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Communication concepts

Communication concepts. Week 2 Lecture 1. Introduction to key concepts. Physical media Analogue & Digital transmission Multiplexing Circuit & Packet Switching Bandwidth & Latency.

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Communication concepts

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

  4. Media (electromagnetic spectrum) Wireless 802.11 & 802.16 2-60ghz range Copper cable Optic fibre 0 3khz 300ghz Power Voice Radio Infrared light UV light X, Gamma rays Visible light

  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)

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