ctive action. To a degree, link control procedures will continue to be the most important kind of protocol in the years ahead, since they are SO basic to the functioning of data networks. However, such procedures rep- resent only one layer of a hierarchy of protocols, including ones to ensure tramsmission integrity over mutiple links, or through multiple networks, and others to allow processes within one computer to com- municate with processes in other computers. 3.2 PROTOCOL HIERARCHY A protocol hierarchy has been systematized in a reference model cre- ated by the International Organization for Standardization (ISO) to PROTOCOLS AND CODES 33 guide standards efforts involving computers and communications. The multi-level approach has a number of advantages. First, the pro- tocol hierarchy (Fig. 3-1) provides a separation of functions, which is useful in designing a complex system. Also, it permits evolutionary changes of the protocol since each level acts as a separate module. There are four levels of protocol standards adopted, and seven defined between data terminal equipment (DTE) and data circuit equipment (DCE). At Level I, there is a physical interface between the DTE and data circuit equipment (DCE). In the past, this inter- face has been implemented using RS-232. In the future, new versions of the electrical interface such as RS-422 will be employed. At Level 2, there is the link control procedure, and here the prime example is the standard high-level data link control (HDLC). At Level 3, there is the network control protocol. Within X.25, the Level 3 protocol defines the packet formats and the control procedures for exchang- ing information between a DTE and the public packet-switched data network. At Level 4 there are the user-to-user protocols connecting one DTE to another DTE on an end-to-end level. Such protocols have not yet been standardized internationally, but will eventually focus on terminal-to-terminal communications. As information moves through the network from one protocol layer to the nex