CSCI1412Lecture 17

Networks 3Network Hardware

Dr John Cowell

phones off (please)

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OverviewNetwork hardware

workstationsmultiplexors, hubs, bridgesgateways and routers

Interface devicesNIC’smodems

Interconnection mediadirectindirect

Network Hardware

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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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BridgesInterconnect LANS of

same/similar typesi.e. protocol

Convert data type

Level 3

Bridge

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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

Interface Devices

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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

Interconnection Media

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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

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SummaryNetwork hardware

workstationsmultiplexors, hubs, bridgesgateways and routers

Interface devicesNIC’smodems

Interconnection mediadirectindirect