Using ISDN/PRI For PBX Trunking

Many users of large telephone systems, commonly called PBX systems, are confronted with the question as to whether or not to use Integrated Services Digital Network/Primary Rate Interface or ISDN/PRI for the central office trunking. The following may answer some of your questions.

Q: What is ISDN/PRI?

A: This is a central office offering that is used to replace the existing analog trunk lines, whether they be direct-in-dial, outgoing or combination trunks, with the new offering, Primary Rate Interface trunking.

PRI is true digital trunking with features not normally available with either analog or regular digital trunking. One desirable feature that is often included as part of the package when you purchase PRI is Automatic Number Identification or ANI. This feature imprints the calling number with the call coming into your system and this is a critical feature when you want to use CTI applications in a call center.

Q: Do all Local Exchange Carriers [LEC] offer ISDN/PRI?

A: No. You must check with those LEC companies servicing your area to determine whether or not they offer this service. You will note that the tariffs are substantially different, also.

Q: Is ISDN/PRI different than just regular digital trunking?

A: Yes, digital trunking does not normally offer PRI features, but digital trunking does have excellent quality of transmission and it is also brought in via T-1 Carrier circuits, as is ISDN/PRI.

Q: Is ISDN/PRI always brought to the campus via T-1 Carrier circuits?

A: Yes.

Q: Will the LEC use copper or fiber medium to bring ISDN/PRI to our campus?

A: That is the choice of the LEC. We suggest that you negotiate with the LEC to have them bring fiber in as part of the deal. The Information Services people at your facility will love you if you can pull this off.

Q: What are the real advantages of ISDN/PRI?

A: There are at least three advantages and they are:

1. The quality of transmission is much superior to analog trunking. You can actually hear the difference when you are on the line.

2. ISDN/PRI offers other desirable features such as the previously mentioned ANI. This feature is very desirable if you are going to use CTI/Call Center Applications.

3. The cost of ISDN/PRI can be very attractive, depending on the LEC and your location. We have found that ISDN/PRI can often be installed at a competitive cost when compared to analog trunking. In some instances, the service has cost more.

Q: Can I strip off individual trunks from the ISDN/PRI or from the T-1 Carrier line for system failure telephones, etc.?

A: Yes, but this is something new. There are now units available that permit individual trunks to be split out for system failure trunks and for other applications. Not all PBX vendors are aware of these units, as they are relatively new. A company called Gordon Kapes, Inc manufactures one unit we have used. Each unit will strip off a specific number of trunk lines, so you may need more than one of these units, based on your particular requirements.