Gateway And Timing Function - Avaya Communication Server 1000 Installation And Commissioning Manual

Planning
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 per-AP
basis.
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 two-thirds if G.729 is in use. Most of the variability of call capacity is removed as
rate scaling effects are eliminated. Therefore, you can get more predictable call capacity at the
expense of maximum number of calls under optimum conditions. Note that with this option
enabled, throughput for 802.11b data devices is severely impacted by even one or two voice
calls.
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.
To gain this type of predictability, engineer the maximum calls per AP based on 1 Mb/2 Mb rate
selections in the handsets, configure that number as the call limit on the WLAN IP Telephony
Manager 2245, and then configure the actual rate of the handsets (on the WLAN IP Telephony
Manager 2245) to Automatic. That way, the WLAN is engineered for the worst case, but in
optimal conditions, more throughput is left over for other devices to use, because the handsets
use higher data rates.
To summarize, do not use the 1 Mb/2 Mb option, even if the network is engineered to that
type of coverage.

Gateway and timing function

WLAN IP Telephony Manager 2245 units provide both the connection or gateway to the Call
Server for the wireless handsets, and the timing function for active calls. This gateway function
is distributed across the WLAN IP Telephony Manager 2245 group.
The number of active WLAN IP Telephony Manager 2245 units is determined dynamically.
Whenever a WLAN IP Telephony Manager 2245 is added to or removed from the system, the
distribution of timing function for active calls, as well as the gateway function, is affected.
72 Avaya WLAN IP Telephony Installation and Commissioning November 2010