DSLAM Basics

westwindshelley · Posted: Fri Dec 09, 2005 11:30 pm Post subject: 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

Ranger24 · Posted: Tue Jan 03, 2006 4:15 pm Post subject: DSLAM concept 0- In brief

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

nske · Posted: Thu Jan 05, 2006 2:13 pm Post subject:

Great post! It filled me in as well, thanks Smile

dwyane · Occasional Member Joined: Oct 17, 2004 Posts: 40

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

DaLight · Posted: Fri Jan 06, 2006 12:38 pm Post subject:

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.

dwyane · Occasional Member Joined: Oct 17, 2004 Posts: 40

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

Lex_Luthor · Posted: Sun Jan 08, 2006 12:41 am Post subject:

Brilliant post Ranger24, thanks mate Surprised

)

Is it also the DSLAM that splits the signals from data or voice, based on whether they are above or below 4000Hz?

Ranger24 · Posted: Tue Jan 10, 2006 4:07 pm Post subject:

Hi Guys,

Thanks for the comments.

I'll start with Lex_Luthor's question regarding the the voice frequencies.

The Telephone line or copper pair carries 2 signals. At the low end of the frequency spectrum (0 -4000hz) the analogue voice from the customer is carried. Between 128 Khz - 1.1Mhz (ADSL) or 2.2Mhz (Adsl2+) the DSL signal is transmitted. This means we have 2 traffic types to deal with.

So we need to split these 2 signals. The voice frequecies need to be passed on to the PSTN (Public Switched Telephony Network). The Data frequencies need to be passed to the Customer or Line card within the DSLAM

For this a Low Pass Filter Shelf (LPSF) is normally used. The copper pair connects to a port within the LPFS, The port contains a passive splitter that send the 2 singals to 2 outputs. The first output will be a copper pair to the PSTN and the second a copper pair to the customer/ line card.

Normally the LPFS is found int he same rack as the DSLAM.

This also helps begin to answer Dwaynes post. The LP card perfoms the splitting of the signal, while the LT card terminates the customer DSL signal.

New Network Diagram:

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

Hope this help.

westwindshelley

This was an overwhelming experience
I had posted this question a month ago and was not getting replies
Then I had my exams because of which I was unable to go to the internet for a while
But when I did I got this huge message from Ranger and Iam 100% that no one else could have given a better explanation than this
its the most explicit one
thanks again ranger

koslyr · Posted: Tue Jun 20, 2006 10:57 pm Post subject:

At the Ethernet DSLAM, it refers that
"Ethernet frames forwarded to VLAN based trunk interface based on connection table and on to BRAS "
At this case which is the mechanism for the mapping between pvc and vlan. Also i would like to ask about the vlan definition at the ethernet dslam. As i know this is a capability of the ethernet switch (metro ethernet) so how the dslam deals with this one?
The next generation of adsl (adsl2+) requires the transition from the ATM based DSLAM to the Ethernet based DLSAM. This happens because the ethernet could supply a GE bandwidth in contrast with the STM-1 which is the commom uplink at the ATM DSLAM. My question is what about the qos at the IP DSLAM & Metro Ethernet.