Sipping on the Trunk Lines

Telephone trunks have a long history, dating back to the dawn of telephony. A trunk is simply a line that carries multiple phone conversations. Trunks were originally built to connect switchboards and then switching offices together. They could be bundles of wires, analog multiplexed circuits that work something like Cable Television to transport many calls on one cable, or digitally multiplexed circuits that are used today to connect PBX telephone systems to telephone service providers. T1 telephone, ISDN PRI, DS3 and SONET are common services that provide digital telephone trunking. But there is a newcomer on the scene that is gaining in popularity. It's SIP trunking.

The idea behind telephone trunking is that you save money by aggregating phone lines onto channels in a single higher bandwidth connection. T1 lines, including ISDN PRI, start saving money over separate phone lines when you get up to 10 or 12 outside lines. ISDN PRI is a special type of T1 line that gives you up to 23 phone lines plus Caller ID. It's a very popular trunk line for traditional PBX telephone systems.

SIP trunking was developed to serve VoIP telephone systems. SIP stands for Session Initiation Protocol. This is a simple telephone switching protocol that has become popular with enterprise VoIP phone systems. Many newer telephone desk sets have SIP and VoIP capabilities built-in. You simply plug them into your LAN rather than a separate telephone network. An in-house IP PBX also sits on the network and provides the call control between handsets and outside telephone lines.

It's those outside lines that SIP trunking addresses. You can connect with the PSTN or Public Switched Telephone Network yourself by plugging-in individual phone lines to your IP PBX. You are then "terminating" the calls to the PSTN. That's actually a good strategy if you have only a few outside lines. Your internal company calls stay on the network and you pay just the incremental cost of each line for outside access. You also have the option of using a T1 or ISDN PRI line as a digital trunk between your PBX and your telephone service provider. In this case you're transporting your calls digitally rather than analog, which provides a cost savings when you have more than a few phone lines.

SIP trunking goes a step further. A SIP trunk keeps your phone calls in VoIP packet protocol and uses the same SIP signaling and your handsets and IP PBX. Many SIP trunks can be thought of as converged voice and data network connections to your phone service provider. The SIP trunk can carry broadband Internet as well as telephone conversations. Voice packets have priority to ensure call quality. Remaining bandwidth is used for dedicated Internet service. Actual call termination to the PSTN is done at the service provider. They provide this service in bulk to many users, so it stands to reason that they can offer a cost savings on the calls off the network. Calls that stay on the network, including other companies that use the same trunking service, might be offered at no additional charge.

Does SIP trunking sound like a logical service for your enterprise VoIP telephone system? If so, find out how much you can save by using competitive SIP trunking phone service at your company.