eable parts. This provided standard- setting agencies with an architecture that defines communication tasks. The OSI model provides the basis for connecting open systems for distributed applications processing. The term open denotes the ability of any two systems conforming to the reference model and associated standards to interconnect. OSI thus provides a common groundwork for the development of families of standards permitting data assets to communicate. ISO broke data communications down into seven areas or layers, arranged vertically starting at the bottom with layer 1, the input/output ports of a data device. The OSI reference model is shown in Figure 10.20. It takes at least two to communicate. Thus we consider the model in twos, one entity to the left in the figure and one peer entity to the right. ISO and the ITU-T organization use the term peers. Peers are corresponding entities on either side of Figure 10.20. A peer on one side of the system (system A) communicates with its peer on the other side (system B) by means of a common protocol. For example, the transport layer of system A communicates with its peer transport layer at system B. It is important to note that there is no direct communication between peer layers except at the physical layer (layer 1). That is, above the physical layer, each protocol entity sends data down to the next lower layer, and SO on to the physical layer, then across and up to its peer on the other side. Even the physical layer may not be directly connected to its peer on the other side of the "connection" such as in packet communications. This we call connectionless service when no physical connection is set up. 14 However, peer layers must share a common protocol in order to communicate. 14 Connectionless service is a type of delivery service that treats each packet, datagram, or frame as a separate entity containing the source and destination address. An analogy in everyday life is the postal service. We put a letter in the mail and we have no idea how it is routed to it