SIP trunking is a term applied to the services offered by local exchange carriers, independent local exchange carriers, competitive local exchange carriers, and Internet telephony service providers to terminate VoIP calls to the public switched telephone network.

SIP trunking allows enterprises and small businesses to eliminate a PSTN gateway at their site and outsource that function to a carrier. It is typically a lower-cost alternative to primary rate interfaces because SIP trunks can be purchased in single-trunk increments (as compared to 23-channel increments for a PRI).

Other ways in which SIP trunks decrease costs:

• With SIP trunks, a single network can be maintained within the organization, rather than having both a voice and data network.

• Internet bandwidth can be used more efficiently.

• Moves, adds and changes can be completed without major wiring upgrades.

• Typical savings over PRIs range from 40 to 60 percent with the payback period for the equipment required, which may include an upgrade to the IP PBX (News - Alert) and the installation of an enterprise session border controller, has been shown to range from 4 – 12 months.

SIP trunks are delivered in several ways. They can be delivered over the public Internet, which allows any enterprise, anywhere, to adopt SIP trunking and assign some (possibly unused) bandwidth to voice at no extra charge for the connection, and providing the highest ROI.

SIP trunks can also be delivered as a managed service. Carriers supply a dedicated, fully managed connection from their point of presence to the enterprise site. This service offers quality of service guarantees, but is somewhat more expensive.

Carriers can also deliver a managed service using multi-protocol label switching to insure the highest voice quality and reliability.

The voice quality, even over an un-managed public Internet connection, can be excellent. Digital packetization of voice has proven to be better than analog. This is because digital packets do not suffer from audio distortions, loss levels, and other weaknesses. The quality of voice is directly related to the quality of the network, with the "weak link" of VoIP being latency in a slow network. A SIP-capable E-SBC with QoS solves this problem.

With these facts in mind, there is no question that SIP trunking offers compelling advantages for businesses large and small.

Steven Johnson is president of Ingate Systems (News - Alert) (www.ingate.com).