what is the UI and RR mechansim?

Question

The context of the question is modems

Answer 1

The UI stands for "Unnumbered Information" frame, and the RR is for "Receiver Ready". The UI frames are "unnumbered" because they do not have sequence numbers. This is in contrast to I frames (I for information) which do have send/receive sequence numbers. The RR frames are used for ACKs (acknowlegements) to achieve Layer-2 (data link layer) flow control. The UI frames are not flow controlled.