Wireless technology that is used in small computing devices and mobile communications devices is the same radio technology Guglielmo Marconi used to provide an alternative communication means to the telegraph and the telephone. Radio technology is based on the wave phenomenon. A wave is a characteristic of vibrating molecules, which you see whenever you move a knife up and down in the still water of a dishpan (Figure 2-1). The force of the knife against the surface of the water causes water molecules to vibrate and form a wave along the surface of the water. The force used to propel the knife determines the wave height. The greater the force, the higher the wave and the greater the distance the wave travels across the surface of the water. The number of times the knife is moved up and down in the water determines the frequency of the wave. Each time the knife is plunged into the water another wave is generated, causing a rippling effect across the water's surface.
Waves are measured in two ways: by the wave height and by the wave frequency. The wave height is referred to as the wave's amplitude, and the frequency of the wave is simply called frequency, which is measured as the number of waves per second (Figure 2-2). The frequency of a wave causes the wave to take on specific characteristics. For example, a low-frequency wave called a sound wave produces a frequency that can be heard by humans. Sound waves travel a short distance through air. A higher-frequency wave called a radio wave cannot be heard but can travel long distances in all directions and through solid objects. And even higher frequencies called light waves take on other characteristics. Light waves can be seen, travel a long distance in a limited direction, and cannot penetrate solid objects. Waves are grouped according to frequencies that have similar characteristics in the electromagnetic spectrum (Figure 2-3). For example, there is an audio spectrum, a radio spectrum, and a light spectrum. There are also subgroups within each spectrum, each of which has a variation of the characteristics of the spectrum. The radio spectrum has divisions for television, microwave, and X-ray frequencies. The light spectrum has divisions for infrared light, visible light, and ultraviolet light. Many small computing devices and mobile communications devices use radio waves and light waves to transmit and receive information. Radio waves are used by cellular telephones, wireless modems, and wireless personal digital assistants (PDAs) for communication. Infrared light waves are used by PDAs to exchange information between PDAs and laptop/desktop computers and among other PDAs.
Radio signals are transmitted in the frequency range from 10 kilohertz to 300,000 megahertz. A hertz is one wave per second, kilohertz is 1,000 waves per second, and a megahertz is a million waves per second.
Radio transmission consists of two components. These are a transmitter and a receiver, both of which must be tuned to the same frequency. A transmitter broadcasts a steady wave called a carrier signal that does not contain any information (Figure 2-4). Conceptually, you can think of a telephone dial tone as a carrier signal. A carrier signal has two purposes. First, the carrier signal establishes a communications channel with the receiver (Figure 2-5). The receiver knows the channel is open when the carrier signal is detected. The carrier signal also serves as the wave that is encoded with information during transmission. A radio transmitter encodes patterns of sound waves detected by a microphone by modifying the carrier signal wave (Figure 2-6). The receiver decodes the pattern from the carrier wave and translates the pattern into electrical current that directs a speaker to regenerate the sound waves captured by the microphone attached to the transmitter.
Limitations of Radio Transmissions
The distance a radio signal travels is based on the amount of energy used to transmit the radio wave. This is similar to the energy used to plunge the knife into the dishpan of water. Using a relatively small amount of energy causes the wave to barely reach the side of the dishpan. However, plunging the knife into the dishpan with force causes the wave to overflow the sides of the dishpan.
Radio waves are measured in watts. A radio signal transmitted at 50 megawatts travels twice the distance a 25-megawatt radio signal travels. A radio signal gradually loses power the farther it travels away from the transmitter. Radio engineers extend the range of transmission by using a repeater. A repeater (Figure 2-7) is both a radio receiver and radio transmitter, also known as a transceiver. A repeater receives a radio signal and then retransmits the signal, thereby increasing the distance the signal travels. Retransmission introduces new energy to power the signal for longer distances.
Radio transmitters send one message at a time over a communications channel. This is similar to sending one telephone call at a time over a telephone line. Each telephone call is placed in a queue while waiting for the current telephone call to end. As you can imagine, telephone calls could easily back up whenever there are more calls than there are empty telephone lines.
Digital radio networks use packet switching technology to transmit multiple messages simultaneously over a communications channel. Each message is divided into small pieces and placed in an electronic envelope called a packet (Figure 2-8). A packet contains information that identifies the sender and the receiver, a digitized portion of the message, the sequence number of the packet, and error-checking information. To reassemble packets, the receiver uses the packet sequence number. A transmitter continuously sends packets from multiple messages over a communications channel.
Packet switching technology is more efficient than traditional transmission methods because packet switching utilizes pauses in a transmission to send packets. A transmission pause is caused when a message isn't ready for transmission. This is similar to a pause in a telephone conversation.
Software running on the transmitter manages multiple outgoing messages to assure that each message is divided and placed into packets and the packets are transmitted.
