98-428: The data link layer , or layer 2 , is the second layer of the seven-layer OSI model of computer networking . This layer is the protocol layer that transfers data between nodes on a network segment across the physical layer . The data link layer provides the functional and procedural means to transfer data between network entities and may also provide the means to detect and possibly correct errors that can occur in
196-425: A multiple-input multiple-output communications (MIMO) scheme. In wired communication, space-division multiplexing , also known as space-division multiple access (SDMA) is the use of separate point-to-point electrical conductors for each transmitted channel. Examples include an analog stereo audio cable, with one pair of wires for the left channel and another for the right channel, and a multi-pair telephone cable ,
294-423: A shared medium . The aim is to share a scarce resource – a physical transmission medium . For example, in telecommunications, several telephone calls may be carried using one wire. Multiplexing originated in telegraphy in the 1870s, and is now widely applied in communications. In telephony , George Owen Squier is credited with the development of telephone carrier multiplexing in 1910. The multiplexed signal
392-408: A time-multiplexing system of multiple Hughes machines in the 1870s. In 1874, the quadruplex telegraph developed by Thomas Edison transmitted two messages in each direction simultaneously, for a total of four messages transiting the same wire at the same time. Several researchers were investigating acoustic telegraphy , a frequency-division multiplexing technique, which led to the invention of
490-458: A MAC layer in combination with a different LLC layer. In the ITU-T G.hn standard, which provides a way to create a high-speed (up to 1 Gigabit/s) local area network using existing home wiring ( power lines , phone lines and coaxial cables ), the data link layer is divided into three sub-layers (application protocol convergence, logical link control and media access control). The data link layer
588-942: A comprehensive description of networking, the model failed to garner reliance during the design of the Internet , which is reflected in the less prescriptive Internet Protocol Suite , principally sponsored under the auspices of the Internet Engineering Task Force (IETF). In the early- and mid-1970s, networking was largely either government-sponsored ( NPL network in the UK, ARPANET in the US, CYCLADES in France) or vendor-developed with proprietary standards, such as IBM 's Systems Network Architecture and Digital Equipment Corporation 's DECnet . Public data networks were only just beginning to emerge, and these began to use
686-477: A corresponding entity at the same layer in another host. Service definitions, like the OSI model, abstractly describe the functionality provided to a layer N by a layer N−1 , where N is one of the seven layers of protocols operating in the local host. At each level N , two entities at the communicating devices (layer N peers ) exchange protocol data units (PDUs) by means of a layer N protocol . Each PDU contains
784-412: A customer's residential area, but the service provider can send multiple television channels or signals simultaneously over that cable to all subscribers without interference. Receivers must tune to the appropriate frequency (channel) to access the desired signal. A variant technology, called wavelength-division multiplexing (WDM) is used in optical communications . Time-division multiplexing (TDM)
882-709: A format specified by the application layer during the encapsulation of outgoing messages while being passed down the protocol stack , and possibly reversed during the deencapsulation of incoming messages when being passed up the protocol stack. For this very reason, outgoing messages during encapsulation are converted into a format specified by the application layer, while the conversion for incoming messages during deencapsulation are reversed. The presentation layer handles protocol conversion, data encryption, data decryption, data compression, data decompression, incompatibility of data representation between operating systems, and graphic commands. The presentation layer transforms data into
980-572: A function that computes the r (amount of redundant bits) corresponding to each string of N total number of bits. The simplest error detection code is the parity bit , which allows a receiver to detect transmission errors that have affected a single bit among the transmitted N + r bits. If there are multiple flipped bits then the checking method might not be able to detect this on the receiver side. More advanced methods than parity error detection do exist providing higher grades of quality and features. A simple example of how this works using metadata
1078-451: A lack of common protocols. For a period in the late 1980s and early 1990s, engineers, organizations and nations became polarized over the issue of which standard , the OSI model or the Internet protocol suite , would result in the best and most robust computer networks. However, while OSI developed its networking standards in the late 1980s, TCP/IP came into widespread use on multi-vendor networks for internetworking . The OSI model
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#17327869944401176-584: A light pulse. For example, a 1 bit might be represented on a copper wire by the transition from a 0-volt to a 5-volt signal, whereas a 0 bit might be represented by the transition from a 5-volt to a 0-volt signal. As a result, common problems occurring at the physical layer are often related to the incorrect media termination, EMI or noise scrambling, and NICs and hubs that are misconfigured or do not work correctly. The data link layer provides node-to-node data transfer —a link between two directly connected nodes. It detects and possibly corrects errors that may occur in
