(i) Spread spectrum wireless technology
In order to reduce the effects of interfering frequencies, wireless devices use the spread spectrum technology. This technology helps share available frequency bandwidth common to wireless devices. It also helps prevent jamming of radio signals due to strong interference from another source of radio frequency. Instead of using a fixed frequency, such as that used with radio and television broadcasts, wireless networks use a spectrum of frequencies. The sender uses a number of narrow-band frequencies to communicate with the receiver. Each narrow band of frequencies contains only a part of the signal. The receiver correlates the signals received at different frequencies to retrieve the original information. Spread spectrum technology synchronizes wireless signals using one of the following methods:
Frequency Hopping Spread Spectrum (FHSS)
FHSS is the method of transmitting RF signals by rapidly switching frequencies according to a pseudorandom pattern, which is known to both the sender and the receiver. FHSS uses a large range of frequency (83.5 MHz) and is highly resistant to noise and interference. The amount of time the signal spends on any frequency is known as dwell time, and the amount of time it takes from switching one frequency to another is known as hop time. FHSS signals are difficult to intercept because the signals usually appear as noise. FHSS works in the unlicensed frequency range of 2.4 GHz and is used in Home RF and Bluetooth. It has a limited speed of transmission that ranges from 1.6 to 10 Mbps.
Direct Sequence Spread Spectrum (DSSS)
DSSS is a modulation technique used by wireless networks. It uses a wide band of frequency and it divides the signal into smaller parts and is transmitted simultaneously on as many frequencies as possible within a particular frequency band. DSSS adds redundant bits of data known as chips. The ratio of chips to data is known as spreading ratio. The higher the spreading ratio, the higher the immunity to interference. DSSS is faster than FHSS and ensures data protection, because chips are redundant and simultaneously transmitted. It utilizes a frequency range from 2.4 GHz to 2.4835 GHz and is used in 802.11b networks.
(ii) Infrared
Infrared technology employs electromagnetic radiations that use wavelengths that are longer than the visible light but shorter than radio frequency. This technology is used in night-vision equipment, thermography, digital cameras, and digital communication systems. Common examples of Infrared devices are the remote controls used by TVs and audio systems. The Infrared technology is standardized by the Infrared Data Association (IrDA). The following are some of the key characteristics of IrDA wireless communication technology:
• It supports point-to-point wireless communications between two devices.
• Infrared transmission uses a direct line of sight suitable for personal area net-works.
• Infrared waves cannot penetrate walls.
• IrDA wireless communication technology supports data transfer speeds ranging from 10 to 16 Mbps.
• Infrared devices consume very low power.
• Infrared frequencies do not interfere with radio frequencies.
• IrDA wireless communication technology provides a secure wireless medium due to the short distance (usually 3 to 12 feet) between devices.
In order to reduce the effects of interfering frequencies, wireless devices use the spread spectrum technology. This technology helps share available frequency bandwidth common to wireless devices. It also helps prevent jamming of radio signals due to strong interference from another source of radio frequency. Instead of using a fixed frequency, such as that used with radio and television broadcasts, wireless networks use a spectrum of frequencies. The sender uses a number of narrow-band frequencies to communicate with the receiver. Each narrow band of frequencies contains only a part of the signal. The receiver correlates the signals received at different frequencies to retrieve the original information. Spread spectrum technology synchronizes wireless signals using one of the following methods:
Frequency Hopping Spread Spectrum (FHSS)
FHSS is the method of transmitting RF signals by rapidly switching frequencies according to a pseudorandom pattern, which is known to both the sender and the receiver. FHSS uses a large range of frequency (83.5 MHz) and is highly resistant to noise and interference. The amount of time the signal spends on any frequency is known as dwell time, and the amount of time it takes from switching one frequency to another is known as hop time. FHSS signals are difficult to intercept because the signals usually appear as noise. FHSS works in the unlicensed frequency range of 2.4 GHz and is used in Home RF and Bluetooth. It has a limited speed of transmission that ranges from 1.6 to 10 Mbps.
Direct Sequence Spread Spectrum (DSSS)
DSSS is a modulation technique used by wireless networks. It uses a wide band of frequency and it divides the signal into smaller parts and is transmitted simultaneously on as many frequencies as possible within a particular frequency band. DSSS adds redundant bits of data known as chips. The ratio of chips to data is known as spreading ratio. The higher the spreading ratio, the higher the immunity to interference. DSSS is faster than FHSS and ensures data protection, because chips are redundant and simultaneously transmitted. It utilizes a frequency range from 2.4 GHz to 2.4835 GHz and is used in 802.11b networks.
(ii) Infrared
Infrared technology employs electromagnetic radiations that use wavelengths that are longer than the visible light but shorter than radio frequency. This technology is used in night-vision equipment, thermography, digital cameras, and digital communication systems. Common examples of Infrared devices are the remote controls used by TVs and audio systems. The Infrared technology is standardized by the Infrared Data Association (IrDA). The following are some of the key characteristics of IrDA wireless communication technology:
• It supports point-to-point wireless communications between two devices.
• Infrared transmission uses a direct line of sight suitable for personal area net-works.
• Infrared waves cannot penetrate walls.
• IrDA wireless communication technology supports data transfer speeds ranging from 10 to 16 Mbps.
