A blog with tips, tricks and tutorials to help you prepare your CCIE Wireless lab exam.

Sunday, August 2, 2009

WCS Planning mode and VoWLAN

What is a good VoWLAN design?
-67 dBm RSSI at the edge of the cell, aiming for less than 1% packet loss to get a MOS (with G711 codec) of 4.1 or better. Typically speed in the cell would be set to 24 Mbps or more (lower speed disabled) for 802.11a or 802.11g, and 11 Mbps or more for 802.11b/g. This last figure makes that some people would say "12 Mbps or more for 802.11g", to have 802.11g value close to the 802.11b/g value.
The main point is that if RSSI is good at the edge of the cell, and if SNR is good at that point (25 dB or more), then call quality would be good.

Now take the WCS, in planning mode, and ask it for Voice service... the network will be designed with the following predictive RSSI at the edge of the cell:
- Aggressive = Minimum [-78 dBm (802.11a/b/g)]
- Safe = Medium [-75 dBm (802.11a/b/g)]
- Very Safe = Maximum [(-72 dBm (802.11a/b/g)]
- 7920_enabled = [(-72 dBm (802.11a); -67 dBm (802.11b/g)]
All these values are way below all Cisco recommendations... you'll need to manually tune the Planning Mode result to get an acceptable predictive coverage...

1 comment:

  1. Obviously predictive planning is limited and not always accurate and a full blown survey is always recommended or at the very least; a predictive with walls and attenuation zones added to increase accuracy. I know Airmagnet and Ekahau are a bit more detailed in how they do their predictive surveys, but my question is around location predictions using WCS (Now NCS/Prime Infrastructure). In addition to VoWLAN, it is also able to predict location based services. I was wondering if there are any tweaks or caveats that should be noted for doing a location based predictive survey with this software?

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