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February 2008 | Volume 11/ Number 2
Packet Voice Over Wireless

802.21 — Cross Network Roaming Improvements

UMA and perhaps later VCC have adequately solved the problem of moving voice sessions back-and-forth between cellular circuit switched and WiFi (News - Alert) connections for consumers. Data sessions don’t benefit from these technologies, nor do enterprise voice connections that have to support PBX features.

For voice service that combines the mobility features of cellular service with the enterprise PBX (News - Alert), there are proprietary mobility controllers from numerous companies like DiVitas, Tango, Aruba, Firsthand, Cisco, Avaya and RIM.

A technology that can complement all these solutions on the data side has been gestating in the IEEE (News - Alert) since 2003. It is called 802.21. The standard is due to be approved in March 2008.

The founders of 802.21 were frustrated that their laptops would often try to associate with the wrong 802.11 network when faced with a choice. They conceived of a mechanism that automatically led the PC to associate with the most appropriate network.

802.21 specifies a server that gleans information about networks from multiple sources, applies rules, and returns to the clients advice about which is the best network to be using at any given moment. It doesn’t cover the actual handover process, only the network discovery and selection, and the setup of the next link. It doesn’t just apply to 802 networks like WiFi and WiMAX (News - Alert), but to any wireless network, including cellular networks.

Since each of these network specifications already provides for handover, the 802.21 working group has had to work with them to retrofit protocol changes to accommodate the 802.21 protocol. The changes to 802.11 are being handled in 802.11u, the changes to WiMAX are being handled in 802.16g. 3GPP has also indicated that it will put provisions into cellular protocols to transport the 802.21 messages. The IETF is adding 802.21 support to its Mobile IP protocol work.

All cellular voice currently goes over TDM circuits. TDM is more bandwidth-efficient than VoIP, and the carriers have vast investments in it. As radio technology continues to evolve there will come a time when it will make sense to phase out the TDM and use VoIP for cellular voice service. When this happens, 802.21 will be useful even for voice session handoffs between cellular and 802 networks.

All this means that we will soon be hearing more about 802.21; mobility controller vendors will have this as a checklist item in their brochures, and ideally we will find that roaming between cellular, WiFi, WiMAX and other networks becomes seamless, swift and reliable. IT

Michael Stanford has been an entrepreneur and strategist in VoIP for over a decade. In his current consulting practice, he specializes in VoIP wireless networks, both WiFi and WiMAX. Internet Telephony Magazine recognized him as one of “The Top 100 Voices of IP Communications” and VoIP News named him one of “The 50 Most Influential People in VoIP”.

» Internet Telephony Magazine Table of Contents

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