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64-567: Based on the OSI model , HART resides at Layer 7, the Application Layer. Layers 3–6 are not used. When sent over 4–20 mA it uses a Bell 202 for layer 1. But it is often converted to RS485 or RS232. According to Emerson, due to the huge installation base of 4–20 mA systems throughout the world, the HART Protocol is one of the most popular industrial protocols today. HART protocol has made

128-501: 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 the OSI model has well-defined functions, and the methods of each layer communicate and interact with those of

192-448: A compiled language , interpreter , and script language . In the Unix operating system, most types of input and output operations are considered to be streams of bytes read from a device or written to a device. This stream of bytes model is used for file I/O, socket I/O, and terminal I/O in order to provide device independence. In order to read and write to a device at the application level,

256-428: 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

320-709: 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

384-430: A good abstraction layer allows for easy reuse by distilling a useful concept or design pattern so that situations, where it may be accurately applied, can be quickly recognized. Just composing lower-level elements into a construct doesn't count as an abstraction layer unless it shields users from its underlying complexity. A layer is considered to be on top of another if it depends on it. Every layer can exist without

448-459: A good transition protocol for users who wished to use the legacy 4–20 mA signals, but wanted to implement a "smart" protocol. The protocol was developed by Rosemount Inc. , built off the Bell 202 early communications standard in the mid-1980s as a proprietary digital communication protocol for their smart field instruments. Soon it evolved into HART and in 1986 it was made an open protocol . Since then,

512-501: 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

576-584: 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

640-461: 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

704-468: 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

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768-532: A new device starts with the maximum preamble length (20 bytes) and is later reduced once the preamble size for the current device is determined. Preamble is: "ff" "ff" "ff" "ff" "ff" (5 times ff) This byte contains the Master number and specifies that the communication packet is starting. Specifies the destination address as implemented in one of the HART schemes. The original addressing scheme used only four bits to specify

832-454: 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

896-448: 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

960-421: A range of pressures, and temperature can be sent digitally over the same wires. In point-to-point mode, the digital part of the HART protocol can be seen as a kind of digital current loop interface . In multi-drop mode the analog loop current is fixed at 4 mA and it is possible to have more than one instrument on a signal loop. HART revisions 3 through 5 allowed polling addresses of the instruments to be in

1024-465: 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. Abstraction layer In computing , an abstraction layer or abstraction level is a way of hiding the working details of a subsystem. Examples of software models that use layers of abstraction include

1088-468: 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

1152-564: 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

1216-442: Is absent for the master and is two bytes for the slave. This field is used by the slave to inform the master whether it completed the task and what its current health status is. Data contained in this field depends on the command to be executed. Checksum is composed of an XOR of all the bytes starting from the start byte and ending with the last byte of the data field, including those bytes. Each manufacturer that participates in

1280-411: 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

1344-403: 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 the OSI model started in the late 1970s to support

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1408-478: 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

1472-466: Is often deliberately misquoted with "abstraction" substituted for "indirection." It is also sometimes misattributed to Butler Lampson . Kevlin Henney 's corollary to this is, "...except for the problem of too many layers of indirection." In a computer architecture , a computer system is usually represented as consisting of several abstraction levels such as: Programmable logic is often considered part of

1536-438: 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

1600-556: 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

1664-423: 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

1728-411: 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,

1792-549: 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

1856-521: The OSI model for network protocols , OpenGL , and other graphics libraries , which allow the separation of concerns to facilitate interoperability and platform independence . In computer science , an abstraction layer is a generalization of a conceptual model or algorithm , away from any specific implementation. These generalizations arise from broad similarities that are best encapsulated by models that express similarities present in various specific implementations. The simplification provided by

1920-489: 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

1984-400: 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

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2048-488: The HART convention is assigned an identification number. This number is communicated as part of the basic device identification command used when first connecting to a device. OSI model The Open Systems Interconnection ( OSI ) model is a reference model from the International Organization for Standardization (ISO) that "provides a common basis for the coordination of standards development for

2112-668: 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

2176-499: 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

2240-402: The capabilities of the protocol have been enhanced by successive revisions to the specification. There are two main operational modes of HART instruments: point-to-point (analog/digital) mode, and multi-drop mode. In point-to-point mode the digital signals are overlaid on the 4–20 mA loop current. Both the 4–20 mA current and the digital signal are valid signalling protocols between

2304-558: 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

2368-416: The controller and measuring instrument or final control element. The polling address of the instrument is set to "0". Only one instrument can be put on each instrument cable signal pair. One signal, generally specified by the user, is specified to be the 4–20 mA signal. Other signals are sent digitally on top of the 4–20 mA signal. For example, pressure can be sent as 4–20 mA, representing

2432-459: The device address, which limited the number of devices to 16 including the master. The newer scheme utilizes 38 bits to specify the device address. This address is requested from the device using either Command 0, or Command 11. This is a one byte numerical value representing which command is to be executed. Command 0 and Command 11 are used to request the device number. Specifies the number of communication data bytes to follow. The status field

2496-551: 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

2560-485: 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 a comprehensive description of networking,

2624-555: 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

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2688-661: 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

2752-407: 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

2816-413: 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

2880-576: The hardware, while the logical definitions are also sometimes seen as part of a device's software or firmware. Firmware may include only low-level software, but can also include all software, including an operating system and applications. The software layers can be further divided into hardware abstraction layers, physical and logical device drivers, repositories such as filesystems, operating system kernels, middleware, applications, and others. A distinction can also be made from low-level programming languages like VHDL , machine language , assembly language to

2944-518: The layers above it, and requires the layers below it to function. Frequently abstraction layers can be composed into a hierarchy of abstraction levels. The OSI model comprises seven abstraction layers. Each layer of the model encapsulates and addresses a different part of the needs of digital communications, thereby reducing the complexity of the associated engineering solutions. A famous aphorism of David Wheeler is, "All problems in computer science can be solved by another level of indirection ." This

3008-452: 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 the development of the Internet . It assumed the presence of generic physical links and focused primarily on

3072-776: 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

3136-502: 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

3200-446: 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

3264-426: 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

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3328-435: 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)

3392-442: The program calls a function to open the device, which may be a real device such as a terminal or a virtual device such as a network port or a file in a file system. The device's physical characteristics are mediated by the operating system which in turn presents an abstract interface that allows the programmer to read and write bytes from/to the device. The operating system then performs the actual transformation needed to read and write

3456-482: 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

3520-434: 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 the flow of data in a communication system into seven abstraction layers to describe networked communication from the physical implementation of transmitting bits across

3584-460: The range 1–15. HART revision 6 allowed addresses 1 to 63; HART revision 7 allows addresses 0 to 63. Each instrument must have a unique address. The request HART packet has the following structure: Specifies slave, Specifies Master and Indicates Burst Mode Currently all the newer devices implement five byte preamble, since anything greater reduces the communication speed. However, masters are responsible for backwards support. Master communication to

3648-436: 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

3712-718: 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

3776-585: 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

3840-557: 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 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

3904-412: The stream of bytes to the device. Most graphics libraries such as OpenGL provide an abstract graphical device model as an interface. The library is responsible for translating the commands provided by the programmer into the specific device commands needed to draw the graphical elements and objects. The specific device commands for a plotter are different from the device commands for a CRT monitor , but

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3968-433: 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

4032-619: 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

4096-628: 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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