This first chapter from the book Linux Unwired by Roger Weeks et al. introduces radio waves, antennas, connections without wires, Bluetooth, cellular data and infrared. (O'Reilly Media, ISBN: 0596005830, 2004.)
Wireless networks use radio waves to move data without wires and they have been around in one form or another for decades. Teletype, or telex, systems were established worldwide in the early 1920s. These systems used copper lines to connect two or more teletype machines. Government investments in military radios lead to innovations in radio; teletype over radio (TOR), or radioteletype, replaced many teletype systems, particularly in third-world countries that lacked copper-wire infrastructures. In many parts of the world, TOR is still used as the primary communications medium for governments. TOR uses the high frequency (HF) radio band. We’ll cover the types of radio bands later in this chapter.
In 1970, Norm Abramson, a professor of engineering at the University of Hawaii, developed a radio-based communications system known as ALOHANET. This was the world’s first wireless packet-switched network, which allows multiple devices to transmit and receive data simultaneously. The research behind ALOHANETwas used by Bob Metcalfe to develop the Ethernet standard for wired networking.
Presently, there are many types of wireless networks in use around the world. The 802.11 protocol set, popularly known as Wi-Fi, includes wireless network standards that allow data transmission up to a theoretical 54 Mbps. The Global Positioning System (GPS) uses a wireless connection from a receiver to a series of satellites to fix a location precisely on the planet. There are several wireless networking standards in the mobile-phone world, including General Packet Radio Service (GPRS) and Code Division Multiple Access (CDMA) 1xRTT (1x Radio Transmission Technology). Subsequent chapters will discuss all of these in detail.
Radio waves are created when electrically charged particles accelerate with a frequency that lies in the radio frequency (RF) portion of the electromagnetic spectrum. Other emissions that fall outside of the RF spectrum include X-rays, gamma rays, and infrared and ultraviolet light. When a radio wave passes a copper wire or another electrically sensitive device, it produces a moving electric charge, or voltage, which can be transformed into an audio or data signal.
Radio waves can be depicted mathematically as a sinusoidal curve, as shown in Figure 1-1.
Figure 1-1. A sine wave representing a radio wave
The distance covered by a complete sine wave (a cycle) is known as the wavelength. The height of the wave is called the amplitude. The number of cycles made in a second is known as the frequency. Frequency is measured in Hertz (Hz), also known as cycles per second. So, a 1 Hz signal makes a full cycle once per second. You should be familiar with this unit of measurement: if your new computer’s CPU operates at 2 GHz, the internal clock of your CPU generates signals roughly at two billion cycles per second.
Note that frequency is inversely proportional to the wavelength: the longer the wavelength, the lower the frequency; the shorter the wavelength, the higher the frequency. The wavelength of a 1 Hz signal is about 30 billion centimeters, which is the distance that light travels in one second. A 1 MHz signal has a wavelength of 300 meters.
Radio Frequency Spectrum
To regulate the use of the various radio frequencies, the Federal Communications Commission (FCC) in the United States determines the allocation of frequencies for various uses. Table 1-1 shows some of the bands defined by the FCC (see http://www.fcc.gov/oet/spectrum/table/fcctable.pdf).
10 kHz to 30 kHz
Very Low Frequency (VLF)
30 kHz to 300 kHz
Low Frequency (LF)
300 kHz to 3 MHz
Medium Frequency (MF)
3 MHz to 30 MHz
High Frequency (HF)
30 MHz to 328.6 MHz
Very High Frequency (VHF)
328.6 MHz to 2.9 GHz
Ultra High Frequency (UHF)
2.9 GHz to 30 GHz
Super High Frequency (SHF)
Table 1-1.Range of frequencies defined for the various bands.