COMP1121 Computers and Computer Networks

1. COMP1121 Computers and Computer Networks Richard Henson University of Worcester April 2008

2. Week 10 – The Physical Layer: Network Hardware • By the end of this session you should be able to: • Identify and select network hardware devices for a variety of purposes • Explain what a hardware driver is how plug-and-play works • Explain how to install a network card and ensure it provides network connectivity

3. Transmission Media • “Connection” between computers on a network is essential for sending/receiving digital data • doesn’t need to be overtly physical • 0’s and 1’s can be transmitted without an apparent physical medium… • Suggestions?

4. Hardware requirements: • A networking “subsystem” on each computer, which links in with the motherboard in some way • at one time this always happened via a network card connecting the motherboard via ISA or PCI connector • nowadays, the networking hardware is often built onto the motherboard • A plug/socket arrangement with the physical medium • USUALLY… this is a “telephone-type” arrangement to insulate twisted pair copper cable • known as RJ45 • AT ONE TIME… co-axial cables and connectors like a TV aerial system were used • known as BNC

5. Network Cardsoftware requirements • Software interfaces effectively with level 3 protocol • nowadays usually IP • part of operating system networking component • requires allocation of IP address • Level 2 software converts packets into “frames” of data • Level 1 software converts the binary frames into electricity (high/low voltage) and sends them out onto the physical medium

6. Network Cardsoftware requirements • Each card gets a unique identifier during manufacture • known as the MAC address – where “MAC” (media access control) is part of the data link layer • Provides software for physical signal/binary number interconversion • OSI layer 1 signals - converts to layer 2 frames • OSI layer 2 frames converted to layer 1 signals

7. Binding Network Card Software to an operating system “protocol stack” • Two way process: • Down: Level 3 IP packets need to be converted to frames • Up: Level 2 frames need to be converted to IP-compatible packets • As with other hardware devices… • achieved through software called “drivers” • Plug-and-play hardware can tell the operating system what it is and provide the relevant software on network card ROM • otherwise, driver software will be needed…

8. Transmission Media • From the network card/adaptor port, data can be physically transmitted in a number of ways: • via insulated copper cable • via optical fibre cable • via “wireless” media e.g. radio waves, microwaves, infra-red beams, etc. • Transmission medium type greatly affects the overall speed and resilience of the network and the number of packets that get corrupted

9. Transmission Media - Cabling • Two types of cables have historically been used in LANs: • thin Coaxial – up to 200 metres per cable run • thick Coaxial – up to 500 metres per cable run • Most cabled networks nowadays use either: • Unshielded Twisted Pair (UTP) – up to 100 metres • Fibre-Optic – up to 1 km per cable run

10. Transmission Media – Wireless • “Wireless” means transmission using electro-magnetic radiation • means an electro-magnetic wave vibrating with a specified frequency and moving forwards at the speed of light… • light itself is electromagnetic radiation • We take wireless for granted now, but it was first used to transmit and receive signals only about 100 years ago • theoretically shown to be possible by Maxwell, UK • invention usually attributed to Marconi, Italian

11. Discovery of Electro-magnetic Radiation • Potential existence of radio waves were predicted in 1864 • an amazing piece of maths… • Started almost 30 years before Marconi with Cambridge professor James Clerk Maxwell • successfully predicted most of the physical laws about propagation and speed of radio waves • noted their resemblance to light waves • showed how they could be reflected, absorbed and focused like the beam from a torch • and could change the very nature of the object on which they were focused

12. Putting Theory into Practice • Hardly anybody believed Maxwell in 1864! • BUT his theory was quantified by Oliver Heaviside into two equations • Over the next 30 years they became a physical reality • in 1879, Prof. David Hughes walked up Portland Place, London with a device that picked up transmitted radio waves • in 1887, German scientist Heinrich Hertz carried out a famous set of experiments that proved • that Maxwell had been right all along • that some materials reflected radio waves back… • in 1894, the British scientist Oliver Lodge transmitted wireless signals over 150 yards

13. First Data Transmission by Radio waves • An Italian in London… • ref: http://www.connected-earth.com/Galleries/Telecommunicationsage/Awirelessworld/Theoriginsofradio/index.htm • Marconi arrived in 1895, 21 years old, with a new system of 'telegraphy without wires' • had already approached the Italian government - but it showed no interest. • 1896: • called upon the Engineer-in-Chief of the Post Office to demonstrate his system • allowed him to set up his transmitter on the roof of the Central Telegraph Office, and a receiver on the roof of a building 300 yards away. • On July 27 succeeded in sending morse code signals between the two locations - world's first recorded wireless message. By 1901, signals had crossed the Atlantic!

