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

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

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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. 4

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 5

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

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

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

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 9

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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 11

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 12

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 B1-64-1D assigned by manufacturer card is configured in Control Panel/Networks 13

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 14

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. 15

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. 16

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 17

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

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 20

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 21

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 22

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

Footprints Earth’s surface could be totally covered by just 3 satellites if the surface was smooth! 24 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

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. 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 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 Corpas Sarsat search and rescue beacons 25

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 26 BT Tower, Birmingham

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