Wireless Bandwidth

One of the biggest problems with Wireless LAN technology is that the actual bandwidth available is much lower than the rated one. Plus, it's shared among users, so a single user can hog up all the bandwidth unless some control measures are put in place. 

There is also an inverse relation between bandwidth and the distance. The relation can be expressed mathematically as: transmitted power (Pt) = signal strength(s) X 4 X 3.14 X distance X distance. The bandwidth decreases with the increase in distance. The real issue is to balance the bandwidth between users at varying distance. 

A WLAN uses multiple access points to cover larger distances and make adequate bandwidth available. However adding multiple access points does not increase the overall bandwidth, rather it provides better fault tolerance. In order to balance the bandwidth two options can be followed. A user can be associated with a particular access point rather than connecting to an access point with maximum signal strength. 

The second option is to configure access points on different frequency channels. Almost all wireless managers, which accompany access points will support the second option, but only a few will support the first one. Some enterprise access points can also limit the number of users that can connect to it. It is better therefore to select an access point, which offers limiting users and a wireless manager that supports load balancing. 

We will show manual calculation that will help to determine two things: number of users and the bandwidth for each user. For instance, you have an 802.11g access point, which has a data speed of 54 Mbps. You can calculate the number of users that the access point can support. Assume that the wired LAN can support maximum throughput of 40 Mbps and desired minimum throughput per wireless user is 3 Mbps. The number of users that can be supported are 40 Mbps/3 Mbps = 13 users. However, the available bandwidth to 13 users depends upon the type of applications, radio interference (walls, doors and ceiling), antennae power, etc.

To calculate maximum bandwidth that each user can have, assume that the access point supports 20 users. Maximum bandwidth per user therefore is (54 Mbps/2)/ 20 users. This is a commonly used formula. The calculation comes out to 1.3 Mbps bandwidth. The total bandwidth thus required is 1.3 X 20 = 26 Mbps. The next step is to place the access point. A site survey will help you find the access point's range, obstruction encountered, etc. 

A good idea is to place access points on walls where minimum obstruction and maximum throughput is available. A number of commercial WLAN analyzers such as Wild Packets and Airmagnet are

there. There are also free WLAN analyzers such as Boingo and Kismet. 

With inputs from Parag Arora,CICSO