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Trunk Interface

Sip Phone And Sip Trunking

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SIP phone and SIP trunking

Session Initiation Protocol (SIP) offers an advantage for small businesses that have moved to VoIP. To understand the advantage, we must first look at the issue of using SIP trunking versus PRI (Primary Rate Interface).

In traditional telephone systems, physical wires are installed on site, but a SIP trunk replaces this with a SIP trunking provider that offers virtual provisioning. As a result, the need for a local Public Switched Telephone Network (PSTN) gateway is eliminated, as is the need for the Primary Rate Interface (PRI), which can be costly, especially to small business. To deploy SIP trunking, your IP PBX will need to have a SIP trunking connectivity port, and most modern IP PBXes do include this functionality as a standard feature. At the same time, your firewall should be SIP-enabled to allow for VoIP calls to be protected. Again, most modern firewalls are SIP-capable.

There are actually three components to a SIP environment, starting with the SIP-enabled IP PBX. The Internet telephony service provider (ITSP) forms the second component, providing a connection between the IP network and the PSTN. Lastly, the border element (such as a SIP-enabled firewall) connects the enterprise IP network, the PSTN, and an external carrier network. The service provider usually includes SIP trunking as part of the phone package. Before contracting with a SIP trunking service provider, review the provider's quality of service terms and service level agreement to determine responsiveness, call quality, security guarantees and other important factors.

A T1 line is a high-bandwidth line often used to connect a midsize business to the Internet, and it is also used in offices that have a high call volume. The Primary Rate Interface divides that T1 line into 23 channels, with each channel carrying a single phone call. The number of channels required for the office is dependent on the peak number of simultaneous calling, rather than the physical number of phone extensions.

The downside to the PRI approach is that the PRI is provisioned in groups of 23 channels, which for a small business, may result in overprovisioning and unneeded expense. Both approaches are scalable, but the PRI approach is scalable only in larger quantities. SIP trunk scalability is more incremental. SIP uses the same T1 line, but does not divide it into channels, and so an entire PRI is not necessary for an incremental upgrade. As a result, there is no need to purchase unneeded channels when only a small upgrade is required.

Some businesses have resisted moving to SIP trunking as opposed to PRI because the PRI did, at one time, offer a superior level of call quality. In addition, PRI was often chosen over SIP trunking because it offered the ability to deploy Direct Inward Dial (DID) numbers for simpler call routing. SIP trunking however, today does offer the same capability, and call quality is equivalent.

There is a cost advantage, both for small and large companies. Smaller companies may be more concerned with incremental scalability. However, a bigger concern is installation. The PRI approach calls for a physical installation of the T1 copper pair, while SIP involves a "virtual" deployment—and this therefore makes installation and maintenance costs significantly less.

About the Author

THE OFFICE PHONE www.theofficephone.com is Colorado leading resource helping businesses to find the best fit and value for the phone system they are looking to purchase or lease.Since 2005, THE OFFICE PHONE has been committed to test and evaluate all the biggest names you know to the small but trustworthy brands just waiting to be found. With THE OFFICE PHONE, you can find the right IP or VOIP Phone  system at the right price that will scale and grow with your business.

Question about VLAN and trunking between switch and router?

I've got a simple little network I need to setup that is going to have a router and a switch + 3 wireless access points.

many feel you should set up a trunk port anyway - the value of it is that you can add others later if the need arises

Cisco IOS - switchport mode trunk