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New Member

DSL over ISDN? Why?

Hi guys,

I have seen this ADSL over ISDN SOHO 96 and 836 routers. Why would you want to do that?

My thinking? ISDN is up to 128k so if the ADSL Exchange is more than the normal ADSL distance limitation then you can run ADSL over ISDN to the central office where they have some sort of kit that can take the signal off the ISDN B channels and present it to a DSL box? Is that BS?

Much appreciate some pointers since cco does not seem to make this clear.


Cisco Employee

Re: DSL over ISDN? Why?

That is correct..ADSL over ISDN is called "IDSL". IDSL is a cross between ISDN and xDSL. As with ISDN, it uses a single wire pair to transmit full-duplex data at 128 Kbps and at distances of up to the Revised Resistance Distance range of 15,000 to 18,000 feet. IDSL runs over ISDN-ready lines. IDSL can be transmitted on lines as long as 36,000 feet from the central office withan ISDN repeater and up to 18,000 feet without one. IDSL also uses a 2B1Q line code to enable transparent operation through the ISDN "U" interface. IDSL is essentially a leased line ISDN Basic Rate Interface (BRI), or an ISDN BRI that is not switched and does not contain signaling (a D channel). IDSL and ISDN BRI use the same 2B1Q line modulation. On the router, this equates to putting the BRI interface in a leased line configuration. The line can be configured for a speed of 64 Kbps, 128 Kbps, or 144 Kbps.

The frames that are going across the wire are standard High-Level Data Link Control (HDLC) frames. IDSL can be configured with Point-to-Point Protocol (PPP) or Frame Relay encapsulation for the leased line BRI interface. The easiest way to think about it is as if the BRI interface was a slow speed synchronous serial port. Also, existing Customer Premises Equipment (CPE) (ISDN BRI terminal adapters, bridges, and routers) can be used to connect to the central office.

New Member

Re: DSL over ISDN? Why?

Thanks for the reply.

However, I still say why? If the DSL signal can be carried over ISDN B channels, so to speak, is the DSL signal limited to 128K or 64K, depending on whether the two B channels are bonded? If so, why bother with DSL? Why not just put in a 801 router instead since you are limited to 128K?



Re: DSL over ISDN? Why?

Hi Steve,

from what I've read about IDSL, it is definitely not the same as ADSL over ISDN and hence the SOHO96, 826 and 836 are NOT IDSL routers.

From an end user perspective, ADSL over ISDN is the same thing as ADSL over POTS, except the phone line that the ADSL is carried over is an ISDN line.

So you'll need a different kind of splitter: one that splits off the ISDN signal (U interface) instead of the POTS signal, obviously.

To this splitter you connect the ADSLrouter and your NT1.

Why would you want to do that? If you already have an ISDN line and you don't want to downgrade it to POTS for the ADSL installation, e.g. because you have an ISDN phone/fax/whatever.



New Member

Re: DSL over ISDN? Why?

Okay !!

I've got it now. Thanks very much for helping me to understand this.

Best regards,



New Member

Re: DSL over ISDN? Why?

OK, I'm currently playing with a SOHO 96 and this is what I know so far:

The SOHO 96 works on ADSL over ISDN. That is, the ADSL signal is superposed over the ISDN signal, and you need a splitter (which is essentially a lowpass/highpass filter) to connect the SOHO96.

The Dutch telco provides an U bus (2 wire).

For ADSL, the U bus first goes to the splitter, and the high-frequency signal goes to the ADSL port of the modem.

In the Netherlands, I typically get 1.5 mbit down and 256 kbit up on ADSL, substantially more than the 2x64k of ISDN. Like with ADSL-over-POTS, ASDL-over-ISDN doesn't influence the ISDN service at all.

ISDN works as usual, that is 8 phone numbers, 2 B channels, etc. In fact, the low-pass port

of the splitter goes to a Network Terminator that translates the U bus to S bus (this is different in the USA), and phones, ISDN-to-analog-POTS converters and the like are connected to the S bus.

The SOHO 96 also has an S bus connection which one apperently can use to dial in and manage the thing. I have not tried that yet but am about to start playing with that.

AFAIK, the S port of the SOHO96 CANNOT be used to dial in/out and provide IP connectivity, just to provide access for management. Does anybody know whether this is true? Being able to dial out would be a big plus in case of ADSL outages, which do happen here every so often and are a problem for a remote office.


Re: DSL over ISDN? Why?

AFAIK the main differences between the SOHO96 and the 836 are hardware encryption and isdn backup, so I don't think you can do it on the SOHO... or maybe it's just a marketing trick to sell the 836 :)

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