WLAN IP Telephony Manager 2245 planning 63
The handsets encapsulate their voice payloads in SVP for QoS. The handsets further synchronize communications, so that the handsets are able to avoid collisions with each other more effectively than the usual
802.11collision avoidance mechanisms. Each handset maintains a list of up to four APs as potential candidates for roaming. The handsets are aggressive in roaming to other APs, which tends to prevent them from using a suboptimal data rate when another AP can provide better service. The handsets also communicate with the WLAN IP Telephony Manager 2245 and discover which APs are at full call capacity, so that the handsets can direct their calls through an AP that has call capacity available.
Under optimal conditions, meaning no interference and all devices in proximity of the AP, up to 10 voice calls from a handset are supported on a single AP 2330. When configuring the maximum call parameter of a WLAN IP Telephony Manager 2245, never configure it above 10. A more realistic rule of thumb that allows for devices to move about and rate scale accordingly is anywhere from six to eight calls per AP. A noisy RF environment can impact the numbers further.
To provide data devices some amount of guaranteed bandwidth, lower the maximum voice calls per AP to prevent voice calls from consuming all available throughput. For example, limiting the maximum calls per AP to seven allows data traffic to reserve up to 30 per cent of media capacity. If the network supports other handset calls on the 802.11b network, you must leave adequate capacity for those calls too. Note that the call admission control function of the WLAN IP Telephony Manager 2245 cannot serve to limit those other voice calls on a
There is an alternative control on the WLAN IP Telephony Manager 2245 that affects call capacity across APs. This control allows the WLAN IP Telephony Manager 2245 to fix the data rates that handsets use. The options are Automatic and 1 Mb/2 Mb only. When you choose the latter, maximum call capacity drops by slightly more than half if G.711 is in use, or by slightly more than
If the automatic option to have higher potential capacity is selected, there is a risk of occasionally being oversubscribed under the worst conditions. For example, if eight calls is the configured limit on the WLAN IP Telephony Manager 2245, and if all eight calls are from handsets on the edge of coverage, the cell is oversubscribed. If five calls is the configured limit and handsets are restricted to 1 Mb/2 Mb, capacity is wasted when most handsets are close that could otherwise be used by other data devices.
Nortel Communication Server 1000
WLAN IP Telephony Installation and Commissioning
Release 5.0 15 June 2007
