Calculating bandwidth per application for access point coverage

Q: What’s the best way to calculate the amount of bandwidth that our enterprise applications will need for each access point? – Harvey, Los Angeles

Dan Simone, Trapeze Networks:

It sounds like you’re asking about capacity planning for your WLAN, a critical issue to consider since wireless bandwidth is both shared and lower than wired bandwidth. Planning tools from some wireless vendors assist in this process, but described below is the manual (albeit complex) method.

To understand the amount of WLAN bandwidth you’ll need, which determines the number and transmission speeds of the access points you’ll use, you need to characterize the applications in question. How much bandwidth will they require to serve users well? Will all the users in a given WLAN coverage area need access to the same applications? If you don’t already know the bandwidth consumption for a given set of applications, you can use a wired packet analyzer to provide some application performance analysis.

Once you know the bandwidth requirements for a set of applications, multiply that bandwidth by the number of users in an area to be covered by the WLAN. Now you’ll have the total amount of bandwidth that you need the WLAN to service. You may wish to reduce this amount by an “over-subscription” factor, since it’s unlikely that all users will require the maximum bandwidth at the same time.

Now comes the tricky part – because distance and the RF attenuation factors in your building environment matters. In a perfect world, each data association rate has a perfect circle of coverage and isn’t impacted by walls and doors. You will need to estimate the circle size for various target data rates. The circle of coverage for a particular data rate of an access point decreases as the data rate increases, thus lowering the number of users that are serviced at that data rate.

When doing this manually, it’s easiest to pick an association data rate and, knowing its coverage area, divide that access point coverage area into the total coverage area to determine the number of access points needed for coverage. Now check that this access point count will actually give you sufficient bandwidth. Take the number of access points needed for coverage and multiply by half of the association data rate. This is the total throughput capacity of the system. We multiply by half of the association data rate because protocol overhead in 802.11 cuts actual data throughput in half.

For example, an 802.11b AP and single client with the maximum 11M bit/sec association will get an average of 5.5M bit/sec of throughput. On 54M bit/sec 802.11a networks, expect about 25 to 30M bit/sec. On 802.11g networks, throughput can vary widely in the presence of 802.11b clients, so it’s safest to use 802.11b bandwidth. The throughput capacity number needs to be equal or higher than the total bandwidth requirement number calculated previously. If it isn’t, increase the number of access points by raising your association data rate – but don’t forget that this also reduces an access point’s coverage area.

This manual method makes several simplifying assumptions. The worst assumption is not accounting for the RF attenuation factors in your particular building. Fortunately, RF capacity planning is already available and made automatic in some WLAN system planning management tools. The better planning tools assign these factors based upon your floor plan and calculate the number, placement, and channel and power settings for access points based on your input of user bandwidth requirements. Another alternative is to hire a firm to perform an RF site survey, but be sure they are asking the right questions regarding your bandwidth requirements.

Israel Drori, Legra Systems:

Build profiles of current usage per user, then profile their locations. Then it’s a simple database query to figure out how to provision your bandwidth. There are also packaged applications to help you plan your bandwidth usage based on the number of users, number of access points, etc. As you calculate bandwidth usage now, however, remember that your system should be able to automatically adjust to changes dynamically. You should be able to move users freely anywhere in the building. They might get a lower megabyte per second rate, but the user capacity should remain the same.