used by the Romans to aid their military. Frontinus said; the Greeks conveyed the names of the victors at the Olympic Games to various cities using homing pigeons. In the early 19th century, the Dutch government used the system in Sumatra, and in 1849, Paul Julius Reuter started a pigeon service to fly stock prices between Aachen and Brussels, a service that operated for a year until the gap in the telegraph link was closed. In the Middle Ages, chains of beacons were used on hilltops as a means of relaying a signal. Beacon chains suffered the drawback that they could only pass a single bit of information, so the meaning of the message such as "the enemy has been sighted" had to be agreed upon in advance. One notable instance of their use was during the Spanish Armada, when a beacon chain relayed a signal from Plymouth to London. In 1792, Claude Chappe, a French engineer, built the first fixed visual telegraphy system between Lille and Paris.
However semaphore suffered from the need for skilled operators and expensive towers at intervals of ten to thirty kilometres. As a result of competition from the electrical telegraph, the last commercial line was abandoned in 1880. On 25 July 1837 the first commercial electrical telegraph was demonstrated by English inventor Sir William Fothergill Cooke, English scientist Sir Charles Wheatstone. Both inventors viewed their device as "an improvement to the electromagnetic telegraph" not as a new device. Samuel Morse independently developed a version of the electrical telegraph that he unsuccessfully demonstrated on 2 September 1837, his code was an important advance over Wheatstone's signaling method. The first transatlantic telegraph cable was completed on 27 July 1866, allowing transatlantic telecommunication for the first time; the conventional telephone was invented independently by Alexander Bell and Elisha Gray in 1876. Antonio Meucci invented the first device that allowed the electrical transmission of voice over a line in 1849.
However Meucci's device was of little practical value because it relied upon the electrophonic effect and thus required users to place the receiver in their mouth to "hear" what was being said. The first commercial telephone services were set-up in 1878 and 1879 on both sides of the Atlantic in the cities of New Haven and London. Starting in 1894, Italian inventor Guglielmo Marconi began developing a wireless communication using the newly discovered phenomenon of radio waves, showing by 1901 that they could be transmitted across the Atlantic Ocean; this was the start of wireless telegraphy by radio. Voice and music had little early success. World War I accelerated the development of radio for military communications. After the war, commercial radio AM broadcasting began in the 1920s and became an important mass medium for entertainment and news. World War II again accelerated development of radio for the wartime purposes of aircraft and land communication, radio navigation and radar. Development of stereo FM broadcasting of radio
In telecommunication, frame synchronization or framing is the process by which, while receiving a stream of framed data, incoming frame alignment signals are identified, permitting the data bits within the frame to be extracted for decoding or retransmission. If the transmission is temporarily interrupted, or a bit slip event occurs, the receiver must re-synchronize; the transmitter and the receiver must agree ahead of time on which frame synchronization scheme they will use. Common frame synchronization schemes are: Framing bit A common practice in telecommunications, for example in T-carrier, is to insert, in a dedicated time slot within the frame, a noninformation bit or framing bit, used for synchronization of the incoming data with the receiver. In a bit stream, framing bits indicate the end of a frame, they occur at specified positions in the frame, do not carry information, are repetitive. Syncword framing Some systems use a special syncword at the beginning of every frame. CRC-based framing Some telecommunications hardware uses CRC-based framing.
In telemetry applications, a frame synchronizer is used to frame-align a serial pulse code-modulated binary stream. The frame synchronizer follows the bit synchronizer in most telemetry applications. Without frame synchronization, decommutation is impossible; the frame synchronization pattern is a known binary pattern which repeats at a regular interval within the PCM stream. The frame synchronizer aligns the data into minor frames or sub-frames; the frame sync pattern is followed by a counter which dictates which minor or sub-frame in the series is being transmitted. This becomes important in the decommutation stage where all data is deciphered as to what attribute was sampled. Different commutations require a constant awareness of which section of the major frame is being decoded. Asynchronous start-stop Phase synchronization Self-synchronizing code Superframe This article incorporates public domain material from the General Services Administration document "Federal Standard 1037C". J. L. Massey.
"Optimum frame synchronization ". IEEE trans. Comm. com-20:115-119, April 1972. R Scholtz. "Frame synchronization techniques", IEEE Transactions on Communications, 1980. P. Robertson. "Optimal Frame Synchronization for Continuous and Packet Data Transmission", PhD Dissertation, 1995, Fortschrittberichte VDI Reihe 10, Nr. 376 PDF
A duplex communication system is a point-to-point system composed of two or more connected parties or devices that can communicate with one another in both directions. Duplex systems are employed in many communications networks, either to allow for simultaneous communication in both directions between two connected parties or to provide a reverse path for the monitoring and remote adjustment of equipment in the field. There are two types of duplex communication systems: half-duplex. In a full-duplex system, both parties can communicate with each other simultaneously. An example of a full-duplex device is a telephone; the earphone reproduces the speech of the remote party as the microphone transmits the speech of the local party, because there is a two-way communication channel between them, or more speaking, because there are two communication channels between them. In a half-duplex system, both parties can communicate with each other, but not simultaneously. An example of a half-duplex device is a walkie-talkie two-way radio that has a "push-to-talk" button.