1274-464: A major advance in the standardisation of network concepts. It promoted the idea of a consistent model of protocol layers, defining interoperability between network devices and software. The concept of a seven-layer model was provided by the work of Charles Bachman at Honeywell Information Systems . Various aspects of OSI design evolved from experiences with the NPL network, ARPANET, CYCLADES, EIN , and
1372-518: A network made up of people speaking, i.e. a conversation, they will each pause a random amount of time and then attempt to speak again, effectively establishing a long and elaborate game of saying "no, you first". The Media Access Control sublayer also performs frame synchronization , which determines the start and end of each frame of data in the transmission bitstream . It entails one of several methods: timing-based detection, character counting, byte stuffing, and bit stuffing. The services provided by
1470-477: A network-layer protocol, if the encapsulation of the payload takes place only at the endpoint, GRE becomes closer to a transport protocol that uses IP headers but contains complete Layer 2 frames or Layer 3 packets to deliver to the endpoint. L2TP carries PPP frames inside transport segments. Although not developed under the OSI Reference Model and not strictly conforming to the OSI definition of
1568-459: A payload, called the service data unit (SDU), along with protocol-related headers or footers. Data processing by two communicating OSI-compatible devices proceeds as follows: The OSI model was defined in ISO/IEC 7498 which consists of the following parts: ISO/IEC 7498-1 is also published as ITU-T Recommendation X.200. The recommendation X.200 describes seven layers, labelled 1 to 7. Layer 1
1666-486: A peak bit rate of 54 Mbit/s, thus increasing the total peak bit rate by the factor k . Different antennas would give different multi-path propagation (echo) signatures, making it possible for digital signal processing techniques to separate different signals from each other. These techniques may also be utilized for space diversity (improved robustness to fading) or beamforming (improved selectivity) rather than multiplexing. Frequency-division multiplexing (FDM)
1764-451: A physical layer can be described in terms of the network topology . Physical layer specifications are included in the specifications for the ubiquitous Bluetooth , Ethernet , and USB standards. An example of a less well-known physical layer specification would be for the CAN standard. The physical layer also specifies how encoding occurs over a physical signal, such as electrical voltage or
1862-471: A remote database protocol to record reservations. Neither of these protocols have anything to do with reservations. That logic is in the application itself. The application layer has no means to determine the availability of resources in the network. Multiplexing In telecommunications and computer networking , multiplexing (sometimes contracted to muxing ) is a method by which multiple analog or digital signals are combined into one signal over
1960-814: A single event loop , with system calls like poll and select (Unix) . Multiple variable bit rate digital bit streams may be transferred efficiently over a single fixed bandwidth channel by means of statistical multiplexing . This is an asynchronous mode time-domain multiplexing which is a form of time-division multiplexing. Digital bit streams can be transferred over an analog channel by means of code-division multiplexing techniques such as frequency-hopping spread spectrum (FHSS) and direct-sequence spread spectrum (DSSS). In wireless communications , multiplexing can also be accomplished through alternating polarization ( horizontal / vertical or clockwise / counterclockwise ) on each adjacent channel and satellite, or through phased multi-antenna array combined with
2058-539: A switched star network such as a telephone access network, a switched Ethernet network, and a mesh network . In wireless communication, space-division multiplexing is achieved with multiple antenna elements forming a phased array antenna . Examples are multiple-input and multiple-output (MIMO), single-input and multiple-output (SIMO) and multiple-input and single-output (MISO) multiplexing. An IEEE 802.11g wireless router with k antennas makes it in principle possible to communicate with k multiplexed channels, each with
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#17327869944402156-481: A transport stream and/or container is commonly called a multiplexer or muxer . A demuxer is software that extracts or otherwise makes available for separate processing the components of such a stream or container. In digital television systems, several variable bit-rate data streams are multiplexed together to a fixed bit-rate transport stream by means of statistical multiplexing . This makes it possible to transfer several video and audio channels simultaneously over
2254-468: Is 1500 bytes, the minimum size of a TCP header is 20 bytes, and the minimum size of an IPv4 header is 20 bytes, so the maximum segment size is 1500−(20+20) bytes, or 1460 bytes. The process of dividing data into segments is called segmentation ; it is an optional function of the transport layer. Some connection-oriented transport protocols, such as TCP and the OSI connection-oriented transport protocol (COTP), perform segmentation and reassembly of segments on
