13.6. Metro-Area Links
The use of MAN (metropolitan area network) technologies is increasing, as the quality and reliability of radio and wireless optical systems is rising , and the need for campus-scale networking is driving adoption of 802.11 radio Ethernet. Free space optics (FSO) is a wireless bridging technology that uses infrared and laser band transmissions. These technologies are described in Chapter 4. WiMax is a newer radio bridging solution that deals with many of the shortcomings of 802.11.
When it comes to survivability in a voice scenario, these systems exhibit the same characteristics as they do in traditional networking. 802.11 is a data link layer technology family, so it has no QoS capabilities. FSO is a physical layer technology, and it has the same shortcoming. This makes it hard to tell when an 802.11 or FSO link is at capacity in order to make a prioritization decision or bandwidth reservation. Consequently, these notions just don't exist in wireless Ethernet.
Failing over from one 802.11 link to another is also complicated. Using 802.11 (or FSO) by itself provides no way of knowing when an automatic failover is necessary, because there are no built-in alert mechanisms that tell you when a link is down. One possible solution is to place routers on either end of an 802.11 link pair, and then use load-splitting or dynamic routing to deal with one of the links going down. Beware of the potential for substantial jitter on 802.11 links, too. For a much more elegant (and detailed) discussion of wireless networking, check out O'Reilly's 802.11 Wireless Networks: The Definitive Guide .