Topics covered in this tutorial

• Microwave communications
• Infrared
• Radio

Wireless Networks

Wireless systems differ from cabled systems in that they use the atmosphere as a conductor. In recent years, the price of wireless technology has fallen considerably, and as a result, it has become an affordable option. These systems are often used in a hybrid environment comprising some cable and some wireless technology. The advantages of wireless systems are as follows:

• Machine mobility
• They can be used in obstructed or difficult locations for cabling and locations where cabling is undesirable
• They support long distances

For local area networks (LANs) the transceiver (or access point) acts as connection between the cabled network and the wireless computers.

Wireless Transmission Techniques

The main wireless network technologies are:

• Microwave
• Infrared
• Narrow band radio
• Spread spectrum radio

The IEEE 802.11 category defines wireless LANs.

Microwave

Microwave is currently one of the most popular long distance transmission technologies. The use of the low GHz frequency range provides higher bandwidths than those available using radio waves. Typically, bandwidth values of 8Mbps are available.

The cost of these systems is relatively high and technical expertise is required to install them, as accurate alignment is required. Often, this service is leased from a service provider, which reduces installation costs and provides the required expertise. However this service can prove expensive in the long term.

The attenuation of microwave systems is highly dependent on atmospheric conditions; for example, both rain and fog can reduce the maximum distance possible. Higher frequency systems are usually affected most by such conditions. The systems are not particularly resistant to EMI and protection for eavesdropping can only be achieved by encryption techniques.

There are two types of microwave system:

Terrestrial

Terrestrial systems are often used where cabling is difficult or the cost is prohibitively expensive. Relay towers are used to provide an unobstructed path over an extended distance. These line-of-sight systems use unidirectional parabolic dishes that must be aligned carefully.

Satellite

Satellite systems provide far bigger areas of coverage than can be achieved using other technologies. The microwave dishes are aligned to geostationary satellites that can either relay signals between sites directly or via another satellite. The huge distances covered by the signal result in propagation delays of up to 5 seconds. The costs of launching and maintaining a satellite are enormous, and consequently customers usually lease the services from a provider.

Infrared

Infrared systems carry data between devices using infrared light. The light source can be a Light Emitting Diode (LED) or an Injection Laser Diode (ILD). Photodiodes are required to receive the signals.

Infrared uses the THz (1,000GHz) range of frequencies, and therefore provides high bandwidths of up to 20Mbps. Unfortunately, these high frequency waves provide no penetration ability and are readily diluted by sunlight. The cost of infrared systems varies considerably but, for long distances, the systems can be very expensive. Like Microwave systems, the attenuation depends heavily on atmospheric conditions and any minor obstructions. The systems must have a high output to overcome intense sunlight.

Infrared technology may be subdivided into a number of different types:

Unidirectional (Point-to-Point)
These are line-of-sight systems using a focused laser to transmit over distances of a half a mile (kilometer) or more. The careful alignment required by these systems means that the installation could be difficult.

Omnidirectional (Broadcast)
These systems use a 'scattered IR' technology. The light is allowed to reflect off all available surfaces providing an environment for mobile devices. The scattering results in lower data rates of less than 1Mbps and the technology is affected by strong sunlight. These systems are simple to install, as no alignment is required.

Reflective
This system uses optical transceivers located near to devices that transmit the data to a common location for redirection to a receiving device.

Radio - Single Frequency and Spread Spectrum

Radio systems use a transmitter and a receiver tuned to a specific frequency for the transfer of signals. The range of radio frequencies (RF) extends from 10KHz to 1GHz and these are subdivided into LW, MW, SW, VHF and UHF. A government authority regulates these frequencies and the use of a frequency requires a license from this body. The license ensures no one else can transmit that frequency within a particular area. There are unregulated frequencies that do not require a license; for example, 2.4 GHz is unregulated on an international basis. However, there is a limit on power output, which means coverage is restricted, so the use of the system is limited. Many of the networking systems operate in unregulated ranges.

Ideally, the higher frequency radio waves (typically, low GHz) are favored as the bandwidths available are greater than for lower frequency systems. The disadvantage of such systems is that they lack penetrating power.

Three different radio technologies are used for networking:

Low-power Single Frequency

These low power systems provide up to 10Mbps bandwidth, but have a limited range of up to almost 100 ft (30m) because of the low transmitter power. Ideally, the systems should be used in open environments as obstructions can reduce range markedly. The devices are often fully configured so they are simple to install. These systems suffer from EMI and the problem is particularly acute for low frequency systems as many EMI sources produce noise in this range.

High-power Single Frequency

This system provides bandwidths up to 10Mbps and has an extended range because of the increased transmitter power. It can be used indoors, outdoors and for mobile devices. The outdoor capabilities include line-of-sight applications and also 'beyond horizon systems' that employ the earth's ionosphere to reflect the signals back to earth.

The equipment required for this form of radio transmission is sophisticated and expensive. It is also complex to install, requiring expert operators and technicians. Like the low-power system, it is susceptible to EMI.

Spread Spectrum

This technology uses multiple frequencies simultaneously. There are two modulation schemes:

• Direct-sequence modulation
• Frequency hopping

Direct-sequence modulation
This is the more common technology that divides the data into chips. Each chip is transmitted using a different frequency. The receiver must be aware of the cycle of frequency changes so that it can reassemble the data correctly. These systems make eavesdropping extremely difficult and often decoy data is also transmitted at other frequencies to confuse the process.

Frequency hopping
This system rapidly switches between predetermined frequencies known as hops for a specified period of time. The receiver and transmitter must be synchronized to ensure data is correctly transferred. Some systems use multiple frequencies simultaneously to increase bandwidth.

These systems are inexpensive and reasonably simple. They provide speeds of up to 5Mbps, but they often have a limited range because they use low power transmitters. They are less sensitive to EMI than other radio technologies because they use a range of different frequencies.

Wireless bridges
Spread spectrum radio can be used to provide a wireless bridge between two buildings as an alternative to using leased lines. The distance supported by a wireless bridge may be up to 3 miles. Long range wireless bridges are able to support communications over a distance of up to 25 miles.

Bluetooth

Devised by Ericsson Mobile Communication, Bluetooth is a short-range radio based wireless communications system to be found on an increasing number of devices, such as PDAs, mobile phones and laptops.

It operates at relatively modest speeds (64Kbps). This is not a significant problem, as Bluetooth is not designed to replace LAN communications, but rather enhance communications from mobile devices with devices on a LAN.

It doesn't require a line of sight between compatible devices, but they must be within 15 yards (10 meters) or so.