2352-506: Is a reference model from the International Organization for Standardization (ISO) that "provides a common basis for the coordination of standards development for the purpose of systems interconnection." In the OSI reference model, the communications between systems are split into seven different abstraction layers: Physical, Data Link, Network, Transport, Session, Presentation, and Application. The model partitions
2450-412: Is a class of techniques where several channels simultaneously share the same frequency spectrum , and this spectral bandwidth is much higher than the bit rate or symbol rate . One form is frequency hopping, another is direct sequence spread spectrum. In the latter case, each channel transmits its bits as a coded channel-specific sequence of pulses called chips. Number of chips per bit, or chips per symbol,
2548-564: Is a data link layer protocol that can operate over several different physical layers, such as synchronous and asynchronous serial lines. The ITU-T G.hn standard, which provides high-speed local area networking over existing wires (power lines, phone lines and coaxial cables), includes a complete data link layer that provides both error correction and flow control by means of a selective-repeat sliding-window protocol . Security, specifically (authenticated) encryption, at this layer can be applied with MACsec . The network layer provides
2646-405: Is a digital (or in rare cases, analog) technology that uses time, instead of space or frequency, to separate the different data streams. TDM involves sequencing groups of a few bits or bytes from each individual input stream, one after the other, and in such a way that they can be associated with the appropriate receiver. If done sufficiently quickly, the receiving devices will not detect that some of
2744-439: Is a novel method for polarized antenna transmission utilizing a differential technique. Orbital angular momentum multiplexing is a relatively new and experimental technique for multiplexing multiple channels of signals carried using electromagnetic radiation over a single path. It can potentially be used in addition to other physical multiplexing methods to greatly expand the transmission capacity of such systems. As of 2012 it
2842-811: Is a technique in which each channel transmits its bits as a coded channel-specific sequence of pulses. This coded transmission is typically accomplished by transmitting a unique time-dependent series of short pulses, which are placed within chip times within the larger bit time. All channels, each with a different code, can be transmitted on the same fiber and asynchronously demultiplexed. Other widely used multiple access techniques are time-division multiple access (TDMA) and frequency-division multiple access (FDMA). Code-division multiplex techniques are used as an access technology, namely code-division multiple access (CDMA), in Universal Mobile Telecommunications System (UMTS) standard for
2940-411: Is closest to TCP, although TCP contains functions, such as the graceful close, which OSI assigns to the session layer. Also, all OSI TP connection-mode protocol classes provide expedited data and preservation of record boundaries. Detailed characteristics of TP0–4 classes are shown in the following table: An easy way to visualize the transport layer is to compare it with a post office, which deals with
3038-405: Is inherently an analog technology. FDM achieves the combining of several signals into one medium by sending signals in several distinct frequency ranges over a single medium. In FDM the signals are electrical signals. One of the most common applications for FDM is traditional radio and television broadcasting from terrestrial, mobile or satellite stations, or cable television. Only one cable reaches
Data link layer - Misplaced Pages Continue
3136-523: Is likewise also true for digital subscriber lines (DSL). Fiber in the loop (FITL) is a common method of multiplexing, which uses optical fiber as the backbone . It not only connects POTS phone lines with the rest of the PSTN , but also replaces DSL by connecting directly to Ethernet wired into the home . Asynchronous Transfer Mode is often the communications protocol used. Cable TV has long carried multiplexed television channels , and late in
3234-461: Is not practical (such as where there are different sources using a single transponder ), single channel per carrier mode is used. In FM broadcasting and other analog radio media, multiplexing is a term commonly given to the process of adding subcarriers to the audio signal before it enters the transmitter , where modulation occurs. (In fact, the stereo multiplex signal can be generated using time-division multiplexing, by switching between
3332-483: Is not usually a fatal problem. The OSI connection-oriented transport protocol defines five classes of connection-mode transport protocols, ranging from class 0 (which is also known as TP0 and provides the fewest features) to class 4 (TP4, designed for less reliable networks, similar to the Internet). Class 0 contains no error recovery and was designed for use on network layers that provide error-free connections. Class 4
3430-412: Is often divided into two sublayers: logical link control (LLC) and media access control (MAC). The uppermost sublayer, LLC, multiplexes protocols running at the top of the data link layer, and optionally provides flow control, acknowledgment, and error notification. The LLC provides addressing and control of the data link. It specifies which mechanisms are to be used for addressing stations over
