communications channel & transmission media. bandwidth, measures the transmission rate of a...
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Communications Channel & transmission media
Communications Channel & transmission media
• Bandwidth, measures the transmission rate of a communications chanel. Speed at which data is transmitted is expressed as bits per second, bps.
• Broadband media, transmits multiple signals over a single communication line, UTP (Unshielded Twisted Pair) e.g. CAT5 100 mbps.
• Physical transmission media
• Coaxial cable• Shielded from interference• Needs terminator at end of cable• Used by cable television because it can be laid
over longer distances than twisted-pair cable
Communications Channel & transmission media
• Physical transmission media, contd.
• Twisted-pair cables
• Cable containing twisted-pair wires bundled together, graded as eg. Cat 5, 100 mbps or Cat 5 e
• Wires twisted together to reduce noise.
Transmission Media
Physical Transmission
• Fibre-optic cable
• Made of glass
• Optical, not electrical – little signal fade and no danger of sparks in hostile environments
• Multiple signals on a single fibre
• Resists EMI
Transmission Media
Physical Transmission, Fibre-optic cable• Core is as thin as a human hair• Not very flexible – needs thick coat• VERY fast, carrying more signals than wire cables• Less susceptible to noise or interference• Very secure (can’t be tapped or snooped)• Disadvantages:
• Costs more than twisted pair or coax• Difficult to install and modify
Transmission Media, c/f transfer rates
Transmission Media - Wireless
1 Broadcast radio• A wireless transmission medium distributing radio
signals through the air over long and short distances• Bluetooth and Wi-Fi communications technologies
use these radio signals• Need a transmitter to send and a receiver to accept
signals
• Bluetooth short range radio waves• Maximum distance 10 mtrs• Data transfer rate of 2mbps c/f wireless 54mbps• Good for use with hand-held devices, eg. PDA’s
Transmission Media - Wireless
Broadcast radio, contd.
Wi-Fi networks• Data is transmitted as radio signals between wireless
NICs and base stations• relatively short distances (e.g. 80m-200m), reduced
by obstacles• speeds are increasing – now about 11Mbps• encrypted communications to prevent eavesdropping• many PCs can connect to a base station• PCs can “roam” and will automatically connect to the
base station that has the strongest signal
Transmission Media - Wireless
2 Cellular radio• A form of broadcast radio used widely for mobile
communications, wireless modems & mobile phones• Users connect their notebook computer or other
device to a mobile telephone to access the web 7 email
• 2G, transmitted digital data at speeds of 9.6-19.2 kbps
• 3G, speed from 144 Kbps to 2.4 Mbps; display multimedia and graphics
Transmission Media – Wireless – cellular radio
2 Cellular radio
Transmission Media - Wireless
3 Microwave, radio waves providing high-speed signal transmission, 150 Mbps• Use microwave stations to transmit data as high
frequency radio signals from one microwave station to another
• Use line-of-sight transmission, stations must be located relatively close together, (<50kms) & on top of mountains or buildings
• Used in wide-open areas, eg. deserts, lakes, b/w buildings
• Handles high data rates over short distances
Transmission Media - Wireless
3 Microwave
Transmission Media
4 Communications Satellite• A space station that receives microwave signals
from an earth-based station, (microwave station) and amplifies the signals and broadcasts back over a wide area
• Devices such as hand help computers and GPS receivers also function as earth-based stations
• Air navigation, TV and Radio broadcasts, paging & GPS use communications satellites.
• Speed of downloading from internet faster than uploading
• More expensive than cable internet but may be only high speed option in remote areas
Transmission Media - Wireless
5 Infra-red
• Uses light waves and requires line-of-sight access• Uses an IrDA port to connect• Effective over short distances, up to 5 metres• Slow data transfer c/f cable, 115 Kbps – 4 Mbps