In this stem you will learn about:

• processing of data in communication systems, including
• encoding and decoding analog and digital signals
• formation of data packets
• routing
• error checking
• parity bit
• checksum
• cyclic redundancy check
• encryption and decryption
• Data speed as measured in Baud rates and BPS
• The similarities and differences between the Internet, intranets and extranets

Processing

In a communication system, processing data is the manipulation of the data. After the data is collected, it must be converted into a form for transmission. This process involves encoding and decoding. 

Encoding and decoding

Encoding involves converting data from its original form into another form for transmission. Decoding is the reverse process. It converts data from the form used for transmission back into the original form. For example, a telephone encodes the sounds from your voice into electrical pulses that can be transmitted along the telephone lines.The telephone receiving the call decodes the electrical pulses back to the sounds of your voice. The type of encoding and decoding depends on whether the original data is in analog or digital form.

Analog data is represented by using continuous variable physical quantities, such as voltages.

• For example, a clock that displays the time by hands moving smoothly around the clock face is an analog device.
• At any moment, the clock will give you an instant measure of the exact time, since you can estimate parts of a second as the second hand moves. 
• Most natural events in the real world, such as temperature, light and pressure, are in analog form. 
• They change smoothly and slowly like the hands of the clock. 
• Sounds, images and video are naturally in analog form. 
• Analog signals are pulses, usually electrical or optical, in the form of a continuous wave.

Digital data is represented in the form of digits or numbers. 

• For example, a digital clock shows the time as a certain number of hours and minutes. 
• The time changes in a series of jumps. 
• Numbers, text and other characters are naturally in digital form. Information technology works with data in a digital form. 
• Digital signals are represented as a series of 0s and 1s.
• The form of the data (either analog or digital) and the transmission signal (either analog or digital signal) affects the quality of the data received and the cost of transmission. The quality of analog data depends on maintaining the exact wave as it moves through a wire or space. If it is corrupted in any way, there is no way of regenerating the wave. However, digital data is transmitted as a series of 0s and 1s, and it is possible to regenerate data that has been corrupted. That is, to reconstruct the data, it is only necessary to distinguish between a 0 and a 1. 

As discussed previously, common methods for error detection include parity checking, checksum and cyclic redundancy check.

There are four encoding and decoding possibilities in transmission:

• Analog data to analog signal. 
• The wave shape of the data is encoded into the signal. 
• A telephone encodes analog data in the form of sounds into analog signals suitable for the telephone line. 
• If the signal is corrupted, there is no way of restoring the original analog data.
• Digital data to analog signal.
• A series of 0s and 1s is encoded into a continuous wave. 
• A modem encodes (or modulates) digital data from a computer into analog signals for the telephone line. 
• When the analog signal is received by another modem, it decodes (or demodulates) the analog signal into digital data (see Figure 3.18).
• Digital data to digital signal.
• A series of 0s and 1s is transmitted by sending it through a channel as a series of on and off pulses. 
• Data transmitted in a LAN is digital data using a digital signal. 
• There is a low error rate for this type of transmission. 
• Digital data is encoded (changed) into a digital signal by the computer or a specific peripheral device.

• Analog data to digital signal—
• the wave shape of the data is encoded into a series of 0s and 1s. 
• This process of generating digits or numbers is called digitising. 
• Images are digitised using such devices as scanners, and sounds are digitised using a process called sampling.
• The transmission of television using a cable is an example of analog data to digital signal.