ADSL Plumbing

ADSL service enters your residence/business over a simple copper phone pair. It comes from the telephone company (telco) Central Office (CO) or remote CO, and enters the NID, which is a box, typically on the side of your building. The telco owns this box, but it sure looks like your own personal box sitting there all by itself on the side of your house. Inside this box, you will find at least one RJ11 connector. If you pull this connector, you have disconnected your inside telephone wires from the telephone network. You could then plug a normal phone into the jack from which you just extracted the plug - in this way you will eliminate all inside wiring in your house. This is the way in which you can perform trouble shooting by isolating inside wiring (typically owned by you) from the wiring owned by the telco, which leaves this NID and travels all the way to the local telephone office, or at least the nearest remote telephone office.

In earlier ADSL installations, and the most premium still, there is a "whole house splitter" which sits between the actual inside wiring of your house and the posts which are connected to the RJ11. This splitter is typically mounted inside the NID, if there is room, otherwise outside in its own personal little NID. Later ADSL installs don't use this whole house splitter, but instead use splitters, called "filters" on each individual phone inside the house. This allows the homeowner to install the ADSL service all by himself [the homeowner is of course incapable of installing the splitter in the NID.......]. This is supposed to make the ADSL install much simpler, and eliminates the "truck roll" to have the telco technician install the whole house splitter.

Once the telephone line leaves the NID, it travels of course to all your phones in the house, and also to an ADSL "modem" of some sort. Now there can only be a single ADSL modem sitting on this phone line, at least only one at a time. This is decidedly unlike old analog modems, sitting inside all your PCs, which can all be connected to this phone line at the same time. So remember that - only one ADSL modem can be active (on) at a time if you happen to have more than one.

Cable modem service enters your house via the Cable TV coaxial cable service of course, where it is split off from the TV signal by a special splitter device, mounted someplace outside or inside your house. The TV service goes to the TV set top box, and the cable modem service goes to the cable modem. On the down side of the cable modem is an Ethernet and/or a USB interface, much like the ADSL modem. We will deal only with Ethernet interfaces in this document. From here the services look pretty much identical.

There are four basic setups for ADSL and cable modem service. By looking at this diagram you can see that A shows a simple Cable Modem service, and B shows a simple ADSL service served by external ADSL modem. The output of both of these modems is, in the USA, ordinary Ethernet [described in some detail here], the ADSL modem having "adapted" between the ATM cells over the ADSL physical layer and the Ethernet frames [all very complicated stuff....], and the Cable Modem having done similar duty for the RF physical layer. The output of this modem can then be wired, via a normal ethernet cable [RJ45 connectors on cat5 cable] directly to the ethernet port on a PC. But this is not the best way to configure your ADSL or Cable Modem service. You will need some extra software on your PC in order to sustain this configuration. This software will be dial-up like software (pppoE client) for ADSL; a software firewall is also mandatory. Cable modem service typically uses the normal DHCP client in the PC, and thus requires no special "dialup" software, but will still require a software firewall. The importance of installing a firewall cannot be overstated.

The correct way to install ADSL and cable modem service is to install a NAT/router behind the modem, as shown in C. This protects you against incoming connection attempts. Figure D shows that oftentimes you can buy a combined NAT/router/hub, or combined NAT/router/hub/wireless access point. For ADSL, you can sometimes buy a combined ADSL modem/NAT/router.

In order to have multiple PCs behind the NAT/router, you may need a hub, if the NAT/router doesn't come with multiple LAN ports. By using a Wireless Access point, you can connect it to the hub, and then install a wireless NIC in your PC, and have wireless communication, in addition to the wired communication using the Ethernet NICs. In addition, some of the newer NAT/routers will have a wireless capability, so that you don't need to purchase a separate Wireless Access Point. This page discusses hubs and switches.

This site contains a lot of FAQs, including many wiring diagrams showing NIDs, homeruns, etc.

This page contains basic home networking education and information, including wireless.