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Voice over Wireless LAN Solution Guide v1.0 December 2005
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Page 47
types of devices side by side. The other major source of problems relates to security capabilities
among clients.
2.6.1.1 Separating data and voice applications
In addition to having different QoS features, PDAs and PCs present another unique challenge
that handsets do not have to deal with, namely supporting data and voice on the same device.
The WSS 2300 has the ability to prioritize on a per-packet basis through ACLs, which significantly
helps to enable a converged WLAN. So in the downstream direction (from WSS to AP), the WSS
can support multiple applications to a single device over the same radios. The upstream direction
still has many client and application dependencies. Ultimately WMM and 802.11e are keys to
building upstream QoS for a mix of applications on the same device, but the applications
themselves must be designed to make use of the separate access classes. In essence this issue
is half resolved today by the WLAN 2300 Series. The remaining issues reside with the client and
applications.
2.6.1.2 Admission control
The WTM 2245 is currently the only device in the solution capable of admission control in terms
of call capacity. Yet it only accounts for calls placed from WLAN Handsets 2210/11/12, not from
IP Softphone 2050, MVC 2050, or MCS Client devices. Therefore, having a mix of WLAN
Handsets 2210/11/12 and PC/PDA voice applications on the same WLAN completely
compromises the entire admission control capability due to the fact that only some calls are
counted. For example, suppose you have the WTM 2245 set to allow only up to six calls per AP
because you want to allow room for a couple of PDA calls. If there are no PDA calls active, then
you are wasting capacity that handsets could otherwise be using. On the other hand, you can still
have too many PDA calls active on an AP and impact your handset calls. So there is no true
protection in this environment. The only way to handle this situation is to revise the number of
handset calls per AP to a low enough number to safely accommodate a certain number of PDA
calls per AP. This problem is more likely to occur between handsets and PDAs, but can also
happen with PCs with a soft client. The reason is that PDAs generally are 802.11b-only devices
today and thus they must share the same radio spectrum with handsets. PCs may have 802.11b
only or 802.11g NIC and in such cases they compete for the radio frequency as well. But PCs
might also have an 802.11a NIC, in which case you could allow only PCs to use the 802.11a
radio, thus mitigating the mixed admissions control issue described here. In summary, PCs have
enough flexibility for the design phase to resolve the issue of compromised admission control of
handsets and WTM 2245, but PDAs do not (today).
2.6.1.3 Prioritization
It was previously discussed that queuing behavior depends on the WMM setting and that ACLs
are used to mark the priority of packets. Also when WMM is disabled (a requirement to support
WLAN Handsets 2210/11/12), packets identified as CoS 6 or 7 are put in the SVP queue, which
implements zero backoff. If non-handset voice is added, you must decide whether to leave other
voice traffic at a lower priority or to put it in the SVP queue where zero backoff is implemented.
The answer to this question partly depends on the radio being used by this other voice traffic. For
example, PCs may be using an 802.11a radio with WMM enabled. In this case, such traffic is not
put in an SVP queue. Another example is a PDA using an 802.11b radio. In this case, handset
and PDA voice calls are shared on the same radio.
2.6.1.4 Security
Another common issue is the lack of consistent security feature support across varieties of
devices. Handsets are the most limited in terms of security options, and laptops generally have
the most options. The situation is a lot better now that the WLAN Handsets 2210/11/12 support
WPA-PSK and WPA2-PSK. In most cases, a common encryption scheme can be selected that
meets the security requirements of the network. If the data network will use WPA2 for encryption,