unnumbered acknowledgement (HDLC) (UA)

The Unnumbered Acknowledgement(UA), the acknowledgement without sequence number, is used in the HDLC protocol to confirm that a connection has been established on the link layer and that the transmission is in spontaneous mode.

The UA message is used to confirm and accept U commands, such as Set Asynchronous Balanced Mode( SABM) and Disconnect( DISC). These commands basically execute a change to a different state than the previous one, from Asynchronous Balanced Mode( ABM) to Asynchronous Disconnect Mode( ADM) or vice versa.