DSLAM Basics

Brilliant post Ranger24, thanks mate )

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

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.

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

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.

Great Mr Ranger24 superb explanation

Hi Guys,

Back in December Koslyr posted some more questions to this thread and I completely missed them. So here are my answers...and apologies!

1)What is the mechanism for the mapping between pvc and vlan?

Simply put this is just a case of configuration. The atm cells will arrive at the client port and be reassembled in to a AAL5 frame which contains the ethernet frame. The frame can then be extracted and the VLAN tag added by the trunk interface as the frame leaves. The appropriate trunk interface will be chosen according to the configuration of the client port.

2)How do DSLAMs deal with VLANs (paraphrased)?

DSLAMs should (must???) deal with VLANs exactly the same as a metroethernet switch. Typically the DSLAM is at the end of a VLAN so will be responsible for tagging upstream frames and forwarding downstream frames (removing the VLAN tag first).

3) What about the qos at the IP DSLAM & Metro Ethernet?

Big question. In my experiance DSLAMs will implement basic QoS at 1 of 3 levels:

a) ATM Traffic Descriptors mapped to IP Class of Service (EF, AF or DF)

b) VLAN Priority tags mapped to IP Class of Service

c) IP Class of Service mapped to IP Class of Service.

The chosen mechanism will depend upon the metro ethernet.

I hope this helps....

R