DSLAM Basics

Really informative....Thnx

Hello Ranger24

I am using the network topology that u mention at a previous post.
PC --> CPE --> DLSAM --(ATM / Ethernet network)-- -->BRAS --> WWW

Here it will be clearer to refer also the transmission (layer 1). Between CPE – DSLAM we have ADSL and between DSLAM – BRAS (backbone network) is using SDH (or DWDM).
At the next phase of ADSL there is at the last lime the upgrade to ADSL2+ (up to 24 Mbps) and to the backbone the implementation of NG-SDH (MSPP nobes).
The really big change is the transition from ATM to Metro Ethernet. These two protocol’s refer to Layer 2 (data transportation, aggregation).
But I have a question about the usefulness of ATM (or MEF). A possible scenario could be to connect direct the uplink of DSLAM to SDH network (ADM multiplexer). The SDH network with the capability of cross connect has the ability to transfer the data to the corresponding BRAS, so why we need the ATM switches? I have tried to find some answers at this question and I work out that the main advantage of using Layer 2 protocol (ATM or Ethernet) is the ability to construct the services (fast internet, VoIP, IPTV).The ATM cell with the overhead information VC, VP offers the right to discriminate the data flow into different services based on VP. The same is happening with the Ethernet where in this case the VLAN is the main concept. Am I right about these thoughts?
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Koslyr,

Interesting question. In order to answer this we need to understand how SDH ADM Multiplexiers workj and how ATM switches work.

SDH ADM Mux
ADM stands for Add / Drop Multiplexier. This kind of SDH mux takes advantage of pointers within the SDH header. An SDH frame (eg STM-1, 155mb/s) is basically a collection of lower order traffic frames such as E1 (2 mb/s). Each E1 is a combination of 32 x 64kb/s timeslots. The pointers within the header mean that any single timeslot can be removed from the ADM (Dropped) and replaced with another (Added).

This make SDH ADMs highly disrable pieces of kit in a circuit based network i.e. the Public Switched Telephony Network (PSTN). However this is not so good when the SDH frame contains ATM. While the header is the same the payload will consist of entirely atm cells. So the pointers probably wouldn't work meaning the ADM would only be able to transfer the traffic not add or drop.

ATM Switch
ATM switches contain an atm switching fabric that allows all cells to be connected across the switch to any of the available ports. The SDH frame arrives, the header is removed leaving the atm cells. The vpi / vci of each cell is read and based upon the switch configuration the cells are given a navigation tag. The navigation tag is read by the switching fabric to determin where the cell is sent out. On the out ward port it can be added to a new SDH frame / ethernet frame or what ever is used.

So both work differently and are not really interchanagble. in addition to this I belive SDH ADMs are far more expensive then ATM switches.

I am pretty sure all this is correct but if anyone knows different please say so.

Regards,

R

Mind Blowing, how on earth did that get put in Basic Concepts, lol.

Brillient explanations Ranger24, guessing you are in that industry then

Take it easy

Wayne

Yup Smurf,

I'm a Broadband Network support, design & training bloke.

I find my head gets full of this stuff and Firewall.cx is a great place to dump it!

R

hi ranger
any good site where i can gather some more useful info on dslam?