14. What vibrates, in electro-magnetic waves? • Put simply: • electricity through a coil produces a magnetic effect • magnetism through a coil generates electricity • If the electricity is varied, or “pulsed”, the magnetic field will also pulse at the same rate • magnetism travels even through a vacuum • can be used to carry a signal • Maxwell’s brainwave was suggesting that this effect could be used to transmit signals without wires • need a energy source, & transmitter/receiver coils

15. Transmission Media – Wireless • Data carried most efficiently nowadays on extremely high frequency radio waves: • patented as “radar”; now called microwaves • used in ww2 - bounced off enemy planes • http://www.fi.edu/weather/radar/video/ /historyrad.mov • Less bandwidth & lower reliability than optical fibre or cable, but becoming very popular… e.g. • Cellular Mobile Phone networks • Connecting mobile phones to each other & the Internet • Satellite microwave • Data to/from satellite in geocentric orbit (22300 miles up!)

16. Transmission Media - Wireless • Point-point microwave • data transmitted either across roofs of adjacent campus buildings, or “line of sight” point-point across open land (up to 30 miles away) • Radio wave • Either “spread-spectrum” or “narrow-band” • Useful for connectingmobile laptops to a LAN

17. Mechanism of data transfer • Coaxial or twisted pair: • data is transmitted by electrical conduction • cabling system consist of two (or groups of two) conducting wires • Fibre optic • Data transmitted by light internally reflected through a thin fibre-glass tube • Data can be safely transmitted separately in both directions

18. Cabling and Crosstalk • Two parallel wires, as used in domestic electrical cabling, cannot be used for data • Reason - “crosstalk”: • electrical interference between signals in the two wires • signals jumping from one wire to the other • The longer the cable, the greater the chance of crosstalk • Therefore there will always be a limit on cable run between data storage devices

19. Crosstalk and Coaxial Cabling • Magnetic fields produced by electricity in the two wires tends to cancel out • This greatly reduces, but does not eliminate cross talk • There is a recommended maximum length for Ethernet cables for this reason: • thin Ethernet - 185 metres • thick Ethernet - 500 metres

20. Thin Coaxial Cable • Also known as: • Thin Ethernet • Base band • Cable consists of: • single copper central wire covered with a layer of insulation • itself covered by wire braiding (a patchwork made of very thin copper wire) • Whole arrangement wrapped in a (usually black) plastic tube

21. Thin Coaxial Cable • Also known as IEEE 10base2 • IEEE - Institute of Electrical and Electronic Engineers (more on this erudite body later) • Not very flexible because of the nature of its construction • Available in a range of different qualities • Generally used for networks using a bus topology • Recommended maximum data transmission rate - 10Mbits/sec

22. Connecting Thin Coaxial Cable • Computersconnected to a thin coaxialbus network need a network card with a BNC socket • Such network cards are becoming very rare… • The coaxial bus connects to the network card through a metal BNC “T connector” • The coaxial cable itself must be “terminated” at each end with a BNC “terminator” that completes the electrical circuit

23. Thick Coaxial Cable • Also known as Thick Ethernet or broadband cable • very expensive and cumbersome to use… • Includes two shielding layers between the wires to allow for “harsh” environments (lots of electrical “noise” caused by nearby electric motors, etc.) • Superior to thin Ethernet is two ways: • Higher data transmission rates (100Mbits/sec recd maximum) • Larger cable lengths (500 metres recd. maximum)

24. Twisted Pair Cable • The current standard in most LANs • Compared to coaxial: • cheaper • much more flexible • easy to use • doesn’t need BNC T connectors or terminators • Twisted Pair construction tends to cancel out magnetic fields - greatly reducing cross talk (but not as effectively as coaxial)

