Last week, we pointed out that with services like Sprint’s Airave and T-Mobile’s HotSpot @Home, callers can use their mobile handset connected to a femtocell or Wi-Fi router to place a voice call using the Internet for access to the PSTN. Under these two plans, minutes of use don’t count against the subscriber’s usage. But, as with any new service a few issues remain to be solved. For example, how can caller locations be identified for E-911 services when users roam between a femtocell, Wi-Fi, and cellular network?

The issue of locating callers placing an emergency call when using VoIP has been largely solved as we’ve discussed. For more information on FCC mandates for locating callers on a VoIP call, click here; for a discussion on how a VoIP caller location is identified, we refer readers to our newsletter here.

In a wireless world, the user’s location depends on the kind of transmission technology. GSM networks like those deployed by AT&T and T-Mobile use triangulation hardware and software that locate the handset based on signal strength recognized from multiple cell towers. CDMA networks like those used by Verizon and Sprint use Assisted GPS (A-GPS) that supplements triangulation with a GPS “Assistance Server.” As with VoIP calls, the FCC has also mandated a benchmark for reliability that was most recently discussed in an FCC statement.

Under the mandate, a GSM caller’s location must be identified within 50 meters 67% of the time and within 100 meters 95% of the time, according to Brian J. McNiff, who leads the marketing, business development and product management teams at TechnoCom. TechnoCom is a leading-edge provider of solutions to enable wireless location services and ensure their ongoing performance.

But back to our opening question: Is a mobile handset connected to a femotcell wireless router a VoIP caller or a wireless caller, and how should the user’s location be identified? We’ll hear more from McNiff on the topic and disclose how Sprint and T-Mobile address the question in our next newsletter.

Steve Taylor is president of Distributed Networking Associates and publisher/editor-in-chief of Webtorials. Larry Hettick is a principal analyst at Current Analysis.