We connect to the internet using ADSL installed on the existing phone system (copper wires) and via several ISPs. Every subscriber, from his ADSL modem/router, can log in to any ISP (other than his ISP) if he has a username/password to the other ISP.

The following example clarifies the issue:

I have a 1-Mbps subscription with ISP-1, so I have my username/password to login to my account using my ADSL modem installed on my phone line. My friend has a 2-Mbps subscription with ISP-2, so he has his username/password to login to his account using his ADSL modem installed on his phone line.

When my friend is logged out, and when I replace my username/password, in my modem, with my friend's one, my modem will log in to my friend's account and takes his bandwidth 2-Mbps on ISP-2 !!!, Noting that the rest of the modem settings are the same.

Typically, I think that every ISP should have a separate DSLAM in the phone company, but in this case, it doesn't seem like that. I need to understand How it works and what the topology used in this case.

1 Answer 1


With ADSL economical issues dominate the technical implementation: Most of the time, the "copper owner" will handle ADSL termination (which explains the identical modem settings) and forward the packet stream to the correct upstream carrier ("your ISP") with a POP at the facility based on RADIUS user ID.

This has the desireable property, that when a subscribe switches ISP, no physical or configuration changes to the system need to be made - by chosing a different user ID on the modem and acceptance of those credentials by the new ISP the switchover is done.

This system is a lot cheaper than attaching each subscriber to a different termination endpoint. The copper owner is payed for lease of his line anyway, so the small additional overhead of him managing the termination is significantly less than what the write-down of setup costs of manual reattachment of a subscriber would cost.

  • Does that mean plan bandwidth limits are enforced by ISP's routers and not by the DSLAM? (With our ISP the ADSL modem directly negotiates what's allowed by the plan, e.g. it shows "speed: 8 Mbps down, attainable speed 14 Mbps down" – I assume that wouldn't work in OP's case...)
    – user1686
    May 7, 2020 at 10:03
  • 1
    Of course the attainable speed limit of the line is a hard limit, while the ISP limit is soft.In the OP case he sees 2 Mbit when logged in with the higher plan, so obviously his line and modem are inside the the physical envelope. In addition to that, modern ADSL implementations will fall back to a lower negotiated speed if a higher speed is possible but not needed to trade (needed) stability and robustness against (unneeded) speed. May 7, 2020 at 10:12
  • So would that mean the modem renegotiates the speed after PPPoE login, or would that mean the modem always negotiates the maximum link speed and the ISP limits the actual speed elsewhere?
    – user1686
    May 7, 2020 at 10:16
  • The speed negotiation (within the limits set by configuration) is normally independent of login, but different implementations might use different ways to deal with speed handshaking May 7, 2020 at 10:48
  • In the OP case, does ISP-2 have the ability to recognize whether the logged account is coming from the one who has the contract BASED on the calling phone number? If it does, why doesn't ISP-2 filter it?
    – was.chm
    May 8, 2020 at 14:21

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