Best Practices for Voice over Wi-Fi – Part 2
2. Roaming Coverage
Appropriate cell coverage overlap is key to having a successful VoWLAN deployment. But the typical, minimal cell overlap between APs people think about is not sufficient. For this reason, coverage design for roaming between Access Points has to expand beyond typical cell overlap. This section will first cover single band cell coverage and then cover band overlap coverage.
In a single band design, handsets make a determination to roam in less than half the overlapping coverage area from a neighboring AP. Therefore, the coverage area must be adequate enough so that when a voice user is moving, the handset has time to discover, associate with and connect to the next AP before the signal on the currently connected AP becomes too weak. You will need to understand what impacts RF coverage and cell size and how much cell overlap is required to properly design and configure your VoWLAN.
The usable cell size of an AP is dictated by the frequency, signal power level, minimum data rate, and objects that attenuate the signal. A properly designed Wi-Fi network will position APs with sufficient overlapping coverage to ensure there are no coverage gaps, or “dead spots” between them.
The result is seamless handoff between APs and excellent voice quality throughout the facility. Sufficient overlapping coverage is usually considered 15% to 20% signal overlap between AP cells in a deployment utilizing maximum transmit power for both handsets and APs. The WLAN layout must factor in the transmission settings that are configured within the APs. The transmission of voice requires relatively low data rates (in low RF signal strength areas) and a small amount of bandwidth compared to other applications.
The 802.11 standard includes automatic rate switching capabilities so that as a user moves away from the AP, the radio adapts and uses a less complex and slower transmission scheme to send the data. The result is increased range when operating at reduced transmission data rates.
When voice is an application on the WLAN, APs should be configured to allow lower transmission rates in order to maximize coverage area. If a site requires configuring the APs to only negotiate at the higher rates, the layout of the WLAN must account for the reduced coverage and additional APs will be required to ensure seamless overlapping coverage.
The 15% to 20% of signal overlap between AP cells generally works well with a typical walking speed of the user (the average walking speed of an individual is 3 mph). If the speed of the moving user is greater (such as a golf cart, fork lift or running/jogging) or the cell size is smaller than a different overlap strategy may be necessary for successful handoff between APs. The amount of time needed to find a new AP is a fixed constant. Smaller cells or faster roaming speeds will need larger overlap percentages due to the need to maintain an overlap area that still allows time to find the next access point.
For example, SpectraLink handsets perform Dynamic Channel Assessment (DCA) in between the transmission of voice and control packets to learn about neighboring APs. It takes a little over one second for a DCA cycle to complete. In order to ensure a DCA cycle can complete within the assessment area (see Figure 1), a person moving through the assessment area must be within the area for at least 2-3 seconds to make sure the DCA starts and ends within the assessment area. Failure to complete the DCA cycle within the assessment area can lead to lost network connectivity resulting in a hard handoff, lost audio, choppy audio or potentially a dropped call.
Figure 1 - Dynamic Channel Assessment (DCA)
The handset compares the signal strength of neighboring APs to determine whether to roam from the current AP. In order to roam, the handset has to determine whether another AP should be roamed to, it must be 10 (ten) decibels stronger than the current AP’s signal. Corners and doorways pose a particular design issue. The shadowing of corners can cause steep drop-offs in signal coverage. This is particularly true of the 5 GHz band. Make sure to have adequate cell overlap at and around corners so that the audio stream is not impacted by a user going around corners. This may require placement of an AP at corner locations to ensure appropriate coverage to prevent RF shadows.
In a dual band deployment, cell overlap is considered from both different APs and between different radios on the same AP. To the phone, different radios on the same AP are really just two different APs. The stronger signal will be given the higher priority. In the follow diagram this is easy to see why.
Figure 2 - Roaming
In the case of an AP with both bands enabled and the phone enabled for band roaming, the stronger band will almost always be selected. Such things as attenuation caused by the phone being close to the callers head will impact 5 GHz more than 2.4 GHz so if 2.4 GHz is the weaker signal, there are time it may be selected since 2.4 GHz has better penetration.
Currently there is no way to give a preference to either band. The phone will pick the strongest signal it sees. Therefore, the case for enabling band roaming can be made.
Added capacity, areas of difficult access to provide coverage, such as stairways and elevators, and mixed infrastructure with older equipment, that doesn’t provide the 5 GHz band, are more reasons to enable band roaming.
In the case of capacity, if a radio becomes saturated with calls the AP will tell a phone trying to request access for a call will be told to use another AP. That other AP be another physical AP or it can be the other radio on that AP.
When deploying voice on 5GHz, there are times that getting adequate coverage in stairways and elevators becomes a challenge. Either APs are not allowed in those spaces or the structure is such that the signal is too attenuated in portions of the space no matter where they are placed to provide complete coverage. Using 2.4 GHz to cover these areas may be a solution. The APs can be placed outside these spaces and antenna selection and placement can be used to penetrate better than 5 GHz can achieve.
We will continue our discussion with Signal Strength in part 3 of this series ...