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In this tutorial, we will see the types of serial communication and clear idea about it.

Types of Serial Communication:

Even thought the parallel communication got lot advantages over the serial communication, unfortunately the serial communication got popular for its less cost. Because of the fact, that the not all peripherals that are connected to computer or processors required high speed data communication.
Based on the format of data to be transmitted or received over the serial communication, it is divided into three types.
a.   Synchronous Serial Communication
b.   Asynchronous Serial Communication
c.   Isochronous Serial Communication

Synchronous Serial Communication: In this, the two or more devices which are involved in communication must be synchronized with a clock signal. By doing so, all the devices can receive or transmit the data in sync with clock signal. For example just like, in group dance, all the dancers sync their dance moments (~data) with respect to music beat (~clock signal) as shown in the figure.

Figure 1: Group Dancers sync their moment with respect to Music

Generally, in data communication, the data transmission and reception can be represented by using timing diagrams as shown below.

Figure 2: Timing diagram for Synchronous serial communication protocol

One may have still doubt, how this synchronous serial communication will work?

For this, let us consider parallel to serial shift register (transmitter) at one end and serial to parallel shift register (receiver) at other end. Both are driven with same clock signal to synchronize the data transmission as shown in the below figure. In this communication, we can do block transfer of bits.

Figure 3: Synchronous Serial communication protocol implementation using shift registers

Asynchronous Serial Communication: In Synchronous serial communication, the end devices are driven with common clock and because of that the receiver knows about the data transfer.

But in Asynchronous serial communication protocol, there is no common clock used to synchronize between transmitter and receiver. So the receiver doesn’t know about when the data will come from transmitter and when to receive it. Hence in this protocol, along with data, some extra bits also transferred to make the receiver to identify weather the valid data is coming or not. Usually, in simple Asynchronous serial communication, a start bit and stop bit are added to actual data to indicate the start and end of the data. Then collectively, the start bit, data bit and end bit are referred as data frame or data packet or protocol data unit (PSU) as shown in the below figure. This technique of adding extra bits are referred as framing and extra bits are called frame over head.

Figure 4: Asynchronous Serial Data Frame 

When the receiver device finds the valid start bit, then it understands about the data incoming and receives the data bits. Finally once the receiver finds the stop bit, it will truncate the receiving data bits.

Synchronous Serial Communication protocol
Asynchronous Serial Communication protocol
All the devices are driven with Common Clock.
No Common Clock
Block transfer – Stream of bits
Packet transfer – framing required
Less complex to implement the hardware with simple shift registers
Design of hardware is complex to identify the valid start bits, data bits and stop bits.
Ex:- Mouse connection with PC
Ex:- Com ports on PC, ATM (asynchronous transfer mode) switching.
Data (one or multiple bytes) can be sent in block with block header. Synch characters will be sent till the next block is ready. Break characters are sent to indicate the end of the transmission.
The time interval between successive frames is not fixed
Relatively high speed
Relatively low speed due to number of bits and time interval or gap between frames
Comparatively costly
Comparatively cheap
Data block is not fixed
Frame size is fixed

Isochronous serial communication protocol:

It is the combination of both synchronous and asynchronous protocols. In which we can transfer the block of data as asynchronously at random intervals. For example, video streaming uses this isochronous serial communication protocol. This protocol doesn't have error correction mechanism. Error correction may introduce the jitter in such applications.


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