Communication Channels

Physics: Cambridge International A Level, topic 10.1.1

Transmission Media

Transmission media are different ways of physically transferring data from one place to another.

The bandwidth of a wave is the range of frequencies in the wave.
Bandwidth can also be thought of the amount of data that can be transferred in a given time.

Inside the cable, there are eight wires which are twisted into pairs. This is to reduce interference from other signals and improve transmission speed.
Twisted cables have different ratings to indicate how quickly data can be transferred.
E.g. Cat-5e can transmit at 1 Gbps.
E.g. Cat-6 can transmit at 10 Gbps.

Fibre-optic cables are made of thin strands of glass which transmit binary data as pulses of light.
Fibre-optic cables have many advantages:
Do not suffer interference.
Very high bandwidth (100Tbps).
Because of the lack of interference, fibre-optic cabling is appropriate for long distance communication such as undersea cabling.

Radio waves are the transmission media of wireless networking.
E.g. WiFi.
E.g. Bluetooth.
The amplitude (strength) of a radio wave decreases as it moves further from its transmitter. This is why WiFi signal loses strength further away from a WAP.
Radio waves are also subject to interference from other radio signals of similar frequency. Wireless access points (WAPs) often check for frequency channels that are least congested before starting a broadcast.

Coaxial cable is a wired transmission media where an insulated copper wire is surrounded by a metal mesh to protect it from interference.
This is most often used for cable television.

Satellites allow us to transmit information over very large distances

A geostationary (or geosynchronous) satellite is one that always appears in the same place in the sky, no matter what the time of day. For this to happen, the satellite must:
Have an orbital period of exactly one day (23 hours 56 mins 4.09 secs).
Be in a circular orbit very near to the equator.
Be orbiting in the same direction as the Earth is rotating.
The geostationary orbit may be calculated to be 35 790 km above the Earth.

Polar orbiting satellites pass over the north and south poles of the Earth over the course of one orbital period.
Polar orbiting satellites tend to be approximately 800 km above the Earth.
This means they have an orbital period of approximately 100 minutes.

Polar orbiting satellites experience less delay in communication times than geostationary satellites as their orbit is low.
Polar orbiting satellites scan the whole surface of the Earth, whereas geostationary satellites are tied to one location.
Aerials connected to geostationary satellites don't have to be moved as their position remains fixed in the sky.