3528-728: Is possible (just as in statistical multiplexing ), that the wide bandwidth allows poor signal-to-noise ratio according to Shannon–Hartley theorem , and that multi-path propagation in wireless communication can be combated by rake receivers . A significant application of CDMA is the Global Positioning System (GPS). A multiplexing technique may be further extended into a multiple access method or channel access method , for example, TDM into time-division multiple access (TDMA) and statistical multiplexing into carrier-sense multiple access (CSMA). A multiple-access method makes it possible for several transmitters connected to
3626-420: Is still in its early research phase, with small-scale laboratory demonstrations of bandwidths of up to 2.5 Tbit/s over a single light path. This is a controversial subject in the academic community, with many claiming it is not a new method of multiplexing, but rather a special case of space-division multiplexing. Code-division multiplexing (CDM), code-division multiple access (CDMA) or spread spectrum
3724-486: Is still used as a reference for teaching and documentation; however, the OSI protocols originally conceived for the model did not gain popularity. Some engineers argue the OSI reference model is still relevant to cloud computing . Others say the original OSI model does not fit today's networking protocols and have suggested instead a simplified approach. Communication protocols enable an entity in one host to interact with
3822-418: Is the spreading factor . This coded transmission typically is accomplished by transmitting a unique time-dependent series of short pulses, which are placed within chip times within the larger bit time. All channels, each with a different code, can be transmitted on the same fiber or radio channel or other medium, and asynchronously demultiplexed. Advantages over conventional techniques are that variable bandwidth
3920-556: Is the function of the payload that makes these belong to the network layer, not the protocol that carries them. The transport layer provides the functional and procedural means of transferring variable-length data sequences from a source host to a destination host from one application to another across a network, while maintaining the quality-of-service functions. Transport protocols may be connection-oriented or connectionless. This may require breaking large protocol data units or long data streams into smaller chunks called "segments", since
4018-423: Is the layer of the OSI model that is closest to the end user, which means both the OSI application layer and the user interact directly with a software application that implements a component of communication between the client and server, such as File Explorer and Microsoft Word . Such application programs fall outside the scope of the OSI model unless they are directly integrated into the application layer through
Data link layer - Misplaced Pages Continue
4116-411: Is the lowest layer in this model. The physical layer is responsible for the transmission and reception of unstructured raw data between a device, such as a network interface controller , Ethernet hub , or network switch , and a physical transmission medium . It converts the digital bits into electrical, radio, or optical signals. Layer specifications define characteristics such as voltage levels,
4214-418: Is transmitted over a communication channel such as a cable. The multiplexing divides the capacity of the communication channel into several logical channels, one for each message signal or data stream to be transferred. A reverse process, known as demultiplexing, extracts the original channels on the receiver end. A device that performs the multiplexing is called a multiplexer (MUX), and a device that performs
4312-488: Is transmitting the word "HELLO", by encoding each letter as its position in the alphabet. Thus, the letter A is coded as 1, B as 2, and so on as shown in the table on the right. Adding up the resulting numbers yields 8 + 5 + 12 + 12 + 15 = 52, and 5 + 2 = 7 calculates the metadata. Finally, the "8 5 12 12 15 7" numbers sequence is transmitted, which the receiver will see on its end if there are no transmission errors. The receiver knows that
4410-458: The Fourier transform principle. In computer programming , it may refer to using a single in-memory resource (such as a file handle) to handle multiple external resources (such as on-disk files). Some electrical multiplexing techniques do not require a physical " multiplexer " device, they refer to a " keyboard matrix " or " Charlieplexing " design style: In high-throughput DNA sequencing ,
4508-534: The IEEE 802.11 WiFi protocols, ATM and Frame Relay . In the Internet Protocol Suite (TCP/IP), the data link layer functionality is contained within the link layer , the lowest layer of the descriptive model, which is assumed to be independent of physical infrastructure. The data link provides for the transfer of data frames between hosts connected to the physical link. Within the semantics of
4606-567: The International Network Working Group ( IFIP WG6.1). In this model, a networking system was divided into layers. Within each layer, one or more entities implement its functionality. Each entity interacted directly only with the layer immediately beneath it and provided facilities for use by the layer above it. The OSI standards documents are available from the ITU-T as the X.200 series of recommendations. Some of
4704-444: The X.25 standard in the late 1970s. The Experimental Packet Switched System in the UK c. 1973 –1975 identified the need for defining higher level protocols. The UK National Computing Centre publication, Why Distributed Computing , which came from considerable research into future configurations for computer systems, resulted in the UK presenting the case for an international standards committee to cover this area at
