Quality of Service for a WLAN is a very tricky thing. Predictability is the key to good voice communications. Voice services are very sensitive to digital transmission errors. When voice is transmitted, a specific compression/decompression (CODEC) scheme is used to compress the outgoing traffic, which is reconstructed on the opposite end of the transmission by a matching CODEC. During transmissions, however, lost packets, random network delays, and retransmissions increase "jitter" (or variation in timing, or time of arrival, of received signal) and signal loss, causing clicks, silent periods, and poor voice quality. When more advanced compression techniques are employed, the problems become even more pronounced because these techniques further delay transmission of the signal.
VoIP systems use buffers to smooth out minor variances in packet transmission rates, and these buffers work fairly well in wired networks. But WLANs feature wider variances in transmission speeds, and delivering packets predictably is where traditional WLANs fall short for mobile VoIP in an enterprise.
Because of their roots in Ethernet technology and the fact that they are basically networks of giant Ethernet hubs, WLANs today face some significant hurdles that, unless addressed by the WLAN system, make them unsuitable for use as a voice transmission network across an enterprise: they can't prioritize and predictably deliver data packets both to and from the client, and they cannot maintain predictable performance between the client and an access point (AP) as the client roams from one AP's coverage area to another's. The former point is because of interference in large enterprise networks and the inherent "random" nature of Ethernet (a collision causes a random back-off and a retry). The latter is a delay caused by the handshake and re-association methods used by 802.11.
So now you're left with "ok, so what do we do?". Some would argue that the 802.11e standard for QoS is the answer. If your problem is a small-scale network with a limited number of APs (5 or so), then 802.11e will help quite a bit. When scaling beyond that, 802.11e has a problem. Because it is unable to actually reserve bandwidth for higher-priority traffic and is actually (in a basic sense) a very aggressive contender grabbing the channel really fast it doesn't solve the predictability for larger networks. A small number of 802.11e clients all being aggressive at the same time will cause collisions and retries and overall no guarantees of quality. In this case, WLAN systems employed need to provide you with a clear way to A) differentiate between high-priority packet and a regular one and B) prioritize the channel access for those clients ROUND-TRIP. The key phrase being "ROUND-TRIP" or both to and from the client. Some systems will claim wireless QoS via Packet Prioritization – which helps with the downstream flow (from AP to client) but does nothing for the upstream flow (from client to AP). Without that, you're going to be disappointed.
Last point, make sure the system is Wi-Fi certified. The last thing you want to do is put in a WLAN system that's going to require client enhancements.
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