Airespace ‘wizard’ sheds light on partner QoS roles

I recently chatted with Bob O’Hara, a founder and director of systems engineering at wireless LAN start-up Airespace (and a Network World “Wireless Wizard”).

I wanted to learn how the quality-of-service (QoS) prioritization capabilities in some of the WLAN switch vendors’ products – like Airespace’s – work with the SpectraLink Voice Priority (SVP) protocol to support wireless VoIP. In other words, what role might each company’s respective products play?

FYI, SVP is wireless phone system vendor SpectraLink’s longstanding, widely deployed mechanism for prioritizing voice packets in WLANs. It has come in particularly handy as the 802.11e QoS enhancements continue to encounter delays.

Generally speaking, SpectraLink handles the transmission synchronization and prioritization of voice packets in the radio-access network. And the WLAN switch-maker – in this case, Airespace  – makes sure that voice packets are prioritized and delay minimized up to the edge of the radio access network and into the wired network.

Here’s a bit more detail about Airespace-plus-SpectraLink from O’Hara:

Generally, in mid- and large-size configurations, there are apt to be many simultaneous calls on a single wireless access point.  SpectraLink adds a voice server to the configuration to coordinate the activity of wireless phones on each individual AP.

The server makes sure the phones are in sync when sending voice packets to and from an AP. They send packets isochronously, one immediately behind the other. They coordinate in both the downlink and uplink directions. In cases where G.711 codecs are used, for example, there will be a burst of downlink traffic from an AP every 20 milliseconds, and 10 milliseconds after the burst, the phones will send a burst of uplink traffic.

This eliminates the possibility that uplink and downlink traffic will collide, O’Hara explains.

SVP has two requirements on the AP: zero random backoff after a transmission for voice packets and packet filtering to identify a voice packet to put it at the head of the transmission queue.

In lieu of the SVP zero random backoff, which Airespace feels violates the 802.11 standard, Airespace supports the 802.11-standard optional point coordination function (PCF). Using PCF, an access point grants a client access to the wireless medium by polling it during contention-free periods. Clients aren’t allowed to transmit until they are polled.

O’Hara notes that part of what needs “fixing” in the forthcoming 802.11e optional standard enhancement for QoS is that “PCF described how AP and mobile device worked over air didn’t include scheduling above the MAC. 11e adds that.”