Data Transmission Media Notes

Wireless Transmission Media

Unguided media transports electromagnetic waves without using a physical conductor.
This type of communication is often referred to as wireless communication.
Signals are broadcast through the air, making them available to anyone with a device capable of receiving them.

Transmission and reception are achieved using an antenna.
- Directional: Transmitting antenna produces a focused beam.
  - Transmitter (sender) and receiver must be aligned, requiring careful alignment.
- Omnidirectional: Signal spreads out in all directions.
  - Can be received by many antennas.

Transmitter uses a parabolic dish, mounted as high as possible.
Focused beam.
Requires an unobstructed line-of-sight between the source and receiver.
Microwave relay towers are used to achieve distance.
Used for long-haul telecommunications.
The curvature of the earth requires stations (repeaters) to be approximately 30 miles apart.
Higher frequencies provide higher data rates but require smaller antennas.
Attenuation increases with rainfall.

Advantages
- No cabling is needed between sites.
- Wide bandwidth is available.
- Supports multi-channel transmissions.
Disadvantages
- Requires line-of-sight.
- Expensive towers and repeaters are needed.
- Subject to interference from passing airplanes and rain.

A satellite is a microwave relay station in space that can relay signals over long distances.
Satellite systems consist of ground-based transmitter and receiver dishes with an orbital satellite circuit (transponder).
Geostationary satellites remain above the equator at a height of 22,300 miles (approximately 36,000 km) to maintain line of sight.

Earth stations communicate by sending signals to the satellite on an uplink.
The satellite then repeats those signals on a downlink.
Signals are transmitted to the orbiting satellite, which relays it back to another ground station/downlink.
The broadcast nature of the downlink makes it attractive for services such as television programming distribution.
Satellite receives on one frequency, amplifies or repeats signal, and transmits on another frequency.

The cost of satellite links is still very expensive, and is primarily used for intercontinental links.

Television distribution: A network provides programming from a central location.
Direct broadcast satellite (DBS).
Long-distance telephone transmission.
High-usage international trunks.
Private business networks.

Advantages
- Can reach a large geographical area.
- Low cost per user for services like PAY TV.
- High bandwidth.
- Cheaper over long distances.
Disadvantages
- High initial cost in launching a satellite.
- Susceptible/prone to noise and interference.
- Receiver dishes and decoders are required.

Omnidirectional: no need for rigidly mounted dish antennas.
Requires line of sight.
Examples include FM radio and UHF/VHF television.
- Advantage: Less affected by rainfall.
- Disadvantage: Suffers from multipath interference due to reflections.

Impairments exist in all forms of data transmission.
Analog signal impairments result in random modifications that impair signal quality.
Digital signal impairments result in bit errors (1s and 0s transposed).