I would like to know, how the synchonization of ADSL (Asymmetric Digital Subscriber Line) actually works, something a bridging DSL modem does, for example.

I could only find out very general things about it yet (primarily using this page), including:

  • the transmission properties of DSL modem and DSLAM need to be determined using some handshake procedure
  • these parameters include line quality, data rate and latency path (fast/interleaved)
  • the parameters remain until the loss of synchonization (e.g. because of a disconnection or power failure)
  • the process is rather complicated

But I would really like to get much more specific information, including:

  • What data is actually transmitted? Is some kind of protocol used you can find a RFC about? Or is it even officially globally standardized from an organization (e.g. IANA/IETF/IEEE)? Is this information publicly available at all? I guess it should not be ISP dependent, because any modem I used in the past was able to synchonize with any ISP.
  • Is it possible for a modem in general to (de)modulate data without any previous synchronization? When the answer is yes, then: If I had a modem that would only modulate and demodulate data and not synchonize, could I connect it to a normal PC and let the operating system (e.g. Linux) do the synchronization (if it's not part of the kernel, ideally with an open source program)? Regarding data that's transmitted this would of course require answers to questions in the previous block.

I am not sure if that's important, but I am using ADSL (yes, ADSL, not ADSL2 or ADSL2+) from the Deutsche Telekom, the most common German internet service provider.

2 Answers 2


The ITU define the standard for basic ADSL in G.922.1:


There is also a USA version defined by ANSI, T1.413


The standards for the vaious variants of DSL are maintained by the ITU, https://www.itu.int/ITU-T/worksem/asna/presentations/Session_6/asna_0604_s6_p4_palm.pdf lists the standard numbers for the various varients. From some googling it looks like the standards can be downloaded freely in pdf from from the itu's website.

What people loosly call "syncronisation" is really a series of steps, broadly speaking the modems must first detect each other, then they must characterise the line to decide how many sub-channels to use and what data rate to use in each sub-channel and finally they must syncronise the de-modulation clocks to the modulation clocks. You can't modulate and de-modulate without first determining the modulation parameters and establishing clock-sync.

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