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.