Session Initiation Protocol (SIP) trunking services from IP telephony service providers have been on the market...
since 2005, but deployments have been limited for a couple of reasons. First, it seems many network managers I talk with do not fully grasp what a SIP trunk is, exactly, and how they work. Second, for those who do understand the concept, only now do I feel that the business case, which can be very compelling, is finally being understood. Organizations looking to deploy Voice over Internet Protocol (VoIP) should take a serious look at SIP trunks as part of their deployment plans.
Let me start by explaining what a SIP trunk is. In traditional telephony, the phone company delivers telephony services over a wire, or a "trunk," that connects the corporate private branch exchange (PBX) to the public switched telephone network (PSTN). This physical trunk carries the phone calls from the corporation to the PSTN providing the corporation with phone service. A SIP trunk allows companies to replace physical cables with telephone service over a data network -- whether it's through a dedicated line, a shared connection with a data service or even by using the Internet for connectivity.
A SIP trunk can deliver much more value to a company than a traditional PSTN trunk. First, there's no real limit to the number of voice sessions that can be carried over a SIP trunk (other than bandwidth), whereas a traditional PSTN trunk is limited to the number of channels available. Also, SIP can deliver many of the unified communications (UC) functions that companies are looking at today. Chat services, presence, conferencing capabilities, the ability to share applications, etc., can all be delivered over a SIP trunk, allowing companies to look at purchasing UC as a service rather than investing in premise-based hardware and software.
SIP trunks also allow organizations to extend VoIP past the physical LAN, where most of the deployments are today. This removes the need for organizations to purchase costly gateways, bridges or other equipment that help connect the corporate VoIP environment to the PSTN. That connectivity is done within the IP telephony service provider's network, which means the enterprise does not have to incur the cost.
To deploy a SIP trunk you really need only three things: An IP PBX with a SIP-ready trunk, a device that sits at the edge of the enterprise that can talk SIP and a service provider that offers a SIP-trunking service (not all do today). The device is similar to the gateway needed for PSTN connectivity, but it's typically much cheaper on a per-user basis than a gateway and it can be a gateway to many more services, other than just voice, providing a much faster return on investment than with a traditional trunk.
Overall, a SIP trunk is a simple, cost-effective method of increasing the value of the investment you've made in VoIP. There are many other benefits as well, such as increased user productivity, convergence of multiple lines, better bandwidth utilization and, if deployed correctly, more uptime than traditional telephony (I'll get into these in future columns).
I urge anyone who has deployed VoIP or has a rollout under way to take a serious look at using SIP trunks to maximize the benefits of your investment.