As a matter of fact, large enterprises, particularly those in financial services and those running call centers, are already starting to deploy SIP as a way of tying together their disparate PBXs. Not only that, but the major carriers are now starting to roll out SIP trunking services to these very same enterprises.

While there are differences in the dialects of SIP supported between the various brands of PBX, there is generally sufficient commonality for trunk-side interoperability at least. (Line-side features tend to be a different matter, as this is where the vendors add their special sauce.)

Deployment of SIP trunking, both between PBXs and to the service provider, gives rise to two sets of requirements:

1. You need a Session Border Controller to provide border security for VoIP and other real-time traffic (including video and IM), as conventional firewalls can't handle the real-time traffic.

2. You need a SIP Session Manager to do call routing between the various systems, dial-plan management and any necessary protocol fixup that remains. And that's just a start; a good Session Manager can:

• Hook into enterprise directories to do calling name dips and convert SIP URIs into phone numbers, and vice-versa;
• Use Web Service interfaces into CRM or other systems to automatically pull up, say, client details as calls are delivered to the desktop;
• Intelligently route calls based on network QoS conditions;
• Provide centralized policy control over who can make calls to whom, at what times of day, and using what network bandwidth;
• Provide a link between IM & Presence systems and PBXs, to enable call control from IM clients and to reflect on-the-phone status back to the client.

(You will not be surprised to learn that Covergence makes a SIP Session Manager that addresses both these sets of requirements.)

The reason enterprises are deploying SIP trunking is that there are huge cost savings to be made by keeping calls off the PSTN, particularly with call centers. In a call center, a call typically hits an IVR and is subsequently transferred to a live agent, who may be located in a different city or even country. The cost of taking back and transferring the call is high -- certainly higher than that of making a regular long distance call. If such transferred calls can be moved off the PSTN to the corporate network, the savings are significant.

However, doing trunk-side integration is just the first stage. What we're seeing corporate voice architects moving towards is in effect adopting an enterprise version of IMS. In this architecture SIP endpoints gain access to the network through a Call Session Control Function (CSCF, aka the SIP Session Manager), which provides security, policy control over access and call routing. In turn the CSCF relies on information provided by corporate directory servers and other systems. PBXs become application servers, running alongside call center software, IVRs, IM & Presence servers and other communications-enabled applications. The glue holding all this together will be a combination of SIP and Web Services (SOAP/XML).

So my answer to you is that in the short term current IP PBXs from different vendors can certainly be integrated, using SIP. Do we need additional SIP-based standards? Well, they come along all the time through the IETF and the 3GPP/3GPP2/TISPAN projects. Some take hold, while others gain little traction in the market. However, a bigger question is how enterprise communications will be architected in future, and how it will mesh with carriers' mobile offerings.

Rob Welbourn
Senior Product Manager
Covergence, Inc.
Maynard, MA