Frame Formats of the Control Field
In HDLC frame formats, the frame format of the Control field decides the HDLC frame type.
The frame formats of HDLC are as follows:
Information format, which is also called I format.
Supervisory format, which is also called S format.
Unnumbered format, which is also called U format.
Figure 1 Frame format of the Control field
The meaning of each field is as follows:
N(S): Send Sequence Number
N(R): Receive Sequence Number
P/F: Poll Bit command frame/Final Bit response frame
M: Modifier Function
X: Reserved
S: Supervisory Function
In the Control field, the first bit or the first and second bits indicate the transmitted frame type, namely, I format, S format and U format. The fifth bit of the Control field is P/F option, namely, Poll/Final option.
Information format, I format for short, is used to transmit valid information or data. An I format is identified by the first bit of the Control field that is a binary number 0.
N (S) in the Control field is used to reserve the transmitted frame sequence so that the transmitter can send multiple frames continuously without waiting for confirmation. N (R) is used to reserve the sequence of the next frame that the receiver expects to receive. Both N (S) and N (R) are 3-bit binary options, ranging from 0 to 7.
An S format is used for error control and traffic control. An S format is identified by both the first and second bits of the Control field that are a binary number 10.
An S format does not contain the Information field and has only 6 bytes, namely, 48 bits. In an S format, the third and fourth bits of the Control field are S format codes. The codes of the S format are as follows:
00
Indicates Receiver Ready (RR) sent by a primary node or a secondary node.
The primary node can use RR S formats to poll the secondary node, expecting that the secondary node transmits the I format whose code is N (R). If such a frame exists, transmission can be performed.
The secondary node can use RR S formats to respond, expecting to receive the next I format whose code is N (R) from the primary node.
01
Indicates Reject (REJ) sent by a primary node or a secondary node. REJ formats are used to require the sender to retransmit all the frames since the frame with code as N (R), indicating that I formats before this N (R) format are received normally.
10
Indicates Receiver Not Ready (RNR).The code 10 indicates that the I format with code less than N (R) is received. However, the receiver is busy, not ready to receive the I format whose code is N (R).
RNR S formats are used to control the traffic of the link.
11
Indicates Selective Reject (SREJ) that requires the transmitter to send an I packet whose code is N (R) and implies that I packets whose code is not N (R) have been confirmed.
RR S formats and RNR S formats have the following functions:
They are used to indicate whether a secondary node is ready to transmit information.
They can confirm all the received I packets whose code is less than N (R).
REJ S formats and SREJ S formats are used to notify the peer node of errors.
REJ S formats are used to request retransmitting all frames since N (R) and the frames before N (R) have been confirmed. After an I packet whose N (S) is equal to the N (R) of a REJ S format, the Reject state can be cleared.
SREJ S formats are used to select retransmission policy. After an I packet whose N (S) is equal to the N (R) of a SREJ S format, the Selective Reject state can be cleared.
The Control field of an Unnumbered format, U format for short, does not contain N (S) or N (R).
U format is used to set up, remove and control links. These control functions are defined by five M bits (M1, M2, M3, M4 and M5), which are also called revision bits. The five M bits can define 32 additional commands or 32 additional response functions, some of which are not used presently.
