1. Communication concepts Week 2 Lecture 1

2. Introduction to key concepts Physical media Analogue & Digital transmission Multiplexing Circuit & Packet Switching Bandwidth & Latency It will be at a broad principle level. We need to get these definitions clear before we can proceed. Some might find it simplistic.

3. Wide Area Network Local Area Network Home ISP ISP & Telco (IBP) The last kilometre Business WiFi Network

4. Media (electromagnetic spectrum) Power Voice Radio Infrared light UV light X, Gamma rays Visible light 03khz300ghz Copper cable Wireless 802.11 & 802.16 2-60ghz range Optic fibre

5. Twisted pair Colour coded pairs of insulated copper wires twisted around each other One carries the signal, the other is grounded and absorbs interference Electrical noise is a big issue – the tighter the twist the more resistant the cable is to noise Lots of variations STP shielded twisted pair UTP unshielded twisted pair Quality of copper, number of twists, length of segment, devices on the line Limit of 90 metre lengths

6. UTP Categories CAT 3 used for Ethernet segments to 10mbps, CAT 5 – higher grade copper, more twists, used for fast Ethernet up to 100mbps – 4 wire pairs CAT 5E and CAT 6 – current standards, up to 1000mbps

7. Fibre optic cable Consists of One or more glass fibre at the core Clad by a glass layer that acts as a mirror Layer of plastic Braiding of Kevlar Plastic jacket Data is transmitted by a pulsating light generated by a laser or LED

8. Fibre characteristics Currently used as a LAN backbone or in the WAN for high capacity links It is marginally more expensive – starting to replace copper to the desktop Basically unlimited bandwidth – up to 1gbps at present Cannot be easily tapped Does not need repeaters or amplifiers Does not transmit in both directions –2 strands NIC and Hubs more expensive

9. Wireless Big range of complex products being introduced here We will look at Mobile telephones Packet Switched Data Networks Point to point links Wireless LANs - 802.11 - WiFi Last kilometer – 802.16 - WiMax Satellites

10. Mobile Network Generations 1G – Analogue – phased out in Oz 2G – Digital GSM in Europe & Asia – other technologies in the US Data to 9.6kbps, SMS messages to 160 characters 2.5G - Digital Higher data rates at lower intro cost than 3G, GPRS (General Packet Radio Service) can go up to 115kbps, Oz intro first GPRS network at 24kbps to go to 48kbps

11. Mobile Network Generations (cont.) 3G – Expected to offer data rates up to 2mbps Frequencies sold recently by auction in Europe & Oz Three competing protocols (CDMA) each backwards compatible with the 3 main 2G technologies 4G Some small implementations have been made Expected to go up to 10mbps Maybe data only

12. Other wireless products Packet Switched Data Networks Used in the US and some other areas at low data rates One new US carrier expects to offer up to 128kbps Point to point links Usually line of sight from rooftops Micro wave links have been in use for many years Laser links now offer up to 1gbps for 2km

13. Other wireless products (cont.) Last kilometer – give access to local exchanges – may be very significant to countries without wired infrastructure 802.16 WiMax Wireless LANs – allow a LAN to operate within a building without wires up to 11mbps WiFi Wireless Personal Area Networks – Bluetooth – allow devices to be connected within 10 metres without wires

14. Satellites GEO – Geo-synchronous Earth Orbit MEO – Medium Earth Orbit LEO – Low Earth Orbit In use downstream in OZ now Main problem is latency 250 mille-seconds for GEO 10 to 100 mille-seconds for LEO

15. Analogue Transmission Until recently telephone networks were analogue Sound is carried along the wire in sine wave form Put simply, there are three attributes, the height and length of the wave, known as amplitude, frequency and phase. Loudness varies the amplitude and pitch varies the frequency Data can be carried by varying – Amplitude, Frequency or Phase

16. Amplitude Shift Keying ASK Frequency Shift Keying FSK

17. Phase Shift Keying PSK ASK – susceptible to noise degradation FSK – has band width limitations PSK – can transmit multiple bits per cycle

18. Digital Transmission Digital signals do not use the sign wave It turns the electrical signal on and off. “On” representing 1 and “Off” Zero Usually represented as a square wave form Not as clear as On or Off because of noise and voltage variation

19. - Problem of synchronisation if a long row of zeros or ones sent - This is one of a number of solutions – Return to Zero

20. Movement to Digital Telephone converting to digital ISDN – Integrated Services Digital Network available for some years at 64kbps ADSL – Asymmetric Digital Subscriber Line Asymmetric means that downstream is faster than upstream, mirroring the usual Internet pattern

21. ADSL Downstream up to 1.5mbps Upstream up to 256kbps Telephone on the same line “Always on” Internet connection Availability & bandwidth dependent on Quality of copper line Length of segment Existence of devices on the line to the exchange

22. Multiplexing Transmission capacity of the media is often much greater than the needs of any one user This capacity can be shared by allowing simultaneous transmission of multiple signals on a single data link This technique is known as multiplexing There is one device to combine the signals – a mux and one to separate them again – a demux – at each end of the link

23. Mux Demux

24. Approaches There are a range of approaches FDM – Frequency Division Multiplexing divides the frequency into sub frequencies – used in ADSL to divide channel into Up and Down stream and Telephone channels TDM – Divides the time into fixed sized frames and then into slots. A connection is then allocated a slot within the frame. Gives a fixed bandwidth whether used or not

25. Multiplexing (Cont.) Statistical Multiplexing allocates capacity to those tasks that have data to transmit WDM – Wave Length Modulation is used in fibre- optic cable, which allows multiple signals with different wavelengths to be transmitted simultaneously CDMA – Code Division Multiple Access is used in mobile phone networks. Allocates a code to each bit and sends them at a higher rate – called a chip rate. See Kurose page 436 for details.

26. Serial & Parallel In Serial transmission the bits follow each other down the one path In parallel multiple paths of wires are used to transmit all the bits in a byte at the same time or in parallel Parallel is now largely limited to printer cables and they are now being replaced by the USB (Universal Serial Bus)