DSLAM

A DSLAM takes incoming DSL connections from the local loop and multiplexes into a larger ATM channel. In bigger setups, this will then connect into an AMUX which multiplexes multiple DSLAM's into an OC3. From there you connect into various ATM switches.

Lucent makes the DSLAM that we use and it seems to be pretty reliable.

A Digital Subscriber Line Access Multiplexer. It is a box that sits in leased space in the local office of your local telephone utility. It takes a bunch of (relatively) puny DSL lines and MUXes them into a fiber pipe that gets sent up to bigger fish in the internet pond.

If a potential job description mentions DSLAMS, you probably don't want it, because you'll have to work with phone companies, which are pretty evil.

Im not going to mention what DSLAM stands for again, and instead i am going to show what role it plays in DSL connectivity. And now, some ASCII art

             Local Loop
 _________________|___________________
|                                     |
                             _________
   SNI                      |         |
    |                       |         |
    |                       |         |
/--\|    _      ______      |         |
|  |]   | |    |      |     |         |
|  |____| |____|      |_____|         |
---------------------------------------
 ^       ^       ^             ^ Central Office
 |       |       |-- Crossbox
 |       |----Pedastal
 |-- Customer Premise

The DSLAM is a piece of equipment that sits in the Central Office. Next on to the anatomy of the central office.

   _________________________________
  |                        |        |
  |     ATM Edge Switch->  |        |
  |                        |        |
  |_____                   |________|
  |     |                     |     |
  |     | <- Voice Switch     |     |
  |     |            _________|__   |
  |_____|           |ATU-C -> || |  |
  |  |     DSLAM -> |       __|| |  |
  |  |              |      |     |  |
  |  |              |      |     |  |
  |  |   Lim Port --|---> ||     |  |
__|_|_|_____________|_____||     |  |

Ok, my poopy ASCII art out of the way, I am gonna talk through this. The local loop, as I have had it explained to me is a pair of copper wires that runs between the SNI and the CO (Central Office). When the pair hits the CO, voice traffic is routed to a voice switch, while DSL traffic heads for the DSLAM. The first hit it makes in the DSLAM (usually) is a LIM port. From the LIM port, it gets sent to an ATU-C. The ATU-C converts the signal into ATM cells, which then get sent to the ATM Edge Switch. The signal then gets sent through a myriad of switches referred to as the ATM Cloud. Then the signal hits the ISP, and goes out to the internet. Reverse for downstream.

My simplified drawing shows a DSLAM with one lim port and one ATU-C. In reality there are racks of LIM ports and ATU-Cs. That's where the multiplexer part comes in. The DSLAM takes a bunch of DSL signals, converts them into ATM cells, and sends it off to the Edge Switch.

The DSLAM I work with is a Cisco 6100. I also work with Lucent DSLAMs, though only rarely. We refer to the Lucent ones as stingers, and they are typically used to extend DSL availibility. The main difference between the two is the Ciscos are typically in the CO, while the Lucents are out in the field.

Another point worth mentioning is that it is possible to chain muliple DSLAMs together. The process is called subtending and allows a DSL provider to allow more DSL connections through a CO