Capacity Analysis Key to Deploying VoWLAN Systems

vowlan_book_cover_sm.jpgThis tutorial is excerpted with permission from the book, Deploying Voice over Wireless LANs  (Cisco Press), written by Jim Geier.

Download Chapter 1, VoWLAN Applications and Benefits, free of charge. This chapter explains how to justify deploying voice over wireless LANs in various environments. 


Download Chapter 2, VoWLAN System Components, free of charge. This chapter describes the various components that comprise a voice over wireless LAN system.


Prior to determining the placement of access points, you must consider the coverage area (radio cell size) that each of the access points will produce. Traditionally, for data-only applications, companies install the access points configured in a manner that produces the largest radio cell size. This is done to minimize the number of access points needed and reduce costs for hardware. With relatively lightly loaded networks, which is generally the case with email, Web browsing, and wireless bar code scanners, this results in plenty of capacity to provide good performance to users. The influx of heavy web video users, however, may begin to strain wireless networks deployed for maximum signal coverage.


A wireless LAN supporting voice applications, however, usually requires greater capacity than typical data applications, such as email and Web browsing. As a result, you must take into account the maximum number of simultaneous voice devices that will need to operate on the network, which will be necessary to determine the density of access points necessary to support all of the voice (and data) traffic. In some cases, where there are few simultaneous calls being made on the network, you can install access points with maximum radio cell sizes in order to minimize costs. In other cases, especially when there are many voice users, it may be necessary to reduce the radio cell size of each access point in order to increase the capacity of the wireless network. This effectively results in having each access point service fewer users, which improves capacity and performance.


When analyzing capacity, refer to the requirements defined for the number and placement of voice users and their corresponding utilization. For example, a hospital may require a total of a hundred users be equipped with wireless IP phones. The challenge is to determine where these users will operate the phones and how many will be placing calls simultaneously from the same location. This is difficult to do because users are mobile, and it may not be well understood where the users will roam and how many will be active at any given time. Make your best estimate, however, based on interviews with potential users. Also, consider planning for some additional capacity to support emergency situations. For a hospital, for example, you may want to support disasters that my require supporting a large triage operation outside the hospital.


To calculate access point density, determine how many simultaneous voice users will be within a given area. An area to use as the basis for this calculation is the typical coverage area (radio cell size) of the access points that you plan to deploy. For 802.11b, this is area is approximately 10,000 to 15,000 square feet. You may find, for instance, that the majority of the 100 hospital users will be distributed within the hospital where it’s likely that a maximum of 3 voice users will be simultaneously making calls in each of the 10,000 square feet areas within the hospital. Based on this example, you could deploy the access points with maximum radio cell size. This would result in having approximately 3 simultaneous voice users associated with each access point. As discussed in Chapter 3, the number of calls that an 802.11b access point can support is 8 (802.11n can support much more, depending on the configuration). Thus, the use of maximum sized radio cells will provide good performance.


Now, assume that the hospital will also require the simultaneous use of 12 wireless IP phones from the corporate conference room, which is roughly 600 square feet in size. For this area, you’ll need to deploy at least two 802.11 access points to support the users. Each access point will service up to 8 users, so two will service up to 16. This is plenty of capacity for that area and even gives some room for expansion. Both access points would be installed in the conference room and set to different RF channels. In addition, the radio cell size each access point should be made smaller to just fit within the conference room in order to avoid users from outside the conference room from connecting to the access points.


If requirements call for a great deal of simultaneous voice users, then the radio cell size of all access points may need to be made smaller to increase the capacity to the required level. For example, a conference facility may predict that there could be as many as 20 users simultaneously placing calls with their wireless IP phones within every 10,000 square feet of the conference facility. If you’re deploying 802.11b, then you’ll need three access points covering each of the 10,000 square foot areas in order to produce enough capacity to support the calls. Each access point will service up to 8 simultaneous calls, so the total supported will be 24. In order to make this work effectively, you’ll need to reduce the radio cell size of each access point to cover approximately one-third of the 10,000 foot area.


Jim Geier provides independent consulting services and training to companies developing and deploying wireless networks for enterprises and municipalities. He is the author of a dozen books on wireless topics.

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