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Transcript
Media for Transmitting Data
Optical Communications
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Description of optical communications:
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Advantages:
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They use light as a carrier of information (as opposed to electrical
signals)
Light waves can be used for the direct transmission of signals.
Use of Fibre Optics
High bit rate – handles both speech and data
Better quality service as it is less susceptible to interference
Unaffected by electro magnetic disturbance (voltages, clicks,
atmosphere)
Difficult to tap into them - secure
Disadvantages:
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Only economical when the bandwidth is fully utilised
High cost of installation
Examples of Optical Communications
Computer networks – also utilise infra red
Computer controlled telecommunications
Transmitting signals in aircraft
Measuring Instruments – devices that use
laser to measure small distances
• High voltage installations
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Fibre optic cable
Summary
• Sends pulses of light
rather than electricity.
• Expensive.
• Runs over long distances.
• Is very quick.
• Can be used for:
– video conferencing
– interactive services.
Wireless LANs
Communications that take place without the use of wires or cables
– Mobile phones
– Networked computers
– Television broadcasts etc
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satellite
radio signals
infrared light beams
lasers
Each workstation and file server has some sort of transceiver/ antenna
to send and receive the data.
Wireless LANs
Advantages
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mobility and elimination of unsightly cables.
It's fast (11 - 125Mbps)
It has a long range (5,000 feet in open areas, 250 to
400 ft / 76 to 122 m in closed areas)
It's easily integrated into existing wired-Ethernet
networks.
Are great for allowing laptop computers or remote
computers to connect to the LAN.
Also beneficial where it may be difficult to install cables.
Wireless communication methods
Disadvantages
potential for radio interference due to
• weather,
• other wireless devices,
• obstructions like walls.
Communications media
• Unshielded Twisted Pair (UTP) cable.
used in telephone network and LANs
– cheap to purchase
– flexible and easy to install
– used to provide telephone lines to offices
– eases installation and management of the
cabling
– easy to upgrade (future networks will
support this media)
Other Communications media
• Coaxial cable
– high quality, well insulated cable
• Communications satellite
– in geosynchronous orbit
CONNECTION OPTIONS FOR WANS
Dial up, using analogue modems
• Cheap option
• Have to connect every time used
• Low bandwidth (54 kilobits)
CONNECTION OPTIONS FOR WANS
Leased line
• High bandwidth (can be gigabit with fibre
optic cable)
• Always on
• High cost (several £1000s per annum)
CONNECTION OPTIONS FOR WANS
ADSL
Asymmetric Digital Subscriber Line
modem or cable modem
• High bandwidth for price (e.g. 2 mb upload for £350 pa)
• Always on
• Not available in all areas
ISDN
Integrated Services Digital Network
• The line is more expensive to rent than a standard line
• There is no need for a modem as the signal is sent in
digital form.
• Uses existing telephone cables.
Factors affecting rate of data
transmission
• The speed of the modem
– Different modems vary in data transmission rates, typically from
9K to 56K bps (bits per second).
• The nature of the transmission line
– A digital line such as an ISDN line has a much higher
transmission speed than an analogue line.
• The type of cable used
– Twisted pair has a transfer rate of 10Mbps; fibre optic cable is
about 10 times as fast.
• The type of transmission
– synchronous or asynchronous.
Definitions
• asynchronous transmission
– In modem communication, a form of data
transmission in which data is sent intermittently, one
character at a time, rather than in a steady stream
with characters separated by fixed time intervals.
Asynchronous transmission relies on the use of a
start bit and stop bit(s), in addition to the bits
representing the character (and an optional parity bit),
to distinguish separate characters.
• synchronous transmission
– Data transfer in which information is transmitted in
blocks (frames) of bits separated by equal time
intervals.