August 1999
Intermediate to advanced
328 pages
8h 50m
English
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THE PROTOCOL DATA UNIT ON THE LINK BETWEEN THE USER AND THE ISP
The traffic that is sent between the user and the ISP is known by various names. The most general term is protocol data unit (PDU). Most people in the industry call a L_2 PDU a frame. Figure 6-6 shows the format of this frame. The frame may contain three L_2 headers, and the HDLC trailer. If this implementation is used, the headers and the trailer perform these functions:
HDLC header: Contains a specific set of bits (the flag) that are used at the receiver to detect the beginning of the frame.
LAPM or LAPB header: Contains sequence numbers, flow control bits, ACK and NAK bits, as well as bits that identify the type of frame (user data, control frame, etc.).
PPP header: Contains information ...