ADSL-over-ISDN. What is the advantage?

Is there any advantage of having ADSL over ISDN instead of PSTN?

I don’t think the question makes sense, as written. DSL over ISDN is known as IDSL, and that isn’t ADSL.

DSL stands for Digital Subscriber Line. ISDN stands for Integrated Services Digital Network. The A in ADSL stands for assymetric. That means that the bandwidth (speed) of the connection is different upstream than downstream (usually, faster downstream). The I in IDSL stands for - wait for it - ISDN.

So perhaps your question is - Is there any advantage of having DSL over ISDN instead of PSTN?

And the answer is - about the only advantage of IDSL is that it is more widely available, and available over a greater distance from the telco central office.

If you have a choice, go with ADSL over IDSL. If you have no choice, IDSL is better than dialup (but equivalent in speed to ISDN-BRI dialup with two B-channels active).

I was always under the impression that IDSL was 144kbps and dual ISDN 128kbps.

Like I said, equivalent in speed. I’d bet my next paycheck that given the two, side by side, you couldn’t tell the difference.

Or hey, if you include the D channel, you can get to 144 Kbps on the ISDN-BRI!

AZCowboy, I believe that you and the OP are talking about two different things. IDSL is basically an ordinary ISDN data line, and it is compatible with existing ISDN cards; the main difference with standard ISDN is that it’s a dedicated data line with no call setup.

ADSL over ISDN, on the other hand, is the same thing as ADSL over a standard analogue telephone line, except that the phone line on which the ADSL signal is piggybacked happens to be an ISDN line. It’s still asynchronous; I am typing this over an ADSL-over-ISDN line with a (theoretical) speed of 8Mbps downstream and 1Mbps downstream. This is the same speed which I would have if I had an analogue line, so there’s no speed advantage there.

So, is there an advantage of ADSL-over-ISDN versus ADSL-over-analogue? As far as the ADSL part is concerned: no. Of course, you still have all the other advantages of ISDN, but most of them are of limited value once you have ADSL for your Internet connection. Personally, I chose ISDN 7 years ago because ADSL was not yet available then, but now that I have ADSL for my always-on broadband Internet needs, and use a mobile phone for 99% of my telephone use, I find myself wondering very often why I am still paying extra for ISDN when it does not have any added value for me.

Bottom line: if you currently have an analogue phone line, go with ADSL-over-analogue. If you currently have ISDN, at least consider switching to analogue first and then choosing ADSL-over-analogue.

That should be “8Mbps downstream and 1Mbps upstream”, of course.

Would someone please spell out ASDL, ISDN, IDSL, and PSDN?
As you can see, I need to catch up. :slight_smile:

Public Switched Telephone Network: your ordinary analogue phone line, also known as POTS (Plain Old Telephone Service). Not very good for Internet access, unless used in combination with ADSL.

Asynchronous Digital Subscriber Line: a high-speed broadband Internet connection piggybacked on top of an ordinary phone or ISDN line. Currently the most popular form of consumer broadband Internet, at least over here in Europe.

Integrated Services Digital Network: a digital telephony network which was (at least here in Europe, again) heavily marketed as the successor to PSTN. It has two 64kbps data lines, which can be used as two independent voice channels, or voice+data, or combined into a single 128kbps data line. Once the best way to access the Internet, but nowadays ISDN by itself is not very much better than PSTN (except for the fact that you are still reachable by phone while netsurfing) and very much inferior to ADSL. You can have ADSL in combination with ISDN, which theoretically gives you the best of both worlds, but as I mentioned most of ISDN’s advantages are made obsolete by ADSL anyway. Unless they make the price of ISDN equal to that of POTS and simply stop installing new POTS lines, I don’t see ISDN having much of a future left for consumer telephony, but it’s still popular with businesses (in addition to the basic two-line ISDN I described, you can also get 10 or more independent voice/data lines).

ISDL: a form of ISDN where the two data lines are dedicated exclusively and permanently to networking, as opposed to being available for voice or data calls as required. The bandwidth is not as high as ADSL, but it is available in places where ADSL is not. I’ve never heard of ISDL being used for consumer-grade Internet access, really, but it may still be a viable option for business purposes because it is generally considered more reliable than ADSL.

Martin Wolf is correct. I was not talking about IDSL.

So I guess there’s no need to get ISDN for my home connection.

Also a slightly related question. Is ISDN really needed to use a PBX? Can you use a PBX over PSTN lines?

Thanks, Martin. That clears up a lot of things. I just signed up with ADSL on Sprint and it works pretty well. A little slow on graphics, though.

Normally a PBX will work fine with PSTN lines but it usually works out cheaper to get ISDN then several PSTNs.

This is not my area of expertise so I’m basically guessing here, but you may want to consider the following:

I’m sure that any old PBX will be able to manage outgoing PSTN lines just fine. However, what you will probably want is for all of your incoming lines to be reachable at a single number. Or, even better, to have all your X incoming lines reachable at Y different phone numbers, without a direct mapping between phone lines and phone numbers, so that you can then route incoming calls to internal extensions through the PBX. Simply having a lot of separate PSTN lines available is not enough; you also need the assistence of the phone company to let you do your own switching/routing of incoming calls. That is, after all, why it is called a Private Branch Exchange: because part of the stuff that would normally be done by the phone company is now under your own control.

Whereas with ISDN, that is a standard feature. So if I were the phone company and you came to me with that request, I would either make you pay through the nose or tell you to use ISDN. In other words: while I’m sure that you will be able to find a PBX which proudly exclaims “Supports both ISDN and POTS!” on the box, you’d better call the local telco before making the decision.