25. Twisted Pairv Thin Coaxial - Disadvantages • Use of the “twisted pairs” - wrapping the individual wires around one another - does not reduce cross talk as effectively as coaxial cable • More susceptible to “ harsh” environments (especially rapidly changing magnetic fields) • Extra insulation of twisted pair cable recommended in such circumstances • Cable therefore becomes more expensive

26. Topology - Twisted Pair Cable • Normal use - Star topology • Connections could go directly from network card sockets to hub ports • using RJ45 plastic end connectors • similar to RJ11 telephone line connectors (but not the same!) • In practice, for flexibility, a combination of CAT5 cables, connectors, patch leads/sockets used to connect network cardsto hubs • Most modern network cards are known as “combo” - this means they have sockets for both BNC (coaxial) and RJ45 (twisted pair)

27. EIA/TIA Cabling Standards for Twisted Pair cable • EIA - Electronics Industries Association • TIA - Telecommunications Industries Association • EIA/TIA is a Cabling standards body • joint venturebetween the EIA & TIA • The EIA/TIA 568 standard covers five types of unshielded twisted pair cable known as (in increasing quality) CAT1 through to CAT5 • CAT5 standard has evolved to CAT5f

28. Existing EIA/TIA 568 standards • CAT1 is OK for voice communications, but not suitable for digital data • CAT2 can only support digital data transfer rates of up to 4 Mbits/sec • CAT3 can only support digital data transfer rates of up to 10 Mbits/sec (this is the lowest standard for IEEE 802.3 10BaseT Ethernet networks - next week’s session) • CAT4 can only support digital data transfer rates of up to 16 Mbits/sec • CAT5 can officially handle up to 100 Mbits/sec, although it is being used on faster (e.g. 155Mbit/sec) FDDI networks • CAT6 can handle faster rates, theory up to 1 Gigabit

29. Features of Twisted Pair Cable • Most popular type currently known as Category 5 UTP (unshielded twisted pair) • 5e still widely used • 5f, 6current preferred standards • CAT5e upwards can carry data at high transmission rates (up to 200 Mbits/sec) • CAT5f, CAT6: even higher (1 Gigabit/sec) • Because of the greater susceptibility of twistedpair to cross talk, the maximum recommended cable length for CAT5 is 100 metres

30. Optical Fibre Cable - construction • The cable itself consists of: • a glass or plastic central light conductor • surrounded by a further layer of glass or plastic cladding • and a protective outer casing • The cable must be connected directly to: • light emitter (one end) • light detector (other end)

31. Optical Fibre Cable • First of all, the electrical signal needs to be converted into light pulses. This done by: • either an LED (light emitting diode) • or a Laser • The light pulses are then directed into the central tube • The light is repeatedly totally internally reflected as it passes along the inside of the tube - as if it were on the inside of a mirror

32. Optical Fibre Cable (continued) • Thanks to total internal reflection, a cable can carry light considerable distances, including round bends, without significant energy loss • 1 km cable run quite possible… • cable must be bent carefully, otherwise internal structure could be damaged… • On emerging from the cable, the pulse is converted back into an electrical signal by a photodiode • More detail: • http://www.arcelect.com/fibercable.htm

33. Optical Fibre Cable - advantages • Speed of transmission (up to Gbits/sec) • Ability to support voice and digital data along the same cable • Security (very difficult to tap) and reliability of transmission (almost immune to electrical interference) • A pair of optical fibres can simultaneously carry light/data in each direction (full duplex) with no danger of signal attenuation • With copper cables, signals in adjacent cablescould interfere with each other

34. Optical Fibre Cable - disadvantages • Expensive • Expensive to install • Not as flexible to use in tight areas of twisted pair • Needs expensive hardware to reliably convert light into electricity and vice versa

35. Health, Safety & Transmission Media • Some people object to cable being visible • apart from aesthetic reasons, also a potential safety hazard • Different types of twisted pair cable are availablefor different environments (e.g. under carpets or in the plenum space above a “lowered” ceiling) • This usually increases the likelihood of crosstalk, and effectively reduces the recommended minimum cable length • Standards laid down by IBM in the 1980s…

36. IBM Cabling Categories