Building wireless bridges
Monday, April 17, 2006 10:40 AM
In this second instalment of wireless networks, George Ou looks at wireless bridging in greater detail.
Wireless bridges allow buildings to connect wirelessly when wiring is too expensive or a second redundant connection is needed as a backup for a wired connection.
Figure 1
802.11 devices are commonly used for this application as well as optical line-of-sight Wireless bridges. 802.11 radio based solutions are usually much cheaper and don't require a line-of-sight between the antennas to operate, but are significantly slower than optical solutions. 802.11 solutions typically operate in the 5 to 30 mbps range while optical solutions operate in the 100 to 1000 mbps range. Both types of bridges can operate beyond 10 miles although the radio based solution is more likely to reach these distances because it doesn't require line-of-sight. The down side to radio based solutions is the lower speed and the possibility of RF (radio frequency) interference while optical solutions aren't affected by RF. The down side of optical solutions is the higher entry price and the fact that line-of-sight isn't always possible between two locations.
Figure 2
Figure 2 illustrates a typical scenario where a Wireless Bridge is used to wirelessly extend an Ethernet network. There are many types of 802.11 bridges and some of them use proprietary methods of interlinking and may not always offer the best compatibility or security. A preferable solution is to use a simple Wireless Bridge that can connect to any common infrastructure-type Access Point as a regular WPA client to provide a secured wireless connection. Once bridged, an additional Ethernet Switch can be used to expand the port capacity and link multiple Wired Ethernet computers to the LAN over the wireless bridge. This is a cheaper option than outfitting multiple fixed position Desktop PCs with wireless Ethernet cards and having to configure them.
Medium-sized Wireless LANs
Figure 3
Medium-sized businesses have traditionally used a simple design where they simply put up multiple Access Points throughout their facilities where they needed Wireless coverage. This particular approach is probably the most common because it has a low cost of entry although it becomes difficult to manage once the number of Access Points gets beyond a hand full. Most of these types of Wireless LANs allow you to roam from Access Point to Access Point because they're configured on the same Ethernet subnet and SSID.
From a management standpoint, each Access Point is managed individually as well as the managed port that it's connected to. In more advanced implementations where multiple Virtual SSIDs are supported, VLAN trunking is used to connect the Access Point to multiple subnets over a single Ethernet connection to a managed switch port. The Switch in this case needs to be configured to support multiple VLANs over a single port. Although it's possible to use a template to configure multiple Access Points, it can still become difficult to manage a large number of Access Points when firmwares and configurations need updating.
From a security standpoint, each Access Point must be configured to handle its own Access Control and Authentication. RADIUS servers make this task easier because the Access Points can delegate Access Control and Authentication to the centralized RADIUS servers which in turn can be tied in to a central user database such as Windows Active Directory. But even so, a RADIUS relationship still needs to be built between each Access Point and each RADIUS server which can be complex if the number of Access Points are high.
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