Telephone companies have installed Digital Network wide area high speed to handle communications between their headquarters.
However, the link between user (Subscriber) and network continues to be an analog line (local loop.) The challenge is to make these digital lines, digital subscriber line, without changing the existing local loop. The local loop is a twisted pair cable with a potential bandwidth of 1 MHz or more.
The asymmetric digital subscriber line (ADSL, Asymmetric Digital Subscriber Line) Is asymmetric, Which means that provides higher bit rates in the input direction (from the telephone exchange to the subscriber's home) in the outbound direction (from the house of the subscriber to the telephone).
This is usually what people want subscribers (Users) want to receive large files online quickly, but usually have to send small files such as e-mails short.
ADSL divides band width a twisted pair cable in three bands.
The first band Usually between zero and 25 KHz, is used for regular phone service (known as POTS telephone service.) This service normally uses only 4 kHz in this band, the rest used as guard band to separate the voice channel data channels.
The second band Usually between 25 and 200 KHz, is used for outbound communication. The third band Usually between 250 KHz and 1 MHz, is used for inbound communication. Some implementations overlap the range of input and output to provide more bandwidth in the direction of entry. Figure 8.32 shows the bands.
Most implementations ADSL originally used a technique called amplitude modulation / phase without carrier (CAP, Carrier less Amplitude / Phase). Subsequently, other ANSI standardized modulation technique known as discrete multitone (DMT, Discrete Multi-Tone).
The modulation / phase without carrier (CAP) Is a modulation technique similar to QAM But with one important difference: it removes the carrier signal. However, this technique is more complex than QAM and not standardized.
The discrete multitone modulation technique (DM1) combines QAM and FDM. The available bandwidth in each direction is divided into 4 KHz channels, each of which has its own carrier frequency.
The bits created by the source are passed through a host of series-parallel converters, where a block of N bits is divided into N parallel streams, each of which consists of a bit. The QAM signals created from each stream are multiplexed together in frequency and the result is sent through the line.
