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WLAN IP Telephony Manager 2245 planning 57
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.
Roaming and handover
Roaming is the ability of the wireless handset to go anywhere in the WLAN
Extended Service Set RF signal coverage area, and to make and receive
calls. Handover is the ability of the wireless handset to maintain an active
Nortel Communication Server 1000
WLAN IP Telephony Installation and Commissioning
NN43001-504 03.04 Standard
23 September 2008
Copyright © 2004–2008 Nortel Networks
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