The Flag Field

The
Flag field
is a single octet (eight bits) that indicates the beginning and end of each frame; it has
a unique pattern of 01111110. Sometimes, a single flag ends one frame and begins the next. But, as
can be seen in Figure 24.1, distinct start and end frames are also found. Both of these examples use
the same pattern.
Bit stuffing
is used to make this pattern unique. Bit stuffing is a technique that
alters patterns in the user data that appear the same as the frame delimiter or other framing information.
For example, if the sequence 010101111110100 appeared representing two characters,
the protocol would interpret this as the start of a frame—01111110. Bit-stuffing will re-represent the
characters by altering this flow so that the 01111110 pattern remains unique.