VoIP services are rapidly becoming the bread and butter of enterprise voice networks, as roughly 72% of all enterprise voice lines shipped by vendors in 2007 were IP-capable. Now that companies are definitively moving away from the traditional Time-Division Multiplexing (TDM) voice networks and into Session Initiation Protocol-based (SIP) VoIP networks, we examine the VoIP industry's most pressing issues, including SIP interoperability, TDM-to-SIP transition services and VoIP security issues.

Is SIP interoperability still a major concern?

As Nemertes Research analyst Irwin Lazar puts it, "it's more a hindrance than major flaw at this point. Three years ago, a team of iLab engineers found that while different VoIP vendors could ensure basic connectivity between their SIP-based phones and devices, there were also "significant failure rates" for enterprise VoIP features and standard security parameters. Lazar says that today key basic VoIP features such as caller ID, message waiting lights, hold and three-way calling are "pretty well standardized," but that more advanced features such as multiple line appearances, call bridging and intercom still face significant SIP interoperability issues between vendors.

Jeff Brandt, the general manager of IT infrastructure design and engineering for business processing firm Sutherland Global Services, expresses a similar viewpoint and says that he has concerns about a lack of SIP interoperability for advanced carrier-level features for his company's call centers, noting in particular that there are limits to advanced enterprise options that have strong SIP interoperability. Brandt says that his main concerns are features such as interfacing with percent allocation capabilities, as well as general bandwidth capacity concerns for SIP-based systems.

"In a contact-center space, it's difficult to predict spikes in the network that are unforeseen," he says. "After AT&T launches the 3G iPhone, for instance, who knows what that will bring to our call centers? The SIP world has not fully matured yet to handle that kind of flux in traffic."

Brandt also says that some companies might experience SIP interoperability problems simply because SIP is a relatively new technology for a lot of enterprises and that IT departments don't yet have the same familiarity with SIP-based systems as with TDM systems.

"Everyone understands how TDM works, but in the SIP environment there has to be some more effort to get people to understand how it operates," he says.

But Marc Tolbert, the volunteer IT coordinator for Bullitt County Adult & Community Education in Shepherdsville, Ky., says that the big SIP interoperability problems are largely experienced by large enterprises, and that small and midsize businesses with more basic VoIP needs will have very trouble-free experiences with their SIP systems.

"Previously, a lot of people would experience problems because of a lack of standards, but that was a few years back," he says. "When it comes down to it, unless you're doing something really bizarre and funky with your implementation, you won't

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