Optimizing network performance for WLAN real-time applications

Lisa Phifer, Contributor

Wireless LANs (WLANs) are now well on their way to replacing wired Ethernet as the primary enterprise network access method. As organizations navigate this migration, making significant network infrastructure upgrades, it is important to consider optimizing network performance to meet needs associated with real-time applications like voice over IP and video conferencing.

Most 802.11n access points (APs) can support data rates up to 300 Mbps, with 450 Mbps now emerging. These higher data rates increase total WLAN capacity, making it possible to support more users per AP as well as high-throughput applications such as video streaming. But these changes are not necessarily good for real-time applications, which require frequent and predictable network access. In this guide, we look at what it takes to support these applications, and how those needs impact wired/wireless network integration.

Understand real-time applications traffic prior to WLAN optimization

Real-time applications are sensitive to latency (the time it takes for a packet to travel from point A to point B) and jitter (variation in packet arrival time). When media is streamed – for example, watching a YouTube video or listening to a podcast – arriving packets can be buffered to compensate for latency and jitter. However, real-time applications like voice calls, video conferencing, instant messaging, unified communication, and on-line gaming cannot depend upon buffers to smooth over those network "speed bumps." Real-time application users expect immediate communication, without odd drop-outs or lags in what they see and hear.

Under the covers, this requires the ability to send packets frequently, at fixed intervals, with consistently fast delivery. Voice over IP always sends short fixed-length packets that carry digitized voice, compressed by a codec (e.g., G.711, G.728) to produce a low-bandwidth stream (e.g., 64 Kbps, 16 Kbps). Video sends longer packets, but frame size and compression still depends on encoding (e.g., MPEG-4, MPEG-2) which determines minimum acceptable throughput (e.g., 5 Mbps, 20 Mbps). However, because latency and jitter have such a significant impact, it is not enough to ensure that your network can transfer 64 Kbps per call or 5 Mbps per video.

As a result, real-time video and voice goals are typically represented and measured using quality metrics. For voice, mean opinion score (MOS) and R-value are used to rate call quality (ranked from 1-5 and 1-100 respectively). For video, a common quality metric is media delivery index (MDI), which is composed of media loss rate (MLR) and delay factor (DF). Design targets for these metrics can vary, depending on the application, content encoding, device type, and what you are trying to achieve. For example, Veriwave's WLAN Site Assessment – Best Practices guide recommends laptops aim for MLR and DF better than 1% and 150 ms, while smartphones should try for MOS values of 4 or better (considered toll-quality voice).

Optimizing network performance to meet real-time goals…Where to begin?

Clearly, optimizing network performance for real-time applications involves many components and parameters, integrated and tuned to meet or beat these design goals. Let's consider a few critical pieces to this puzzle.

Before even reaching the WLAN, mobile devices that run real-time applications require strong, reliable connectivity. This piece can be safely assumed for a desktop or laptop cabled to an Ethernet LAN, but varies over time for wireless devices – including laptops in motion and handsets carried by users standing still but experiencing signal fluctuation due to RF interference. To deliver VoIP-grade wireless connectivity, most handset and AP vendors specify WLAN design goals, including maximum AP transmit power, minimum signal strength and signal to noise ratio (SNR), and delivery traffic indication message (DTIM) interval. Even though other real-time applications may have less stringent requirements, designing WLAN coverage to meet these goals gives you a fighting chance at achieving desired quality scores. To see an example, consult Cisco's Voice over WLAN Checklist.

Prioritize network traffic by optimizing network performance

Inside the WLAN, traffic must be prioritized to ensure that voice gets frequent predictable media access to keep jitter under about 10 milliseconds. Because video requires more bandwidth, it should be prioritized under voice, but above best-effort data or background applications. This AP traffic queue and airtime prioritization can be accomplished by setting 802.11e (Wi-Fi Multimedia, WMM) defined access classes. When that traffic hits the wired network, WMM priorities should be mapped to 802.1p (Ethernet frame) or DiffServ Code Point (IP packet) header markings. Beware of "blind spots" in this mapping – for example, when frames are tunneled from a WLAN controller to edge switch to core switch through a trunk port that does not provide 802.1p. To learn more, view Aerohive's End-to-End QoS video.

WLAN optimization saves real-time applications from latency

Even with prioritization, excessive competition for scarce resources can still doom real-time applications. Here again, it's important to evaluate this factor end-to-end, all the way from the channel to AP to controller, through Ethernet switches and WAN hops that must be traversed to reach the call manager, media server, or other real-time traffic destination.  For example, channel competition can be managed by assigning voice handsets to their own SSID and frequency, using AP load balancing and Call Admission Control to restrict the number of active calls per AP. When those packets hit Ethernet, they can stay segregated by mapping SSIDs to VLANs.

However, those VLANs still compete for shared media inside the wired network when relayed from APs to controllers and from controllers to switches. To address this, minimize the number of wired network hops a real-time packet must traverse, and make sure that each hop provides sufficient capacity. For example, employ distributed network architectures that permit direct client-to-client communication without tunneling real-time traffic back to a distant Controller. Estimate aggregate load on backhaul links and upgrade to Gigabit Ethernet where required to meet aggregate video throughput demands. Eliminate low-speed or unreliable WAN link traversal by installing distributed real-time application servers in branch offices.

In addition to packet flow optimization and capacity planning, look for other sources of latency and jitter that could be deal-breakers for real-time applications. For example, when mobile users roam from one AP to another (or enter/exit a dead zone) they may be required to re-authenticate. In a WLAN secured with 802.1X, this roam time can easily introduce unacceptable delay. Possible steps include virtual-cell solutions (which avoid channel changes), opportunistic key caching, 802.11r fast roaming, or using a faster method like WPA2-PSK to authenticate real-time clients.

On the wired side, minimize dependency on a central authentication server, take steps to reduce latency at intermediate choke points like firewalls, and consider whether and how to use IP multicast for streaming. The latter might surprise you – many WLANs do not handle IP multicast very well because data rates drop to satisfy the weakest (oldest or most distant) receiver. Even if you use IP multicast to reduce wired network load, consider WLAN products that convert wired-side multicast frames into wireless unicast frames that are uniquely-addressed to each client. This might be counterintuitive, but transmitting many frames at much faster data rates can actually consume less airtime, reducing latency for all clients using that WLAN.

Get the most out of real-time applications by optimizing network performance

All of these pieces have a role to play in producing voice MOS values, video MDI scores, and other real-time application performance metrics. Optimizing a network to support these demanding applications can be difficult, especially in a large enterprise network that includes a wide variety of client devices and applications. For best results, use real-time-aware performance test tools to measure MOS and MDI, both before and after network upgrades and expansions. If you cannot see what is going on, it is very hard to tune individual components and parameters to predictably and consistently achieve real-time application goals.

About the author:
Lisa A. Phifer is president of Core Competence Inc. She has been involved in the design, implementation and evaluation of data communications, internetworking, security and network management products for more than 20 years and has advised companies large and small regarding security needs, product assessment and the use of emerging technologies and best practices.

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