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back to back ADSL


i have two 2651XM and both the router installed with WIC-1ADSL card. now my question is can we connect the both the router back to back using ADSL connectivity? i tried to do the same but it was not working so i want to make sure whether its possible or not?

i also want cable specification as well as configuration for the same.

i m trying in my lab to figure out one deployment...




Re: back to back ADSL

Hi, devang

Unlike a normal ethernet connection, in which there are four wires between two devices, ADSL only have 2 wires between them. In the four wires situation ,you have 2 wires for transmitting data and other 2 wires for receiving data. To connect two devices back-to-back, you just need to crossover the wires on the other side, then they can talk.But this strategy no more work for ADSL connection because ADSL has only two wires. ADSL uses a different strategy to make the communication work: it devides the frequency band of the wires into 3 category: low band, medium band, and high band. Then one device (we call it ATU-C:ADSL Termination Unit Central) uses the high band to transmit data to the other side(we call it ATU-R:ADSL termination Unit Remote)

and uses the medium band to receive data from the ATU-R.

For your case, you connect two ADSL cards (both of them are ATU-R) back to back. They will send data in the same band (a kind of collision) and expect to receive data in the same band (but no one will send data on that band). It just like you connect two NICs by using a straight through cable. It won't work just at the LAYER 1.

To make the connection successful at Layer 1, you will need to connect each of them to a ATU-C on the other side. (Typically on a ISP site, where they have a device called DSLAM which binds many ATU-Cs in one device.) Only after that you can have the possibilty to make them talk on Layer 2 and Layer 3.

Here is a webpage you can refer to.

Hope this help


New Member

Re: back to back ADSL


I don't know if it is possible for ADSL but I know for sure that is possible for SHDSL.

In attach you can find the configs for a SHDSL B2B situation.

So it is possible without a real DSLAM(ATU-C).

The CO config is the ATU-C and the CPE config is, obvious, the ATU-R.

Maybe you can addapt the config for ADSL.



Re: back to back ADSL


thank you very much for your response. can i have more good link or material so i can read more about the DSL?

your information helps me lot thank you very much again.



New Member

Re: back to back ADSL

must explain why two people cannot talk over a telephone then ?

Back to back should work. do a search into dsl over dry wire