to identify the type of the frame. In addition, it includes sequence numbers, control features and error tracking according to the frame type. Modes of operation – LAPB works in the Asynchronous Balanced Mode (ABM). This mode is balanced (i.e., no master/slave relationship) and is signified by the SABM(E)/SM frame. Each station may initialize, supervise, recover from errors, and send frames at any time. The DTE and DCE are treated as equals. FCS – The Frame Check Sequence enables a high level of physical error control by allowing the integrity of the transmitted frame data to be checked. Window size – LAPB supports an extended window size (modulo 128 and modulo 32768) where the maximum number of outstanding frames for acknowledgment is raised from 7 (modulo 8) to 127 (modulo 128) and 32767 (modulo 32768). Protocol operation LAPB has no master/slave node relationships. The sender uses the Poll bit in command frames to insist on an immediate response. In the response frame this same bit becomes the receivers Final bit. The receiver always turns on the Final bit in its response to a command from the sender with the Poll bit set. The P/F bit is generally used when either end becomes unsure about proper frame sequencing because of a possible missing acknowledgment, and it is necessary to re-establish a point of reference. It is also used to trigger an acknowledgment of outstanding I-frames. Node addressing The following table shows which addresses are placed into the LAPB frame when issuing commands and responses from DTE to DCE and DCE to DTE using single link operation or multilink operation: Protocol commands and responses See also ISDN Frame Relay Synchronous Data Link Control References External links ITU-T Recommendation X.25 ITU-T Recommendation X.222 Link protocols Logical link control Link access protocols X.25