Sunday, October 14, 2007

SIP vs PRI Telephone Trunking

Telephone trunking is the means of transporting multiple telephone conversations across metro or long distances on a single line. The first approach to this used analog frequency division multiplexing in an approach known as carrier telephony. The noise and crosstalk associated with carrier telephony has long since been replaced by digital trunk lines using time division multiplexing techniques. But now TDM trunk lines are being challenged by an even newer technology called SIP trunking.

The impetus for SIP trunking is the move from the traditional circuit switched telephone world to packet switching. In other words, the traditional PBX system is giving way to the iPBX or VoIP telephone system. Does that mean that T1 telephone lines are yesterday's news? Not by a long shot.

The dirty little secret is that just about all business telephone systems with more than half a dozen phone lines are using T1 trunk lines or their big brother, the T3 line or DS3 connection. That includes the latest IP PBX systems, even those that are connected to their phone service providers via SIP trunking. The reason is simple. T1 lines are the most universally available and cost effective carriers commonly available.

The real discussion is what format T1 line to employ. Business office PBX systems and call centers usually want a variant known as T1 PRI. The PRI stands for Primary Rate Interface, a term that comes from a set of telecommunication standards called ISDN or Integrated Services Digital Network. What PRI adds to a standard channelized T1 line is a dedicated signaling channel that makes call setup and teardown faster, plus the ability to provide Caller ID and ANI (Automatic Number Identification.) A T1 PRI line gives you 23 channels that can each carry one telephone call plus the data or signaling channel. If you need more phone lines, simply add another T1 PRI line to your system. If you need a lot more phone lines, say more than 4 or 5 T1 PRI lines worth, you may find it more cost effective to upgrade to a T3 line or DS3 service on fiber optic carrier.

SIP trunking also uses a T1 line, but one configured differently than a T1 PRI line. In this case, channelization is unnecessary because the phone conversations are assigned to packets, not channels. SIP or Session Initiation Protocol is a switching system used for IP telephony, also known as VoIP. SIP is becoming more popular, supplanting other switching protocols in VoIP phone systems.

Both T1 PRI and SIP trunks connect between your phone system and your telephone service provider. Which you select depends on your system's capabilities and the provider that terminates your calls to the PSTN or Public Switched Telephone Network. It is not necessarily true that a new VoIP phone system will automatically use a SIP trunk for service. In fact, many IP PBX systems are ordered with T1 PRI interface cards because the best service deal may well be from a competitive supplier providing PRI local and/or long distance telephone service.

Not sure what to order or if you are getting the best possible deal from your current service provider? Let our expert consultants help you get the best lease rates on the most cost effective voice and/or data services for your business needs.

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