Asymmetrical Digital Subscriber Line
1.Links subscriber and internet service provider only for residential users.
2.Uses existing local loops.
3.Uses currently installed twisted pair cable.
4.Can carry broader spectrum of 1MHz or more.
The twisted pair local loop is actually capable of handling bandwidths up to 1.1Mhz, but the filter installed at the end office of the telephone company where each local loop terminates limits the bandwidth to 4Khz(for voice comm.). If the filter is removed the entire 1.1Mhz is available for data and voice comm. but practically it is not the case.
1.ADSL allows transmission of high bit-rates over local loop.
n It does not require any changes in the wiring.
2.In addition, it uses the local loop for dual purpose.
n A user can use the telephone for analog voice communication and at the same time transmit data or stream video.
3.It requires a splitter and an ADSL modem
n The splitter separates voice/fax signals from data stream.
n PC to ADSL modem: typically USB.
n ADSL is asymmetric: it provides a higher bit-rate downstream than upstream.
n Downstream: 32 kb/s to 6.4 Mb/s (more typically, 1.5 Mb/s)
n Upstream: 32 to 640 kb/s (more typically, 256 kb/s)
n Asymmetry is acceptable when high bitrate data is transmitted to the user.
n E.g.: Video-On-Demand, Internet radioÂ¦
n In some cases, symmetric communication is preferable.
n To achieve high bitrate transmission, ADSL uses a sophisticated technology:
n It is adaptive: ADSL modems at the two ends probe the line between them to find its characteristics, and then agree to communicate using techniques that are optimal for that line.
n Depending on the characteristics of the wiring, different bit-rates can be achieved.
Discrete Multitone Technique
The modulation technique that has become standard for ADSL is called the DISCRETE MULTITONE TECHNIQUE.
n DMT combines QAM and FDM.
n Multiple carrier signals at different frequencies
n Some bits on each channel
n 4kHz subchannels with 15 bit modulation.
n Send test signal and use subchannels with better signal to noise ratio
n 256 downstream subchannels at 4kHz (60kbps)
n Frequencies from 138kHz to 1104kHz.