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