In a digital time division multiplexing communication system, in order to correctly separate the time slot signals, the sending end must provide the start mark of each frame, and the process of detecting and obtaining this mark at the receiving end is called frame synchronization. There are two types of frame synchronization: start-stop synchronization method and insert special synchronization code group method.
Start-stop synchronization method: The code group itself has its own grouping information by using a special code element encoding rule at the sending end.
Inserting special synchronization code group method: It refers to inserting several synchronization codes for frame synchronization in the transmitted symbol sequence.
There are mainly two types of centralized insertion and interval insertion. Centralized insertion, also known as coherent insertion, requires that the frame synchronization special block has excellent autocorrelation characteristics. A commonly used frame synchronization code group is the Barker code. Interval insertion. Also known as distributed insertion, a simple periodic cyclic sequence is usually used as the frame synchronization code, and is uniformly inserted into the information code stream.
The reason why the data link layer combines bits into frames and transmits them is that when an error occurs, only the frame with errors can be retransmitted, and it is not necessary to retransmit all the data, thereby improving the efficiency. Checksums are usually calculated for each frame. When a frame arrives at the destination, the checksum is calculated again, and if it is different from the original checksum, an error can be detected.
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