ugh lines radiating from the elipse in Figure 12.8. Relay systems apply the layers as the functions are executed to provide a successful to both packet and circuit switching and can function in either the communication to the destination AP. connection-mode or connectionless-mode depending upon the spe- This transfer of information between the systems involves the cific operational configuration. The circles in Figure 12.1 along the construction and forwarding of control information and data units paths interconnecting the end systems are examples of intermediate that convey the actions and substance of the communication. Figure relay systems that participate in OSIE to support the communica- 12.9 presents the basic data unit structure that passes layer-by-layer tions. within a system. A service data unit (SDU) is the data that is passed to the (N)-layer from the (N+1)-layer for forwarding to the peer 12.3.6 Identifiers (N+1)-entity in the destination system. Therefore, the (N)-layer is responsible for transferring the SDU transparently, SO that it is The principles of naming and addressing have been included in the delivered to the destination (N+1)-entity exactly as it was received OSI structure to facilitate the mapping of the information flow by the (N)-entity in the source system. through the layers within a system and to find the path between The SDU is received by the (N)-entity as a parameter of a request systems for transmission of the data to the destination system. The primitive issued by the (N+1)-entity. The (N)-entity then produces conceptual port between an (N)-layer and an (N+1)-layer is called the protocol control information (PCI), which initiates the required a Service Access Point (SAP), which is shown in Figure 12.7. The functions and provides the appropriate parameter information for location of the SAP is identified by an (N)-address, which may be execution by the peer (N)-entity in the destination system. 12/8 Open systems interconnection reference model Multiplexing involves the mapping o