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why we need adsl modems to connect to ISP

Frequent Contributor
Frequent Contributor

Hi all, can anyone tell me wha we need an adsl modem for to connect to the isp, is this because analogue signals can only be sent over the telephone line, Im gathering my home modem connects to another modem in the exchange, and this would be the DSLAM?

5 Replies 5



"Im gathering my home modem connects to another modem in the exchange, and this would be the DSLAM?"

As per my knowldge if there is master modem at exchange then its a back to back connectivity ....(leased line)

..Master modem ==== slave modem (at your permises) this setup DSLAM will not come in picture..

if its ADSL modem then it will be terminating to DSLAM.


Hello Uij,

actually a DSLAM is a concentration of master modems:

each linecard can handle 8 to 16 lines

A DSLAM is an enginereed solution it takes tens to hundreds of copper wire pairs:

at the exchange CO going through passive filters base band telephony PSTN is separated from DSL signals:

DSL signals are sent to DSLAM linecards via wire pairs.

The DSLAM then has an ATM or IP high speed uplink to ISP broadband RAS or routers.

a good note is that actually a DSL link is a point-to-point link between two devices:

ATU-R : modem in your home

ATU-C : CO modem

ATU-C transmits on the frequencies where ATU-R receives and viceversa

This is also the reason why we cannot connect back-to-back two ADSL modems they are both two ATU-R.

Hope to help


Hi Giuseppe,

Wah wonderfull explanation ...very informative concept was wrong ...thanks for the valuable information ...i ve rated ...


Hi there, so does the DSLAM have built in modem facilities ? else how does the signal at the exchange get demodulated ?

Hello Carl,

yes the DLSAM has buil-in modem facilities to be able to terminate tens to hundreds of DSL lines, the POTS splitter divides PSTN from DSL and only DSL signals go to the DSLAM chassis using 25 wire pairs cables (TELCO connector) each cable plug provides 25 pairs/up to 25 lines.

Inside the chassis each pair is routed to a modem on a linecard, the ATM cells are extracted after demodulation and in traditional DSLAM with ATM uplinks they are switched (ATM cells) to some other device that will terminate the ATM connection.

After ATM and PPP termination IP packets are sent to the internet.

The opposite happens for downstream return traffic:

packets for your public (or private this depends on provider) ip address are sent inside a PPP pipe, then inside an ATM PVC and then through the DSL line to your home modem

Hope to help


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