Asynchronous data transmission
Serial data communication generally employs either synchronous or asynchronous
communication scheme. This two scheme used different techniques for synchronizing in
the circuits in sending and receiving end.
In asynchronous transmission each character is transmitted separately, that is one
character at a time. The character is preceded by a start bit, which tells the receiving end
where the character coding begins, and is followed by a stop bit, which tells the receiver
where the character coding ends. There will be intervals of ideal time on the channel
shown as gaps. Thus there can be gaps between two adjacent characters in the
asynchronous communication scheme. In this scheme, the bits within the character frame
(including start, parity and stop bits) are sent at the baud rate.
The START BIT and STOP BIT including gaps allow the receiving and sending
computers to synchronise the data transmission. Asynchronous communication is used
when slow speed peripherals communicate with the computer. The main disadvantage of
asynchronous communication is slow speed transmission. Asynchronous communication
however, does not require the complex and costly hardware equipments as is required for
synchronous transmission.
Advantages and Disadvantages of Asynchronous Transmission
The advantage of asynchronous transmission is that it does not required any local storage
at the terminal or the computer and is thus cheaper to implement.
Major disadvantage of asynchronous transmission is that the transmission lines is idle
during the time intervals between transmitting characters.
Serial and Parallel data transmission
The need to provide data transfer between a computer and a remote terminal has led to
the development of serial communication.
Serial data transmission implies transfer data transfer bit by bit on the single (serial)
communication line.
In case of serial transmission data is sent in a serial form i.e. bit by bit on a single line.
Also, the cost of communication hardware is considerable reduced since only a single
wire or channel is require for the serial bit transmission. Serial data transmission is slow
as compared to parallel transmission.
However, parallel data transmission is less common but faster than serial transmission.
Most data are organized into 8 bit bytes. In some computers, data are further organized
into multiple bits called half words, full words. Accordingly data is transferred some
times a byte or word at a time on multiple wires with each wire carrying individual data
bits. Thus transmitting all bits of a given data byte or word at the same time is known as
parallel data transmission.
Parallel transmission is used primarily for transferring data between devices at the same
site. For eg : communication between a computer and printer is most often parallel, so
that entire byte can be transferred in one operation.
Synchronous Communication
In Synchronous communication scheme, after a fixed number of data bytes a special bit
pattern is send called SYNC by the sending end.
Data transmission take place without any gap between two adjacent characters., however
data is send block by block. A block is a continuous steam of characters or data bit
pattern coming at a fixed speed. You will find a Sync bit pattern between any two blocks
of data and hence the data transmission is synchronized.
Synchronous communication is used generally when two computers are communicating
to each other at a high speed or a buffered terminal is communicating to the computer.
Advantages and Disadvantages of Synchronous Communication
Main advantage of Synchronous data communication is the high speed. The synchronous
communications required high-speed peripherals/devices and a good-quality, high
bandwidth communication channel.
The disadvantage include the possible in accuracy. Because when a receiver goes out of
Synchronization, loosing tracks of where individual characters begin and end. Correction
of errors takes additional time.