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This supplement was originally printed
in MacOS monthly. DSL - How it Works Corby here. This page, a supplement to the June 2000 Shop Talk (reprinted from MacOS monthly) is intended to give you a general overview of how DSL works. Your ISP and DSL provider may work differently than this. Contact them for specific setup instructions.
1. At your place you connect your Mac to your DSL hardware (the industry term for your DSL hardware is CPE - Customer Premises Equipment) using the ethernet crossover cable provided by the company providing the DSL line or "transport." Your Mac sends out data in the form of IP packets which is what the internet communicated with. When the packets hit your CPE they are converted into ATM cells. This will be explained later. You then connect a piece of phone line between your CPE and a phone jack that has the DSL on it.
Some transport providers require that you put special filters on all other extensions in your place other than the extension your DSL is on. Some have to run a special piece of phone line just for the DSL and put the filer in the box on the out side of your house for the rest of the extensions.
2. At the central office (CO) all of the phone lines for that part of town come together. The lines that don't have DSL all connect into the Public Switched Telephone Network (PSTN). a) Lines that are setup for DSL typically connect into the first stage
of a Digital Serial Line Access Multiplexer (DSLAM) where your phone line
is split. The frequencies for voice communication connect into the PSTN
while the frequencies continue their journey through the DSLAM. b) The second stage of the DSLAM is the Line Interface Module (LIM).
If you have ever called tech support you may have heard the term "training."
The LIM is the part of the DSLAM where your line trains up. Some transport
providers employ modem pooling for DSL. This means that they will have
(typically) four customers for every one LIM. If you are connected to
a modem pooling LIM and they are are full, this means that you will not
be able to connect until an opening becomes available. Most transport
providers set a limit for how long someone can be connected which means
everyone has a fair chance at getting on line. c) The third stage of the DSLAM is the "modem." It isn't really a modem.
Technically a modem is a modulator/demodulator (mo-dem) which takes digital
signals from your computer and turns them into an analog signal for transmission
over standard phone lines. In reality, people use the term "modem" to
refer to the piece of hardware that connected them to the internet. CPE
manufacturers decided to stick with the term modem so that it would be
easier for consumers to relate. So, just like your trusty old 56K modem
would dial up and connect with another modem at your ISP, your DSL modem
does essentially the same thing; it connect to a "modem" at the CO. d) The Network Interface Card (NIC) is what puts the "M" in DSLAM (too many letters!). All of the DSL lines coming into the DSLAM are concentrated or multiplexed into one connection out to the ATM cloud.
3. Between the CO your DSL line connects into and your ISP is the ATM cloud. ATM = Asynchronous Transfer Method. It's a very high speed form of networking designed for voice, video, and data. Earlier I said that your CPE turns the IP packets into ATM cells. This is where that becomes important. Because the network that connects your CO to your ISP is an ATM network the information has to be in a format that the network understands. A large network like this is often referred to as a cloud because it is this large, undefinable mass of stuff (like a cloud!). An IP packet can vary in size. An ATM cell is always the same size; 53 bytes. This consistency in size is what contributes to ATM being so fast. If an IP packet is bigger than a cell, the packet is broken up across however many cells it takes to pass the data along. Typically the path your connection to your ISP takes is permanent. It's called a PVC (Permanent Virtual Circuit). It's a predefined path through the ATM cloud that never changes. This path is negotiated with a VPI/VCI (Virtual Path Identifier/Virtual Circuit Identifier). This path isn't a dedicated piece of wire, it's virtual. At the other end of the cloud is your ISP.
4. The last and final leg of your journey out to the internet is your ISP. Your ISP has a series of gateways, routers, servers, etc. which ultimately connect out to the backbone of the internet. Typically one of these pieces of equipment converts the ATM cells that have traveled all the way from your CPE back into IP packets for transmission out on the internet. The whole process goes in reverse when information comes back to you.
So, there it is. DSL in a Nutshell. Again, your transport provider and your ISP may work very differently from what is outlined above, but this is generic enough to give you an idea of how it all works. Pretty complicated, eh? The next time it breaks try to be a bit more patient and understanding with the three or four different offices you speak with to get your connection back up. There are many pieces involved with different people responsible for the different pieces. They are only human and are limited in what they have access to. Here's hoping nothing breaks. If you have any questions or comments please drop me a line at corby@macosjournal.com. Thanks! |
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