OSI Layer

SECOND CLASS LECTUREOSI Seven-Layer ModelIn the 1980s, the European-dominated International Standards Organization (ISO),began to develop its Open Systems Interconnection (OSI) networking suite.OSI has two major components: an abstract model of networking (the BasicReference Model, or seven-layer model), and a set of concrete protocols.The standard documents that describe OSI are for sale and not currently available online.Parts of OSI have influenced Internet protocol development, but none more thanthe abstract model itself, documented in OSI 7498 and its various addenda.In this model, a networking system is divided into layers. Within each layer,one or more entities implement its functionality. Each entity interacts directly onlywith the layer immediately beneath it, and provides facilities for use by the layer above it.Protocols enable an entity in one host to interact with a corresponding entity at thesame layer in a remote host.The seven layers of the OSI Basic Reference Model are (from bottom to top):communications media, as well as the electrical properties and interpretationof the exchanged signals. Ex: this layer defines the size of Ethernet coaxial cable,the type of BNC connector used, and the termination method.transmitted on a particular medium. Ex: this layer defines the framing,addressing and checksumming of Ethernet packets.data links can deliver data between any two nodes in a network. Ex: this layerdefines the addressing and routing structure of the Internet.Ex: this layer defines if and how retransmissions will be used to ensure data delivery.1. The Physical Layer describes the physical properties of the various2. The Data Link Layer describes the logical organization of data bits3. The Network Layer describes how a series of exchanges over various4. The Transport Layer describes the quality and nature of the data delivery.than the packets handled by lower layers. Ex: this layer describes how requestand reply packets are paired in a remote procedure call.Ex: this layer describes how floating point numbers can be exchanged betweenhosts with different math formats.Ex: this layer would implement file system operations.The original Internet protocol specifications defined a four-level model,and protocols designed around it (like TCP) have difficulty fitting neatly intothe seven-layer model. Most newer designs use the seven-layer model.\begin{soapbox}The OSI Basic Reference Model has enjoyed a far greater acceptance than theOSI protocols themselves. There are several reasons for this. OSI's committee-baseddesign process bred overgrown, unimaginative protocols that nobody ever accusedof efficiency. Heavy European dominance helped protect their investments in X.25(CONS is basically X.25 for datagram networks). Perhaps most importantly, X.25data networks never caught people's imagination like the Internet, which, with astrong history of free, downloadable protocol specifications, has been loath toembrace yet another networking scheme where you have to pay to figure howthings work.And why should we? OSI's biggest problem is that doesn't really offer anything new.The strongest case for its implementation comes from its status as an "international standard",but we already have a de facto international standard - the Internet. OSI protocols willbe around, but its most significant contribution is the philosophy of networking representedby its layered model.If the Internet community has to worry about anything, it's the danger of IETF turninginto another ISO - a big, overgrown standards organization run by committees,churning out thousands of pages of rubbish, and dominated by big business playersmore interested in preserving investments than advancing the state of the art.5. The Session Layer describes the organization of data sequences larger6. The Presentation Layer describes the syntax of data being transferred.7. The Application Layer describes how real work actually gets done.Open System Interconnection Reference ModelLayer 7—ApplicationLayer 6—PresentationLayer 5—SessionLayer 4—TransportLayer 3—NetworkLayer 2—Data linkLayer 1—PhysicalAll—Application layerPeople—Presentation layerSeem—Session layerTo—Transport layerNeed—Network layerData—Data link layerProcessing—Physical layerThe Open System Interconnection (OSI) reference model describes howinformation from a software application in one computer moves through a networkmedium to a software application in another computer. The OSI reference model isa conceptual model composed of seven layers, each specifying particular networkfunctions. The model was developed by the International Organization forStandardization (ISO) in 1984, and it is now considered the primary architecturalmodel for intercomputer communications. The OSI model divides the tasksinvolved with moving information between networked computers into sevensmaller, more manageable task groups. A task or group of tasks is then assigned toeach of the seven OSI layers. Each layer is reasonably self-contained so that thetasks assigned to each layer can be implemented independently. This enables thesolutions offered by one layer to be updated without adversely affecting the otherlayers. The following