Data Communication

 

Definition.
-Is concerns the transmission of digital messages to devices external to the message source.Have electrical transmission system.
e.g: (Telephone lines and satellites)


Basic requirements for working of a communication
system

• The sender (source) who creates the message to be transmitted.
• A medium that carries the message.
• The receiver (sink) who receive the message.

Data : A collection of facts in raw form forms that become information after processing.
Signals : Eletric or electromagnetic encoding of data.
Signaling : Propagation of signals across a communication medium.
Transmission : Communication of data achieved by t he processing of signals.

Data sequencing : A long message to be transmitted is broken into

smaller packets of fixed size for error free data transmission.

Data Routing : It is the process of finding the most efficient route

between source and destination before sending the data.

Flow control : All machines are not equally efficient in terms of speed.

Hence the flow control regulates the process of sending data between

fast sender and slow receiver.

Error Control : Error detecting and recovering is the one of the main

functions of communication software. It ensures that data are transmitted

without any error.

Data transmission mode

Simplex : The communication can take place in one

direction. The receiver receives the signal from the transmitting device.

The flow of information is Uni-directional. Rarely

used for data communication.  

 

e.g : television / radio station because always transmit the signak to its listenners and never allow transmitting data.

Half-duplex : The communication channel is used

in both directions, but only in one direction at a time. Thus a half-duplex

line can alternately send and receive data.

e.g : walky talkie.

 

Full-duplex : The communication channel is used in both

directions at the same time. Use of full-duplex line improves the

efficiency as the line turn around time required in half-duplex arrangement

is eliminated.

 

e.g: telephone line.

Digital And Analog Transmission

-Data is transmitted from one point to another point by means of electrical signals

  that may be in digital and analog form.

 

Analog signal:

-The transmission power varies over a continuous range with respect to sound, light and radio waves.

 E.g. computer and computer related equipment. Measured in Volts and its frequency is in Hertz (Hz).

Digital signal:

-A digital signal may assume only discrete set of values within a given range.

-A sequence of voltage represented in binary form. When digital

data are to be sent over an analog form the digital signal must be converted to

analog form.

• Digital signal is converted to analog signal – Modulation.
• Analog signal is converted to digital signal –Demodulation

The device is used is Modem.

Asynchronous and Synchronous Transmission

-Data transmission through a medium can be either asynchronous or synchronous.

Asynchronous transmission :

-Data is transmitted character by character as you

go on typing on a keyboard.

-There is irregular gaps between characters.

-It is cheaper to implement, as you do not have to save the data before

sending.

• Serial transmission – Transmits all the bits one after another ona single line. This style is a bit slower compare to parallel. 

  

Synchronous transmission :

-The saved data is transmitted block by block.

-Each block can contain many characters.

-well suited for remote communication between a computer and related devices like card reader and printers.

• Parallel transmission – a group of bits is transmitted simultaneously by using a separate line for each bit.