Checklist for Planning a Voice Over Wi-Fi Network: Wi-Fi Coverage (Part 2 of 4)
02-06-2012 04:57 PM
Back to the future with this Airheads Online article from 2007.
A number of considerations for the Wi-Fi infrastructure:
- Although nearly all currently-available Wi-Fi phones support only 2.4 GHz radios (802.11b and 802.11g), the infrastructure should support a/b/g everywhere, as the 5 GHz band (802.11a) has many more available channels and is less prone to interference.
- Wi-Fi coverage for voice requires more planning than for data users, as it is important to avoid dead-spots, and people will make voice calls from places where they would never think of using their PC. Best practice today is to study a floorplan, upload it into the WLAN vendor’s planning tool to identify AP locations, then make a ‘walk-around’ site survey to identify any special situations: large metal objects, thick reinforced concrete walls, etc, which may need special consideration.
- For voice, it is important there is continuous coverage, but the APs should not be too close together. Very closely-spaced APs result in extra handover events and can make it more difficult for the client to make a good handover decision. The usual parameters for a planning tool would be for a minimum data rate of 6 Mbps (802.11g) with a 50% overlap between cells (in the Aruba planning tool, set overlap to 150%).
- We often prefer a 4-channel plan for the 2.4 GHz band (802.11b/g). This normally uses channels 1, 4, 8, 11. Although this results in some channel overlap, it has proven to be a more successful model than a 3-channel plan using channels 1, 6, 11. It is not necessary to overlap channels in the 5 GHz band.
- Although generalizations are odious, for office environments optimum AP spacing will often be about 20-25 metres (60-75 feet) for data-only networks and 15-20 metres (45-60 feet) where voice is used. Modern WLANs automatically adjust AP transmit power and channels for optimum coverage once installed, so the objective should be to install more densely than would be necessary if all APs were running at maximum power (consistent with the 6 Mbps minimum data rates suggested above). Our experience is that older WLAN designed for data service only do not often require adjustment when voice service is added.
- All modern WLANs incorporate some form of dynamic RF configuration that periodically calculates and optimizes the channel assignment and transmit power of every AP, taking interference and other factors into account. This feature is particularly important in ensuring consistently good voice quality, so it should be examined critically as part of any WLAN vendor evaluation: we believe the Aruba RF Management (ARM) feature is the most complete available.