ocks are not divided into separate bytes because many types of information, such as graphics, is not presented as a set of bytes. Unique start-of-frame and end- of-frame sequences or flags are used to provide frame synchronization. These flags should be unique and actual data must not include similar data sequences. One common method used to avoid frame misalignment is to use bit stuffing or zero insertion, as shown in Figure 6.3. Consider a flag (01111110) used in the popular high-level data link control (HDLC) proto- col. After the start-of-frame flag the sequence of six subsequent 1's is not allowed in the data section of the frame. To avoid that, a 0 is inserted in the end of each sequence of five subsequent 1's. In the receiver each 0 following five subsequent 1's is discarded. If binary 1 follows five subsequent 1's, the frame is declared to be finished (end-of-frame flag) [1]. Synchronous transmission requires that the bit timing information be inserted into the data stream itself with the help of line coding because frames are very long. As an example, many LANs use the Manchester line code that we described in Chapter 4. The principles we have discussed above are contained in physical layer and data link layer definitions in the data communication architecture -01111110 11011001 11111101 11111100 01111110 I Start-of-frame End-of-frame Data to be transmitted Transmitter Delimiter Receiver generation Zero bit Frame Zero bit Enable/Disable insertion synchronizer deletion Inserted bits 01111110 11011001 11110110 1111101 01111110 Start-of-frame Data to the line End-of-frame Figure 6.3 Bit stuffing or zero insertion. Introduction to Telecommunications Network Engineering described in Section 6.3. As we will see later, these two protocol layers deal with aspects of how data communications over a physical connection between two machines are arranged. Connection from a computer to another through the data communica- tions network requires a switching function, which routes data frames or packets from the source h