nd computer traffic grew. Data communications became full-time jobs for communications-savvy engineers and technicians. A significant computer standard, developed in the late 1970s, remains in wide use today. The open systems interconnect (OSI) stack1 defines a hardware section beneath a software section in a total of seven layers, bottom to top. The OSI stack makes a good framework for communi- cations networks, including the Internet. Figure 4-1 shows the two- section, seven-layer stack with a brief explanation about what it repre- sents and how it is applied. (see the section on "Layering as Used in Computer and Communication Networks," Chapter 5.) Later in this and other chapters you'll find references to the OSI stack. When the OSI stack was introduced, computers were just beginning to change from stand-alone islands into distributed processing Application Presentation Session Transport Network Data Link Physical FIGURE 4-1 OSI Stack: Hardware and Software Sections PRACTICAL IP AND TELECOM FOR BROADCAST ENGINEERING AND OPERATIONS systems connected by data networks. The basic idea behind the stack concept is that each layer interfaces and interacts or communicates with the one immediately above and below, except, of course, the bottom and top layers for obvious reasons. If each layer successfully accomplishes its functions, then the system it's applied to should operate top to bottom. Attempts to map Internet and Telecom func- tions or processes to the stack are made from time to time, but in isolated ways such as a reference to layer 2 switching, or layer 3 routing, or even layer 2/3 switching or routing. These references seem to be more of a way to characterize a particular switching or routing function in terms of the OSI stack, rather than applying the OSI stack to communications networks in general. Furthermore, it would seem to be useful in analyzing and structuring or designing networks capable of carrying disparate, converged traffic types on common access or transport facility. BASIC NETWORK ELEMENTS