 link Physical Physical Physical Physical (a) Network layer protocol Network 1 or Network n or (b) Data link layer protocol subnetwork 1 subnetwork n (c) Physical layer protocol Physical medium for interconnection Figure 6.10 The OSI reference model. Data Communications the lowest end-to-end layer and it uses the network to implement the service for the session layer. When we look at what kind of functions each layer performs, we notice that the lower the layer we look at, the more functions we see that are related to the network technology used for the actual data transmission. The more we look at the upper layer, the more we see common functions available for software applications running in hosts. As we see in Figure 6.10, layers 4 through 7 are all implemented only in the communicating end machines. We do not need layers 4 through 7 at all for actual end-to-end data transmis- sion; this is accomplished by layers 1, 2, and 3. The only purpose of the uppermost layers is to help software applications, and to do this they provide more sophisticated services than just a stream of data. As an example, this stream of data from the network layer may contain some errors. Each appli- cation software designer should design an error recovery scheme in his appli- cation if this service is not provided by the transport layer protocol. Note that data link layer and physical layer may be completely different in each network or subnetwork according to OSI terminology [2]. For exam- ple, in Figure 6.10, host A could use ISDN for connection of network 1 and host B could use Ethernet technology in network N. 6.3.3.2 Physical Layer The physical layer is concerned with transmitting bits over one hop in a com- munications channel. The main design issue is to make sure that when one side sends a 1 bit, the other side receives is as a 1, rather than as a 0 bit. Typi- cal specifications of the physical layer are the duration of a bit in microsec- onds, the number of volts used to represent a 1 and a 0, a number of pins, and the connec