They request and suspend transmission, report on status, and acknowledge the receipt of I frames. They do not have an information field. They are used for control purposes. For example, they are used to initialize secondaries. Depending on the function of the unnumbered frame, its control field is 1 or 2 bytes.
Some unnumbered frames have an information field.
The CRC calculation is redone in the receiver. If the result differs from the value in the sender's frame, an error is assumed. LAPB was created to ensure ongoing compatibility with HDLC, which had been modified in the early 1980s. IEEE 802.2 is a modification of HDLC for LAN environments. Also, like SDLC, HDLC supports synchronous, full-duplex operation.