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ISDN PRI vs SIP Trunks For Business Telephony
Does SIP or ISDN PRI trunking make the most sense for your business phone needs?

By: John Shepler

Enterprise telephony long ago advanced beyond analog business lines in favor of PBX telephone systems and ISDN PRI trunking. Now there’s another option that may offer cost and performance advantages for companies that want to improve their voice services. It’s called SIP Trunking. Let’s compare and contrast SIP Trunks with ISDN PRI to see how to choose between them.

It's All Digital
You should know that we are still talking exclusively about digital telephony. There is no analog transport in either a SIP Trunk orPRI. What differentiates the two is that ISDN PRI is based on a switched circuit model and SIP Trunking is based on packet switching.

Both Trunking Systems Work
For many companies it could be six of one and a half-dozen of the other. That’s because either trunking service can be made to work with your in-house phone system. The idea behind digital trunking is to carry a half-dozen or more outside phone lines on a single digital line. Usually that’s one or more T1 lines, although DS3, OC3 fiber or Ethernet can also be used.

How Many Lines In An ISDN PRI?
ISDN PRI uses a T1 line configured into 23 separate voice channels plus one data and control channel. Most PBX systems and the newer IP PBX phone systems make it easy to connect to ISDN PRI. They either offer one or more PRI ports built-in or accommodate a plug-in adaptor card.

Expanding ISDN PRI
If you need more than 23 lines, you simply add more ISDN PRI lines incrementally. Each additional line can bring in up to 23 more business phone lines. You’ll have to check what’s available for your phone system, but it is not uncommon to have 2 to 4 PRI lines plugged into a PBX system.

The Telco Legacy
The technology used for ISDN PRI is defined by the Integrated Systems Digital Network specification. PRI is the Primary Rate Interface. There is also a lower performance BRI or Basic Rate Interface that isn’t seen much anymore. This is a legacy telephone technology designed to support the idea of separate digitized telephone lines, one per channel. There’s no chance of interference between calls because the channels are strictly defined and timed using Time Division Multiplexing.

SIP On The Network
SIP Trunking is more of a network than a telephone based technology. It is a packet switched network line, much like you would use to connect two business locations. What SIP Trunking can do is directly support a converged voice and data network. You can run SIP Trunks between business locations to expand your voice and data network geographically.

Why SIP?
The SIP in SIP Trunking stands for Session Initiation Protocol. That’s the switching technology used in VoIP telephone systems. In SIP trunking the control signals for the phone system are transported in packets that travel down the same network pipe as the voice packets.

ISDN PRI isn’t so much about convergence
It’s really about having separate telephone and data lines leave the premises, although you are free to converge your in-house network and let an IP PBX system on the network handle call routing, including outside calls on the ISDN PRI trunk line.

SIP Trunking doesn’t have to be about convergence, either
You can simply use a SIP Trunk to connect your PBX system to a telephone service provider rather than using an ISDN PRI. Depending on which CODEC or coding scheme you use, you may be able to handle more than 23 lines on a SIP Trunk. If you are using an Ethernet connection to your provider, you may be able to support dozens of phone lines depending on the bandwidth available.

The SIP Trunk Voice and Data Option
For small businesses, SIP Trunks can be configured to transport both voice and data to your service provider. Most often this is used to bring in telephone lines and broadband Internet service on the same SIP Trunk. There’s a cost advantage, even if you are not running a converged voice and data in-house.

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