al Layer Physical Layer Figure 1.3 The OSI reference model INFORMATION THEORY can communicate with its counterpart in that node (e.g. physical layer with physical layer, transport layer with transport layer). This means that all messages are exchanged at the same level/layer between two network elements, and this is known as a peer-to-peer protocol. All data exchange in a mobile network uses a peer-to-peer protocol. The Seven Layers of the OSI Reference Model Layer 1: Physical Layer The physical layer is SO called because of its 'physical' nature. It can be copper wire, an optical fibre cable, radio transmitter or even a satellite connection. This layer is responsible for the actual transmission of data. It transmits the information that it receives from layer 2 without any changes except for the information needed to synchronise with the physical layer of the next node where the information is to be sent. Layer 2: Data Link Layer The function of this layer is to pack the data. Data packaging is based on a high-level data link control protocol. This layer combines the data into packets or frames and sends it to the physical layer for transmission. Layer 2 does the error detection and correction and forms an important part in protocol testing, as the information from layer 3 is sent to layer 2 to be framed into packets that can be transferred over layer 1. Layer 3: Network Layer This layer is responsible for giving all the information related to the path that a data packet has to take and the final destination it has to reach. Thus, this layer gives the routing information for the data packets. Layer 4: Transport Layer This layer is a boundary between the physical elements and logical elements in a network and provides a communication service to the higher layers. Layer 4 checks the consistency of the message by performing end-to-end data control. This layer can perform error detection (but no error correction), and can cater for a reduced flow rate to enable re-transmission of data. Thus, layer 4 provides flow co