CSCI1412 Lecture 17

1. phones off(please) CSCI1412Lecture 17 Networks 3Network Hardware Dr John Cowell

2. Overview • Network hardware • workstations • multiplexors, hubs, bridges • gateways and routers • Interface devices • NIC’s • modems • Interconnection media • direct • indirect

3. Network Hardware

4. Modems and Multiplexers • A modem is a device that converts data bits into sound so that data can be transmitted over public telephone lines • it is an acronym for modulator / demodulator • A multiplexer is a device which allows two or more signals to be sent over the same line. They can be separated because they occupy different frequencies. • A demultiplexer separates the signals.

5. Multiplexers - MUX • Several low-speed terminals share a high-speed data line • Signals from terminals combined into a single data channel • separated again at the other end

6. Hubs • Allow simple interconnectivity of workstations • peer-to-peer network • little/no processing in hub • bus architecture • level 1

7. Switches • Allow faster interconnectivity of workstations • peer-to-peer network • routing in switch • any two machines can talk simultaneously • level 2

8. Bridges • Interconnect LANS of same/similar types • i.e. protocol • Convert data type • Level 3

9. Gateways and Routers • Interconnect networks • convert protocols • Gateways • interface to Internet • level 7 • Routers • operate within Internet • decide pathway for signals • usually level 3 • NB the term gateway is also used by network engineers for any device that interconnects networks, or subnetworks

10. Interface Devices

11. Interface Devices • Some device must physically connect a computer to a network • Three principal devices are • network interface cards • a card that plugs into a PC expansion slot to provide a direct (permanent) connection to a network • modems • a device that converts bits into sound to be transmitted through the public telephone network • terminal adaptors • a device that converts bits into format required for a digital telephone service such as ISDN

12. Network Interface Card • Network interface card - NIC • an expansion card plugged into a PC expansion slot so that the computer can connect to a network • often now on the motherboard • it is a direct (permanent) physical link to a network • Each NIC (usually) • plugs into one sort of expansion slot • e.g. ISA, PCI, PCA • supports one type of network protocol • e.g. Ethernet, Token Ring • connects to one type of media • e.g. twisted-pair (cat 5), wireless

13. Ethernet Cards • An ethernet NIC contains protocol control firmware and Ethernet controller needed to support the media access control (MAC) data link protocol used by Ethernet • each card has its own unique MAC address • eg 00-01-02-B1-64-1D • assigned by manufacturer • card is configured in Control Panel/Networks

14. Modulation and Demodulation • Standard telephones work by analogue signals • sounds are converted into electrical signals of varying amplitude (volume) and frequency (pitch) • Computers work by digital signals • encoded somehow: e.g. 0 low voltage; 1 high voltage • Modulation is the process of encoding digital signals in some way into analogue signals • Demodulation is the opposite process of converting the analogue signals back to digital

15. DSL • A Digital Subscriber Line (DSL) modem connects a single computer to a DSL. • If more than one computer is connected a DSL router is needed. • A DSL modulate data into high frequency tones which are transmitted to a Digital Subscriber Line Access Multiplexer (DSLAM). • The DSLAM separates the voice and data components. • A DSL modem uses frequencies for data between 25KHz and about 1Mhz. • Voice is below 4KHz.

16. ADSL • Asymmetric Digital Subscriber Line (ADSL) • Distinguished from DSL because data transfer rates from the Internet to the computer are much faster than those in the other direction. • Why? The DSLAM at the ISP site is the noisiest part of the circuit. This is where the signal from the customer is weakest and that to be downloaded at its strongest. • ADSL is full-duplex. • ADSL filters are needed at each phone socket – a low pass filter for voice and a high pass filter for data. • More than 5 filters cause a degradation of the signal.

17. ADSL • Range about 4km. • Data rates 1.5 to 9 Mbps when receiving data • downstream rate • Typically 16 to 640 Kbps when sending data • upstream rate

18. Interconnection Media

19. Interconnection Media • Directly connected networks • unshielded twisted pair • fibre optic • co-axial cable • now rarely used • Indirectly connected networks • microwave • radio • infra-red • laser

20. Unshielded Twisted Pair • UTP - strands of wire twisted in pairs • ‘cat 5’ • Two insulated copper wires are twisted around each other • this reduces crosstalk and interference (noise) • Different cable lengths can cause signals to be received out of phase • Each twisted pair can carry a single signal • Usually four pairs in outer sheathing • Cheapest form of cabling

21. Fibre Optic • Optical fibre • strand of glass fibre • not prone to electromagnetic interference • Coherent beam of mono-frequency light • produced by light-emitting-diode (LED) or laser • Coated to prevent loss of signal by refraction • Multiple strands are bundled together • thousands of signals can be carried by each strand • mostly used for long distance cabling • more expensive than UTP cable • signal repeaters required after several miles

22. Broadcast Networks • An early broadcast radio network was Aloha • used a packet radio system • established connection between Hawaiian Islands • Originally a user could transmit at any time • had to retransmit if a collision occurred • this was fine when network traffic was very light • Later, ‘slotted Aloha’ reduced the chances of collision by requiring users to only start transmitting at the beginning of fixed time slots

23. Satellite Networks • Arthur C. Clarke discussed use of geo-stationery satellites for telecommunications in 1945

24. Footprints • Earth’s surface could be totally covered by just 3 satellites • if the surface was smooth! • Iridium system used 66 in Low Earth Orbit  250 miles • now being left to decay their orbits and burn • US GPS system • 24 satellite military system • now widely used for navigation • Galileo • European 30 (eventually) satellite system • purely for civilian users

25. Galileo System • Four different navigation service: • The Open Service (OS) • Free with an accuracy of <4 m horizontally and <8 m vertically. • The encrypted Commercial Service (CS) • Not free. Hasan accuracy of better than 1 m. Can be complemented by to give an accuracy of < 0.1m. • The encrypted Public Regulated Service (PRS) and Safety of Life Service (SoL) • Protection against jamming and the reliable detection of problems within 10 seconds. • Similar accuracy to the OS • Aimed at the police, military and safety-critical transport applications (air-traffic control, automated aircraft landing, etc.), respectively. • Global Maritime Distress Service. • Able to detect and report signals from the CorpasSarsat search and rescue beacons

26. Microwave • Microwaves • line of sight • parabolic dish reflector • electromagnetic radiation • Other electromagnetic systems • in general the longer the wavelength, the further the signal will travel • laser (visible light) • infra-red • radio BT Tower, Birmingham

27. Summary • Network hardware • workstations • multiplexors, hubs, bridges • gateways and routers • Interface devices • NIC’s • modems • Interconnection media • direct • indirect