t, the lower-level protocol accepts all the data and control information of the higher-level protocol and then performs a number of functions upon it. In most cases, the lower-level pro- Software Hardware Hardware Software Levels Physical RS-232 Physical electrical electrical Fig. 3-1. Interface standards have been developed for levels of protocols. 34 TELECOMMUNICATIONS tocol takes all the data and control information, treats it uniformly as data, and adds on its own envelope of control information. For instance, the host-to-host protocol will take the application data from the user program and add on a protocol header. As this message is sent through a computer network, the subscriber-to-net- work protocol will add its own control information. Then the link control procedure will add on more information and, in addition, the network will add on subnetwork control fields. It is in this format of messages flowing through a communications network that the concept of the protocol hierarchy is most evident. 3.3 PHYSICAL ELECTRICAL INTERFACE Early in the history of computers, the EIA organization saw that unless there was a common interface built into all equipment, com- puter manufacturers would soon be building their own. The Elec- tronic Industries Association (EIA), established a set of Level 1 physical standards which has now come to be known as the RS-232-C interface (international equivalent is CCITT V.24). Basically RS-232-C ensures three things: 1. That voltage and signal levels will be compatible. 2. That the interface connectors may be plugged together (mated) with identical pin wiring and corresponding pin connection. 3. That certain control information supplied by one device must be understood by the other device. The RS-232-C interface includes 25 pins. Few systems use all of them; however, Table 3-1 gives the most commonly used pins and their functions. It is important to make the distinction between data sent from the terminal (DTE) and data from the data communication equipment (DCE). DTE refers to t