 command frame with the P bit set to 1. Of course, the F bit is used to acknowledge an incoming P bit. A station may not send a final frame without prior receipt of a poll frame. As can be seen, P and F bits (a) I Format control field (b) S Format control field (c) U Format control field Figure 10.26 The three control field formats of HDLC. REVIEW EXERCISES are exchanged on a one-for-one basis. Thus only one P bit can be outstanding at a time. As a result the N(R) count of a frame containing a P or F bit set to 1 can be used to detect sequence errors. This capability is called check pointing. It can be used not only to detect sequence errors but to indicate the frame sequence number to begin retransmission when required. Supervisory frames, shown in Figure 10.26b, are used for flow and error control. Both go-back-n and continuous (selective) ARQ can be accommodated. There are four types of supervisory frames: 1. Receive ready (RR): 1000 P/F N(R); 2. Receive not ready (RNR): 1001 P/F N(R); 3. Reject (Rej): 1010 P/F N(R); and 4. Selective reject (SRej): 1011 P/F N(R). The RR frame is used by a station to indicate that it is ready to receive information and acknowledge frames up to and including N(R) - 1. Also, a primary station may use the RR frame as a command with the poll (P) bit set to 1. The RNR frame tells a transmitting station that it is not ready to receive additional incoming I frames. It does acknowledge receipt of frames up to and including sequence number N(R) - 1. I frames with sequence number N(R) and subsequent frames, if any, are not acknowledged. The Rej frame is used with go-back-n ARQ to request retransmission of I frames with frame sequence number N(R), and N(R) - 1 frames and below are acknowledged. Unnumbered frames are used for a variety of control functions. They do not carry sequence numbers, as the name indicates, and do not alter the flow or sequencing of I frames. Unnumbered frames can be grouped into the following four categories: 1. Mode-setting commands and responses 2. Information t