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A short note on LAP-B:What is LAP-B Protocol

February 10th, 2011 Remo Leave a comment Go to comments

LAPB (Link Access Protocol Balanced) is a data link layer protocol that manages communication and packet framing between DTE and DCE devices. LAPB is a bit-oriented protocol that ensures that frames are correctly ordered and error-free.

LAPB uses three frame format types:

Information (I) frame- These frames carry upper-layer information and some control information (necessary for full-duplex operation). I-frame functions include sequencing, flow control, and error detection and recovery. I-frames carry send- and receive-sequence numbers.

Supervisory (S) frames- These frames provide control information. They request and suspend transmission, report on status, and acknowledge the receipt of I frames. They do not have an information field.

Unnumbered (U) frames- These frames are not sequenced. They are used for control purposes. For example, they can initiate a connection using standard or extended windowing (modulo 8 versus 128), disconnect the link, report a protocol error, or similar functions.


Ø Flag- Delimits the LAPB frame. Bit stuffing is used to ensure that the flag pattern does not occur within the body of the frame.

Ø Address- Indicates whether the frame carries a command or a response.

Ø Control- Provides further qualifications of command and response frames, and also indicates the frame format (U, I, or S), frame function (for example, receiver ready or disconnect), and the send/receive sequence number.

Ø Data- Carries upper-layer data. Its size and format vary depending on the Layer 3 packet type. The maximum length of this field is set by agreement between a PSN administrator and the subscriber at subscription time.

Ø Frame check sequence (FCS)- Ensures the integrity of the transmitted data.

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