ADSL Modems
ADSL Signal Encoding
Traditional POTS uses a bandwidth of 4 KHz for voice. As was mentioned in the dial-up modems' chapter, it limits the available data rate to 56 kbps. In order to get higher data rate (up to 9 Mbps), ADSL uses a wider frequency range: 20 KHz – 1.2 MHz:
* 0 – 4 KHz is used for POTS
* 25 KHz – 138 KHz is used for ADSL Upstream
* 138 KHz – 1.1 MHz is used for ADSL Downstream
DMT Line Code
The line code standard used for ADSL is Discrete Multitone (DMT).
DMT divides the ADSL bandwidth (138 kHz – 1.2 MHz) to 256
sub-channels (tones). Each tone has a bandwidth of 4 KHz and spacing of 4.3
KHz,
each supporting up to 15 bits. High frequency tones are more vulnerable to noise
and attenuation, and therefore represent less than 15 bits.
ADSL Modulation and Error Correction
ADSL uses quadrate amplitude modulation (QAM) to achieve the 15-bit maximum that any single tone can carry.
One of the most important technologies that enabled ADSL to reach its high bandwidth is a coding/encoding method called constellation encoding and decoding. With this method, the information on the line may be damaged and still be repaired by the decoder with a very high reliability.
Another useful method to increase reliability is
called Forward Error Correction (FEC), which is based on Reed Solomon coding
method.
Error Bit-Rate
ADSL has a fixed error rate (BER of 10-7) and a variable data rate. During training, the modems evaluate the quality of the line by measuring noise and attenuation, and then decide on the highest data rate in order to keep the fixed error rate. This behavior is different from other digital technology, such as ATM and ISDN, which have a fixed data rate and a variable error rate.
ADSL Modems
Network Architecture and Physical Connectivity
ADSL Signal Encoding