4802-400: The teardown , between two or more computers, which is called a "session". Common functions of the session layer include user logon (establishment) and user logoff (termination) functions. Including this matter, authentication methods are also built into most client software, such as FTP Client and NFS Client for Microsoft Networks. Therefore, the session layer establishes, manages and terminates
4900-504: The 20th century began offering the same services as telephone companies . IPTV also depends on multiplexing. In video editing and processing systems, multiplexing refers to the process of interleaving audio and video into one coherent data stream. In digital video , such a transport stream is normally a feature of a container format which may include metadata and other information, such as subtitles . The audio and video streams may have variable bit rate. Software that produces such
4998-689: The ISO meeting in Sydney in March 1977. Beginning in 1977, the ISO initiated a program to develop general standards and methods of networking. A similar process evolved at the International Telegraph and Telephone Consultative Committee (CCITT, from French: Comité Consultatif International Téléphonique et Télégraphique ). Both bodies developed documents that defined similar networking models. The British Department of Trade and Industry acted as
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#17327869944405096-456: The OSI model has well-defined functions, and the methods of each layer communicate and interact with those of the layers immediately above and below as appropriate. The Internet protocol suite as defined in RFC 1122 and RFC 1123 is a model of networking developed contemporarily to the OSI model, and was funded primarily by the U.S. Department of Defense. It was the foundation for
5194-505: The OSI model started in the late 1970s to support the emergence of the diverse computer networking methods that were competing for application in the large national networking efforts in the world (see OSI protocols and Protocol Wars ). In the 1980s, the model became a working product of the Open Systems Interconnection group at the International Organization for Standardization (ISO). While attempting to provide
5292-583: The OSI network architecture, the protocols of the data link layer respond to service requests from the network layer , and perform their function by issuing service requests to the physical layer . That transfer can be reliable or unreliable ; many data link protocols do not have acknowledgments of successful frame reception and acceptance, and some data link protocols might not even perform any check for transmission errors. In those cases, higher-level protocols must provide flow control , error checking, acknowledgments, and retransmission. The frame header contains
5390-462: The airport ticket desk back to the airline data center are also installed. Some web proxy servers (e.g. polipo ) use TDM in HTTP pipelining of multiple HTTP transactions onto the same TCP/IP connection . Carrier-sense multiple access and multidrop communication methods are similar to time-division multiplexing in that multiple data streams are separated by time on the same medium, but because
5488-499: The application layer, known as HTTP, FTP, SMB/CIFS, TFTP, and SMTP. When identifying communication partners, the application layer determines the identity and availability of communication partners for an application with data to transmit. The most important distinction in the application layer is the distinction between the application-entity and the application. For example, a reservation website might have two application-entities: one using HTTP to communicate with its users, and one for
5586-446: The capacity to carry several HDTV channels in one multiplex. In digital radio , a multiplex (also known as an ensemble) is a number of radio stations that are grouped together. A multiplex is a stream of digital information that includes audio and other data. On communications satellites which carry broadcast television networks and radio networks , this is known as multiple channel per carrier or MCPC . Where multiplexing
5684-458: The case of CCITT7 signaling systems and 30 voice channels for customer-connected Q931, DASS2, DPNSS, V5 and CASS signaling systems. Polarization-division multiplexing uses the polarization of electromagnetic radiation to separate orthogonal channels. It is in practical use in both radio and optical communications, particularly in 100 Gbit/s per channel fiber-optic transmission systems . Differential Cross-Polarized Wireless Communications
5782-425: The circuit time was used to serve another logical communication path. Consider an application requiring four terminals at an airport to reach a central computer. Each terminal communicated at 2400 baud , so rather than acquire four individual circuits to carry such a low-speed transmission, the airline has installed a pair of multiplexers. A pair of 9600 baud modems and one dedicated analog communications circuit from
5880-558: The connections between the local and remote application. The session layer also provides for full-duplex , half-duplex , or simplex operation, and establishes procedures for checkpointing, suspending, restarting, and terminating a session between two related streams of data, such as an audio and a video stream in a web-conferencing application. Therefore, the session layer is commonly implemented explicitly in application environments that use remote procedure calls . The presentation layer establishes data formatting and data translation into
5978-436: The data link layer are: In addition to framing, the data link layer may also detect and recover from transmission errors. For a receiver to detect transmission errors, the sender must add redundant information as an error detection code to the frame sent. When the receiver obtains a frame it verifies whether the received error detection code matches a recomputed error detection code. An error detection code can be defined as
