Aruba Networks Version 3.3 manual Voice Functionality and Features, Traffic Prioritization

call quality. Jitter buffers are used in VoIP networks to smooth out this effect, but they add delay and must be as small as possible. Aruba Mobility Controllers adjust network settings to minimize jitter and maximize voice quality.

Traffic Prioritization

Aruba Mobility Controllers use traffic prioritization as one method to address delay and jitter. Traffic prioritization assures that voice packets have preferential access to the media and are moved ahead of best-effort traffic in buffers during congestion.

Application-based prioritization requires stateful inspection; this capability is a crucial difference between an Aruba solution, and competing wireless solutions. Competing solutions prioritize based on a wireless SSID, meaning that all traffic transmitted on a particular SSID is treated the same. This precludes support for voice applications running on multi-function devices such as laptops or PDAs, since these devices use multiple protocols. Aruba Mobility Controllers contain a policy enforcement firewall that statefully identifies, tracks and dynamically prioritizes traffic based on the application flow, e.g., giving higher priority to a SIP session than an HTTP session, even from the same device.

Network Wide QoS

While the Aruba Mobility Controllers can handle much of the heavy lifting by identifying, properly tagging, and scheduling packets into the network, the rest of the components must also be ready to handle QoS. If the access, distribution, core, and data center switches and routers are only providing best effort delivery, voice quality will suffer. At each level in the network, devices that will be forwarding QoS tagged traffic must be configured to properly prioritize traffic above data and background traffic.

Voice Functionality and Features

Voice Service Module features provide deep visibility into the session, such as, viewing the call progression and voice quality of a SIP based VoIP call. Advanced voice-over-WLAN features such as Call Admission Control (CAC), voice-aware RF management, and voice-specific diagnostics allow the Mobility Controller to deliver enterprise class mobile VoIP capabilities.

Voice-Aware RF Management

As discussed in Chapter 7 on page 55, Aruba’s Adaptive Radio Management (ARM) is normally configured to adjust channel and transmit power levels of wireless APs based on nearby interference and other RF conditions. Client devices will react to a channel change by scanning for a new AP and then re-associating as though they were roaming. Most data applications will not be noticeably affected by this action. Voice is highly susceptible to packet loss, however, and a channel change during a voice call will very likely cause packet loss and audible disruption to the call.

Because Aruba Mobility Controllers statefully follow voice protocols, they will not allow a channel change while voice calls are taking place. If a channel change is required, the controller will wait until that AP is no longer handling active voice calls before initiating the channel change.

Call Admission Control

Typical voice codecs (Coder/Decoder) used in VoIP do not consume large amounts of bandwidth. Even with G.711, which uses 64Kbps per call, a typical 802.11b access point could theoretically support nearly fifty simultaneous calls based purely on bandwidth. In practice, the limiting factor is contention for the wireless medium because 802.11 uses a collision-avoidance algorithm that makes timely access to the wireless media a challenge for delay-sensitive devices. Due to this limitation, the number of

60 Voice over Wi-Fi Campus Wireless Networks Validated Reference Design Version 3.3 Design Guide