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About Wireless Networks
Wireless networks depend on how the spectrum works, is allocated, and used in cellular and mobile situations. Each network utilizes specific ranges of electromagnetic spectrum. The spectrum of airwaves is ordered by frequencies that increase with high energy, extremely short waves. When you listen to FM radio at 88.5 on your dial, the station is transmitting to your radio at 88.5 megahertz (MHz) or 88.5 million cycles per second. Cellular networks operate in one band around 800 MHz and another around 1900 MHz. The difference between traditional broadcast radio and the cellular network is that broadcast towers speak, cellular towers listen.
Cellular bands are licensed by the FCC to guarantee the owner exclusive use. An operator with a wireless license selects an air interface such as TDMA, CDMA, GSM, or W-CDMA that yields the most subscribers possible. An air interface is a radio transmission protocol for devices and base stations. Spectrum is expensive. Europeans have spent more than $100 billion for 20-year 3G licenses. On the other hand, some parts of the spectrum are unlicensed, such as the 2.4 GHz band known as Instrument Medical Scientific (ISM) and the 5 GHz band known as Unlicensed National Information Infrastructure (U-NII). Unlicensed spectrum opens the market to any transmitting device and is where LAN and PAN devices operate.
Three important principles of cellular networks are important to understand: cellular subdivision, handoff, and spectrum reuse. Cellular networks continually subdivide any given area, adding more low-power transceivers to reuse the spectrum better. This increases the number of subscribers in an area. A subscriber call must transfer or hand off as the person moves from one transmitter to the next, as shown in Figure 2, where a car moves from cell tower A1 to cell tower B1. The vacated "call space" in A can then be reused. A person moving in a wireless 802.11b office also needs handoff service.
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Figure 2 Cellular handoff.
Wireless networks are evolving. The telecommunication network has traditionally been circuit-switched to deliver only voice. It connects devices with phone numbers. Circuits guarantee a two-way connection and are used in two-way voice calls. In an ordinary cell phone call, a person enters a phone number and the network builds up a circuit to the calling party. The circuit is vigorously maintained and torn down at the end of the call. The newer Internet is packet-switched to deliver data and voice. It connects devices with IP addresses. To make an Internet connection, a person enters an Internet Protocol address (IP address). The network finds the host network and device that matches the address and responds by routing packets. Packet switching is efficient at sending data immediately, since there is no circuit setup time. Packets are used in wireless Ethernet, paging, and modern cellular networks such as i-mode.
Paging was at first a one-way wireless communication network. The first large citywide public paging service was launched in New York in 1961. The original pagers were one-way and produced a tone alert. If you got a page, you had to find a telephone and respond. The Netherlands started the first national public paging service in 1963. The first popular U.S. pager was the 1974 Motorola Pageboy, which beeped a tone. Soon tones gave way to numeric paging codes, and paging network engineering became the control channels for cellular phones.
Since all wireless networks use radios, the tower and the device both have antennas. Omnidirectional transceivers connect to antennas to radiate power and receive any signal in a sphere around the tower. All cellular WAN systems have a control channel part of the spectrum that works like a paging channel, probing and signaling devices about to make, take, or hand off a call. When devices are ready to talk, the tower transfers the control channel to the transmission channels. The two transmission channels for WAN phones are the send (uplink) and receive (downlink) channels. Once a device is located and transmission takes place, the continuous use of an omnidirectional control antenna wastes power. Directional transceivers can focus antenna power in a sector. New diversity antennas with two aerials and digital signal–processing hardware and software radically conserve energy efficiency and improve subscriber capacity.