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It’s not easy ensuring quality on wireless LANs. Because of a WLAN’s limited capacity and shared medium, you can’t overprovision it like a wired Ethernet LAN. And WLAN traffic patterns can be unpredictable, so enforcing quality of service is tricky, especially when wireless traffic crisscrosses the wired Ethernet infrastructure.
On a landline Ethernet and IP network, you can use QoS prioritization schemes–802.1d and DiffServ, for example–or throw more bandwidth at the problem to ensure time-sensitive applications like voice and video get sufficient headroom. As these applications–which are susceptible to unpredictable bandwidth, high latency and jitter–move to WLANs, so too must QoS schemes.
Quality-Hungry Apps
VoIP (voice over IP) will likely be the most common time-sensitive application for atypical enterprise, and the one most in need of QoS. Enterprise adoption of VoIP hasn’t exploded, of course, but wireless VoIP makes it more appealing, especially if a WLAN infrastructure is being deployed anyway for conventional data applications.
Wireless VoIP gives workers who aren’t regularly at their desks more flexibility, and it can reduce operational costs associated with the use of cell phones and private radio walkie-talkie systems in the enterprise. Warehouse, distribution, retail and health-care operations have used wireless VoIP for a number of years, and it will become increasingly popular over the next several years as both VoIP and WLAN technologies mature and their prices decrease. In fact, the percentage of large enterprises deploying wireless VoIP is predicted to increase from the single digits today to 33 percent in 2006, according to Infonetics Research.
Wireless QoS can augment other applications as well, such as guest access, where you offer your clients, customers or other visitors wireless access over your WLAN while they’re on site. QoS here gives your internal users higher priority than visitors, and likewise, some internal users take precedence over others.
Then there’s the consumer market, where Wi-Fi has been a huge hit as a home networking technology for sharing printers, files and broadband Internet connections. QoS lets home users prioritize how bandwidth gets split among these kinds of operations. WLAN QoS also is necessary for multimedia home entertainment, where moving digitized multimedia content across home systems is gaining steam.
But QoS is more complex in the enterprise because applications are more varied and the physical scale of WLANs is greater. With QoS standards slow to emerge, some organizations have taken pragmatic approaches to ensuring their apps get the bandwidth they need. Some hospitals, for example, deploy multiband 802.11 a/b/g network infrastructures: They dedicate the 5-GHz 802.11a system to data applications and the 2.4-GHz 802.11b/g system to voice.
Consider the Options
WLAN infrastructure vendors, including Airespace, Cisco Systems and Colubris Networks, offer proprietary wireless QoS. They link traffic prioritization to users and groups by assigning priority to either the user’s authenticated identity or the 802.11 ESSID (Extended Service Set Identifier).
We recently tested technology from Colubris in Network Computing’s Syracuse University Real-World Labs® that provides QoS by ESSID, using what Colubris calls “service-aware” Wi-Fi QoS. We connected three Dell Latitude notebooks with embedded 802.11b NICs to our lab Ethernet LAN over a Colubris CN320 access point. We used NetIQ Chariot to transfer files to each of the clients. Not surprisingly, each client shared the available bandwidth and achieved comparable throughput of about 1.6 Mbps. We then reconfigured the AP with multiple 802.11 ESSIDs and gave each a different priority assignment, resulting in higher performance for prioritized users (the chart on page 83 shows how each client performed).
Although this approach doesn’t address all QoS problems, it’s easy to implement. It’s simple to assign ESSIDs and associated security policies to phone devices so that they get priority over conventional data traffic. The catch is that though AP-centric prioritization schemes let you prioritize traffic from the AP to the client and from the AP to the wired segment, they don’t do QoS over the airwaves. So if the wireless network experiences severe congestion, VoIP clients may not gain predictable access to the network for upstream traffic. That’s because all WLAN clients adhere to the same rules for gaining access to the medium–those of CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance)–which are designed for fairness rather than prioritized access.
SpectraLink Corp.’s proprietary SVP (SpectraLink Voice Priority) technology gets around this problem by making media access less fair. VoIP phones supporting SVP can jump to the head of the line. Most WLAN infrastructure vendors have implemented SVP in their devices.
SVP provides wireless QoS by specifying a back-off value of zero for voice packets. That gives voice packets priority over normal data packets. The downside is that if multiple SVP VoIP phones attempt to transmit at the same time, you’ll get collisions. If too many phones exist in a given cell, the phones can monopolize available bandwidth and thus starve out conventional data users. To get around this, limit the number of handsets per cell to a dozen or less. For the nuts and bolts of how media access is controlled on 802.11 networks, see “How 802.11 Media Access Works,” below.
On the wired side, SVP also adds priority queuing to the AP–using a unique protocol type in the IP header–to give voice packets priority.
