Data Communications Networking Devices - 4th Ed.pdf

1.13 STANDARDS ORGANIZATIONS, ACTIVITIES AND THE OSI REFERENCE MODEL _____ 73Layer 3Ðthe network layerThe network layer level 3) is responsible for arranging a logical connectionbetween a source and destination on the network to include the selection andmanagement of a route for the ¯ow of information between source and destinationbased upon the available data paths in the network. Service provided by this layerare associated with the movement of data through a network,to include addressing,routing,switching,sequencing and ¯ow control procedures. In a complex networkthe source and destination may not be directly connected by a single path,butinstead require a path to be established that consists of many subpaths. Thus,routing data through the network onto the correct paths is an important feature ofthis layer.Several protocols have been de®ned for layer 3,including the ITU X.25 packetswitching protocol and the ITU X.75 gateway protocol. X.25 governs the ¯ow ofinformation through a packet network while X.75 governs the ¯ow of informationbetween packet networks. In the TCP/IP protocol suite the Internet Protocol IP)represents a network layer protocol. Packet switching networks are described inChapter 2.Layer 4Ðthe transport layerThe transport layer level 4) is responsible for guaranteeing that the transfer ofinformation occurs correctly after a route has been established through thenetwork by the network level protocol. Thus,the primary function of this layer isto control the communications session between network nodes once a path hasbeen established by the network control layer. Error control,sequence checking,and other end-to-end data reliability factors are the primary concern of this layer.Examples of transport layer protocols include the Transmission Control ProtocolTCP) and the User Datagram Protocol UDP).Layer 5Ðthe session layerThe session layer level 5) provide a set of rules for establishing and terminatingdata streams between nodes in a network. The services that this session layer canprovide include establishing and terminating node connections,message ¯owcontrol,dialog control,and end-to-end data control.Layer 6Ðthe presentation layerThe presentation layer level 6) services are concerned with data transformation,formatting and syntax. One of the primary functions performed by the presentationlayer is the conversion of transmitted data into a display format appropriatefor a receiving device. This can include any necessary conversion between differentdata codes. Data encryption/decryption and data compression and decompressionare examples of the data transformation that could be handled by this layer.