BTEC nationalBTEC national

Unit 25 Data communication and Networks

Communications technologyCommunications technology

Learning objective:

To understand the principles of digital communications technology

Before you start…Before you start…There are four main transmission media:

Radio waves: wireless networks, Bluetooth-enable mobile devices and satellite uplinks.

Electrical signals: cabled networks, broadband/ADSL (asymmetric digital subscriber line) high-speed link and the older analogue system used by a dial-up modem.

Microwaves used between the buildings of large organisations to share data and telephone communications.

Light, as in the use of fibre optic cables to send high-speed signals over a distance and infrared connection between a laptop and a printer or a mobile phone.

Signal theory Signal theory The sine wave has two properties:

Amplitude

Frequency

Amplitude Amplitude

Radio and microwave use the same method of transmission: radio wave.

All cable rely on electrical current, and its strength is measured in volts.

Light: the brighter the light source, the stronger the signal.

Frequency Frequency

The different media all operate at different frequencies based on the properties of the electromagnetic spectrum.

A low-frequency signal has a small number of cycles per second. A higher frequency signal can have billions of cycles per second.

Common frequency Common frequency measurementsmeasurements

Hz 1- 999 cycles per second

KHz 1,000 – 999,999 cycles per second

MHz 1,000,000 – 999,999,999 cycles per second

GHz 1,000,000,000 - 999,999,999,999 cycles per second

THz 1,000,000,000,000 + cycles per second

ExamplesExamples

700 MHz = 700,000,000 cycles per second

2GHz = 2,000,000,000 cycles per second

Task Task Visit

www.its.bldrdoc.gov/fs-1037/images/frqcharc.gif

And find out where infrared light appears on the electromagnetic spectrum.

Visit

entertainment.howstuffworks.com/hearing.htm

and search for information on the human ear. How does the terminology used on this website relate to frequency and amplitude.

Digital signalling methodsDigital signalling methods

All computer use binary.

0 represents off state and 1 represents on state.

For example, 01001011 is a single byte that represents the decimal value 76 or the ASCII value of ‘v’

EncodingEncodingSending data from one computer to another is called encoding and various formats exist according to the system used (wireless, fibre or electric cable).

Manchester encoding and Huffman coding is the most common formats.

Encoding is based on a digital ‘square’ wave, which is an adaptation of the sine wave.

Square WaveSquare Wave

Encoding rulesEncoding rules

All binary zeros (off) are sent at high amplitude so that there is no confusion with the ‘power off’ of no signal being sent.

All binary ones (on) are sent at a mid-range amplitude to contrast with the rule for the binary zero.

Task Task Visit

http://www.asciitable.com/

And convert each letter of the phrase ‘the cat on the mat’ from ASCII to its decimal equivalent (remember that you have spaces in the phrase and space character has a value).

Using the result, create a CRC where each value can be computed with the formulae (4 * ASCII)2