to a destination in network(s) Provides means to transfer data between network Data Link entities and to detect and possibly correct errors Concerned with transmission of unstructured bits over a Physical physical medium Figure 8.4 OSI seven-layer functions Packetised Data Communications through the network. It also provides for end-to-end error control. The network layer (L3) routes data packets forward between network nodes, and manages data flow. The data-link layer (L2) is responsible for data transmission integrity between nodes, including error con- trol. Finally the physical layer (L1) is concerned with physical modulation of data onto an analogue link. Note a physical-only connection exists at layer 1, through which physical sig- nals carry data to the next node in the network. Each layer generally adds a header to the packet data which carries protocol information necessary for the receiver device and its layers to correctly extract the data contained within the packet. All transmit-side layers are paired with their counterparts on the receiver side, physically at layer 1 and logically at layers 2-7. When packets pass through intermediate relay points, the relay device extracts network layer header information in order to determine the next node to which the packet is to be sent. Packet flow from the application layer down to the physical layer and on to its destination through two intermediate nodes is shown in Figure 8.5. Note the logical pairing between layers across the network, and how an end-to-end virtual circuit is established. As discussed above, the OSI architecture defines the interfaces between layers and therefore different software and hardware vendors may design and develop products for data communica- tions networking. The standardised interface facilitates interoperability between different vendors' products. Device A Device B Intermediate Intermediate Application Application Presentation Presentation Session Session Transport Transport Network Network Network Network Data Link Data Link