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#17327869944406076-451: The data link layer is analogous to a neighborhood traffic cop; it endeavors to arbitrate between parties contending for access to a medium, without concern for their ultimate destination. When devices attempt to use a medium simultaneously, frame collisions occur. Data-link protocols specify how devices detect and recover from such collisions, and may provide mechanisms to reduce or prevent them. Examples of data link protocols are Ethernet ,
6174-556: The data link layer is described in more detail with media access control (MAC) and logical link control (LLC) sublayers; this means that the IEEE 802.2 LLC protocol can be used with all of the IEEE 802 MAC layers, such as Ethernet, Token Ring , IEEE 802.11 , etc., as well as with some non-802 MAC layers such as FDDI . Other data-link-layer protocols, such as HDLC , are specified to include both sublayers, although some other protocols, such as Cisco HDLC , use HDLC's low-level framing as
6272-401: The data link layer. In the Internet Protocol Suite (TCP/IP), OSI's data link layer functionality is contained within its lowest layer, the link layer . The TCP/IP link layer has the operating scope of the link a host is connected to, and only concerns itself with hardware issues to the point of obtaining hardware (MAC) addresses for locating hosts on the link and transmitting data frames onto
6370-430: The development of the Internet . It assumed the presence of generic physical links and focused primarily on the software layers of communication, with a similar but much less rigorous structure than the OSI model. In comparison, several networking models have sought to create an intellectual framework for clarifying networking concepts and activities, but none have been as successful as the OSI reference model in becoming
6468-551: The dispatch and classification of mail and parcels sent. A post office inspects only the outer envelope of mail to determine its delivery. Higher layers may have the equivalent of double envelopes, such as cryptographic presentation services that can be read by the addressee only. Roughly speaking, tunnelling protocols operate at the transport layer, such as carrying non-IP protocols such as IBM 's SNA or Novell 's IPX over an IP network, or end-to-end encryption with IPsec . While Generic Routing Encapsulation (GRE) might seem to be
6566-576: The fact; the reverse of the traditional approach to developing standards. Although not a standard itself, it was a framework in which future standards could be defined. In May 1983, the CCITT and ISO documents were merged to form The Basic Reference Model for Open Systems Interconnection , usually referred to as the Open Systems Interconnection Reference Model , OSI Reference Model , or simply OSI model . It
6664-518: The flow of data in a communication system into seven abstraction layers to describe networked communication from the physical implementation of transmitting bits across a communications medium to the highest-level representation of data of a distributed application . Each intermediate layer serves a class of functionality to the layer above it and is served by the layer below it. Classes of functionality are implemented in software development using established communication protocols . Each layer in
6762-670: The form that the application layer accepts, to be sent across a network. Since the presentation layer converts data and graphics into a display format for the application layer, the presentation layer is sometimes called the syntax layer. For this reason, the presentation layer negotiates the transfer of syntax structure through the Basic Encoding Rules of Abstract Syntax Notation One (ASN.1), with capabilities such as converting an EBCDIC -coded text file to an ASCII -coded file, or serialization of objects and other data structures from and to XML . The application layer
6860-407: The functional and procedural means of transferring packets from one node to another connected in "different networks". A network is a medium to which many nodes can be connected, on which every node has an address and which permits nodes connected to it to transfer messages to other nodes connected to it by merely providing the content of a message and the address of the destination node and letting
6958-413: The functions of communication, as is the case with applications such as web browsers and email programs . Other examples of software are Microsoft Network Software for File and Printer Sharing and Unix/Linux Network File System Client for access to shared file resources. Application-layer functions typically include file sharing, message handling, and database access, through the most common protocols at
7056-472: The last number received is the error-detecting metadata and that all data before is the message, so the receiver can recalculate the above math and if the metadata matches it can be concluded that the data has been received error-free. Though, if the receiver sees something like a "7 5 12 12 15 7" sequence (first element altered by some error), it can run the check by calculating 7 + 5 + 12 + 12 + 15 = 51 and 5 + 1 = 6, and discard
7154-492: The link. The link-layer functionality was described in RFC 1122 and is defined differently than the data link layer of OSI, and encompasses all methods that affect the local link. The TCP/IP model is not a top-down comprehensive design reference for networks. It was formulated for the purpose of illustrating the logical groups and scopes of functions needed in the design of the suite of internetworking protocols of TCP/IP, as needed for
