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

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comp1121 computers and computer networks

COMP1121 Computers and Computer Networks

Richard Henson

University of Worcester

April 2008

week 10 the physical layer network hardware
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
transmission media
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?
hardware requirements
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
network card software requirements
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
network card software requirements1
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
binding network card software to an operating system protocol stack
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…
transmission media1
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
transmission media cabling
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
transmission media wireless
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
discovery of electro magnetic radiation
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
putting theory into practice
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
first data transmission by radio waves
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!
what vibrates in electro magnetic waves
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
transmission media wireless1
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!)
transmission media wireless2
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
mechanism of data transfer
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
cabling and crosstalk
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
crosstalk and coaxial cabling
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
thin coaxial cable
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
thin coaxial cable1
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
connecting thin coaxial cable
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
thick coaxial cable
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)
twisted pair cable
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)
twisted pair v thin coaxial disadvantages
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
topology twisted pair cable
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)
eia tia cabling standards for twisted pair cable
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
existing eia tia 568 standards
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
features of twisted pair cable
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
optical fibre cable construction
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)
optical fibre cable
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
optical fibre cable continued
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
optical fibre cable advantages
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
optical fibre cable disadvantages
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
healt h safety transmission media
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…