Session Initiated Protocol(SIP) trunking is a technology that supports phone calls traveling as Internet Protocol (IP) signals from beginning to end, allowing enterprises to communicate over IP beyond the boundaries of their networks.The technology (and the technologies it’s replacing) is fraught with jargon, so we’re going to break down the details before we look at what the shift to SIP looks like.

The Basics

First, let’s outline the various technologies that are involved in making and receiving phone callsin a non-SIP environment. Legacy phone systems send and receive calls using time-division multiplexing (TDM) signals. Those TDM signals are sent over a primary rate interface (PRI) circuit via integrated service digital network (ISDN) lines. Each PRI has 23 channels, meaning they can support 23 simultaneous voice calls. Many companies go one step further and dedicate specific PRIs to handle inbound traffic, with other PRIs carrying only outbound traffic.This eliminates potential competition for the fixed number of available lines. From the organization’s internal phone system, PRIs connect to the public switched telephone network (PSTN), where calls are routed to their destinations.

How Does SIP Trunking Work?

In a nutshell, SIP trunking replaces an enterprise’s PRI circuits. The technology eliminates the fixed size and function of a PRI in favor of a more open communication pathway. SIP trunks offer greater flexibility than PRIs—for example, enterprises are no longer limited to purchasing services in 23-channel increments—and can support voice, data and video traffic in a single pipe. Bill Long, vice president of Enterprise Voice Services at Broomfield, Colo.’s Level 3 Communications, describes SIP as an equivalent to one of the voice channels carried within a PRI, but one that has “the right to send and receive a phone call at any given point in time.” Unlike PRIs, which are typically assigned a single type of voice call, SIP trunks can support a range of services simultaneously, such as local inbound and outbound calling, long distance and international traffic and toll-free calls.

SIP trunks are increasingly popular with businesses, as evidenced by research conducted by Infonetics, which revealed that a third of enterprises use SIP trunks today, and a total of 42 percent plan to have them implemented by 2014. “Essentially there is an IP pipe that the SIP trunk is delivered over, and then generally there is some sort of premise equipment,” says Charles Studt, vice president of Product Management at IntelePeer in San Mateo, Calif. The flavor of on-site equipment varies depending on each organization’s telephony infrastructure. In many cases, it consists of either a TDM-based or IP-based phone system, but the number of companies using unified communications (UC) platforms to handle their voice traffic is growing.

Saving Money Through SIP

Companies making the switch to SIP often do so to reduce their monthly phone bills, but how much can you really expect to save? It depends on a number of factors, including what sort of prices you were able to negotiate for your legacy telecom services, as well as your company’s network architecture. “We generally find that our customers save on average about 30 percent over their traditional PRI telecom costs,” Studt says. Voice traffic can be consolidated for

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