Transmission Modes | Synchronous and Asynchronous

In this tutorial, we are going to see how communication between two machines can be done in different ways (Synchronous and Asynchronous). Since there are problems in parallel transmission, serial transmission is the most used. However, since only one wire carries the information, there is a problem of synchronization between the transmitter and the receiver, which means that the receiver cannot a priori distinguish the characters (or generally the sequences of bits) because the bits are sent successively. There are therefore two types of transmission that overcome this problem:
 

 

Asynchronous Transmission:

The asynchronous transmission, in which each character is transmitted at random times (for example, a user sending characters entered on the keyboard in real-time). So, let’s imagine that a single bit is transmitted during a long period of silence… the receiver could not know if it is 00010000, or 10000000, or 00000100…

In order to solve this problem, each character is preceded by a piece of information indicating the beginning of the character being transmitted (the information at the beginning of the transmission is called the START bit) and ended by sending a piece of information at the end of the transmission (called the STOP bit, there can be several STOP bits).
 

 
 

Synchronous Transmission:

Synchronous transmission, in which the sender and receiver are clocked at the same rate. The receiver continuously receives information (even when no bits are transmitted) at the same rate as the sender sends it. This is why it is necessary that the sender and receiver are clocked at the same speed. In addition, additional information is inserted to ensure that there are no errors during transmission.
 

 
During a Synchronous transmission, the bits are sent in a successive way without separation between each character, it is thus necessary to insert elements of synchronization, so we talk about synchronization at the character level.

The main disadvantage of synchronous transmission is the recognition of the information at the level of the receiver because there can be differences between the clocks of the transmitter and the receiver. This is why each data sent must be done over a long enough period for the receiver to distinguish it. Thus, the transmission speed cannot be very high in a synchronous link.
 
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