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QoS and voice over Wi-Fi

Sep 27, 20044 mins
Network SecurityWi-Fi

Q: With more than enough bandwidth available, would QoS be critical to implement for voice over Wi-Fi applications? – Kevin J.

The Wizards gaze into their crystal ball and respond:

Steve Smith Sr., Meru Networks

Absolutely. Many wired networks today have been deployed to provide dedicated 100M bit/sec for each user with Gigabit links for the backbone. Analysis of these networks typically shows 10% to 20% usage on these backbones at peak times, so QoS is important to a lesser degree on these networks because of the extra bandwidth available.

Unfortunately, WLANs are different because they operate as shared, half duplex mediums. The current maximum bandwidth available is 54M bit/sec with 802.11a or g. As such, all clients are contending for the overall bandwidth, as was the case with wired Ethernet hubs. Shared mediums and high user counts are inversely proportional: as the user count increases, the overall throughput degrades. QoS is therefore imperative for WLANs, because you must be able to ensure adequate bandwidth for priority traffic even when the overall throughput degrades with additional users.

Ideally, a WLAN should provide QoS control over the air traffic on both downstream and upstream flows, with downstream being from the access point to the client and upstream being from the client to the access point. You can prioritize packets on the downstream by giving them a priority queue, but for upstream QoS, the access point must control the quality of the link and coordinate that with other clients in the air as well as with other access points. To establish and maintain a large number of voice calls, the WLAN system must have the ability to provide downstream and upstream QoS on those voice flows as data flows continue in the background.

Seth Goldhammer, Roving Planet

I would be interested in hearing what makes you think you have more than enough bandwidth available. Even with 802.11a or 802.11g, you should still think of access points as nothing more than Layer 1 hubs. Without full duplex traffic, or the ability to switch traffic, all bandwidth is shared among the common devices associated to that access point. Keep in mind that the effective ‘real world’ throughput of an access point is a little more than half of the quoted throughput – the quoted of throughput of 11M bit/sec for 802.11b is in reality 6M to 6.5M bit/sec, and 54Mbit/sec quoted for 820.11a is really about 22M to 23M bit/sec, which can be reduced to 10M to 11M bit/sec in the presence of 802.11b clients. Also, the role of an access point in 802.11 is to set the timing of the network. Devices that are farther in proximity to the access point are not allowed to talk as frequently as others, which as a result influences the amount of bandwidth available to that device. Site surveys can assist in determining that saturation is adequate in your space. Even still, voice applications needs traffic consistency, because data networks are asynchronous, and voice packets need to arrive relatively quickly together in order to prevent jitter and drop-offs. Providing QoS is still critical to ensure the user-experience is consistent for both data and voice applications.

Dan Simone, Trapeze Networks

Bandwidth is not the only consideration to implement voice over Wi-Fi. Voice applications also require low latency and low variation in latency (known as jitter). If voice is combined with data applications that are bursty in nature over Wi-Fi, without sufficient QoS capabilities the voice applications will experience increased jitter. Voice users may hear clicks and cutouts or calls may drop.

Scott Haugdahl, WildPackets

In short, yes, unless the users are willing to be subjected to random sub-par quality compared to their standard telephone. Some form of QoS – the emerging IEEE 802.3e standard or a proprietary solution – is required to ensure uninterrupted and satisfactory VoIP service. It only takes a couple of users performing a file transfer over the same 802.11 wireless channel to disrupt voice quality. Larger jitter buffers (at the VoIP end listening devices) can help, but can tend to create a more walkie-talkie-like conversation. VoIP is like an IV – it expects delivery at a nice steady pace.