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Telephone Trunk Service For VoIP Phone Systems
How telephone trunking works and what options are offered for your phone system.

By: John Shepler

Any business with more than a few outside phone lines knows that trunk lines are the way to go. The next question is what type of trunk service to get. You might think that with a VoIP telephone system, the obvious choice is SIP Trunking. But is it? Take a closer look at what’s available and then decide.

The Need For Telephone Tunks
The idea of telephone trunking originated with the phone companies back when Ma Bell ran the industry. When phones were new, a couple of wires were all you needed to connect from your location to the local phone company. Then everybody wanted a phone and businesses started installing switchboards to handle all the lines they needed. You know those old-timey photos of phone line congestion in the cities. That’s what happens when you build out one line at a time.

One Line, Many Calls
The mass of subscriber lines was gotten under control using multi-pair cable, often buried. But long distance lines between switching offices were another matter. The more simultaneous calls you carried, the more wires were needed. That gets expensive fast when you’re talking about dozens or hundreds of miles. Wouldn’t it be nice if one pair of wires couldcarry multiple conversations? That's the niche that trunk lines fill.

Analog Trunking
The early systems were set up like radio stations on the dial, except over wires. Each call modulated its own carrier frequency that was spaced far enough in the spectrum from adjacent carriers so that they wouldn’t interfere. This was analog carrier telephony. The switch from analog to digital technology came with the T1 line. It’s been around for at least half a century and is still going strong.

T1 Trunking
T1 telephone trunks are divided into 24 separate channels, each transporting one two-way telephone call. All the dialing and switching is done on these channels, just as it would be on analog phone lines. You can get devices called channel banks to convert T1 to and from 24 analog phone lines at your office. The T1 line itself is two twisted pair running from your building to the nearest phone company central office.

Cost Advantages
The advantage of T1 lines as telephone trunks is that save a lot of wire and can save cost, too. There is no economy of scale when you buy your phone lines one at a time. Soon you have a dozen or two at the price of a dozen or two times the cost of one line. If you convert all those lines into a T1 telephone line (trunk), you’ll generally save money even if you only use half the capacity of the T1. If you need dozens of lines for your call center, T1 can save you a bundle.

Connecting to the PBX
Chances are that you won’t need a channel bank unless your phone system is completely analog. Most PBX systems have T1 ports already installed. Even the new IP PBX systems designed for VoIP telephones usually have T1 ports where you just plug in one or more T1 lines for your outside telephone service.

ISDN PRI Trunking
A closely related service that is even more popular than T1 telephone trunks is ISDN PRI. This service runs on a T1 line and is very similar to a T1 trunk. The difference is that only 23 channels are used for individual phone lines. The remaining channel is used for all the dialing, switching and data such as Caller ID. The switching is faster than with T1 telephony and most everyone wants Caller ID anyway. The port for this is called a PRI, ISDN PRI or T1 PRI an it is likely installed in your system now.

SIP Trunking
The other digital trunk option that is becoming popular thanks to VoIP is the SIP Trunk. SIP is the switching protocol used with IP telephones in a VoIP system. A SIP Trunk, which might also be carried on a T1 line, is not broken up into channels. Instead it is more like your local area network that transports Ethernet packets.

SIP Trunks Support Converged Networks
This is one of the big advantages of a SIP Trunk. It can connect directly to your network without having to convert from one protocol to another. In fact, just like your network carries both VoIP phone traffic and computer data traffic, a SIP Trunk can provide both telephone and broadband service from your service provider. This is the preferred arrangement, as it allows the provider to establish class of service controls to maintain voice quality regardless of other network traffic.

SIP Trunk Options
As you might suspect, an IP PBX system works well with a SIP Trunk. But SIP trunks can also be ordered with customer premises equipment that offers a T1 or ISDN PRI digital handoff or even analog phone lines.

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