A little history about ISDN

A few weeks ago we wrote two newsletters about network-based services (see “Are you ready for network-based services? Part 1” and “Are you ready for network-based services? Part 2”), and we used the emergence of the entire “Networking T-1 Multiplexer” market phenomenon as an example of how the carrier missed a golden opportunity back in the 1980s and essentially lost account control for enterprises. Instead, they were relegated to simply being bit-shippers as opposed to network service providers – and they’re still trying to recover from that market position. Let’s face it: There are no profit margins in providing raw transport.

One of the reader responses to the newsletters mentioned ISDN as yet another missed opportunity. And although we’re not totally convinced that ISDN could have led the way to the type of enhanced services that the service providers would like to sell today, it’s certainly instructive to examine the lessons that may be learned from this service’s ultimate demise.

Let’s face it. ISDN was doomed from the start – but this was not the carriers’ fault.

For those of you who have neither studied nor lived the history of telecom, a few quick facts: ISDN (Integrated Services Digital Network) is/was based on the 64K bit/sec “DS-0” of the traditional telephony network. In the U.S., it was offered in two flavors: Primary Rate Interface (PRI) and Basic Rate Interface (BRI). One format of the PRI interface consisted of 24 DS-0s, and looked a lot like a channelized T-1 circuit. The PRI is similarly defined for international use where the fundamental historical transport bit rate is 2.048M bit/sec, sometimes called “E1.” In this case, one of the 32 DS-0s was already used for control purpose, so there were only 31 available DS-0s for ISDN.  Hence, the format was 30B+D.

The major difference between a garden-variety “T-1” and a PRI is that the PRI normally has 23 channels used for transport of digital voice or date conversations (called “Bearer” or “B” Channels), while the 24th channel was used for control (called the “Data” or “D” channel).  Thus, a PRI ISDN circuits is often referred to as 23B+D. “Normally” is inserted above because one “D” channel can control more than 23 “B” channels, so it was also possible to have a “24B” ISDN circuit so long as the control came from another PRI.

*** If you’re wondering about some of the acronyms mentioned, according to Bill Flanagan’s acronym book:

“DS-0” is Digital Signal level 0 (0 = number zero) and “DDS” is Digital Data System, network that supports DATAPHONE Digital Service. And T-1 is Transmission at the DS-1 level. ***

In the next newsletter, we’ll examine the difference between the PRI and the BRI, and we’ll look at some of the particular advantages each brings.