ications issues in computer environ- ments. The standard is named the open systems interconnect (OSI) model and typically referred to as the OSI stack. The communi- cations industry adopted the technique for use in standards and design documents. Two examples of communications stacks are shown in Figure 5-4 along with the OSI stack. The OSI stack has seven layers; the synchronous optical network/synchronous digital hier- archy (SONET/SDH) and Internet stacks are both four layers. Taken in isolated display, there doesn't appear to be much of a match or direct relationship between the three. However, we know that 5 NETWORK ARCHITECTURE, FACILITIES, AND SERVICES Open Systems Interconnect Model Internet Model Application ASCII Presentation Binary Virtual Terminal Session TCP & UDP SONET/SDH Model Transport Network Data Link Section Physical Media FIGURE 5-4 Layering Models Used in Computing and Communications SONET/SDH is an example of the physical layer of the OSI stack, and only the physical layer. The four-layer Internet stack used by many rests on an IP layer, which is a peer to the network layer in the OSI stack. Each of the layers in the OSI stack is unique in function and behavior, and has a specific relationship with its neighbors above and below. The OSI model is subdivided with layer 1 and 2 classified as being hardware-oriented, whereas the upper layers are said to be software- oriented. As an entity, the lower layers are communications-oriented; the upper layers are user- or application-oriented. Another commonly used way of illustrating the layering concept and relating it directly to linking is to draw an end-to-end service model with multiple layers showing multiple virtual channels on top of a physical channel. Figure 5-5 is another example of the use of layering models to show relationships between the layers in communications networks and computers. Figure 5-5 has been redrawn from the three separate models shown in Figure 5-4. It shows these three models and how they relate to each other. PRACTICAL