Some years back there used to be MODEMS and when you recieved a call,the internet used to disconnect,but today it does not disconnect even if I lift the headset of my telephone.How is that achieved? I use the ADSL technology(no idea about the technology just read from the box of my dsl modem)
The modems you remember were designed to use the frequencies that worked on any telephone line. In the early days, you could attach a modem to any phone by setting the handset (after dialing manually) into a mechanical acoustic coupler. This had the advantage of even working with a pay phone.
The phone company did not permit connection of equipment to the public phone network that did not use voice frequencies, and really only guaranteed that frequencies between about 300 Hz and 3400 Hz would be carried. That limited bandwidth is part of the reason for the characteristic sound of a phone call, but isn't a problem for transmission of intelligible speech because most of the important information in the spoken word is carried in that band, even though young, healthy humans can hear tones below 100 Hz and as high as 20000 Hz or higher.
Limiting the signal bandwidth had another advantage. As the trunk lines that carried long distance calls converted to digital signals, they only had to sample the analog call at 8000 samples per second to provide 4000 Hz of bandwidth. Since they used 8-bit samples, that meant that a voice call only required 8000 bytes per second (64000 bits per second) on the trunk.
That bandwidth also limited the bit rate that could be delivered by a modem carried over that channel. The 56 kbaud modems (which were only capable of transmitting their peak rate in one direction, and a much lower rate the other) were about as fast as could be achieved over the voice network.
But people wanted more data and wanted it delivered on wires that already existed because otherwise the cost of installing the "last mile" of wire to each subscriber makes the network too expensive to build. Someone noticed that the mile or two of copper between the end user and their closest phone company machine room (the Central Office or CO) was capable of carrying far more bandwidth than is required for the voice call. And DSL was born.
All of the various DSL schemes have depended on using the spectrum above 4 kHz that is not used by the voice call. They also have to contend with interruptions like the ring voltage (60 to 90 volts AC), the modem-like signaling used for caller ID, and the switching glitches caused by plugging and unplugging phones, lifting the receiver to make a call, and presumably the presence of actual Dial phones on the same line.
When you installed your DSL modem, you isolated the plain telephone equipment from it with filters that pass only voice frequencies to and from the phone equipment. The DSL modem gets to see the whole spectrum, but ignores anything happening in the voice band.