ers to control the data link. The basic control field is 8 bits long and uses modulo 8 sequence numbering. There are three types of control field: (1) I frame (information frame), (2) S frame (supervisory frame), and (3) U frame (unnumbered frame). The three control field formats are illustrated in Figure 10.26. Consider the basic 8-bit format shown in Figure 10.26. The information flows from left to right. If the frame shown in Figure 10.25 has a 0 as the first bit in the control field, the frame is an I frame (see Figure 10.26a). If the bit is a 1, the frame is an S or a U frame, as illustrated in Figure 10.26b and 10.26c. If the first bit is followed by a 0, it is an S frame, and if the bit again is a 1 followed by a 1, it is a U frame. These bits are called format identifiers. Turning now to the information (I) frame (Figure 10.26a), its purpose is to carry user data. Bits 2, 3, and 4 of the control field in this case carry the send sequence number N(S) of the transmitted messages (i.e., I frames). N(S) is the frame sequence number of the next frame to be transmitted and N(R) is the sequence number of the frame to be received. Each frame carries a poll/final (P/F) bit. It is bit 5 in each of the three different types of control fields shown in Figure 10.26. This bit serves a function in both command and response frames. In a command frame it is referred to as a poll (P) bit; in a response frame as a final (F) bit. In both cases the bit is sent as a 1. The P bit is used to solicit a response or sequence of responses from a secondary or balanced station. On a data link only one frame with a P bit set to 1 can be outstanding at any given time. Before a primary or balanced station can issue another frame with a P bit set to 1, it must receive a response frame from a secondary or balanced station with the F bit set to 1. In the NRM mode, the P bit is set to 1 in command frames to solicit response frames from the secondary station. In this mode of operation the secondary station may not transmit until it receives a