[kingcraft] 3.1 link layer - functions of data link layer

3.1 Data link layer functions

The main function of the data link layer is to strengthen the ability of the physical layer to transmit the original bit stream , Transform the possible error physical connection of the physical layer into a logically error free data link .

3.1.1 Services for the network layer

The data link layer can provide the following services for the network layer ：

1. No connection service without confirmation ： The source machine does not need to establish a link connection before sending data frames , The destination machine does not need to play a confirmation when it receives it . For lost frames , The data link layer is not responsible for retransmission, but is handled by the upper layer .
2. Confirmed connectionless service ： The source machine does not need to establish a link connection before sending data frames , But the destination machine must send back the data frame . When the source machine does not receive the return message within the specified time, it retransmits the data frame , So as to improve the reliability of data transmission
3. Connection oriented service with acknowledgement ： The frame transmission of this service is divided into link establishment 、 Transmit frame 、 There are three stages to releasing the data link . The destination machine needs to confirm each received frame , The source machine cannot send the next frame until it receives it

Establishment of data link layer connection 、 The process of maintenance and release is called Link management , It is mainly used for connection oriented services , That is, a connection oriented service with validation .

3.1.2 Frame delimitation 、 Frame synchronization and transparent transmission

When two hosts transmit information to each other , The network layer packets （packet） Packets are encapsulated into frames . Add a piece of data before and after the head and tail to form a frame . An important information in the head and tail is to determine the frame delimitation . Frame synchronization is that the receiver should distinguish the start and end of the frame from the received binary bit stream . For example HDLC In the agreement, it is marked as F（01111110） To identify the beginning and end of the frame , This is Frame delimiter . Each data link layer protocol specifies the maximum length of the data part of the frame —— Maximum transfer unit （MTU）

Start flag	Address 8 position	control 8 position	Information N position （ variable ）	Frame check sequence （FCS） 16 position	Termination sign

But if the same bit combination as the frame delimiter happens to appear in the data , The receiver may mistake the information in the data for the termination flag , Causes the frame to be truncated from the middle . The data link layer protocol can adopt special methods to prevent this kind of situation , That is, no matter what combination of bitstreams are transmitted , The link layer can transmit complete frames , The transmitted information will not be mistaken for the end flag , This is it. Transparent transmission .

3.1.3 flow control

Since the sending and receiving parties will have their own working rates , The sending capacity of the sender may exceed the receiving capacity of the receiver . If the sender's rate is not limited , This will result in a steady stream of incoming frames flooding the receiver , As a result, more and more frames need to be cached by the receiver, and eventually the cache will burst , Cause frame loss . Therefore, flow control is the data flow of the sender , The sending rate shall not exceed the receiving rate .

3.1.4 Error control

Due to channel noise and other reasons , Errors may occur in the transmission of frames . The method that enables the sender to confirm whether the receiver has correctly received the data sent by the sender is called Error control . Usually, errors are divided into Bit error and Frame error . Bit error is the error of some bits in the frame , use CRC Cyclic redundancy check Dislocations can be found , adopt Automatic retransmission request ARQ Method to retransmit the wrong frame . Frame error refers to the loss of frames 、 Repetition or disorder , The introduction of timer and numbering mechanism in the data link layer can ensure that each frame is finally and only once correctly delivered to the destination .