SIP trunking is becoming a very popular way to connect to the public switched telephone network (PSTN). Session Initiation Protocol (SIP) is a signaling protocol widely used to set up and listen to multimedia communication sessions, such as voice and video calls over the Internet. Instead of a line from your local provider (AT & T, Verizon, etc.), you connect to one of several SIP trunk providers, sometimes called an Internet Telephony Provider (ITSP). They can provide phone numbers in most area codes and provide features such as the Direct Inward Dial (DID) and Caller-Id over a SIP trunk. Think of Vonage, but on an enterprise scale.
What confuses a lot of people is that the SIP trunk is a logical connection. This is not like POT, T1 or PRI, which are physical compounds. The most common ways to connect to ITSP are via the Internet or a private MPLS scheme. The biggest limitation to using the Internet is the lack of quality of service (QOS), but it's amazing how well it works.
SIP channels have been supported in Cisco Unified Communications Manager for some time. Configuring SIP trunks directly in UCM is limited. That is why Cisco recently added enhanced support for trunking SIP on its routers. Cisco Unified Border Element (CUBE) is a feature set that can be added to a voice gateway. There is an added value for software, but compared to the cost savings of SIP this can make a lot of sense.
For more information, visit my blog at http://www.VoIP-Tutor.com/blog
