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ISDN = Integrated Services Digital Notworking? (in the US)

Remember ISDN? It was the 1980s digital connect methodology that was going to bring us everything digitally. In truth, ISDN (a.k.a. BRI) is the “little brother” of T-1 (a.k.a. PRI). PRI is actually the signaling scheme, but the relationship is accurate. Whereas a T-1 is 24 x 64kbps voice channels and signaling an ISDN circuit can be 2 x 64 kbps channels (Bearer or B channels) and signalling (a D channel).

In Europe ISDN is quite common, having been a valid alternative to analog POTS lines for many years. In the US it never caught on the same way. It’s common in certain vertical market niches. For example, just about every significant recording studio used ISDN as a means of transferring audio across country in real-time. It’s a channel with assured bandwidth. The two B channels could be bonded yielding a solid 128 kpbs. Throughout the 1990s voice-over talent working from home studios (Hi Alison!) almost always used this sort of a setup.

It’s flexible, too. An ISDN circuit can be a mix of data and voice. When you pick up the phone to make a call one of the 64 kbps B channels gets used by the voice calls but the remaining channel is still available for data. When the call ends the data stream can use both B channels.

It’s not distance bound. Like a T-1 an ISDN line can be installed in most locations where you might have POTS service. It’s not bound by distance in the manner of ADSL. Of course, it’s nowhere near as fast either, but it is assured bandwidth, and fully symmetrical.

Why do I care about this essentially out-moded means of connectivity? Two factors;

1. It’s digital

My thoughts about small FXO interfaces for Asterisk are well known. I think that they can be avoided if at all possible. Staying with a 100% digital circuit has many advantages, call quality and call termination detection amongst them. If you scan through the history of the Asterisk-Users mailing list analog interfaces (FXOs) are one of the most common sources of problems.

2. It’s cheap

Ah, this is the part that’s less known. Part of the pre-1996 regulatory scheme the price of ISDN lines are fixed by regional regulators. In Texas, where I live, an ISDN BRI (2B+D) circuit costs $46/mo. ISDN circuits usually have caller-ID, call waiting and a few other services which typically cost more when added to POTS lines.

So for small businesses & home offices an ISDN circuit can be a cheaper, digital means of providing local lines into a PBX. What’s not to love?

In fact, my employer has a factory in the UK with 40 staff. That facility is serviced by 4 ISDN pair into an Alcatel Omni PCX PBX. In the US that would typically be serviced by a fractional T-1 for a lot more money every month.

US vs ROW Signaling Standards

Here’s the trouble. The US and rest of the world (aka R.O.W.) evolved different signaling standards. Since ISDN is only common outside of the US the ISDN interface cards available for use in Asterisk typically don’t work on US ISDN lines. At least not until recently.

While on a VOIP Users Conference call a couple of months ago someone from Xorcom suggested that the BRI modules for their Astribank USB channel bank should work on US standard ISDN lines. He said that even if they didn’t the differences should be addressable in the driver layer, so not too difficult to solve.

On a later call someone from Pika suggest that their BRI modules should work as well.

The Opportunity To Try It Out

This brings me to an interesting point. I’d love to test this out. Testing would involve borrowing some hardware and getting an ISDN line installed. Xorcom have already offered hardware and it sounds like Pika could be convinced as well. The ISDN line more of a problem. That costs money and may require a commitment to a term.

The bigger issue is that to test this I’d need to have an ISDN line provisioned, probably from AT&T…the ILEC in my area. As you may recall I am about 8 years into a boycott of AT&T for personal reasons. I’m not at all certain that I’m ready to see my funds in AT&T hands.

Further, some of my customers (broadcasters) have reported that ILECs really don’t want to deal with ISDN anymore. They support existing installations but may not provision new ones because they often have less of that sort of gear in the local COs. Not sure about this. I think that ILECs sometimes give you a runaround just because.

In the early 1990s I lived in Belleville, Ontario and worked from home. This being prior to the availability of ADSL I tried to order ISDN from Bell Canada…but they hardly knew what it was. They passed me from office to office until I basically lost interest.

I’m perfectly happy not having any POTS lines. While we made that move to avoid doing business with AT&T after they bought SBC, it’s been a motivating factor driving me to learn more and refine our voip installation.

I’m interested in another connect method for my home, and this might just be worth a try. Maybe there’s a CLEC that can provide me an ISDN circuit? Maybe someone else has already done this in the US? If you have please let me know.

This Post Has 2 Comments
  1. Very interesting post.. there alot of “forgotten” technologies which are still viable today. I work with alot of Cisco voip stuff and I am always surprised that they sell versions of their hardware with ISDN because it is so uncommon to us here in the USA.

    I would love to know more about your 40 person company in the UK. I assume that you can purchase a block of DID’s and have them us all of the channels? Or do you have to purchase a block per 2 BRI channels/1 “ISDN” line?

  2. Yes, we have a large-ish block of DIDs so that everyone has a direct line, regardless of what channel the call rides. It works really well, even thought it’s older and the Alcatel Reflexes phones are crappy.

    In North America we use a hosted IP-PBX through OnSIP. I have that programmed to match all the UK dialing plan. When UK staff visit over here they pick up a phone and dial using the same 3/4 digit internal extensions as if they were a the factory. Those extensions are mapped to SIP URIs corresponding to the UK DID for each desk, and speed dials for cell phones.

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