Wireless LANs getting priorities straight

With wireless LANs starting to be used for more than just data traffic, quality of service is getting taken more seriously.

Some wireless voice-over-IP (VoIP) vendors have created simple proprietary schemes to give voice priority on WLANs. That's critical for voice because without it the quality of calls erodes quickly.

Separately, the IEEE's 802.11e task group is finalizing work on a QoS standard that will let WLAN administrators set a range of priorities for different kinds of packets and control the time intervals between transmitted packets. The ability to schedule traffic flows is critical in transforming a WLAN from a network that hopes for the best to a network that guarantees delivery.

But, as with so much else in WLANs, none of this will be easy or quick.

Dartmouth College in Hanover, N.H., is experiencing a growing need for QoS as its campuswide WLAN gains users and traffic, says Brad Noblet, director of technical services. This fall, Dartmouth - like many other schools with extensive WLANs - was hammered by a range of aggressive network viruses. The Nachia virus, for example, sent out surges of Internet Control Message Protocol (ICMP) messages, fast and repeatedly, over a range of addresses.

The Cisco access points that Dartmouth uses had a vulnerability in some software versions that caused them to stop forwarding traffic when receiving ICMP packets. "Without QoS, you can get head-of-the-line blocking and dropped packets," Noblet says. Dartmouth has worked with Cisco and WLAN switch vendor Aruba Wireless Networks to correct any vulnerabilities and improve the ability to set traffic priorities.

The basic idea behind WLAN QoS is to handle traffic streams based on what kind of packets they are, such as voice, data or video. Class-of-service features categorize the traffic, and QoS features set priorities for delivery.

"There needs to be a mechanism to define traffic types, and then you can set [network] policies against [those types]," Noblet says. "You want to make sure there's no interruption in your voice traffic: You need to make sure that every 20 milliseconds voice traffic gets passed along. Otherwise you drop packets."

"QoS is needed in wireless LANs as more voice and video applications are rolled out," says Lynn Lucas, director of product marketing for Proxim. "I just received a [request for quote] from a university that is requiring support for 24 simultaneous videostreams per classroom. We believe this usage of video over the WLAN network will increase."

A QoS capability also will let network executives manage WLAN bandwidth, which is quickly consumed by voice and multimedia applications, or just big file transfers.

"A lot of our users stream [music or other] files," he says. "They could consume a lot of the access point's bandwidth, which today is really pretty limited. In the best case with 802.11b, we have about 6 megabits per second, tops, of useable bandwidth."

Some vendors, such as SpectraLink and Symbol Technologies, have introduced their own QoS techniques that let access points and client devices such as wireless VoIP phones give voice traffic priority on a network. A number of WLAN equipment vendors, including Proxim and Trapeze Networks, have licensed the SpectraLink protocol to support wireless voice applications.

"[With these schemes] you can boost voice traffic to the highest level of priority, but they don't deal with the problem of getting the packet [time] intervals correct," says Jacob Jorgensen, president and CEO of Broadstone Networks, a start-up beginning trials of an appliance designed to introduce QoS in WLANs. "[Simple prioritization] works in a lightly loaded environment, but in moderate to heavy loads it doesn't work well."

Jorgensen says that these priority schemes also are intended for voice traffic over WLANs. But the most demanding WLAN environment for QoS is one in which both voice and data are being transmitted. The proprietary protocols don't take this into account. In addition, these protocols are not standards, and devices using different vendor-based QoS won't interoperate.

The IEEE 802.11e task group is nearing the end of its work on a QoS standard for WLANs. It could gain final approval at the March IEEE plenary meeting, says Harry Bims, CTO and founder of Airflow Networks, and a member of the 802.11e task force.

Currently, WLANs use a technique for network access called Carrier Sense Multiple Access with Collision Avoidance (CSMA). With this technique each wireless client "listens" to hear if the channel is clear before transmitting. If another client transmits at the same time, a collision occurs, and both pause to wait for the channel to clear. Then they try again.

This best-effort technique can cause big lags for real-time applications such as voice or video, which can't tolerate such delays. "With best effort, you're not guaranteed when you can transmit," Bims says. "The delays can be so long that a real-time application breaks."

There are two main parts to the 802.11e standard, he says. One is a subset called Wireless Multimedia Extensions that some vendors currently are implementing. It is a fairly basic scheme for giving one type of traffic priority. But it doesn't address timing, which is critical for real-time applications.

This is the focus of the second part of the standard, called Wi-Fi Scheduled Multimedia, which can schedule a uniformly spaced series of time periods that are reserved solely for use by a specified traffic flow, Bims says. Each schedule is called a traffic specification, and only packets associated with the designated traffic flow are transmitted.

"This significantly mitigates the possibility of over-the-air packet collisions and creates guaranteed time periods when a device knows it can transmit," Bims says. "In addition, the back-off rules of CSMA are suspended, so that back-to-back packet transmissions are faster."

Broadstone's Jorgensen says users will need more than what the 802.11e standard alone can offer. "You need a QoS infrastructure, not just QoS features in the [media access control] layer [of the network]," he says. Broadstone's QoS appliance, which will incorporate the final 802.11e standard, is designed to give known WLAN users specific bandwidth allocations.

This could permit a fixed number of voice calls, and block or allow videostreaming. Users in one department might have priority over users in another, or priority for certain types of traffic. All these policies are set up via a Web GUI.

Bims acknowledges there are key areas the 802.11e standard ignores and that these will require additional software capabilities created by vendors outside the standard. One area is how neighboring access points coordinate their traffic flow schedules so that a client device can move from one access point to another without having to renegotiate its traffic specification with the new access point. Companies such as Airflow and Broadstone will have to handle this.

Another issue is that the 802.11e standard might include several optional items or features that vendors can implement or not. Different implementations could cause a lack of interoperability in some products.

"It's only going to get worse," says Dartmouth's Noblet, with the resignation of a network executive who's seen it all. "It just takes a lot longer to cook all that [in the standards body] when you have a lot of cooks in the mix."

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