How ADSL breaks the bandwidth barriers existed in the copper loop
ADSL operates at up stream and down stream at 25 to 1100 kHz. Compare to voice communication through copper cable, ADSL bandwidth is increased more than 1000 kHz. Therefore we can go for high capacities over copper loop. It uses another path for data access. In order to transmit data at high rates over copper wire, the hardware must implement to avoid that loss. Therefore ADSL sends “symbols” rather than individual bits, this process known as modulation.
Basically there are two types of modulationused in ADSL. They are,
CAP (Carrier less Amplitude Phase Modulation)
CAP operates by dividing the signals on the telephone line bandwidth into three distinct bands. Voice conversations are carried in the 0 to 4 KHz band. The upstream channel is carried in a band between 25 and 160 KHz. The downstream channel begins at 240 KHz and goes up to maximum of about 1.1 MHz (megahertz). Therefore using this technique we can go for higher bandwidth than voice.
DMT (Discrete Multi Tone Modulation)
In DMT divides telephone line bandwidth into 256 different 4.3125-KHz lines and then add bits to each of them. Each channel is monitored and, if the quality is too impaired, the signal is shifted to another channel. This system constantly shifts signals between different channels, searching for the best channels for transmission and reception. This modulation technique allows transmit more capacity according to line condition and more flexibility on lines of differing quality.
We can increase the bandwidth of the copper cable using these two techniques and break the bandwidth barriers.
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