Data Communication Basics 1 - Communications model and data encoding

Data Communication Basics 1 - Communications model and data encoding

Learning Outcomes:

Communications model and data encoding

1. Understand the simple model of communications.
• Components of the model include agent, input device, transmitter, transmission medium, receiver, and output device.
2. Know the basic concepts of data encoding.
• Distinction between data and signals should be known.
• Students should know that either form of digital and analog data can be coded into either form of digital and analog signal.
3. Understand some error detection methods.
• The error detection methods include parity check, checksum and cyclic redundancy check (CRC). Implementation arithmetic for parity check and checksum should be known.

Simple model of communications, Data encoding

Data vs Signal:

In simple term, data is the information that is in the form you can use or store.

When you want to transfer the data to another party over a network, the data will be converted into signal in order for it to be transferred over a transmission medium.

Analog vs Digital:

Analog data/signal: continuous in nature that varies with time. E.g. Sound wave, electromagnetic wave

Digital data/signal: a set of discrete values. E.g. data representation in computers – 1s and 0s

Transmitter: 

Convert data into signal for transmission. Depending on your data and transmission medium used, an appropriate transmitter is used. E.g. sending a file over an optical fibre means converting a digital data into digital signal for transmission. Broadcasting a CD with analog radio waves means converting digital data into analog signal. Details of the conversion are out of the exam scope.

Putting these into a simple communication model: 

An agent wants to send out something(data) which is stored or input into an input device. A transmitter will convert the data into appropriate signal (could be analog or digital depending on transmission medium) for transmission. The receiving end uses a receiver to convert the signal back into data. The output device can then use or store the data.

Relevant past paper:

DSE ICT Elect B(SP-2017):  2012 3a. 2013 2a.

Error detection methods:

Major source of error: interference or noise in the communication channel.

Parity check:

• Extra bit called parity bit is appended to each unit of data.
  10101010 1
• Odd parity check: the total number of 1s is odd number, as in above.
  Even parity check: the total number of 1s is even number. E.g. 10101010 0

Checksum:

• HKID and ISBN-10 use checksum.
• Example:
• ISBN number: 019852663-6 the last digit is check digit such that the weighted sum is divisible by 11.
• 0 x10 + 1 x9 + 9 x8 + 8 x7 + 5 x6 + 2 x5 + 6 x4 + 6 x3 + 3 x2 + 6 x1 = 231

Cyclic redundancy check:

                Details not required.

Comparison:

• Parity check: simple, but only able to detect odd number of error bits.
• Checksum: ease of computation, small overhead(single check digit attached to original data)
• CRC: able to detect burst error, which is more likely to happen. Easy to implement using hardware.

Relevant past paper:

DSE ICT Elect B(SP-2017):  2012 2c.

Reference:

https://isbn-information.com/the-10-digit-isbn.html

https://www.tutorialspoint.com/data_communication_computer_network/error_detection_and_correction.htm

https://freeitnetwoking.blogspot.com/2017/02/types-of-errors-in-data-transmission.html