7252-502: The network find the way to deliver the message to the destination node, possibly routing it through intermediate nodes. If the message is too large to be transmitted from one node to another on the data link layer between those nodes, the network may implement message delivery by splitting the message into several fragments at one node, sending the fragments independently, and reassembling the fragments at another node. It may, but does not need to, report delivery errors. Message delivery at
7350-449: The network layer imposes a maximum packet size called the maximum transmission unit (MTU), which depends on the maximum packet size imposed by all data link layers on the network path between the two hosts. The amount of data in a data segment must be small enough to allow for a network-layer header and a transport-layer header. For example, for data being transferred across Ethernet , the MTU
7448-426: The network layer is not necessarily guaranteed to be reliable; a network layer protocol may provide reliable message delivery, but it does not need to do so. A number of layer-management protocols, a function defined in the management annex , ISO 7498/4, belong to the network layer. These include routing protocols, multicast group management, network-layer information and error, and network-layer address assignment. It
7546-558: The operation of the Internet. In general, direct or strict comparisons of the OSI and TCP/IP models should be avoided, because the layering in TCP/IP is not a principal design criterion and in general, considered to be "harmful" (RFC 3439). In particular, TCP/IP does not dictate a strict hierarchical sequence of encapsulation requirements, as is attributed to OSI protocols. OSI model The Open Systems Interconnection ( OSI ) model
7644-428: The physical layer. The data link layer is concerned with local delivery of frames between nodes on the same level of the network. Data-link frames, as these protocol data units are called, do not cross the boundaries of a local area network. Inter-network routing and global addressing are higher-layer functions, allowing data-link protocols to focus on local delivery, addressing, and media arbitration. In this way,
7742-435: The physical layer. It defines the protocol to establish and terminate a connection between two physically connected devices. It also defines the protocol for flow control between them. IEEE 802 divides the data link layer into two sublayers: The MAC and LLC layers of IEEE 802 networks such as 802.3 Ethernet , 802.11 Wi-Fi , and 802.15.4 Zigbee operate at the data link layer. The Point-to-Point Protocol (PPP)
7840-482: The protocol specifications were also available as part of the ITU-T X series. The equivalent ISO/IEC standards for the OSI model were available from ISO. Not all are free of charge. OSI was an industry effort, attempting to get industry participants to agree on common network standards to provide multi-vendor interoperability. It was common for large networks to support multiple network protocol suites, with many devices unable to interoperate with other devices because of
7938-402: The received data as defective since 6 does not equal 7. More sophisticated error detection and correction algorithms are designed to reduce the risk that multiple transmission errors in the data would cancel each other out and go undetected. An algorithm that can even detect if the correct bytes are received but out of order is the cyclic redundancy check or CRC. This algorithm is often used in
8036-436: The receiving side; connectionless transport protocols, such as UDP and the OSI connectionless transport protocol (CLTP), usually do not. The transport layer also controls the reliability of a given link between a source and destination host through flow control, error control, and acknowledgments of sequence and existence. Some protocols are state- and connection-oriented . This means that the transport layer can keep track of
8134-422: The reverse process is called a demultiplexer (DEMUX or DMX). Inverse multiplexing (IMUX) has the opposite aim as multiplexing, namely to break one data stream into several streams, transfer them simultaneously over several communication channels, and recreate the original data stream. In computing , I/O multiplexing can also be used to refer to the concept of processing multiple input/output events from
8232-498: The same frequency channel, together with various services. This may involve several standard-definition television (SDTV) programs (particularly on DVB-T , DVB-S2 , ISDB and ATSC-C), or one HDTV , possibly with a single SDTV companion channel over one 6 to 8 MHz-wide TV channel. The device that accomplishes this is called a statistical multiplexer . In several of these systems, the multiplexing results in an MPEG transport stream . The newer DVB standards DVB-S2 and DVB-T2 has
8330-434: The same physical medium to share their capacity. Multiplexing is provided by the physical layer of the OSI model , while multiple access also involves a media access control protocol, which is part of the data link layer . The Transport layer in the OSI model, as well as TCP/IP model, provides statistical multiplexing of several application layer data flows to/from the same computer. Code-division multiplexing (CDM)
8428-736: The secretariat, and universities in the United Kingdom developed prototypes of the standards. The OSI model was first defined in raw form in Washington, D.C. , in February 1978 by French software engineer Hubert Zimmermann , and the refined but still draft standard was published by the ISO in 1980. The drafters of the reference model had to contend with many competing priorities and interests. The rate of technological change made it necessary to define standards that new systems could converge to rather than standardizing procedures after
