em, some marker must be provided to tell the receiver when a frame starts. This "marker" is the unique field. Every data frame starts with a unique field. A generic data frame is shown in Figure 10.7. View the frame from left to right. The first field is the unique field or flag, and it generally consists of the binary sequence 01111110. A frame always starts with this field and ends with the same field. If one frame follows another contiguously, the unique field ending frame No. 1 is the unique field starting frame No. 2, and SO forth. Once a frame knows where it starts, it will know a priori where the following fields begin and end by simple bit/octet counting. However, with some data-link protocols, the information field is of variable length. In this case, the information field length will appear as a subfield in the control field. 8-Bit flag pattern (trailing) 16-Bit frame checks sequence Text of varying length (or fixed length) 8-Bit control field 8-Bit address field 8-Bit flag pattern (leading) Figure 10.7 A generic data frame. Flag pattern = unique field. Text is often called "info" field or information field. DATA COMMUNICATIONS Synchronous data transmission systems not only require frame alignment, but also must be bit-aligned. This was an easy matter on start-stop systems because we could let the receive clock run freely inside the 5 bits or 8 bits of a character. There is no such freedom with synchronous systems. Suppose we assume a free-running receive clock. However, if there was a timing error of 1% between the transmit and receive clocks, not more than 100 bits could be transmitted until the synchronous receiving device would be off in timing by the duration of 1 bit from the transmitter, and all bits received thereafter would be in error. Even if the timing accuracy of one relative to the other were improved to 0.05%, the correct timing relationship between transmitter and receiver would exist for only the first 2000 bits transmitted. It follows, therefore, that no timing error whatsoever can be 