DSLAM Basics

What is the concept of DSLAM

What are the things which constitute a DSLAM

How does it work as a multiplexer as what I know about multiplexers is that it has many inputs and one output and that the output is achieved with the help of the combination of switches

How does it functions with a radius server

Hi,

DSLAM stands for Digital Subscriber Line Access Multiplixer. It's primary function is to provide broadband connections to subscribers based upon various DSL flavours, e.g.;

ADSL - Asymetrical Digital subscriber Line
VDSL - Very High Speed DSL
SH.DSL - Symetrical High Speed DSL

DSLAMs are split in to 2 sides - Customer Side & Trunk Side.

Customer side is where the customer telephone lines are connected and trunk side is where the dslam connects to the ISP network.

On the customer side each line is configured to supply a data connection via DSL to the customer modem. Customer connections are always ATM over DSL - this means the customers ethernet frames (from their PC) are put in to ATM cells which are transmitted vis the DSL on their phone line to the DSLAM.

The trunk side can use various connections depending upon the make & model. Typically trunk connections are ATM, Ethernet or IP.

Trunk interfaces can include:

- STM1 155mb/s
- DS3 (Americas approx 34 Mb/s)
- E1 (European approx 2 Mb/s)
- Gigabit Ethernet 1000 Mb/s
- Fast Ethernet Mb/s

Depending on the trunk connection the DLSAM can make connections between the customer side and the trunk side in a number of ways:

1) ATM DSLAM
- Customer modem packages ethernet frames in to Atm Cells
- ATM cells are transmitted over the phone line using DSL
- DSLAM line card receives ATM cells and passes them to Control card
- Control card switched ATM cells via switching Fabric to trunk interface
- ATM cells are forwarded via ATM network to BRAS - Broadband Remote Access Server (edge router with radius/ authentication functions)

2) ETHERNET DSLAM
- Customer modem packages ethernet frames in to Atm Cells
- ATM cells are transmitted over the phone line using DSL
- DSLAM line card receives ATM cells and passes them to Control card
- Control card re-assembles ethernet frame from ATM cells
- Ethernet frames forwarded to VLAN based trunk interface based on connection table and on to BRAS

3) IP DSLAM
- Customer modem packages ethernet frames in to Atm Cells
- ATM cells are transmitted over the phone line using DSL
- DSLAM line card receives ATM cells and passes them to Control card
- Control card re-assembles ethernet frame from ATM cells
- IP Packets are removed from ethernet frames
- IP Packets are routed to VLAN based trunk interface based on routing table and on to BRAS

A typical network would look like this:

PC --> CPE --> DLSAM --(ATM / Ethernet network)-- -->BRAS --> WWW

(CPE = customer premisies equipment, or DSL Modem)

Please feel free to ask further questions, and please let state where you are in the world as the european implementation is different from the Asian implementation which is different from the Americas implementation

- Ranger

Great post! It filled me in as well, thanks

great post Ranger24, one of the best explanations regarding DSLAM , The only thing i knew earlier was DSLAM seggregates the ISP from the customer

Excellent post Ranger24! All I knew about DSLAMs previously was that they were installed at British Telecom's telephone exchanges for connection to the ISP networks. You've made it so clear.

Do u have any information on the LP ( low pass ) card and the LT ( line termination) card :