What is the difference between bit synchronization (bit synchronization) and frame synchronization?

The most basic synchronization method in data communication is "bit synchronization" (bit synchronization) or bit synchronization. Bits are the smallest unit of data transfer. Bit synchronization (bit synchronization) means that the receiving end clock has been adjusted to the same as the sending end clock, so the receiver receives the bitstream and is able to adjudicate in the middle of each bit (as shown). Bit synchronization (bit synchronization) is intended to properly receive each bit sent by the sending side. This is done at the right moment (usually in the middle of each position) to judge the received level according to the pre-agreed rules. For example, a level above a certain value is 1, otherwise 0.

But just a bit of synchronization is not enough. Because the data is to be sent in frames. If a frame is wrong, the wrong frame will be re-transmitted later. Therefore, a frame should have a definite boundary, that is to say, there must be a frame delimiter. After receiving a bitstream, the receiver must be able to correctly locate the frame delimiter so that it knows which bits make up a frame. The receiver finds the frame delimiter and determines the exact position of the frame, which is the completion of frame synchronization.

In time-division multiplexing communication using PCM, which uses synchronous communication, the receiver is not enough to receive only the bit stream correctly. The receiving end must also accurately distinguish the time-division multiplexing frames. Therefore, a special time slot (including some special bit combinations) is used so that the receiver can determine the position of each division multiplexing frame. This is also called frame synchronization. The two different frame synchronization are given.

The above part of the synchronous communication mode in the telecommunications network is widely used, one of the important feature is that the transmitting side continuously send the bit stream, even if the time slot is not used by the user, but the time slot used for synchronization should also be retained in the corresponding position in the division multiplexing frame. The task of frame synchronization in synchronous communication is to enable the receiving side to determine the location of each time-division multiplexing frame from the received continuous bit stream.

The following part of the figure of the asynchronous communication method in the computer network is used more. We can notice that the time the data frame appears at the receiving end is irregular. Therefore, frame delimitation must be performed at the receiving end. But frame delimitation is also often referred to as frame synchronization. Therefore, when we see "Frame synchronization", we should make it clear whether this is the frame synchronization in synchronous communication or the frame delimitation in asynchronous communication.

Here we emphasize that, in asynchronous communication, the receiver must also receive all the bits in the data frame one at a time, even if it finds the beginning of the data frame. Therefore, the receiver must synchronize bits with each bit in the data frame (this is the synchronization problem in asynchronous communication). Imagine: If the receiver does not know how long each bit to continue, then how can one bit receive it down? Therefore, in order to receive each bit in a bit block, whether synchronous or asynchronous, bit synchronization (bit synchronization) must be performed with bits in the bit block. In asynchronous communication, however, the method of bit synchronization (bit synchronization) is not exactly the same as synchronous communication.

In synchronous communication, the most accurate synchronization method is to make the whole network clock accurately synchronized. The long-term accuracy of the master clock in the whole network requires ±1.0´1011, so atomic clocks (for example, cesium atomic clocks) must be used, but such synchronous networks are expensive (e.g. sdh/sonet networks). In fact, in synchronous communication, it is also possible to use a more economical method to achieve synchronization. This method is to extract the bit-synchronized clock information from the received bit stream at the receiving end (the sender sends a bit stream, and the message sending the clock is already in the bitstream being sent). This type of synchronization is often referred to as quasi-synchronization (plesiochronous). The Manchester code allows the receiver to easily extract the clock information from the received bitstream, making it easy to synchronize bits. In the asynchronous communication with the frame as the transmitting unit, the receiver usually uses the method of extracting the clock information from the received bit stream to realize the bit synchronization.

In the character-based asynchronous communication, because each character only 8 bits, so as long as the two sides of the clock frequency difference is not too large, at the start bit triggered, the bit synchronization of these 8 bits is easy to do, so there is no need to take other measures to achieve bit synchronization (but not equal to say can not be bit synchronization).

What is the difference between bit synchronization (bit synchronization) and frame synchronization?