Wireless WANs satisfy both mobile and stationary applications. Components, therefore, vary depending on the technology and configuration of the wireless WAN. A satellite-based wireless WAN, for example, has different components than a cellular-based system.
Users of wireless WANs operate small and portable devices. This is because access to the network is available over much wider areas, and the users must carry their devices with them. For example, a business traveler can easily carry a small PDA or mobile phone and access e-mail while riding in a taxi from the airport to a hotel. Figure 7-1 illustrates the types of user devices most common to wireless WANs.
The use of wireless WANs to network stationary PCs is not common; however, some applications do exist. The need to set up a portable point-of-sale (POS) register at a remote area, such as a makeshift concert site, could prompt the use of a wireless WAN. A vendor selling t-shirts can process credit cards over the wireless WAN to an Internet-based processing center.
Some mobile phones have integrated wireless WAN radios. Telecommunication companies, such as Verizon and Sprint, offer wireless WAN connections with voice services. The problem, however, is that several different types of wireless WANs exist, making it a challenge for users to find a mobile phone that interfaces with the type of wireless WAN they want to use.
To interface a laptop or PDA to a wireless WAN, you need to purchase an applicable wireless WAN radio NIC. Figure 7-2 shows a radio NIC available for wireless WANs. These cards can look like ones for wireless PANs and LANs; however, the card might contain one of several incompatible technologies.
Along with the purchase of hardware, the vendor generally sells access to the service that the card is designed to interface with. Telecommunications companies spend significant amounts of money to secure frequency spectrum and install hardware over vast areas. As a result, all wireless WAN providers charge for the service. This is different than wireless LAN hotspots, where many hotels and airports are finding it advantageous to offer free Internet access to wireless LAN users. This is made possible because wireless LAN deployment doesn't require much capital.
Be certain to purchase a wireless WAN radio NIC that interfaces with a type of wireless WAN that's available in the areas you need. Consider the coverage area, just as you would with a mobile phone, before deciding on which NIC and service to use.
In the case of using a satellite-based wireless WAN, the purchase of satellite terminal hardware becomes necessary. Mobile versions of satellite terminals have a small-parabolic (dish) reflector antenna and electronics that can fit within a medium-sized briefcase. This type of wireless WAN interface is relatively expensive. Satellite terminals are also available for more permanent installations, such as home or recreational vehicles.
Base stations for wireless WANs typically appear outdoors. In fact, the familiar cell towers shown in Figure 7-3 are seen scattered around cities and country areas. Similar to wireless LANs, these base stations rely on wires to connect to a distribution system that provides switching and an interface to the Internet. In most cases, the towers reside outdoors to provide maximum coverage. However, some large public facilities?such as shopping malls and airports? install cellular base stations indoors to handle larger numbers of subscribers.
Another form of wireless WAN base station is a satellite in orbit, which is actually a repeater in the sky. On the ground, a user aims a dish antenna at the satellite, and the satellite receives the signal and retransmits the signal back to an Earth station. (See Figure 7-4.) A strong advantage of this approach is that less infrastructure is necessary on the ground. The problem, however, is that operators must spend millions of dollars to establish a satellite system for computer traffic. This equates to expensive service charges for users.
Wireless WAN base stations and user devices use a variety of antennae depending on the type of wireless LAN. For cellular systems, the antenna on the user device is generally omnidirectional. Cell towers generally have multiple directional antennae, however, that cover vast distances.
A satellite user has a dish antenna with characteristics as shown in Figure 7-5. The transceiver, located at the focal point, transmits and receives the radio frequency (RF) signal. For example, the RF signal leaves the transmitter side of the transceiver, and the shape of the dish focuses the RF signals in one direction.
No matter how the RF signal hits the dish, the signals leave the dish in the same direction because of its parabolic shape. In fact, the opposite is also true. When the dish receives RF signals, the shape of the dish focuses the RF signals at the receiver, which is at the focal point.
A snow sled disk is the shape of a parabolic reflector.