s destination. The address on the letter serves to route the letter. 10.10 WHAT ARE DATA PROTOCOLS? Application Application layer layer Presentation Presentation layer layer Session Session layer layer Transport Transport layer layer Network Network layer layer layer layer Physical Physical layer layer System A System B Figure 10.20 The OSI reference model. There are seven OSI layers, as shown in Figure 10.20. Any layer may be referred to as an N-layer. Within a particular system there are one or more active entities in each layer. An example of an entity is a process in a multiprocessing system. It could simply be a subroutine. Each entity communicates with entities above it and below it across an interface. The interface is at a service access point (SAP). The data that pass between entities are a bit grouping called a protocol data unit (PDU). Data units are passed downward from a peer entity to the next OSI layer, called the (N layer. The lower layer calls the PDU a service data unit (SDU). The (N 1) layer adds control information, transforming the SDU into one or more PDUs. However, the identity of the SDU is preserved to the corresponding layer at the other end of the connection. This concept is illustrated in Figure 10.21. OSI has considerable overhead. By overhead, we mean bit sequences that are used for logical interfaces or just simply to make the system work. Overhead does not carry revenue-bearing traffic. Overhead has a direct bearing on system efficiency: As overhead increases, system efficiency decreases. OSI layering is widely accepted in the world of data communications even with its considerable overhead. Encapsulation is a good example. Encapsulation is used on all OSI layers above layer 1, as shown in Figure 10.22. (N) layer (N) PDU (N - 1) PCI (N 1) SDU (N 1) layer (N 1) PDU PCI = protocol control information PDU = protocol data unit SDU = service data unit Figure 10.21 An illustration of mapping between data units on adjacent layers. DATA COMMUNICATIONS User data Incoming Outgoing frame fram