HDLC LAPB is a very efficient protocol . A minimum of overhead is required to ensure flow control, error detection and recovery. The concept of a frame window is used to send multiple frames before receiving confirmation that the first frame has been correctly been received. This kind of situation occurs, for instance in satellite communication. The window sizes vary, but are typically 7 frames for most terrestrial lines and up to 128 frames for satellite links.
Supervisory frames perform the flow control and error recovery functions.
Unnumbered frames provide the link initialization and termination. The maximum frame size depends on the number of CRC bytes at the end of the frame. For larger frame sizes, up to 16kbytes, a four byte CRC is used.