Practical TCP/IP and Ethernet Networking- P9: The transmitter encodes the information into a suitable form to be transmitted over the communications channel. The communications channel moves this signal as electromagnetic energy from the source to one or more destination receivers. The channel may convert this energy from one form to another, such as electrical to optical signals, whilst maintaining the integrity of the information so the recipient can understand the message sent by the transmitter | 22 Practical TCP IP and Ethernet Networking through the common upper and lower boundaries by passing physical information through service access points SAPs . A SAP could be compared to a predefined postbox where one layer would collect data from the previous layer. The relationship between layers entities functions and SAPs is shown in Figure . Layering Entity Concepts N 1 Entities N Entities Figure Relationship between layers entities functions and SAPs In the OSI model the entity in the next higher layer is referred to as the N 1 entity and the entity in the next lower layer as N-1. The services available to the higher layers are the result of the services provided by all the lower layers. The functions and capabilities expected at each layer are specified in the model. However the model does not prescribe how this functionality should be implemented. The focus in the model is on the interconnection and on the information that can be passed over this connection. The OSI model does not concern itself with the internal operations of the systems involved. When the OSI model was being developed a number of principles were used to determine exactly how many layers this communication model should encompass. These principles are A layer should be created where a different level of abstraction is required Each layer should perform a well-defined function The function of each layer should be chosen with thought given to defining internationally standardized protocols The layer boundaries should be chosen to minimize the information flow across the boundaries The number of layers should be large enough that distinct functions need not be thrown together in the same layer out of necessity and small enough that the architecture does not become unwieldy The use of these principles led to seven layers being defined each of which has been given a name in accordance with its process purpose. The diagram below shows the seven layers of the OSI model. Networking .
