 Data Link Data Link Physical Physical Physical Physical Physical communications Figure Data flow through different OSI layers (dotted lines are virtual peer-to-peer connections) Broadband Telecommunications Technologies and Management As discussed in the IP and OSI case study, the OSI seven-layer architecture is overtaken by the IP suite in carrying most packet-switched data traffic. However, the model is used widely in the design of telecommunications systems such as mobile telephones and wireless local area networks. In particular, layers 1-3 specifications are used for defining how a transmission medium may be accessed by multiple users. We will return to OSI layer definitions in future chapters, however packet-switched communications is exclusively discussed using the IP below. Internet Protocol The alternative architecture for data communications is the suite of protocols associated with the IP, including TCP, developed at the same time as the OSI protocol. The TCP/IP stack is built in a similar way to that of the OSI model, in that functions are divided across several layers. There are however a number of differences. The most important difference is the fact that TCP/ IP is connectionless, in that no physical or virtual end-to-end connection is established between two applications. Packets are individually sent from a source to a local switch, which decides on where the packet should be forwarded based on the receiver's address. The end-to-end path which the packet takes remains open and transparent to the transmitter and intermediate nodes. Another difference between the internet and OSI protocols is how the layers are specified. The IP is made of fewer layers since many of the higher layer functions are included within the applications. A comparison of the two protocol stacks is shown in Figure 8.6, broadly dividing the functions included in respective layers of the protocols. Internet's application layer covers OSI's application, presentation and part of session layers' functionalities. The application