VoIP Environment with Many Subscribers

Unanswered Question
Oct 8th, 2009


I have the following environment setup:

- 3 Aironet AP (AIR-LAP 1242AG)

- 1 WLAN controller (WLC 2106)

- VoIP device, e.g. IP phone, beltpack

- Security policies [WPA + WPA2][Auth(PSK)]

Please advise:

1. What is the maximum number of subscribers can be supported per AP in one BSS?

2. Why is it limited to that number of subscribers?

3. What is the best choice of IEEE standards for VoIP (IEEE 802.11a, 802.11b or 802.11g)?

4. If we have 1000 subscribers in one big studio in one IP subnet (one WLC), please advise how many Cisco APs must be allocated?

5. What are the things to consider to avoid interference, voice delay and the best way to assign or divide the channel allocation for each AP?

Thank you.

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jeromehenry_2 Thu, 10/08/2009 - 09:37


There is no magic answer to your question without a proper site survey and without knowing your clients specifications, but here are some common values (provided that a site survey was conducted properly) if you were using Cisco 7921:

1. Up to 27 active subscribers in ideal conditions, but as real world is not ideal conditions, aiming at 14 clients on 802.11g, or 7/8 clients on 802.11b/g, 20 clients on 802.11a. This is again IF you did a good site survey and respected the recommended practices. This document will help you:


2. The reasons for this limitation is the wireless space. Each user needs to send a certain number of voice packets per second, so you don't have space for more than a certain number of users without facing the risk of too many packets collisions, retries, and their corresponding delay and jitter issues.

3. 802.11a is usually quieter (less sources of interference) than 802.11/b/g, and is usually preferred for voice deployment... but it also depends on you clients! Are they 802.11a ? b? g? This will dictate what you can do.

4. 1000 users in one subnet? This looks very high to me... this is one broadcast domain, I would split them into smaller subnets... but on the wireless side, if you do the math (and if as usual you do a clean site survey, which will also need to take into account the expected density of users in each area), you would deploy somewhere around 50 APs if you deploy in 802.11a (but this also depends very much on how "big" your studio is, and how many APs you need to cover the physical area).

5. To avoid interferences, many things can be done, the very first one being a site survey. Then, there are usually more channels and less interferences in 802.11a. Then read the document referenced above, there are many things you need to do and understand to do a proper VoWLAN deployment...

Hope it helps



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