To listen to the other person they release the button, which turns on the receiver but turns off the transmitter. Systems that do not need the duplex capability may instead use simplex communication, in which one device transmits and the others can only "listen". Examples are broadcast radio and television, garage door openers, baby monitors, wireless microphones, surveillance cameras. In these devices the communication is only in one direction. A half-duplex system provides communication in both directions, but only one direction at a time. Once a party begins receiving a signal, it must wait for the transmitter to stop transmitting, before replying. An example of a half-duplex system is a two-party system such as a walkie-talkie, wherein one must use "over" or another designated keyword to indicate the end of transmission, ensure that only one party transmits at a time, because both parties transmit and receive on the same frequency. A good analogy for a half-duplex system would be a one-lane road with traffic controllers at each end, such as a two-lane bridge under re-construction.
Traffic can flow in both directions, but only one direction at a time, regulated by the traffic controllers. Half-duplex systems are used to conserve bandwidth, since only a single communication channel is needed, shared alternately between the two directions. For example, a walkie-talkie requires only a single frequency for bidirectional communication, while a cell phone, a full-duplex device, requires two frequencies to carry the two simultaneous voice channels, one in each direction. In automatically run communications systems, such as two-way data-links, the time allocations for communications in a half-duplex system can be controlled by the hardware. Thus, there is no waste of the channel for switching. For example, station A on one end of the data link could be allowed to transmit for one second station B on the other end could be allowed to transmit for one second, the cycle repeats. In half-duplex systems, if more than one party transmits at the same time, a collision occurs, resulting in lost messages.
A full-duplex system, or sometimes called double-duplex, allows communication in both directions, unlike half-duplex, allows this to happen simultaneously. Land-line telephone networks are full-duplex, since they allow both callers to speak and be heard at the same time, with the transition from four to two wires being achieved by a hybrid coil in a telephone hybrid. Modern cell phones are full-duplex. A good analogy for a full-duplex system is a two-lane road with one lane for each direction. Moreover, in most full-duplex mode systems carrying computer data, transmitted data does not appear to be sent until it has been received and an acknowledgment is sent back by the other party. In this way, such systems implement reliable transmission methods. Two-way radios can be designed as full-duplex systems, transmitting on one frequency and receiving on another. Frequency-division duplex systems can extend their range by using sets of simple repeater stations because the communications transmitted on any single frequency always travel in the same direction.
Full-duplex Ethernet connections work by making simultaneous use of two physical twisted pairs inside the same jacket, which are directly connected to each networked device: one pair is for receiving packets, while the other pair is for sending packets. This makes the cable itself a collision-free environment and doubles the maximum total transmission capacity supported by each Ethernet connection. Full-duplex has several benefits over the use of half-duplex. First, there are no collisions. Second, full transmission capacity is available in both directions because the send and receive functions are separate. Third, since there is only one transmitter on each twisted pair, stations do not need to wait for others to complete their transmissions; some computer-based systems of the 1960s and 1970s required full-duplex facilities for half-duplex operation, since their poll-and-response schemes could not tolerate the slight delays in reversing the direction of transmission in a half-duplex line. Where channel access methods are used in point-to-multipoint networks (such as cellular networks
Frame check sequence
A frame check sequence refers to an error-detecting code added to a frame in a communications protocol. Frames are used to send payload data from a source to a destination. All frames and the bits and fields contained within them, are susceptible to errors from a variety of sources; the FCS field contains a number, calculated by the source node based on the data in the frame. This number is added to the end of a frame, sent; when the destination node receives the frame the FCS number is recalculated and compared with the FCS number included in the frame. If the two numbers are different, an error is assumed and the frame is discarded; the FCS provides error detection only. Error recovery must be performed through separate means. Ethernet, for example, specifies that a damaged frame should be discarded and does not specify any action to cause the frame to be retransmitted. Other protocols, notably the Transmission Control Protocol, can notice the data loss and initiate retransmission and error recovery.
The FCS is transmitted in such a way that the receiver can compute a running sum over the entire frame, together with the trailing FCS, expecting to see a fixed result when it is correct. For Ethernet and other IEEE 802 protocols, this fixed result known as the magic number or CRC32 residue, is 0xC704DD7B; when transmitted and used in this way, the FCS appears before the frame-ending delimiter. By far the most popular FCS algorithm is a cyclic redundancy check, used in Ethernet and other IEEE 802 protocols with 32 bits, in X.25 with 16 or 32 bits, in HDLC with 16 or 32 bits, in Frame Relay with 16 bits, in Point-to-Point Protocol with 16 or 32 bits, in other data link layer protocols. Protocols of the Internet protocol suite tend to use checksums. Ethernet frame § Preamble and start frame delimiter Syncword