8526-585: The segments and retransmit those that fail delivery through the acknowledgment hand-shake system. The transport layer will also provide the acknowledgement of the successful data transmission and sends the next data if no errors occurred. Reliability, however, is not a strict requirement within the transport layer. Protocols like UDP, for example, are used in applications that are willing to accept some packet loss, reordering, errors or duplication. Streaming media , real-time multiplayer games and voice over IP (VoIP) are examples of applications in which loss of packets
8624-584: The signals have separate origins instead of being combined into a single signal, are best viewed as channel access methods , rather than a form of multiplexing. TD is a legacy multiplexing technology still providing the backbone of most National fixed-line telephony networks in Europe, providing the 2 Mbit/s voice and signaling ports on narrow-band telephone exchanges such as the DMS100. Each E1 or 2 Mbit/s TDM port provides either 30 or 31 speech timeslots in
8722-426: The source and destination addresses that indicate which device originated the frame and which device is expected to receive and process it. In contrast to the hierarchical and routable addresses of the network layer, layer 2 addresses are flat, meaning that no part of the address can be used to identify the logical or physical group to which the address belongs. In some networks, such as IEEE 802 local area networks,
8820-589: The standard model for discussing and teaching networking in the field of information technology . The model allows transparent communication through equivalent exchange of protocol data units (PDUs) between two parties, through what is known as peer-to-peer networking (also known as peer-to-peer communication). As a result, the OSI reference model has not only become an important piece among professionals and non-professionals alike, but also in all networking between one or many parties, due in large part to its commonly accepted user-friendly framework. The development of
8918-405: The telephone . In telephony , a customer 's telephone line now typically ends at the remote concentrator box, where it is multiplexed along with other telephone lines for that neighborhood or other similar area. The multiplexed signal is then carried to the central switching office on significantly fewer wires and for much further distances than a customer's line can practically go. This
9016-407: The term is used to indicate that some artificial sequences (often called barcodes or indexes ) have been added to link given sequence reads to a given sample, and thus allow for the sequencing of multiple samples in the same reaction. In sociolinguistics , multiplexity is used to describe the number of distinct connections between individuals who are part of a social network . A multiplex network
9114-482: The third-generation (3G) mobile communication identified by the ITU. The earliest communication technology using electrical wires, and therefore sharing an interest in the economies afforded by multiplexing, was the electric telegraph . Early experiments allowed two separate messages to travel in opposite directions simultaneously, first using an electric battery at both ends, then at only one end. Émile Baudot developed
9212-433: The timing of voltage changes, physical data rates, maximum transmission distances, modulation scheme, channel access method and physical connectors. This includes the layout of pins , voltages , line impedance , cable specifications, signal timing and frequency for wireless devices. Bit rate control is done at the physical layer and may define transmission mode as simplex , half duplex , and full duplex . The components of
9310-466: The transmission medium and for controlling the data exchanged between the originator and recipient machines. MAC may refer to the sublayer that determines who is allowed to access the media at any one time (e.g. CSMA/CD ). Other times it refers to a frame structure delivered based on MAC addresses inside. There are generally two forms of media access control: distributed and centralized. Both of these may be compared to communication between people. In
9408-651: The transport layer, the Transmission Control Protocol (TCP) and the User Datagram Protocol (UDP) of the Internet Protocol Suite are commonly categorized as layer 4 protocols within OSI. Transport Layer Security (TLS) does not strictly fit inside the model either. It contains characteristics of the transport and presentation layers. The session layer creates the setup, controls the connections, and ends
9506-471: The two (left channel and right channel) input signals at an ultrasonic rate (the subcarrier), and then filtering out the higher harmonics.) Multiplexing in this sense is sometimes known as MPX , which in turn is also an old term for stereophonic FM, seen on stereo systems since the 1960s. In spectroscopy the term is used to indicate that the experiment is performed with a mixture of frequencies at once and their respective response unraveled afterward using
9604-668: Was published in 1984 by both the ISO, as standard ISO 7498, and the renamed CCITT (now called the Telecommunications Standardization Sector of the International Telecommunication Union or ITU-T ) as standard X.200. OSI had two major components: an abstract model of networking, called the Basic Reference Model or seven-layer model, and a set of specific protocols . The OSI reference model was
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