list details the seven layers of the Open SystemInterconnection (OSI) reference model:Note A handy way to remember the seven layers is the sentence "Allpeople seem to need data processing." The beginning letter of each wordcorresponds to a layer.Figure 1-2 illustrates the seven-layer OSI reference model.Characteristics of the OSI LayersFigure 1-3 illustrates the division between the upper and lower OSI layers.Figure 1-2: The OSI Reference Model Contains Seven Independent LayersThe seven layers of the OSI reference model can be divided into two categories:upper layers and lower layers.The upper layers of the OSI model deal with application issues and generally areimplemented only in software. The highest layer, the application layer, is closest tothe end user. Both users and application layer processes interact with softwareapplications that contain a communications component. The term upper layer issometimes used to refer to any layer above another layer in the OSI model.The lower layers of the OSI model handle data transport issues. The physical layerand the data link layer are implemented in hardware and software. The lowestlayer, the physical layer, is closest to the physical network medium (the networkcabling, for example) and is responsible for actually placing information on themedium.Figure 1-3: Two Sets of Layers Make Up the OSI LayersProtocolsOSI Model and Communication Between SystemsInteraction Between OSI Model LayersThe OSI model provides a conceptual framework for communication betweencomputers, but the model itself is not a method of communication. Actualcommunication is made possible by using communication protocols. In the contextof data networking, a protocol is a formal set of rules and conventions that governshow computers exchange information over a network medium. A protocolimplements the functions of one or more of the OSI layers.A wide variety of communication protocols exist. Some of these protocols includeLAN protocols, WAN protocols, network protocols, and routing protocols. LANprotocols operate at the physical and data link layers of the OSI model and definecommunication over the various LAN media. WAN protocols operate at the lowestthree layers of the OSI model and define communication over the various wideareamedia. Routing protocols are network layer protocols that are responsible forexchanging information between routers so that the routers can select the properpath for network traffic. Finally, network protocols are the various upper-layerprotocols that exist in a given protocol suite. Many protocols rely on others foroperation. For example, many routing protocols use network protocols to exchangeinformation between routers. This concept of building upon the layers already inexistence is the foundation of the OSI model.Information being transferred from a software application in one computer systemto a software application in another must pass through the OSI layers. For example,if a software application in System A has information to transmit to a softwareapplication in System B, the application program in System A will pass itsinformation to the application layer (Layer 7) of System A. The application layerthen passes the information to the presentation layer (Layer 6), which relays thedata to the session layer (Layer 5), and so on down to the physical layer (Layer 1).At the physical layer, the information is placed on the physical network mediumand is sent across the medium to System B. The physical layer of System Bremoves the information from the physical medium, and then its physical layerpasses the information up to the data link layer (Layer 2), which passes it to thenetwork layer (Layer 3), and so on, until it reaches the application layer (Layer 7)of System B. Finally, the application layer of System B passes the information tothe recipient application program to complete the communication process.OSI Layer ServicesA given layer in the OSI model generally communicates with three other OSIlayers: the layer directly above it, the layer directly below it, and its peer layer inother networked computer systems. The data link layer in System A, for example,communicates with the network layer of System A, the physical layer of System A,and the data link layer in System B. Figure 1-4 illustrates this example.Figure 1-4: OSI Model Layers Communicate with Other LayersOne OSI layer communicates with another layer to make use of the servicesprovided by the second layer. The services provided by adjacent layers help a givenOSI layer communicate with its peer layer in other computer systems. Three basicelements are involved in layer services: the service user, the service provider, andthe service access point (SAP).In this context, the service user is the OSI layer that requests services from anadjacent OSI layer. The service provider is the OSI layer that provides services toservice users. OSI layers can provide services to multiple service users. The SAP isa conceptual location at which one OSI layer can request the services of anotherOSI layer.Figure 1-5 illustrates how these three elements interact at the network and data linklFaiygeurrse. 