Software running on the receiver manages incoming packets, reconstructs packets into original messages, and forwards messages to the appropriate application software for future processing.
Radio Data Networks
Radio transmissions are commonly used to broadcast analog voice information on radio waves that travel 360 degrees over the air and through many physical obstructions. However, radio technology is also used to transmit digital information on those same waves.
Information is traditionally encoded as variations of an aspect of the wave. Encoding is achieved by modifying the amplitude of the wave, known as amplitude modulation (AM), or modifying the frequency of the wave, called frequency modulation (FM). Encoding uses many values to represent information using AM and FM.
Hundreds of thousands of radio waves are simultaneously and independently transmitted. Sometimes a radio receiver picks up an erroneous radio signal while tuned to its primary frequency. The erroneous radio signal is known as interference and can disrupt the accurate decoding of the transmitted information.
Today information is digitally encoded using binary values to represent information transmitted on radio waves. Digitizing information enables receivers to accurately decode transmitted information because the degree of uncertainty in the encoded information is far less than experienced in analog encoded information.
Both an analog signal and a digital signal are waves. They differ by the way information is encoded into the signal. Information is represented in an analog signal as many values. The receiver must determine whether each value is a component of the signal or is interference. The same information is represented in a digital signal as one of two discrete binary values. The receiver ignores a signal whose value is not a binary value.
Furthermore, error-checking software in the receiver determines whether an erroneous digital signal is received.
Radio transmitters, repeaters, and receivers are organized to form a radio network that extends transmissions over great distances. Radio networks are scalable because repeaters are placed in the network to increase the distance that the original transmission travels.
There are three types of wireless radio networks: low-power single frequency, high power single frequency, and spread spectrum. Low-power single frequency covers an area of 30 meters, which is about the area of a small building such as a warehouse or a stock exchange trading floor. A high-power single frequency wireless radio network can cover a metropolitan area.
Both low-power single frequency and high-power single frequency radio networks are exposed to the same security risk. Anyone tuned into the radio frequency receives the transmitted signal. Therefore, all transmissions must be encrypted to hinder eavesdropping on the signal.
A spread-spectrum wireless radio network uses multiple frequencies to transmit a signal using either direct sequence modulation or frequency hopping. Direct sequence modulation breaks down information into parts and then simultaneously transmits each part over a different frequency. The receiver must tune to each frequency to receive each part, then reassemble parts into the full message. Frequency hopping transmits information rotating among a set of frequencies. The receiver must be tuned to each frequency according to the transmission rotation.
Most radio frequencies are controlled by the Federal Communications Commission and require an FCC license before a wireless radio network can be established.
Microwave is a sub spectrum of the radio spectrum and has many characteristics of radio waves discussed previously in this chapter. However, microwaves travel in one unobstructed direction. Any obstruction, such as a mountain or building, disrupts microwave transmission.
There are two kinds of microwave networks: terrestrial and satellites. Terrestrial microwave networks transmit a microwave signal over a terrain, such as buildings in an office complex. Satellite microwave networks transmit a microwave signal between a ground station and orbiting satellites and among orbiting satellites (Figure 2-9).
Earth-to-satellite transmissions are slower than terrestrial microwave transmissions, which causes unnatural pauses to occur in the transmission. This is noticeable during a live international television broadcast when a pause occurs between the times a television news anchor questions a reporter and the reporter's response. Therefore, satellite microwave transmission may not be suitable for real-time two-way communications where nearly instantaneous transmission is expected.
A satellite is an orbiting repeater that receives a microwave transmission from an earth station or from other satellites, and then retransmits the signal to a microwave receiver located on the ground or in another satellite. The first generation of satellites used for the military were stationed in geosynchronous orbit at a fixed location 22,300 miles above the surface of the earth. However, the geosynchronous orbit hampers real-time transmission because of the signal delay between earth and the satellite, which makes geosynchronous orbiting satellites unacceptable for commercial two-way real-time communication.
A newer breed of satellite technology, called Low Earth Orbiting Satellite (LEOS), overcame the communications delay by positioning satellites lower than geosynchronous orbit-between 435 miles and 1,500 miles above the earth. LEOS eliminated delays in communication, but introduced two new problems. First, LEOS covers a smaller area of the earth, and therefore more satellites are required to cover the same ground area as covered by geosynchronous satellites. The other problem is the orbital speed. LEOS travels faster than the earth's rotation and requires ground stations to locate LEOS before beginning transmission. Geosynchronous satellites always remain in the same position above the ground station.
In an effort to compromise between LEOS and geosynchronous satellites, another breed of satellites called the Middle Earth Orbit (MEO) was developed. MEO orbits between LEOS and geosynchronous satellites-6,000 to 13,000 miles-and thus has less delay than geosynchronous satellites and poses less difficulty than LEOS for ground stations to locate.
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