1. BTEC national Unit 25 Data communication and Networks

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

3. 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.

4. Signal theory The sine wave has two properties:  Amplitude  Frequency

5. 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.

6. 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.

7. Common frequency measurements Hz 1- 999 cycles per second KHz 1,000 – 999,999 cycles per second MHz 1,000,000 – 999,999,999 cycles per second GHz1,000,000,000 - 999,999,999,999 cycles per second THz1,000,000,000,000 + cycles per second

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

9. 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.

10. Digital 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’

11. 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.

12. Square Wave

13. 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.

14. 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