SDSL Bridging with the ZyXEL 782R G.SHDSL Router

You may have wondered, how do I bridge DSL modems? I can’t be the only one…

A great copper span of several feet
A great copper span of several feet

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I purchased a pair of ZyXEL 782R modems off of eBay (the best place to get all that sort of outdated NOS crap).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:

Serial Terminal Menu Screen
Serial Terminal Menu Screen

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.

Menu 3.2
Menu 3.2

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.

All lights lit up!
All lights lit up!

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!

2 thoughts on “SDSL Bridging with the ZyXEL 782R G.SHDSL Router”

  1. That’s a very clever find. I would never have suspected that some SDSL “modems” could be put into a CO mode. COs – telephone exchanges would use DSLAMs (DSL Access Multiplexers) for there end of their link rather than banks of modems like these. Clever idea of Zyxel then to make these modems dual use. I wonder if that is a common feature of SDSL modems.

    One interesting thing about xDSL modems is that speak a network protocol, ATM (Asynchronous Transfer Mode) with the connection between endpoints being permanently nailed up, a so called PVC (Permanent Virtual Connection) in contrast to the relatively unpopular for ATM, SVC “switched virtual connection” which can be connected to arbitrary destinations and disconnected as required. Originally, DSLAMs were connected to ATM switches, these days cheaper switching technologies are also employed. Some modems offer the ability of supporting multiple PVCs, so you could have connections to more than one end point. If your interested in ATM it is possible to get closer to the interface. Short of proper ATM hardware and a true modem with an ATM physical layer interface that passes the ATM bit stream directly, it appears some USB xDSL modems require the host to implement the ATM stack, and such a stack and ATM API was available for Linux.

    I’m not sure while you’re playing with this though, do you need a long twisted pair run in some environment where you can’t use WiFi? I see the same seller also has some cheaper optic fibre equipment which would have to be traded off against the additional cost of having to deal with fibre. Otherwise, Wifi with directional antennas generally gives you better bang for your buck.

    1. My college radio station has a separate tower from the studios and it’s all audio over IP. We run a single fiber strand up to the tower (we’ve added several others) but we’ll be seeing a bit of downtime due to some building data upgrades and I was interested if it was possible to run data services over the 1 mile long copper pair that used to carry analog audio back in the day. It’s in an area heavily polluted with wifi signals (college campus). We’ve looked into a bunch of different solutions to get data up there and quite honestly the most practical appears to be some license free microwave links from Ubiquity.

      In addition, I’m also interested in various ISP and IP technologies like DOCSIS, GPON, and DSL. DOCSIS won’t bridge, nor will GPON. DSL it is!

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