ADSL is made up of several parts (shown by Figures 1 and 2):
Figure 1: ADSL Block Diagram (POTS Splitter)
<-------Home/Office---------------> <-----Telco Central Office---->
NID
----- -----
2 wire X-----------Voice-=| S | | D |
phone | P | | S |=----- Voice Switch
line | L | 2 wire | L |
| I |=-------------=| A |
| T | Local Loop | M |=----- ISP Connection
10baseT ---------- Data | T | | |
Ethernet X--=| |=----=| E | -----
or ---------- | R |
ATMF ADSL -----
NIC ANT
The Customer Premises Equipment (CPE) for ADSL consists of the ANT and/or NIC card. The ADSL Network Termination (ANT), shown in Figure 1, is located at your home or office, provides an IP connection. ANTs come in several types:
In each case, the ANT/NIC provides the a router address to an ISP. Each Telco will specify the configurations that they will allow. The most desirable configuration for the Linux user is the ANT with a 10baseT Interface, since the cost/setup is the easiest. The other options require special drivers, which have, to date, not been made available for Linux. The bad news is that some providers allow only integrated ANT/NIC PCI cards that do not have Linux Drivers.
Warning! Make sure any third party ANT/NIC you may purchase are compatible with your Telco provider. There are two major line encodings for ADSL (CAP, DMT), and several options for IP encapsulation. Your Telco should provide you a list of allowable options.
The ANT is connected to your house's inside wire (2 wire phone line). This inside wire is connected to the data side of a POTS splitter, or, in the case of the splitterless version, directly connected to the local loop. Figure 1 shows the POTS splitter wiring, and Figure 2 shows a splitterless type.
In my case, I was provided with an Alcatel ANT, which supports a 10baseT (wired as a Crossover) RJ45 jack. I understand that a NIC card that fits directly in the PC will be available sometime in the future.
Figure 2: ADSL Block Diagram (Splitterless Design)
<-------Home/Office---------------> <-----Telco Central Office---->
SNI
- -----
2 wire X-[RJ11]---Voice----| | D |
phone Filter | | S |=----- Voice Switch
line | 2 wire | L |
|=-------------=| A |
| Local Loop | M |=----- ISP Connection
10baseT ---------- Data | | |
Ethernet X--=| |=-----| -----
or ----------
ATMF ADSL
NIC ANT
Somehow, the digital and analog signals need to be separated for all of this to work. Thus, a filter needs to be placed in the signal path at some point. There are two methods for doing this: Using a POTS Splitter or using RJ11 phone jack filters.
First, in the POTS splitter method, device is located on the "side of the house" where the Telco line is connected. The splitter provides two functions. First, it is the "demarcation point" that separates the Telco wiring from the inside wiring. Second, it "splits" the DSL signal from Telco into a separated data channel and a voice channel. The voice channel is a normal analog phone line (2 wire), and the data channel is sent to the ANT. The splitter is a passive, non-powered device, which will allow the voice channel to operate even if the power fails at the home location. The Telco signal is sent to the splitter using an existing 2 wire line to the home. The Splitter is housed in the Network Interface Device (NID) on the outside of your house.
Second, in the splitterless design, shown in Figure 2, the outside local loop is connected directly to the inside wire at the Subscriber Network Interface (SNI), the same box that is used today at your house. At each extension jack where you wish to plug in an analog phone, you place a special jack that contains a filter that removes the digital signal. This is called an RJ11 filter (RJ11 is the official Telco term used for your 4/6 pin phone jack). The extension used for your ANT does NOT use a filter (otherwise it won't work). That's all there is to it! It should also be noted that some low speed ADSLs will not require RJ-11 filters.
The splitterless design is very desirable from the Telco point of view, as they won't have to roll any trucks to do the install work, and allows them to offer ADSL at a lower price. For most users, it doesn't really matter, in fact, the analog phones will still work without the RJ11 filter in place. The only thing is that you will hear a bit of a high pitched whine when you use the phone. However, this is not recommended, as later version may damage the phone or have some other nasty effect.
The DSLAM is the equipment at the Telco Central Office that that splits the signal and connects the user to the Voice Switch and ISP. That's all you need to know from a user perspective.
An ISP connects to the DSLAM via a high-speed data connection, usually ATM over a T3 (45Mbps) or OC-3 (155Mbps). The important thing here is that an ISP must "subscribe" with your Telco to provide this connection.
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