The IEEE recently ratified the long-awaited 802.11r standard for fast handoff, officially named Fast Basic Service Set Transition. 802.11r, in development for four years, is a key component to solving the performance challenges associated with VoIP over Wi-Fi in large-scale networks.
802.11r reduces handoff delays associated with 802.1X authentication by shortening the time it takes to reestablish connectivity after a client transitions from one 802.11 access point (AP) to another while roaming.
A wireless VoIP expert at Polycom (formerly SpectraLink) explains how the mechanism works to balance the QoS requirement for very fast user re-authentication during roaming with 802.11i’s very high standard security levels. Geri Mitchell-Brown, the company’s director of technical business development who also chairs the Wi-Fi Alliance’s Voice over Wi-Fi marketing task group, writes:
“802.11r joins other standard QoS mechanisms, such as packet prioritization and call admission control (CAC), to improve the performance of real-time applications such as voice. It balances the reduced handoff time with high levels of security through three MAC-layer enhancements:
* The 802.1X key exchange is not required on handoffs between two APs within the same ‘mobility domain,’ which is a group of APs configured to support fast transitions between them.
* The four-way handshake for session key establishment is integrated in the already-existing 802.11 authentication/association messages, thereby decreasing the delay after re-association until security negotiation completes, allowing data transmissions to resume faster.
* Call resource requests are bundled into new authentication messages exchanged before the re-association.
“The forthcoming Wi-Fi Alliance Voice-Enterprise certification program (expected to begin in mid 2009) factors in the need for both enterprise-class security and good voice quality. The certification is planned to require support of 802.11r by both APs and client devices. Secure handoffs within the mobility domain must be under 50 milliseconds.”
The completion of this standard is a critical piece to realizing enterprise-grade security and QoS for voice applications, according to Mitchell-Brown. Once these standards are widely implemented, “we anticipate the mainstream adoption of voice over Wi-Fi,” she says.