0, the following octet is an extension of the address field. The address field is terminated
when the least significant bit of an octet is 1. Thus we can see that the address field can
be extended indefinitely.
The control field (C) immediately follows the address field (A) and precedes the infor-
mation field (I). The control field conveys commands, responses, and sequence numbers
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
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 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
The three control field formats of HDLC.