Slashdot Mirror

← Back to Stories (view on slashdot.org)

DMT vs CAP Encoding / DSL Upgrade Downside?

Posted by Cliff on from the what's-the-real-culprit dept.

cmark asks: "I recently 'upgraded' my Qwest DSL from 640/272 to 640/640 to increase the bandwidth of my Counter-Strike server. When I got this upgrade, Qwest sent me new a new router (a Cisco 678 to replace my Cisco 675) and changed the encoding from CAP to DMT. In this process I do not have greater bandwidth and thus less lag on the server but I have increased latentcy (higher pings). My ISP says that it is the encoding, something about the interlacing that they cannot explain. Qwest says that it is not the encoding but must be some routing that my ISP is doing. What is causing the increased latentcy and what can I do about it?"

"A traceroute looks normal to me... If I was hosting a website or using it for some other business purposes I probably would not care but in gaming a low ping is crucial, I have lost the regular players that I had and the server sits empty most times all the while costing me more money! Qwest also says that I cannot go back to CAP even if I downgraded, since they are migrating off of CAP."

4 of 9 comments (clear)

  1. interleave by meanmachine · · Score: 2, Informative

    DMT makes use of interleave to minimize the change of errors (they can recreate the original data as onlyy a portion gets corrupt in case of a disturbance)

    This mechanism de creases the error bitrate from 1 in 10^3 down to 1 10^6
    However, it increaese delays due to the interleaving.

    ALSO the TCP/IP will take care of the retransmissions in case of a problem. Ping times can be as high as 160mS one way and as low as 20mS with interleaving turned of.

    1. Re:interleave by complex · · Score: 2

      i concur with this information. (note: i used to work for verizon dsl)

      meanmachine is right, the bit error rate isn't critical for you. the latency is.

      the solution, for you, luckily, is nearby. the cisco line of dsl routers has changable interleaving settings! all the info i can find now is at dslreports.

      if this doesn't work, then the solution is to get your provider to turn off interleaving. this will be difficult. repeat: this will be difficult. you'll probably have to beg, beg some more, beg to 20 different people, and then they still won't do it. ask to talk to a dsl tech, supervisor, whatvever. but if your cisco can't do it, then getting them to change it at the far end is the only way.

      good luck.

      i'll try to post more if i can find it.

      complex

    2. Re:interleave by complex · · Score: 2

      hope this isn't considered karma whoring, but the cisco 678 documentation will be useful in the course attempting the interleaving change. fully searchable, unlike your dead tree version! (if you even have one, dsl providers typically get the modems sans box, packaging and docs)

      complex

  2. DMT has advantages, disadvantages by isdnip · · Score: 2

    CAP is basically obsolete. It was the first ADSL technology, and has worked okay in some cases. But it has one big disadvantage. CAP causes crosstalk. It puts a lot of energy into some narrow spectra, and it leaks into adjacent wire pairs. The FCC is cracking down on this and CAP might not pass future standards. If a telco deploys one DSL line out of a hundred subscribers, CAP will work, but as the subscsriber density rises, CAP causes trouble

    DMT spreads the signal differently, and thus has less crosstalk. It is thus the "standard". It doesn't always perform as well and is more latent though.