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
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
- 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
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 transfer commands and responses
3. Recovery commands and responses
4. Miscellaneous commands and responses.
The information field follows the control field (Figure 10.25) and precedes the frame
check sequence (FCS) field. The I field is present only in information (I) frames and in
some unnumbered (U) frames. The I field may contain any number of bits in any code,
related to character structure or not. Its length is not specified in the standard (ISO 3309,
Ref. 25). Specific system implementations, however, usually place an upper limit on I
field size. Some versions require that the I field contain an integral number of octets.
Frame check sequence (FCS). Each frame includes an FCS field. This field immedi-
ately follows the I field, or the C field if there is no I field, and precedes the closing
flag (F). The FCS field detects errors due to transmission. The FCS field contains 16 bits,
which are the result of a mathematical computation on the digital value of all bits exclud-
ing the inserted zeros (zero insertion) in the frame and including the address, control, and
What is the basic element of information in a binary system? How much information
does it contain?