HDLC Transparency

The flag sequence must never occur within the content of a frame otherwise it could be confused with an intentionally sent flag. A technique known as 0-bit insertion is used to prevent random data synthesising a flag. The technique is said to make HDLC transparent, since any stream of bits may be present between the open and closing flag of a frame. The transparency is achieved by encoding the data by inserting a 0-bit after any sequence of 5 consecutive 1's within the payload, as shown:

Insertion of a "zero-bit" into the content of a frame to ensure transparency