2.5. Unguided Media
Unguided media relates to data transmission through the air and is commonly referred to as wireless. The transmission and reception of data is carried out using antenna.
There are two main ways that antenna work:
- Directional (in a beam)
- Omnidirectional (all around)
Radio transmission works with or without line of sight. If line of sight is possible then transmission can take place between sending antenna and receiving antenna. The placement of antenna has to take into account the curvature of the Earth with antenna being built taller accordingly. This will also allow for greater transmission distances. If line of sight cannot be implemented then signals can be broadcast to the upper layers or the atmosphere or space and then transmitted back to Earth.
The electromagnetic spectrum used for radio communication consists of eight distinct ranges which are regulated by government authorities:
|VLF||Very low frequency||3 – 30KHz|
|LF||Low frequency||30 – 300KHz|
|MF||Middle frequency||300KHz – 3MHz|
|HF||High frequency||3 – 30MHz|
|VHF||Very high frequency||30- 300MHz|
|UHF||Ultra high frequency||300MHz – 3GHz|
|SHF||Super high frequency||3 – 30GHz|
|EHF||Extremely high frequency||30 – 300GHz|
A satellite is any object that revolves around a planet in a circular or elliptical path. The moon is Earth’s natural satellite at 240,000 miles distant. Other satellites that fulfil this definition are man made and have been launched into orbit to carry out specific functions. These satellites are typically between 100 and 24,000 miles away.
Satellites have many purposes including data communications, scientific applications and weather analysis.
Satellite transmission requires an unobstructed line of sight. The line of site will be between the orbiting satellite and a station on Earth. Satellite signals must travel in straight lines but do not have the limitations of ground based wireless transmission, such as the curvature of the Earth.
Microwave signals from a satellite can be transmitted to any place on Earth which means that high quality communications can be made available to remote areas of the world without requiring the massive investment in ground-based equipment.
The path that a satellite takes round the Earth is called its orbit. There are three types of orbit and each is used for different purposes. The orbits used are:
- Low Earth Orbit (LEO)
- Geosynchronous Earth Orbit (GEO)
- Polar Orbit (PO)
Low Earth Orbit (LEO)
Low Earth Orbit (LEO) satellites are placed at the top of Earth’s atmosphere from between 100 and 1240 miles high. The higher a satellite’s orbit the longer it takes for the signal to be received. The time delay between sending and receiving is called latency. With LEOs this latency is relatively negligible at hundredths of a second.
In order to resist the pull of the Earth’s gravity LEOs must travel at high speeds of about 17,000 miles an hour. Due to this the time taken for a LEO’s period is 90 minutes. A period is the term used to describe how long it takes for a satellite to complete one full orbit of the Earth.
A low Earth orbit is the simplest and most cost effective of satellite placement and provides high bandwidth and low latency. The other types of orbit require much more rocket fuel and energy.
Low Earth Orbit satellite systems require several dozen satellites to provide coverage of the entire planet.
Geosynchronous Earth Orbit (GEO)
Geosynchronous Earth Orbit satellites orbit the Earth at approximately 23,000 miles above the equator and have a period of exactly one day. If looked at from Earth GEOs would look as if they were fixed in one location.
GEO satellites must have their sending and receiving antenna locked to each other to maintain line of sight and to do this they must rotate at the same speed of the Earth. A GEO satellite must also attain a specific orbit around the equator to keep away from other GEO satellites. The distance between GEO satellites is 2 degrees or 1000 miles.
Only three equally spaced GEO satellites are needed to cover the entire surface of the Earth, except or the Polar regions. The main drawback of a GEO satellite is the high latency with a value of 0.24 of a second.
Most of today’s satellites are GEO satellites and their applications include communications and weather.
Polar Orbit (PO) satellites are generally Low Earth Orbit (LEO) satellites that have an orbit near or over the poles. These satellites orbit from north to south at low altitudes of around 600 miles and can pass over the north and south poles many times a day.
Typical applications of polar orbit satellites are for observation of the Earth and reconnaissance.
A Satellite’s Coverage Area
Satellites can cover massive areas of the Earth’s surface and this area is called a satellite’s footprint. The further from the Earth a satellite is the larger its footprint.
Satellites use two frequencies for communications. One band sends while the other receives. Transmission from the Earth to a satellite is called an uplink whereas transmission from the satellite to the Earth called a downlink.
Satellite Advantages and Disadvantage
- High bandwidth
- Coverage over a large geographical area
- Can be cheaper over long distances
- Huge initial cost
- Noise and interference
- Propagation delay
Microwave transmission also requires line of sight in order to work properly. In order to allow two way communications two frequencies are used. However, this does not mean that there has to be two antennas because the frequencies can be dealt with by one antenna at both ends.
The distance covered by microwave signals is based upon the height of the antenna. In order to increase this coverage each antenna has a built-in repeater that regenerates the signal before passing it on to the next antenna in line. The placement of the antenna to do this is approximately 25 miles.
The main drawback of microwave signals is that they can be affected by weather, especially rain.
Principles of Microwave Transmission
Microwave transmission relies on three key elements:
- Use of radio frequency to achieve the transmissions (operating between 1Ghz to 170Ghz)
- Clear line-of-sight with no obstacles in the way
- Regular relay stations required due to line of site and cost considerations
Microwave Advantages and Disadvantage
- No cables needed
- Multiple channels available
- Wide bandwidth
- Line-of-sight will be disrupted if any obstacle, such as new buildings, are in the way
- Signal absorption by the atmosphere. Microwaves suffer from attenuation due to atmospheric conditions.
- Towers are expensive to build