Data link layer

* * the data link layer is mainly responsible for dealing with the communication between hosts * three basic problems: encapsulated into frames, transparent transmission, error checking * * encapsulated into frames: after the network layer packets are sent to the data link layer, they will be divided into small pieces because of packet switching. These small datagrams are added to the head and tail of the frame to form data frames and data transmission units. Transparent transmission: the so-called transparent transmission means that no matter what the data is, it will not affect the judgment information of the frame, that is, the same character bit coding as the control character used to define the frame is not allowed in the data. No matter what bit combination, the data can be transmitted as is. Error checking: errors may occur in the actual data transmission process, so it is necessary to carry out error checking, such as cyclic redundancy check (CRC), which can achieve no bit error in the frame, but the loss, repetition and disorder of the frame are not managed by the data link layer.