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56 Planning
and packet loss must still be maintained. The Layer 3 design guidelines for
having clients and WLAN IP Telephony Manager 2245 in different subnets
does not mean that the WLAN IP Telephony Manager 2245 clustercan
also be separated by routers. All the WLAN IP Telephony Manager 2245
units in the cluster must still be collocated in the same subnet.
WLAN handsets
The WLAN Handsets 2210/2211/2212/6120/6140 support both G.711 and
G.729 codecs, but only using a 30 ms packetization rate.
The WLAN IP Telephony Manager 2245 translates between packetization
rates, meaning that from the WLAN IP Telephony Manager 2245 to the
handset, the call uses the packetization rate specified by the CS 1000 (for
example, 20 ms). Nortel recommends that the CS1000 paketization rate
match the 2245 at 30ms. For BCM, the packetization rate must be 30ms.
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
802.11 collision 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 per-AP basis.
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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