Figure L-1 The fields in the LAPB frame are as follows: • Flag: A specific one-octet field that delimits (i.e., establishes the limits or boundaries of) the beginning and end of the frame.This octet is always 01111110 in binary notation (7E in hexadecimal notation), which is prevented from appearing in the payload by bit stuffing. • Address: A one-octet field that indicates the sending station in command frames and the receiving station in response frames. User addressing is the responsibility of the logical channel identifier (LCI) field contained in the packet layer protocol (PLP), which is one layer above LAPB in the X.25 protocol stack. 74570c12.qxd 9/11/07 12:25 PM Page 278 |