| Session Initiation Protocol (SIP) offers an advantage | | | | a midsize business to the Internet, and it is also used in |
| for small businesses that have moved to VoIP. To | | | | offices that have a high call volume. The Primary Rate |
| understand the advantage, we must first look at the | | | | Interface divides that T1 line into 23 channels, with each |
| issue of using SIP trunking versus PRI (Primary Rate | | | | channel carrying a single phone call. The number of |
| Interface). | | | | channels required for the office is dependent on the |
| In traditional telephone systems, physical wires are | | | | peak number of simultaneous calling, rather than the |
| installed on site, but a SIP trunk replaces this with a SIP | | | | physical number of phone extensions. |
| trunking provider that offers virtual provisioning. As a | | | | The downside to the PRI approach is that the PRI is |
| result, the need for a local Public Switched Telephone | | | | provisioned in groups of 23 channels, which for a small |
| Network (PSTN) gateway is eliminated, as is the need | | | | business, may result in overprovisioning and unneeded |
| for the Primary Rate Interface (PRI), which can be | | | | expense. Both approaches are scalable, but the PRI |
| costly, especially to small business. To deploy SIP | | | | approach is scalable only in larger quantities. SIP trunk |
| trunking, your IP PBX will need to have a SIP trunking | | | | scalability is more incremental. SIP uses the same T1 |
| connectivity port, and most modern IP PBXes do | | | | line, but does not divide it into channels, and so an |
| include this functionality as a standard feature. At the | | | | entire PRI is not necessary for an incremental upgrade. |
| same time, your firewall should be SIP-enabled to allow | | | | As a result, there is no need to purchase unneeded |
| for VoIP calls to be protected. Again, most modern | | | | channels when only a small upgrade is required. |
| firewalls are SIP-capable. | | | | Some businesses have resisted moving to SIP trunking |
| There are actually three components to a SIP | | | | as opposed to PRI because the PRI did, at one time, |
| environment, starting with the SIP-enabled IP PBX. The | | | | offer a superior level of call quality. In addition, PRI was |
| Internet telephony service provider (ITSP) forms the | | | | often chosen over SIP trunking because it offered the |
| second component, providing a connection between | | | | ability to deploy Direct Inward Dial (DID) numbers for |
| the IP network and the PSTN. Lastly, the border | | | | simpler call routing. SIP trunking however, today does |
| element (such as a SIP-enabled firewall) connects the | | | | offer the same capability, and call quality is equivalent. |
| enterprise IP network, the PSTN, and an external | | | | There is a cost advantage, both for small and large |
| carrier network. The service provider usually includes | | | | companies. Smaller companies may be more |
| SIP trunking as part of the phone package. Before | | | | concerned with incremental scalability. However, a |
| contracting with a SIP trunking service provider, review | | | | bigger concern is installation. The PRI approach calls |
| the provider's quality of service terms and service level | | | | for a physical installation of the T1 copper pair, while |
| agreement to determine responsiveness, call quality, | | | | SIP involves a "virtual" deployment-and this therefore |
| security guarantees and other important factors. | | | | makes installation and maintenance costs significantly |
| A T1 line is a high-bandwidth line often used to connect | | | | less. |