Voice over Wireless LAN Solution Guide v1.0 December 2005
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2.4.1.2 IP Softphone 2050 and MCS Client
Providing scalability rules for the IP Softphone 2050 or MCS Client may appear meaningless in
this context because the phone runs on the same PC with other data applications. A pure voice
capacity number is not going to provide much engineering guidance because that same PC can
also send large amounts of data on the medium. However, it is much easier to design a VoWLAN
when you can take advantage of a higher speed NIC in a PC such as an 802.11a interface. The
exact limit has not been determined through testing, but as a rule of thumb you can expect to get
as much as 3.5 times as many calls as you can on an 802.11b network, roughly in the
neighborhood of 30 to 35 calls if all laptops are next to the AP. As mentioned earlier under
channel reuse scenarios, the difference in capacity is 5 to 10 times that of 802.11b under channel
reuse scenarios. Both are significantly lower numbers, depending on network load, than the
corresponding non-reuse call numbers per AP. Whatever the maximum number of calls per AP, it
is wise to use a more conservative number given that the network (upstream packet flow) will be
supporting data at the same priority level as voice and given that PCs in reality will be scattered
across the coverage area and will be rate scaling downward.
Similar numbers can be applied for 802.11g networks if the network is homogeneous with respect
to 802.11g devices, compatibility mode (the mechanism required when 802.11g and 802.11b
clients coexist on the same WLAN) is not currently running, there is no data traffic, and there is
no channel reuse. If the network is running in compatibility mode, the numbers become
ambiguous due to the variety of protection mode mechanisms and possible implementations of
those variants. In a best case, with no active 802.11b traffic or other data traffic, it can still be as
high as 20 to 25 calls from 802.11g devices. But again, like in the 802.11a case, such numbers
quickly become meaningless upper bounds when reality will dictate less optimal conditions.
The IP Softphone 2050 and MCS Client also pose a number of additional challenges, such as
when you use a multimode NIC in combination with other voice devices on the WLAN. You can
plan to have WLAN Handsets 2210/11/12 on the 802.11b WLAN and laptops on 802.11a. The
WTM 2245 is able to accurately count the number of voice calls on the 802.11b radios. If a
multimode NIC on a PC chooses to use the 802.11b radio for some reason, you now have calls
traversing the 802.11b network that are not counted by the WTM 2245. Therefore, Nortel
recommends that when you deploy WLAN Handsets 2210/11/12 together with PC-based voice
applications (IP Softphone 2050 or MCS Client) in the same network, confine the PC-based voice
applications (as much as possible) to the 802.11a channels. You must also account for the QoS
mechanisms of the PC itself, which may not be very robust in terms of prioritizing applications.
Discussion of PC operating system features and NIC device drivers is beyond the scope of this
document.
2.4.1.3 Mobile Voice Client (MVC) 2050
It is difficult to determine a rule of thumb for the MVC 2050 on a PDA because there are multiple
types of hardware and corresponding drivers on which the client can be installed. Not all PDAs
are equal, nor are all drivers for a particular device equal. The MVC 2050 also supports
packetization rates of 20 ms and 30 ms with G.711 A-law and µ-law, so capacity will vary with the
packetization. G.729 and G.723 are not supported codecs today on the MVC 2050. Contrast the
level of options on PDAs with the uniformity of the WLAN Handset 2210/11/12, whose fewer
permutations make engineering easier. For PDAs there is no way to provide a blanket
supportability statement such as “Ten MVC 2050 voice calls per AP 2230.” Any guideline must be
determined per make and model of PDA with driver and codec dependencies. With the latest
drivers installed, Hewlett Packard (HP) iPAQs have been tested with up to eight voice calls per
AP. As a general rule with PDAs, the engineering limit is six to eight calls per AP depending on
model and driver version.
A mixed environment likely has performance metrics proportional to the mix of devices in use. For
example, if half the PDAs are iPAQs and iPAQs are known to scale up to 8 and Dell Axims make