Best Practices for Voice over Wi-Fi – Part 4
4. Deployment Considerations
The 802.11b, 802.11g and 802.11n standards utilize the 2.4 GHz frequency spectrum. 802.11g 802.11n networks that support 802.11b-only clients must run in protected mode to enable backward compatibility.
Protected mode adds considerable overhead to each transmission which ultimately translates into significantly reduced overall throughput. Several voice handsets, for instance SpectraLink 8400 Series Wireless Telephones, support running in a mixed mode. The overhead associated with performing protected mode transmissions largely negates any benefits of transmitting relatively small voice packets at higher 802.11g data rates. For this reason, when handsets are installed on a mixed 802.11b/g network which is already running in protected mode, the handset must be configured for 802.11b & b/g mixed mode. In an 802.11b/g mixed environment a handset that is configured for the 802.11b and b/g mixed mode will only utilize 802.11b data rates and has no 802.11g functionality while this mode is enabled.
The handset operating in 802.11g-only mode must use a WLAN with data rates set so only 802.11g clients can associate. There must be no 802.11b client connected to and using the WLAN. The way to ensure only 802.11g clients use the WLAN is to set to disable all 802.11b data rates (1, 2, 5.5, and 11Mbps). It is important to include these settings for all SSIDs in the handset coverage area and not just the voice SSID, since this impacts the spectrum for the entire area.
5 GHz 802.11a
The 802.11a standard utilizes the 5.1 GHz to 5.350 and the 5.725 to 5.825 GHz Unlicensed National Information Infrastructure (UNII) Spectrum. Although having the same maximum throughput as 802.11g (54 Mb/s), the increased frequency spectrum at 5 GHz offers up to 23 channels, providing the potential for higher AP density and increased aggregate throughput. There is significant variation in channel availability and use between countries, however, which must be considered for any particular 802.11a deployment.
As compared with the 2.4 GHz frequency of 802.11b/g radio deployments, higher frequency RF signals utilized by the 802.11a/n 5GHz band do not propagate as well through air or obstacles. This typically means that an 802.11a network will require more APs than an 802.11b/g network to provide the same level of coverage. This should be taken as a guideline however, as signal propagation may also be impacted by the output power settings of the AP and the antenna type. A comprehensive wireless site survey focusing on VoWLAN deployments should be conducted to identify the specific needs for each environment.
Like the issue of 802.11g clients used alongside 802.11b clients, 802.11n clients must operate in a protected mode when 802.11a clients are co-existing. The same issues apply with the protected mode operation and small packet sizes and as such when sending voice packets the phone will only send using 802.11a. It can receive 802.11n packets from the AP though.
We will continue our discussion with Access Point Configuration in part 5 of this series.