SIP trunking: A primer

TDM trunks have long served to connect corporate PBXs to the public switched telephone network (PSTN). But with more companies moving to VoIP, SIP trunking has become an increasingly popular technology companies can use to simplify their network architecture and save money. Here are the basics:

Let's take it one part at a time. SIP refers to Session Initiation Protocol, the standard developed in the 1990s by the Internet Engineering Task Force that is used to set up and terminate VoIP calls and generate dial tone. A SIP trunk, then, is a broadband Internet link that utilizes SIP to connect a company's IP-based PBX to an Internet telephone service provider (ITSP). Instead of terminating the trunk directly at the IP-PBX, for security sake companies tend to terminate the trunks at a SIP-capable session border control system that acts as a firewall.

The big one is simply whether SIP trunks will be available for all your branches. As Leo notes, businesses with offices in rural or remote areas could have difficulty finding a vendor that covers all their locations: "As service providers ramped up they have addressed large metropolitan areas first. But when you get to more remote locations it's lagging behind. It's only been in the last two years that SIP trunking has become available to enterprises."

SIP trunks provide the most efficiency for businesses that have multiple locations spread out over a wide area. If you only have one central location, or if you have offices that are located in a very concentrated geographic area, then a SIP trunk will probably not be worth your time.