SIP trunking: A primer

How connecting your IP PBX to a SIP trunk can save serious cash

By Brad Reed, Network World
July 12, 2010 12:00 AM ET

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:

Top five SIP trunking gotchas

Just what is SIP trunking?

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.

How does a SIP trunk save money?

By drastically consolidating and simplifying your voice architecture. SIP trunks can support voice, data and video all over IP, meaning a single trunk can replace multiple TDM trunks.

"If you have multiple offices and have a highly distributed network, you're probably going to have a lot of TDM trunking going into those organizations," says Michael Leo, the director of enterprise solutions marketing at Acme Packet. "With SIP trunking you can reduce your traditional amount of TDM connectivity by better utilizing connectivity across the board."

SIP trunking also makes it possible to add capacity during times of high call traffic. If you rely on T-1 lines, for instance, you have to purchase 24 channels even if you only use five of them at any given time. With SIP trunking, if your provider supports the capability, you just assign bandwidth to locations as needed to deal with high call volume. "Without SIP trunking you have to buy extra lines and pay for them all year whether you need them or not," says Nemertes Research analyst Irwin Lazar. "With SIP trunking you can burst call access during special times."

How much money can it save?

Lazar estimates that on average companies that adopt SIP trunking save 20% to 60% from what they pay now for TDM trunks. Citing one particularly dramatic case study, Leo from Acme Packet said he knows of one company that used 1,500 SIP trunks to replace 2,250 trunks, a shift that reduced telecom expenses from $5.4 million per year to $945,000 per year.

What questions should I ask a SIP trunk service provider before investing?

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."

The second big question has to do with interoperability, as many SIP trunking providers will only support a limited number of vendors. In other words, some SIP trunk providers may only support session border controllers from Avaya while others might only support session border controllers from Cisco.

And finally, you'll want to ask about pricing schemes, both in terms of overall installation costs and flexibility to quickly and cheaply add capacity. Since SIP trunks are still a relatively new technology, they don't really have standardized pricing schemes and can vary widely in the services they provide.

"You'll definitely want to ask them how quickly they can get a SIP trunk up and running," says Anne Coulombe, an Avaya executive. "Some of the tier-two service providers are extremely rapid in being able to ramp up SIP trunks, while the big carriers are a little slower."

Finally, is there any business where a SIP trunk is not worth the investment?

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.

"If you're a company that's already oriented toward TDM and most of your calls go between one or another town, then you might not need SIP trunking," Coulombe says. "So a pharmacy with only two branches might not need it, but if that pharmacy grew to have 15 branches then SIP trunking would be really worthwhile for them."

Read more about lans & wans in Network World's LANs & WANs section.