January 24, 2001, 9:53 AM — Some people may say that ISDN is dead, but that's news to the millions who rely on ISDN Primary Rate Interface service for connecting PBXs and dial-up modem banks to the public network.
While it's true that sales of ISDN Basic Rate Interface service aren't as high as those of competing broadband technologies, PRI sales are going through the roof.
As a PBX platform, PRI's offer more flexibility than traditional analog trunks. PRI's are good for ISPs because they let users connect via 56K bit/sec modems or 128K bit/sec ISDN BRI service.
A PRI offers 23 B channels for voice, video and data traffic via a 1.544M bit/sec T-1 line. The 24th channel, the D channel, is used for signaling. When you combine multiple PRI's, you only need to use one signaling channel for the group of lines, freeing at least one additional B channel for regular traffic. Because of their digital nature, PRI's are equally good at handling voice or data traffic. What's more, PRI will certainly be the access method of choice for connecting voice-over-IP traffic to the public switched telephone network.
Breaking down the costs
The cost of PRI service comprises four components: the T-1 line; the B and D channel arrangements; the trunking arrangements; and special features.
The T-1 charge covers the cost to connect your network to the carrier's switch. In the case of some PRI service providers, you can actually collocate your equipment in the same building with the switch, thus eliminating this charge.
The channel arrangement refers to the way in which you multiplex the B and D channels over a T-1, such as 23B + D, 24B and so on. The trunking element covers services such as Direct Inward Dialing, which lets you give each employee his own direct-dial phone number.
Finally, the special features include options such as calling line ID. When you combine all these elements, you arrive at the true cost of PRI service.
Where to buy
One of the most difficult aspects of buying PRI service is determining if a particular carrier has the service in your area. There's no question that regional Bell operating companies provide the greatest coverage of any PRI provider. Interexchange carriers such as AT&T, WorldCom and Sprint also offer PRI's, but these circuits are typically sold for access to long- distance service.