1-5: Service Users, Providers, and SAPs Interact at the Network andData Link LayersOSI Model Layers and Information ExchangeInformation Exchange ProcessThe seven OSI layers use various forms of control information to communicatewith their peer layers in other computer systems. This control information consistsof specific requests and instructions that are exchanged between peer OSI layers.Control information typically takes one of two forms: headers and trailers. Headersare prepended to data that has been passed down from upper layers. Trailers areappended to data that has been passed down from upper layers. An OSI layer is notrequired to attach a header or a trailer to data from upper layers.Headers, trailers, and data are relative concepts, depending on the layer thatanalyzes the information unit. At the network layer, for example, an informationunit consists of a Layer 3 header and data. At the data link layer, however, all theinformation passed down by the network layer (the Layer 3 header and the data) istreated as data.In other words, the data portion of an information unit at a given OSI layerpotentially can contain headers, trailers, and data from all the higher layers. This isknown as encapsulation. Figure 1-6 shows how the header and data from one layerare encapsulated into the header of the next lowest layer.Figure 1-6: Headers and Data Can Be Encapsulated During Information ExchangeThe information exchange process occurs between peer OSI layers. Each layer inthe source system adds control information to data, and each layer in the destinationsystem analyzes and removes the control information from that data.OSI Model Physical LayerOSI Model Data Link LayerIf System A has data from a software application to send to System B, the data ispassed to the application layer. The application layer in System A thencommunicates any control information required by the application layer in SystemB by prepending a header to the data. The resulting information unit (a header andthe data) is passed to the presentation layer, which prepends its own headercontaining control information intended for the presentation layer in System B. Theinformation unit grows in size as each layer prepends its own header (and, in somecases, a trailer) that contains control information to be used by its peer layer inSystem B. At the physical layer, the entire information unit is placed onto thenetwork medium.The physical layer in System B receives the information unit and passes it to thedata link layer. The data link layer in System B then reads the control informationcontained in the header prepended by the data link layer in System A. The header isthen removed, and the remainder of the information unit is passed to the networklayer. Each layer performs the same actions: The layer reads the header from itspeer layer, strips it off, and passes the remaining information unit to the nexthighest layer. After the application layer performs these actions, the data is passedto the recipient software application in System B, in exactly the form in which itwas transmitted by the application in System A.The physical layer defines the electrical, mechanical, procedural, and functionalspecifications for activating, maintaining, and deactivating the physical linkbetween communicating network systems. Physical layer specifications definecharacteristics such as voltage levels, timing of voltage changes, physical datarates, maximum transmission distances, and physical connectors. Physical layerimplementations can be categorized as either LAN or WAN specifications. Figure1-7 illustrates some common LAN and WAN physical layer implementations.Figure 1-7: Physical Layer Implementations Can Be LAN or WAN SpecificationsOSI Model Network LayerThe data link layer provides reliable transit of data across a physical network link.Different data link layer specifications define different network and protocolcharacteristics, including physical addressing, network topology, error notification,sequencing of frames, and flow control. Physical addressing (as opposed tonetwork addressing) defines how devices are addressed at the data link layer.Network topology consists of the data link layer specifications that often definehow devices are to be physically connected, such as in a bus or a ring topology.Error notification alerts upper-layer protocols that a transmission error hasoccurred, and the sequencing of data frames reorders frames that are transmittedout of sequence. Finally, flow control moderates the transmission of data so that thereceiving device is not overwhelmed with more traffic than it can handle at onetime.The Institute of Electrical and Electronics Engineers (IEEE) has subdivided thedata link layer into two sublayers: Logical Link Control (LLC) and Media AccessControl (MAC). Figure 1-8 illustrates the IEEE sublayers of the data link layer.Figure 1-8: The Data Link Layer Contains Two SublayersThe Logical Link Control (LLC) sublayer of the data link layer managescommunications between devices over a single link of a network. LLC is defined inthe IEEE 802.2 specification and supports both connectionless and connectionorientedservices used by higher-layer protocols. IEEE 802.2 defines a number offields in data link