What is IDSL?

IDSL, or ISDN Digital Subscriber Line, is a type of internet connection that combines ISDN and DSL technology to deliver faster speeds over traditional phone lines. It's a solution for those in areas without access to newer broadband infrastructure. If you're seeking a balance between speed and availability, IDSL might be your bridge to the digital world. How does it fit into today's internet landscape?
R. Kayne
R. Kayne

ISDL is a hybrid of ISDN and DSL (Digital Subscriber Line) technology that uses a little of each to deliver a service that is slightly faster than ISDN, but significantly slower than most DSL services. A person might opt for ISDL if standard DSL service is not available in his or her area.

Standard ISDN (Integrated Services Digital Network) service uses existing copper telephone lines to deliver data transfer speeds up to 128 kilobits per second (kbps). It also provides voice channels for telephone and fax, making it superior to standard dial-up, which cannot share an Internet connection with a telephone or external fax machine. Standard dial-up is also much slower than ISDN at about 53 kbps. However, ISDN is still a dial-up service, as opposed to an "always on" service like DSL.

An IDSL filter.
An IDSL filter.

IDSL uses the same technology as ISDN except that it routes the signals through different equipment at the phone company or Telco. While ISDN uses the standard switching equipment, IDSL uses DSL-like equipment. The difference is two-fold: an increase in speed to 144 kbps, and an "always on" connection.

IDSL can be expensive to set up, but may be worth it to businesses or individuals that require faster service without the option of DSL. One of the advantages of IDSL over DSL is that the former is not limited by distance from the Telco. To be a DSL subscriber, the client must be within the limit of 18,000 wire-feet (5,486 wire-meters) from the Telco hub. If the residence or business falls at the outer limits of this range, the maximum speed of the DSL connection will be significantly reduced and signal quality will not be optimal. The closer to the Telco hub, the better the DSL service. IDSL does not have this limitation, allowing clients at further distances to have an "always on" connection.

One drawback of IDSL is that it requires its own telephone line for the IDSL service, while a second line provides phone services. This makes it more expensive than ISDN.

Ascend Communications developed IDSL technology. If you think it might be the right solution for your needs, contact your local telephone company to ask about ISDN or IDSL services in your area.

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Discussion Comments


I have IDSL and I wanted everyone to know that you don’t always need to purchase a second phone line if you have IDSL. Some service providers (like the one I’m a customer of) give you a splitter when they send you your IDSL modem. The splitter plugs into the phone jack on your wall. There’s one jack on the splitter for your Internet connection, and one for your phone line. That way, you can be on the Internet and use your phone at the same time!


@SailorJerry – Yes DSL is pretty slow, but it still has better bandwidth than dialup. The problem with cable is that you have to get a physical cable installed in your house in order to receive cable Internet.

My old apartment had cabling already installed, so it was a cinch to get cable Internet service there. My new place doesn’t have cabling, and the landlord won’t let me install any. So, I have ADSL simply because it uses the pre-existing phone lines and I have no other options.


@anon19301 – Did you mean IDSL, not ISDL? AT&T’s website doesn’t say anything about them offering ISDL service. I have At&T and I’m pretty sure they only offer a variety of DSL packages.

To answer your question, IDSL is slower than ADSL. ADSL has an internet speed of up to 8 megabits per second. That’s over eight times faster than IDSL. Most people don’t need a really fast connection, but if you do a lot of online gaming or watch high definition movies online, you might want to get ADSL. Also, if you upload a lot of large files, you’d be better off with ADSL.

I have a slow IDSL connection (it’s all I can afford at the moment), and it works for me. The only problem I have is that watching online videos can be a pain. The movies can’t download fast enough, so and they pause every ten minutes. If you can afford it, I’d go with ADSL, just because the fast download speeds make Internet use so much easier.


What are the advantages of going with DSL internet? Isn't it pretty much the slowest broadband option?

I had DSL at my old house. It was one of the more reliable services I've had (the only problem we had, I think, turned out to be our modem!) but it was definitely the slowest. When we moved and had cable internet again, we really noticed a difference. Our current cable internet is pretty wonky, but our last provider was rock-solid.


As the article states, ISDL is typically slower than ADSL. Stick with ADSL. People generally only get ISDL if ADSL is not available.


what is the difference between a ISDL line and a ADSL line???>... we are being quoted both from AT&T

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    • An IDSL filter.
      An IDSL filter.