View Full Version : The VoIP Elephant in the Room: SIP Trunking and E911

VoIP News & PR
07-10-2014, 04:52 AM
Historically, public switched telephone network (PSTN) access has been provided by either analog plain old telephone service lines or voice T1s called Primary Rate Interfaces. The line was brought into the building and was only accessible to those making calls from within the building. That made 911 identification simple. If a call came in over one of those lines, you were positive the caller was in that building. As the world moves to unified communications (UC) and Session Initiation Protocol (SIP) trunking, finding the 911 caller is becoming infinitely more complex.

Challenges with E911 and UC

UC raises a number of challenges with 911 and determining a personís location. Through SIP and PSTN SIP trunking, thousands of users who are located all over the United States or even all over the world may make calls out of a SIP trunk coming from a single data center. One way we can attempt to remedy this is by providing location information based on the originating caller ID of the call. However, doing so makes the huge and often incorrect assumption that a person is always making calls from the same location. In reality, one of the great benefits of UC is mobility. Users can appear to be at their desks yet actually be travelling, working from home, at the beach, or anywhere else. Static location information for E911 has serious limitations.

Next there is nomadic E911. This can take one of two forms. First, we can query users and ask them to input their locations. This can be a bit tedious and highly subject to user error because 911 public-safety answering points require very specific formatting for address information. Second, a growing number of SIP trunking providers are mapping the Internet and providing nomadic E911 as a service. Wherever a user plugs in, the location is preidentified in a database and automatically associated with that user when the user makes a call. This is also an imperfect solution considering that mapping the Internet is an impossible job that gets harder all the time with the rapid growth of the Internet.

SIP Trunking

There are also a number of challenges inherent to SIP trunking as a technology. First, if your network is down, then you canít call 911. In some locations not having a backup connection for 911 is actually illegal, and itís frowned upon in all locales. SIP trunking providers also often have peer agreements with other providers or resell othersí services to have a larger effective coverage area. This can create problems with 911 connectivity or how the information is passed between providers.

Always make sure to discuss 911 requirements with your provider and educate yourself about local regulations. Laws and requirements generally differ by state but can also vary from city to city within the same state or even by building size. When in doubt, seek an expert for consultation to make sure your chosen solution meets all local, state, and federal requirements. SIP trunking is a powerful technology and a great fit for many UC strategies, but it needs to be implemented correctly and carefully to ensure success.

About the Author


Alex Lewis MCITP, CISSP, has a background in telecommunications, IT, and consulting, with more than 15 years of experience. He has worked in a wide range of environments, architecting and implementing some of the largest Unified Communications deployments in the world. The author of eight books, including titles on Active Directory, Exchange Server, and Lync, Alex is currently principal consultant and vice president at Modality Systems and an analyst with Studio B (http://www.studiob.com/).