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In telecommunications, a communications protocol is a system of rules that allow two or more entities of a communications system to transmit information via any kind of variation of a physical quantity. These are the rules or standard that defines the syntax, semantics and synchronization of communication and possible error recovery methods. Protocols may be implemented by hardware, software, or a combination of both.〔Licesio J. Rodríguez-Aragón: ''(Tema 4: Internet y Teleinformática )''. retrieved 2013-04-24. 〕 Communicating systems use well-defined formats (protocol) for exchanging messages. Each message has an exact meaning intended to elicit a response from a range of possible responses pre-determined for that particular situation. The specified behavior is typically independent of how it is to be implemented. Communications protocols have to be agreed upon by the parties involved. To reach agreement, a protocol may be developed into a technical standard. A programming language describes the same for computations, so there is a close analogy between protocols and programming languages: ''protocols are to communications as programming languages are to computations''.〔 ==Communicating systems== The information exchanged between devices—through a network, or other media—is governed by rules and conventions that can be set out in technical specifications called communications protocol standards. The nature of a communication, the actual data exchanged and any state-dependent behaviors, is defined by its specification. In digital computing systems, the rules can be expressed by algorithms and data structures. Expressing the algorithms in a portable programming language makes the protocol software operating system independent. Operating systems usually contain of a set of cooperating processes that manipulate shared data to communicate with each other. This communication is governed by well-understood protocols, which can be embedded in the process code itself.〔Ben-Ari 1982, chapter 2 - The concurrent programming abstraction, p. 18-19, states the same.〕〔Ben-Ari 1982, Section 2.7 - Summary, p. 27, summarizes the concurrent programming abstraction.〕 In contrast, because there is no common memory, communicating systems have to communicate with each other using a shared transmission medium. Transmission is not necessarily reliable, and individual systems may use different hardware and/or operating systems. To implement a networking protocol, the protocol software modules are interfaced with a framework implemented on the machine's operating system. This framework implements the networking functionality of the operating system.〔 The best known frameworks are the TCP/IP model and the OSI model. At the time the Internet was developed, layering had proven to be a successful design approach for both compiler and operating system design and, given the similarities between programming languages and communications protocols, layering was applied to the protocols as well.〔Comer 2000, Sect. 11.2 - The Need For Multiple Protocols, p. 177, explains this by drawing analogies between computer communication and programming languages.〕 This gave rise to the concept of layered protocols which nowadays forms the basis of protocol design.〔Sect. 11.10 - The Disadvantage Of Layering, p. 192, states: layering forms the basis for protocol design.〕 Systems typically do not use a single protocol to handle a transmission. Instead they use a set of cooperating protocols, sometimes called a protocol family or protocol suite.〔Comer 2000, Sect. 11.2 - The Need For Multiple Protocols, p. 177, states the same.〕 Some of the best known protocol suites include: IPX/SPX, X.25, AX.25, AppleTalk and TCP/IP. The protocols can be arranged based on functionality in groups, for instance there is a group of transport protocols. The functionalities are mapped onto the layers, each layer solving a distinct class of problems relating to, for instance: application-, transport-, internet- and network interface-functions.〔Comer 2000, Sect. 11.3 - The Conceptual Layers Of Protocol Software, p. 178, "Each layer takes responsibility for handling one part of the problem."〕 To transmit a message, a protocol has to be selected from each layer, so some sort of multiplexing/demultiplexing takes place. The selection of the next protocol is accomplished by extending the message with a protocol selector for each layer.〔Comer 2000, Sect. 11.11 - The Basic Idea Behind Multiplexing And Demultiplexing, p. 192, states the same.〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Communications protocol」の詳細全文を読む スポンサード リンク
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