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Wireless LAN Technology Overview
The most popular protocol for wireless LAN technology today is by far the 802.11 series, commonly known as Wi-Fi. The 802.11 wireless standards are basically an extension of the Ethernet protocol, which is why it interoperates so well with wired Ethernet networks. It uses the frequencies of 2.4GHz for 802.11b and 802.11g and 5GHz for 802.11a to broadcast data signals. These frequencies are general-use spectrum, so you don't have to apply for a license from the FCC to use them. The downside of this is that other consumer devices can use these wavelengths too. Some cordless phones and microwaves are also on the 2.4GHz band, so if you have these devices or other Wi-Fi networks in your area, you may encounter some interference.
This wavelength is perfect for the short range that Wi-Fi is intended for. Its design parameters allow for about 150 feet indoors and over 800 feet outdoors under normal conditions. However, with a high-power antenna and line of sight, you can get up to a 20-mile range, which makes it attractive for office-to-office communications within a city (this assumes you are not in very mountainous terrain and you have access to a rooftop at least several floors up). Table 10.1 describes the four flavors of the 802.11 wireless standard that have emerged.
A Wi-Fi wireless network can operate in one of two modes. Ad-hoc mode allows you to directly connect two nodes together. This is useful if you want to connect some PCs together and don't need access to a LAN or to the Internet. Infrastructure mode lets you set up a base station, known as an access point (AP), and connect it to your LAN. All of the wireless nodes connect to the LAN through this point. This is the most common configuration in corporate networks, as it allows the administrator to control wireless access at one point. Each wireless access point and card has a number assigned to it called a Basic Station System ID (BSSID). This is the MAC address for the access point's wireless side. The access point also has a Station Set Identifier (SSID), which defines the name of the wireless network that all the nodes associate with. This name is not necessarily unique to that access point. In fact, most manufacturers assign a default SSID to APs so they are usable right out of the box. The access point's SSID is needed to connect to the network. Some base stations have additional functionality, including routers and built-in DHCP servers. There are even some integrated units that act as a wireless access point, firewall, and router for home and small business users.
You set up a wireless network node by installing a wireless network interface card (NIC) in a computer. A wireless NIC comes in several forms: It can be a card that goes in a PC slot, a PCMCIA card, an external USB device, and now even a compact flash format for the smaller slots in handheld computers. An 802.11 wireless network in infrastructure mode has an access point that acts as your bridge between the wired Ethernet LAN and one or more wireless endpoints. The access point sends out "beacon" broadcasts frequently to let any wireless node in the area know that it is there. The beacon broadcasts act like a lighthouse, inviting any wireless nodes in the area to log on. These beacon signals are part of the problem with Wi-Fi. It is impossible turn off these signals completely, which makes it hard to hide the fact that you have a wireless network in your office. Anyone with a wireless card can at least see your beacon signals if they are in range, although some sets allow you to limit the amount of information that goes out in these broadcasts.
These signals contain basic information about the wireless access point, usually including its SSID (see Figure 10.3). If the network isn't using any encryption or other protections, then this is all that is required for someone to access to the network. However, even on an encrypted wireless network, the SSID is often transmitted in the clear and the encrypted packets may still be sniffed out of the air and subject to cracking attempts.
Figure 10.3. Wireless Network Operation
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