BTEC national Unit 25 Data communication and Networks

Communications technology Learning objective:  To understand the principles of digital communications technology

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 The sine wave has two properties:  Amplitude  Frequency

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  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 measurements Hz cycles per second KHz 1,000 – 999,999 cycles per second MHz 1,000,000 – 999,999,999 cycles per second GHz1,000,000, ,999,999,999 cycles per second THz1,000,000,000,000 + cycles per second

Examples 700 MHz = 700,000,000 cycles per second 2GHz = 2,000,000,000 cycles per second

Task Visit 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 methods All computer use binary. 0 represents off state and 1 represents on state. For example, is a single byte that represents the decimal value 76 or the ASCII value of ‘v’

Encoding  Sending 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 Wave

Encoding 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 Visit 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