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Is there a way to connect two ADSL modems together directly, to each other's ADSL port?

As far as I can see, this is not likely as they both act as clients, and there needs to be a server. Correct me if I'm wrong.

Assuming a server is required, what kind of system can I build to provide an ADSL service to routers?

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    This smells like an XY problem. What are you actually trying to achieve by connecting the routers in this way?
    – DavidPostill ♦
    Commented Nov 26, 2017 at 20:14

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Is there a way to connect two ADSL routers together directly, to each other's ADSL port?

No, each ADSL modem/router that you have is a slave (or remote) unit. They cannot work together (as you already suspect).

Communication links like xDSL are point-to-point but have asymmetric topology, that is, the modem at your end (called the remote unit or CPE, customer-premises equipment) is a "slave" unit, and the modem/DSLAM at the ISP or central office end (called the line unit) is a "master" unit. (Do not confuse or conflate the asymmetric topology with the asymmetric connection speeds of ADSL.)

In order to use the ADSL link, the line and remote units first have to perform a predefined series transmissions and responses (called training) to evaluate line conditions and set transmission rates, power levels and equalization. Only after training is complete can the ADSL link be used to transmit/receive data.
Bottom line is that you cannot use an ADSL modem only built for use as the remote unit in place of the ADSL line unit.
You don't need a entire DSLAM (a multiplexer for many channels), but you would need a Line (or master) ADSL unit (e.g. Ethernet on one side, ADSL on the other).

Assuming a server is required, what kind of system can I build to provide an ADSL service to routers?

Rather than ADSL, you might be better off with a symmetrical connection (i.e. upstream and downstream speeds are the same), such as a point-to-point VDSL setup.
See How to network two locations through 130m of trees

BTW -- Your use of "server" and "client" are inappropriate. xDSL modems use the master/slave model, not the client/server model.

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    Although VDSL is symmetric in the sense of identical up/down speeds, is it really also symmetric in the sense of not requiring different equipment on both ends?
    – grawity_u1686
    Commented Nov 27, 2017 at 7:12
  • @grawity -- I've tried to avoid such confusion by using "asymmetric" and "symmetrical" as adjectives for either "topology" or "connection" to remove ambiguity. Apparently it didn't help. I did not imply the "it really also symmetric in the sense of not requiring different equipment". Instead I wrote that "xDSL are point-to-point but have asymmetric topology" and "xDSL modems use the master/slave model".
    – sawdust
    Commented Nov 27, 2017 at 19:40
  • @grawity -- There are VDSL modems that can be used at either end, but there is typically a hardware switch that configures the mode, either master (CO) or slave (CPE). See versatek.com/product/vx-veb160r1 So the "equipment" can be the "same" at both ends, but the topology is still asymmetric, i.e. master/slave.
    – sawdust
    Commented Nov 28, 2017 at 22:01
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What kind of system can I build to provide an ADSL service to routers?

You need at least a DSLAM:

A digital subscriber line access multiplexer (DSLAM, often pronounced dee-slam) is a network device, often located in telephone exchanges, that connects multiple customer digital subscriber line (DSL) interfaces to a high-speed digital communications channel using multiplexing techniques.

There will be other bits of infrastructure required as well.

For example, you will something behind the DSLAM that will route traffic between your two devices.

What you are trying do is to become your own ISP ...

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  • Not quite my own isp, but make a small adsl local network to put old routers to use and network between buildings close by. Mainly just the concept of the adsl connection. It’s also easier to get an adsl router and cheaper than a wan router.
    – DevelopedLogic
    Commented Nov 26, 2017 at 21:38
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    You can get yourself a DSLAM or equivalent, and they are not that expensive - especially second hand ones supporting ADSL rather then VDSL.
    – davidgo
    Commented Nov 26, 2017 at 22:49
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    "You need at least a DSLAM" -- You exaggerate.
    – sawdust
    Commented Nov 26, 2017 at 23:35
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    @DevelopedLogic You'd do much better with point-to-point wireless bridges.
    – David Schwartz
    Commented Nov 27, 2017 at 2:12
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    @sawdust Exaggerate? Really?
    – DavidPostill ♦
    Commented Nov 27, 2017 at 6:15

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