Signals

The Transmission of wireless signal waves takes place in the electromagnetic (EM) spectrum. The carrier frequency of the data is expressed in cycles per second called hertz (Hz). Low frequency signals can travel for long distances through many obstacles but cannot carry a high bandwidth of date while high frequency signals waves can travel for shorter distances through few obstacles and carry a narrow bandwidth. “Waves that belong to the wireless spectrum (waves used for broadcasting, cellular phones, and satellite transmission) are neither visible nor audible, except by the receiver”. Regan (2004)

There are four categories of Wireless Signals and are as follows;

1. Radio: 10 KHz to 1 GHz is broken into various bands including AM, FM, and VHF bands. The Federal communications Commission (FCC) regulates the assignment of these frequencies. Frequencies for unregulated use 902 to 928 MHz, these include cordless phones and remote controls. Mobile companies’ use 2.4 GHz 5.72 to 5.85 GHz radio signals to communicate, an example would be a mobile service that exchanges information using two-way radios.

2. Microwave: Microwaves are used to link networks over long distances but the two microwave towers must have a line of sight between them. The frequency is usually 4-6GHz or 21-23GHz. Speed is often 1-10Mbps. The signal is normally encrypted for privacy. Two nodes may exist. Microwave signals are used as a backbone or backhaul carriers in cellular technology, communication with satellites, or a Microwave radio relay link for television and telephone service providers.

3. Satellite: A satellite orbits at 22,300 miles above the earth which is an altitude that will cause it to stay in a fixed position relative to the rotation of the earth using stations on the ground to send and receive signals from the satellite. The signal can have broadcast delays between 0.5 to 5 seconds due to the distances that are involved. The transmission frequency is 11to14GHz with transmission speeds in the range of 1to10Mbps. With communication being such an integral and critical part of the military, Satellite Communication is responsible for the lines of communication which is used to transmit messages over vast distances.

4. Infrared: The Infrared signal uses a visible range of light, with the transmission frequency of 100GHz to1000THz with the distance of transmission in 10's of meters and uses a light emitting diode (LED) or laser to transmit the signal. Infrared signals use point to point transmission and are limited to line of sight transmission because signals cannot travel through objects. The broadcast speed is 100Kbps to 16Mbps and the signal is dispersed so several units may receive the signal at the same time. The unit used to disperse the signal may be reflective material or a transmitter that amplifies and retransmits the signal. Normally the speed is limited to 1Mbps. Installation is easy and the cost is comparatively inexpensive for wireless networking, and it is difficult to eavesdrop on infrared transmissions. Wireless infrared communication systems can be characterized by the application for which they are designed, for example; building to building connections for high-speed network access or metropolitan or campus area networks may use the infrared technology.

Figure 1: The properties of the four signal types

Media		Frequency Range		Cost	Ease of installation		Capacity Range		Attenuation	Immunity for interference and signal capture
Radio	Low power single frequency		Entire RF, a high GHz is most common		Moderate depending on the equipment	Simple		<1 to 10 Mbps		High	Extremely low
	High power single frequency		Entire RF, a high GHz is most common		Moderately expensive	Difficult		<1 to 10 Mbps		Low	Extremely low
	Spread spectrum radio		Entire RF is 902 to 928 in the U.S. and 2.4 is the most common		Moderate depending on the equipment	Simple to moderate		2 to 6 Mbps		High	Moderate
Microwave	Terrestrial Microwave		Uses a low GHz, 4 to 6 or 21 to 23 is the most common		Moderate to high depending on the equipment	Difficult		<1to 10 Mbps		Variable	Low
Satellite	Satellite Microwave		Uses a low GHz, 11 to 14 GHz is the most common		High	Extremely difficult		<1to 10 Mbps		Variable	Low
Infrared	Point to Point Infrared		Uses 100 GHz to 1000 THz		Low to Moderate	Moderate to difficult		<1 to 16 Mbps		Variable	Moderate
	Broadcast Infrared		Uses 100 GHz to 1000 THz		low	Simple		<1 Mbps		High	Low
										Regan (2004)

Conclusion

Today, the use of wireless technology is widespread throughout the United States and growing. About 71 % of America's 108 million households own at least one cell phone, according to Forrester Research Inc. More than 25 million households now own laptop computers, according to Forrester. And 5.3 million households have wireless Internet access and is up from zero a couple of years ago. That is rapid growth. Ask (2008)