• Infrared devices consume very low power.
• Infrared frequencies do not interfere with radio frequencies.
• IrDA wireless communication technology provides a secure wireless medium due to the short distance (usually 3 to 12 feet) between devices.
(iii) Bluetooth
Bluetooth wireless networking technology provides short-range communications between two or more devices. It is a low-cost networking solution widely used in telephones, entertainment systems, and computers. It is designed to overcome the limitations of IrDA technology. The following are some of the key characteristics of Bluetooth-based wireless communication:
• It supports transmission speeds from 1 Mbps (Bluetooth 1.0) to 3 Mbps (Bluetooth 2.0).
• It works over the unlicensed frequency range of 2.4 GHz.
• The devices must be within a short range of less than 10 meters.
• It uses FHSS technology.
• It offers high resistance to electromagnetic interferences.
• Unlike the Infrared signals, it does not require a direct line of sight.
• Bluetooth devices consume very low power.
• Two or more Bluetooth computers form an ad-hoc wireless network.
(iv) Factors that affect wireless services
Wireless services use radio frequencies that travel through the atmosphere. There are several factors that may affect the speed, signal quality, and range of wireless signals. These include interference from other electrical devices, the type of antenna used, and other environmental factors. This section covers a brief discussion of these factors.
Interferences. Atmospheric interferences to wireless signals cannot be prevented, but they can certainly be reduced to achieve optimum performance. Some of the major causes of interference include the following:
• Physical objects such as buildings, trees, concrete and steel walls. These objects can either significantly reduce signals or even completely block them.
• Electromagnetic interference (EMI) generated by high-power electric lines, power transformers, heavy electrical machinery, fans, light fixtures, etc.
• Radio frequency interference (RFI) generated by other wireless equipment working in the same frequency ranges used by computer wireless devices. Examples of these types of equipment are wireless phones, wireless game controllers, or microwave ovens.
Type of antenna. The range of wireless signals depends on the type of antenna used for transmitting radio frequency signals. Selection of an antenna is a critical part of implementing a wireless network. Different shapes and sizes of antennas offer different signal levels. The strength of a wireless antenna (called its gain)is measured in decibels isotropic (denoted as dBi). An isotropic antenna sends signals of equal strength in all directions. A simple rule for calculating effective strength of an antenna is that every 3 dBi of gain almost doubles its output.
Omni-directional antennas send wireless signals in all directions. This type of antenna is useful when the coverage is required equally around the point of trans-mission. On the other hand, directional antennas transmit signals in one direction only. This helps send the entire output of the transmitting device in one direction, in which case, signals are more effectively transmitted.
Environmental factors. Environmental conditions, including weather, significantly affect the speed, range, and coverage of wireless signals. These factors can have a bad impact on wireless signals.
Bluetooth wireless networking technology provides short-range communications between two or more devices. It is a low-cost networking solution widely used in telephones, entertainment systems, and computers. It is designed to overcome the limitations of IrDA technology. The following are some of the key characteristics of Bluetooth-based wireless communication:
• It supports transmission speeds from 1 Mbps (Bluetooth 1.0) to 3 Mbps (Bluetooth 2.0).
• It works over the unlicensed frequency range of 2.4 GHz.
• The devices must be within a short range of less than 10 meters.
• It uses FHSS technology.
• It offers high resistance to electromagnetic interferences.
• Unlike the Infrared signals, it does not require a direct line of sight.
• Bluetooth devices consume very low power.
• Two or more Bluetooth computers form an ad-hoc wireless network.
(iv) Factors that affect wireless services
Wireless services use radio frequencies that travel through the atmosphere. There are several factors that may affect the speed, signal quality, and range of wireless signals. These include interference from other electrical devices, the type of antenna used, and other environmental factors. This section covers a brief discussion of these factors.
Interferences. Atmospheric interferences to wireless signals cannot be prevented, but they can certainly be reduced to achieve optimum performance. Some of the major causes of interference include the following:
• Physical objects such as buildings, trees, concrete and steel walls. These objects can either significantly reduce signals or even completely block them.
• Electromagnetic interference (EMI) generated by high-power electric lines, power transformers, heavy electrical machinery, fans, light fixtures, etc.
• Radio frequency interference (RFI) generated by other wireless equipment working in the same frequency ranges used by computer wireless devices. Examples of these types of equipment are wireless phones, wireless game controllers, or microwave ovens.
Type of antenna. The range of wireless signals depends on the type of antenna used for transmitting radio frequency signals. Selection of an antenna is a critical part of implementing a wireless network. Different shapes and sizes of antennas offer different signal levels. The strength of a wireless antenna (called its gain)is measured in decibels isotropic (denoted as dBi). An isotropic antenna sends signals of equal strength in all directions. A simple rule for calculating effective strength of an antenna is that every 3 dBi of gain almost doubles its output.
Omni-directional antennas send wireless signals in all directions. This type of antenna is useful when the coverage is required equally around the point of trans-mission. On the other hand, directional antennas transmit signals in one direction only. This helps send the entire output of the transmitting device in one direction, in which case, signals are more effectively transmitted.
Environmental factors. Environmental conditions, including weather, significantly affect the speed, range, and coverage of wireless signals. These factors can have a bad impact on wireless signals.
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