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Voice on WLANs turns corner

Apr 05, 20043 mins
Cellular NetworksNetwork SecurityVoIP

* Industry tackles voice over WLAN issues

The wireless LAN industry is working hard to get 802.11-based networks in shape to accommodate the technical idiosyncrasies of VoIP. Among the challenges: 

* Latency-induced VoIP performance degradation as users roam. Traditionally, this has required them to repeatedly re-authenticate to the network each time they associate to a different access point.

* A lower level of security on VoIP handsets than on data devices, leaving the network vulnerable to potential spoofing.

* Scarcity of sophisticated location tracking/E-911 support.

* Rapid drain on handset battery life as phones remain associated with access points “in case” a VoIP packet should arrive.

During the next few newsletters, we’ll discuss progress with each of these issues in turn, starting here with latency.

The original vision of WLANs was as a cable-free way to connect desktop computers to network resources. A single AP radio hooked to a cluster of client radio devices, collectively forming a “Basic Service Set,” or BSS. In this model, inter-AP hand-offs aren’t even in the picture.

When the application for WLANs grew to also embrace mobility, then real-time applications, the latency issue reared its head. Total latency in a voice call should not exceed 150 milliseconds, according to International Telecommunications Union recommendations. In the overall delay “budget,” some sources of delay are variable and some are not. Propagation delay, for example, based on distance and the speed of light, is not alterable.

The delay budget share imposed by the wireless network infrastructure for roaming and authentication should not exceed 50 milliseconds, most industry observers agree.

The latency challenge – as well as the other four others mentioned – is addressed in a software module announced this week by WLAN switch start-up Aruba Wireless Networks. Aruba says its new Secure Voice module for its AirOS WLAN switch centralizes state information about each user, eliminating the need for APs to talk to one another.

The company says this keeps inter-AP handoff times to 10 milliseconds. Inter-switch communications is via the Mobile IP protocol and is 20 milliseconds, according to Aruba.

Meanwhile, Chantry Networks announced this week that it is partnering with wireless-optimization software-maker Propagate Networks to propose a vendor-agnostic tunneling protocol to the IETF Control and Provisioning of Wireless Access Points (CAPWAP) Working Group. The aim of the proposed standard is to enable Layer 3 roaming among APs made by any vendor while keeping latency to less than 10 milliseconds, regardless of topology.

And Proxim’s ORiNOCO Switching System, due to ship this quarter, authenticates users when they initially connect to the WLAN, then automatically pre-authenticates them on neighboring APs to eliminate re-authentication.

Finally, a new, relevant letter has made its way into the 802.11 lexicon. The 802.11 Fast Roaming Study Group has officially become an IEEE working group, I’m told, and aims to build an “IEEE 802.11r” standard for inter-AP handoffs that minimize latency and optimize security. Details to follow as they become available.