Definition
Information frame, usually referred to as I frame, is used to transmit valid information or data. The first or first and second bits of the control field in the HDLC frame format indicate the type of transmission frame: the first bit is "0" for information frames, the first and second bits are "10" for monitoring frames, and "11" is for none Sequence number frame. In the information frame, the N(S) of the control field, that is, the second, third, and fourth bits are used to store the sequence number of the sent frame, so that the sender does not need to wait for confirmation and send multiple frames continuously. The fifth bit is the Poll/Final bit. When it is 1, the polled station is required to give a response. The N(R) of the control field, that is, the 6th, 7th, and 8th bits are used to store the sequence number of the receiver's next expected frame. For example, N(R)=5, which means that the receiver will receive the 5th frame in the next frame. , In other words, the frames before the 5th frame have been received. N(S) and N(R) are both 3-bit binary codes, which can take values from 0 to 7.
Classification
The high-level data link control (HDLC) procedure defined by the International Organization for Standardization ISO is a set of protocols used to transmit data between network nodes. A protocol widely used in the DLL. In the HDLC protocol, data is organized into units called frames, which are transmitted over the network and received by the receiver. At the same time, the HDLC protocol also manages the data stream and the interval between data transmission. The data transmitted in each frame in the HDLC protocol can contain any number of bits, and the start and end of the frame are delimited by the agreed bit pattern (mark), which is a "bit-oriented" protocol. HDLC divides frame types into three categories, namely: I frame (information frame): carries upper layer information and some control information. The functions of the information frame include sequencing, flow control, error detection and recovery. The I frame carries the sequence number for sending and receiving.
S frame (monitoring frame): Carrying control information. The functions of the monitoring frame include request or delay transmission, status report, and I frame reception response. The S frame only carries the receiving sequence number.
U frame (frame without sequence number): carries control information. Functions include link establishment and disconnection, and error reporting. U frames do not carry any sequence numbers.