Networks 3 Network Hardware Dr John Cowell phones off (please) 1.
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Transcript of Networks 3 Network Hardware Dr John Cowell phones off (please) 1.
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OverviewNetwork hardware
workstationsmultiplexors, hubs, bridgesgateways and routers
Interface devicesNIC’smodems
Interconnection mediadirectindirect
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Modems and MultiplexersA modem is a device that converts data bits into
sound so that data can be transmitted over public telephone linesit 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.
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Multiplexers - MUX
Several low-speed terminals share a high-speed data line
Signals from terminals combined into a single data channelseparated again at the other end
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HubsAllow simple
interconnectivity of workstationspeer-to-peer networklittle/no processing in
hubbus architecturelevel 1
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SwitchesAllow faster
interconnectivity of workstationspeer-to-peer networkrouting in switchany two machines can
talk simultaneouslylevel 2
Switch
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Gateways and RoutersInterconnect networks
convert protocolsGateways
interface to Internetlevel 7
Routersoperate within Internetdecide pathway for signalsusually level 3
NB the term gateway is also used by network engineers for any device that interconnects networks, or subnetworks
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Interface DevicesSome device must physically connect a computer to
a networkThree principal devices are
network interface cards a card that plugs into a PC expansion slot to provide a
direct (permanent) connection to a networkmodems
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
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Network Interface CardNetwork 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 networkEach 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
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Ethernet CardsAn ethernet NIC contains
protocol control firmware and Ethernet controller needed to support the media access control (MAC) data link protocol used by Etherneteach card has its own
unique MAC address eg 00-01-02-B1-64-1D assigned by manufacturer
card is configured in Control Panel/Networks
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Modulation and Demodulation
Standard telephones work by analogue signalssounds 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 voltageModulation is the process of encoding digital
signals in some way into analogue signalsDemodulation is the opposite process of
converting the analogue signals back to digital
BT Network
Modem Modem
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DSLA 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.
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ADSLAsymmetric 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.
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ADSLRange about 4km.Data rates 1.5 to 9 Mbps when receiving data
downstream rateTypically 16 to 640 Kbps when sending data
upstream rate
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Interconnection MediaDirectly connected networks
unshielded twisted pairfibre opticco-axial cable
now rarely used
Indirectly connected networks microwaveradio infra-red laser
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Unshielded Twisted PairUTP - strands of wire twisted in pairs
‘cat 5’Two insulated copper wires are twisted around
each otherthis reduces crosstalk and interference (noise)
Different cable lengths can cause signals to be received out of phase
Each twisted pair can carry a single signalUsually four pairs in outer sheathingCheapest form of cabling
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Fibre OpticOptical fibre
strand of glass fibrenot 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 refractionMultiple strands are bundled together
thousands of signals can be carried by each strandmostly used for long distance cabling
more expensive than UTP cable signal repeaters required after several miles
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Broadcast NetworksAn early broadcast radio network was Aloha
used a packet radio systemestablished connection between Hawaiian Islands
Originally a user could transmit at any timehad to retransmit if a collision occurredthis 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
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Satellite NetworksArthur C. Clarke discussed use of geo-stationery
satellites for telecommunications in 1945
23,000 miles
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FootprintsEarth’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
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Galileo SystemFour 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. Has an 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 OSAimed 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 Corpas Sarsat search
and rescue beacons
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MicrowaveMicrowaves
line of sightparabolic dish reflectorelectromagnetic
radiation
Other electromagnetic systemsin general the longer the
wavelength, the further the signal will travel laser (visible light) infra-red radio
BT Tower, Birmingham