the residential subscribers
10.3.3. Principle of Operation of XDSL
XDSL techniques uses greater range of frequencies over telephone cable than the traditional telephone services used. By using frequency range above the telephone bandwidth (300 Hz to 3400 Hz generally), XDSL can encode more data to achieve higher data rates. In order to utilize the frequencies above the voice audio spectrum, XDSL equipment must be installed on both ends. The loading coils used in copper wires between customer premises and local exchange must be removed or avoided to enable the copper wire to pass higher frequencies over the entire route. The loading coils are the inductors added in series with the copper wires to compensate the parallel capacitance of the line. This coil limits the bandwidth in the copper line.
In transmission, the bit rate may differ for upstream and downstream. The upstream is transfer of information from subscriber to local telephone exchange and the down stream is transfer of information from local telephone exchange to the customer premises. If the transfer of information in bit rate is same for upstream as well as down stream, it is referred as symmetric. If the bit rate varies for upstream and downstream, it is referred as asymmetric or rate adaptive or uneven. As the subscriber usually want to receive high volume files quickly, but usually have small files to send, XDSL one mostly asymmetric, that is down stream bit rates are higher than upsteram. XDSL with symmetric bit transfer also available (IDSL).
Various methods of encoding and modulation techniques are available for encoding data onto XDSL circuits. They are Quadrature amplitude modulation (QAM), Pulse code modulation (PCM), Pulse amplitude modulation (PAM), V.90, Carrierless amplitude and phase (CAP) modulation, Discrete multitone (DMT) modulation etc. The common are QAM, V.90, CAP and DMT. In this section, CAP and DMT are summarised. The QAM and V.90 were discussed in section 3.7.
CAP. Carrierless amplitude and phase (CAP) modulation is similar to QAM but more complex than QAM and is not standardized. Both CAP and QAM are single carrier signal techniques. It is a proprietary standard implemented by Globespan semiconductor. While the name specifies that the modulation is "carrierless" an actual carrier is imposed by the transmit band shaping filter through which the outband symbols are filtered. Hence CAP is algorithmically identical to QAM. The upstream symbol rate is 136 K baud on a 113.2 kHz carrier, while the downstream symbol rate is 340 K baud on a 435.5 kHz carrier, 680 K baud on a 631 kHz carrier, or 952 K baud on a 787.5 kHz carrier.
Fig. 10.7 shows the spectral use of CAP.
CAP uses the entire loop bandwidth (excluding 4 kHz) base band analog voice channel to send the bits all at once. CAP has some benefits over DMT and DMT has some benefits over CAP. One advantage of CAP is a lower peak-to-average signal. Power ratio relative to DMT. This means that the drivers and receivers may operate at lower power than DMT. This is so because CAP does not peak signal capacity as required is DMT circuitry. Most of the RBO's started using CAP but have since moved on to DMT.
Discrete Multitone (DMT) modulation. The use of DMT for ADSL was first proposed in 1991. In 1992, ANSI committee TIE1.4 began work toward a standard for ADSL, defined a set of requirements. In March 1993, the DMT system was chosen to be the best of the standard. ADSL uses DMT encoding methods which use QAM to divide the bandwidth of the channel into multiple subchannels, with each channel transmitting information using QAM modulation. DMT uses the frequency spectrum from 26 Hz to 1.1 MHz for broadband data. For POTS, it uses the frequency spectrum from 0 to 4 kHz.
Fig. 10.8 shows the spectral use of DMT.
Fig. 10.8. Spectrum of DMT.
Fig. 10.8. Spectrum of DMT.
DMT modulation separates usable frequency range into 256 frequency bands (or channels). These are intimately connected to the FFT algorithm with DMT used as its modulator and demodulator. By dividing the frequency spectrum into multiple channels DMT is thought to perform better in the presence of interference sources such as AM radio transmitters. The channels 6-31 used for upstream and 32-250 used for downstream. The number of bits per symbol within each channel may be independently selected allowing the modem to be rate adaptive.
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