You may have wondered, how do I bridge DSL modems? I can’t be the only one…
When you go to your friendly neighborhood cable office or phone company to acquire DSL or DOCSIS services, the box that your ISP gives you is referred to in the industry as CPE or Customer Premises Equipment. Most Internet Service Provider technologies are asymmetric, which means that the upload and download rates are different (in addition to being on different frequencies).
This is generally desirable for consumers who wish to download more than they upload. However, this has the added side affect of preventing two modems from communicating to each other. For example, you cannot take two DOCSIS 3.0 modems and connect them back to back because they have to communicate with an entirely different type of unit, called a CMTS or Cable Modem Termination System (which happen to be very expensive).
SDSL stands for Symmetric DSL, and describes set of consumer equipment which is symmetric in speed. Of importance, not all SDSL modems can communicate because the uplink and downlink do not occur on the same frequencies. The modem that is to be the “base station” will still have to be put into CO or Central Office mode in order to communicate with a modem in CPE mode. The assignment is largely arbitrary in this case.
I purchased a pair of ZyXEL 782R modems off of eBay (the best place to get all that sort of outdated NOS crap). They are a 1999 vintage modem, and as such the G.SHDSL standard they abide by is not particulary impressive in speed (2.3 mbps symmetric). However, I’m looking to begin playing more with this and this was a fairly cheap introduction to the technology.
They can be programmed either over Serial Console (default 9600 baud) or some parameters are accessible through a web interface. In either case, the default password is 1234. Go ahead and connect your serial cable to the modem and hit enter to get to the password prompt. You should see a menu like this:
First, press 1 and hit enter and go ahead and name your router. Call it “base.” While you’re there, disable IP routing and enable “Bridge.” You’ll want to disable all “routing” capacities in these routers, as we don’t want them to do anything but push data they receive off to the remote side. This means disable IP routing, IPX, RIP, etc. There are multiple route settings under menu 1, 11.1, etc.
Under menu 3.2, make sure to disable DHCP. You’ll also want to set static addresses for everything used here. I used:
- 192.168.1.1 – “Base” router (set to Server mode under menu 2)
- 192.168.1.2 – Computer connected to “base router” LAN
- 192.168.1.3 – “Remote” router (set to Client mode)
- 192.168.1.4 – Computer connected to “remote router” LAN
We want these set to bridge because we will be connecting these modems up to switches which will take care of only sending the necessary traffic along the pipe.
Once you’ve configured everything as above, go ahead and connect the two modems together with a good old phone cable to test. Within a minute, you should see the xDSL light on the front panel should illuminate indicating connectivity, after which you should be able to ping across the link. I tested transferring a 60 mb file over HTTP and it worked at modest speed.
I’m going to purchase a set of VDSL2 modems which should bring symmetric speeds of up to 100 mbps. I’ll update this when I do!