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11/25/2009

Wireless Mesh

One of the more interesting ways to use Wireless LAN Controllers is a network design called a "Wireless Mesh". This essentially means that Access Points can simultaneously support wireless end clients and wireless backhaul to the wired network.

 

Why would this be useful? Wireless networks are often used in environments where it is difficult or impractical to construct a traditional wired network. Typical examples include warehouses and factory floors as well as older buildings with solid stone or brick walls where running physical cables would either be unsightly or dangerous. Another example is providing outdoor wireless networking far from buildings, in a park for example, or across areas where you cannot secure a cabling right of way.

If these difficult spaces are geographically small you can put Access Points around the perimeter to provide connectivity to devices inside the area. But in many cases these spaces are too large to allow full coverage from the perimeter. You need to have Access Points in the middle of the area. In this case, Wireless Mesh would allow these Access Points in inaccessible spaces to use their radio transmitters to carry user traffic back to the rest of the network.

Another interesting potential use for a Wireless Mesh design is a temporary wireless network. For example, Wireless Mesh would be a natural to the problem of deploying access points around an auditorium for a conference in which participants required network access.

AWPP

Cisco has developed a proprietary protocol called Adaptive Wireless Path Protocol (AWPP), which allows Access Points to dynamically discover the best wireless path back to one or more designated "root" Access Points that connect to the main network.

This protocol works well with Wireless LAN Controllers, because the Controller can centrally coordinate the bridging paths between Access Points. And, because Cisco's Aironet Access Points all support 802.11a/b/g, you can take advantage of the different frequency bands of 802.11a and 802.11b/g to separate the client traffic from the backhaul traffic.

Also the Wireless LAN Controller can dynamically reconfigure the RF properties of each Access Point to minimize cross-talk problems. And, in terms of the user experience, Wireless LAN Controllers allow for seamless roaming throughout the coverage area, which is often useful in these scenarios.

The biggest advantage to AWPP is that it can dynamically select the best backhaul path. So if one of your Access Points goes down or gets congested, or if part of your network suddenly starts to suffer from RF noise problems because somebody turned on the microwave oven, the backhaul traffic will automatically divert around the bottleneck.

Caveats

The two main things to be aware of when building a Wireless Mesh are the bandwidth and latency limitations.

Bandwidth is always going to be an issue with Wireless Mesh. If one Access Point is acting as a bridgehead connecting several others to the wired network, it will quickly run out of wireless bandwidth if all of those downstream Access Points send a burst of traffic at once. This is unfortunately somewhat worse with LWAPP because all client 802.11 packets must traverse the network back to a Wireless LAN Controller located in the wired part of your network. So it is useful to have several Access Points around the perimeter of your wireless domain that can perform this function.

The second issue is latency. The slower a link is the greater the so-called serialization delay. This means that, because you have to send that 1500 byte packet one bit at a time, it will take almost twice as long to send it across a 54Mbps 802.11g link as a 100Mbps Fast Ethernet link. If you then have to relay that packet through several 802.11a hops, each operating at 25Mbps before it finally hits the wired network, it's clearly going to take a lot longer to cross the network. Add to this the fact that at each hop along the way the devices must receive the entire packet before retransmitting it, and possibly put it into a buffer behind other packets, and it's easy to see how latency can increase significantly.

Because of both of these issues, CNG doesn't believe that Wireless Mesh is appropriate for real-time applications such as roaming 802.11 telephone handsets. But it is ideal for low speed and non-latency sensitive data applications.

Conclusion

The net result is that you can build a wireless network anywhere, even if you can't get cable Ethernet connectivity to the Access Points. In situations where it's simply impossible or impractical to run Ethernet cables, Wireless Mesh means that you can still deploy a reliable network.