layer frames that enable multiple higher-layer protocols to share asingle physical data link. The Media Access Control (MAC) sublayer of the datalink layer manages protocol access to the physical network medium. The IEEEMAC specification defines MAC addresses, which enable multiple devices touniquely identify one another at the data link layer.The network layer defines the network address, which differs from the MACaddress. Some network layer implementations, such as the Internet Protocol (IP),define network addresses in a way that route selection can be determinedsystematically by comparing the source network address with the destinationnetwork address and applying the subnet mask. Because this layer defines thelogical network layout, routers can use this layer to determine how to forwardpackets. Because of this, much of the design and configuration work forinternetworks happens at Layer 3, the network layer.OSI Model Transport LayerThe transport protocols used on the Internet are TCP and UDP.OSI Model Session LayerOSI Model Presentation LayerThe transport layer accepts data from the session layer and segments the data fortransport across the network. Generally, the transport layer is responsible formaking sure that the data is delivered error-free and in the proper sequence. Flowcontrol generally occurs at the transport layer.Flow control manages data transmission between devices so that the transmittingdevice does not send more data than the receiving device can process. Multiplexingenables data from several applications to be transmitted onto a single physical link.Virtual circuits are established, maintained, and terminated by the transport layer.Error checking involves creating various mechanisms for detecting transmissionerrors, while error recovery involves acting, such as requesting that data beretransmitted, to resolve any errors that occur.The session layer establishes, manages, and terminates communication sessions.Communication sessions consist of service requests and service responses thatoccur between applications located in different network devices. These requestsand responses are coordinated by protocols implemented at the session layer. Someexamples of session-layer implementations include Zone Information Protocol(ZIP), the AppleTalk protocol that coordinates the name binding process; andSession Control Protocol (SCP), the DECnet Phase IV session layer protocol.The presentation layer provides a variety of coding and conversion functions thatare applied to application layer data. These functions ensure that information sentfrom the application layer of one system would be readable by the application layerof another system. Some examples of presentation layer coding and conversionschemes include common data representation formats, conversion of characterrepresentation formats, common data compression schemes, and common dataencryption schemes.Common data representation formats, or the use of standard image, sound, andvideo formats, enable the interchange of application data between different types ofcomputer systems. Conversion schemes are used to exchange information withsystems by using different text and data representations, such as EBCDIC andASCII. Standard data compression schemes enable data that is compressed at thesource device to be properly decompressed at the destination. Standard dataencryption schemes enable data encrypted at the source device to be properlydeciphered at the destination.OSI Model Application LayerPresentation layer implementations are not typically associated with a particularprotocol stack. Some well-known standards for video include QuickTime andMotion Picture Experts Group (MPEG). QuickTime is an Apple Computerspecification for video and audio, and MPEG is a standard for video compressionand coding.Among the well-known graphic image formats are Graphics Interchange Format(GIF), Joint Photographic Experts Group (JPEG), and Tagged Image File Format(TIFF). GIF is a standard for compressing and coding graphic images. JPEG isanother compression and coding standard for graphic images, and TIFF is astandard coding format for graphic images.The application layer is the OSI layer closest to the end user, which means thatboth the OSI application layer and the user interact directly with the softwareapplication.This layer interacts with software applications that implement a communicatingcomponent. Such application programs fall outside the scope of the OSI model.Application layer functions typically include identifying communication partners,determining resource availability, and synchronizing communication.When identifying communication partners, the application layer determines theidentity and availability of communication partners for an application with data totransmit.When determining resource availability, the application layer must decide whethersufficient network resources for the requested communication exist. Insynchronizing communication, all communication between applications requirescooperation that is managed by the application layer.Some examples of application layer implementations include Telnet, File TransferProtocol (FTP), and Simple Mail Transfer Protocol (SMTP).