Wireless WAN Components
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.
Wireless WAN User Devices
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.
Radio NICs
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.
note
 |
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
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.
Antennae
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.
note
|
A snow sled disk is the shape of a parabolic reflector. |
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