Dot11 Guru

802.11, Design

A Thought Provoking VoWiFi Talk


@Mac_WiFi gave a great talk at #WLPC this year, which has led to some great discussion around our office.  Here´s the link if you haven´t already viewed.

As a quick recap in terms of design, the take away messages are:

  1. Coverage, make sure you meet the vendor RSSI requirements.
  2. Data Rates, If you set a high mandatory data rate, set some of the lower as supported.
  3. Channel Widths, 20MHz is best for voice.
  4. Larger Cells, don´t keep the cells to small to avoid excessive roaming.
  5. Channel Plan, try and minimise your channel plan to absolute maximum 8 channels.

Channel Plan

To minimise the amount of time a client spends off channel probing for other access points, it was recommended to minimise the amount of channels used in your deployment.  Also avoid DFS channels for voice networks to further reduce the time a client is off channel.

Two design scenarios

This in consideration, two scenarios come to mind when using seperate voice clients (eg. not skype running on a PC).

Scenario 1

Voice is important, very important, but it´s not mission critical.  The other applications running on the network are not high bandwidth applications.  For the company I work for, this could be a factory with VoWiFi clients and RF guns.  RF guns are more important than the voice clients.

In this scenario it would make sense to have access points grouped.  UNII1 APs have the voice SSID, and the data SSID.  The other APs have only the data SSID.

Scenario 2

The second scenario is when voice is mission critical, and there are high bandwidth applications in use.  An example might be a customer call center that has gone all-wireless (hopefully not to be seen anytime soon..).

Here it might make sense to use access points which have dual 5GHz radios.  UNII1 Channels are for 5GHz Radio A, and only voice SSID is available.  UNII2 & 3 Channels are for 5GHz Radio B, and only data SSID is available.

This would mean that the clients on the data SSID are able to consume as much airtime as they´d like, without impacting the voice SSID which exists only within UNII1.

I don´t know of a vendor who can assign SSID by radio, maybe there´s somebody already doing this?


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