al that can be synthesized by summing amplitude modulated cosine and sine waves. These two components, being 90 deg out of phase, are called quadrature, hence the name Quadrature Amplitude Modulation. By combining amplitude and phase modulation of a carrier signal, we can increase the number of states and thereby transmit more bits per every state change. CAP Carrierless amplitude and phase (CAP) modulation technique is closely related to QAM in that amplitude and phase are used to represent the binary signal. The difference between CAP and QAM lies in the state representation of the constellation pattern. CAP does not use a carrier signal to represent the phase and amplitude changes. Rather, two waveforms are used to encode the bits. The encoder replaces a stream of digital data with a complex equation that symbolizes a point on the constellation diagram. Thus, for a 32-CAP, there would be 32 possible locations on the diagram, all of which can be represented as a vector consisting of real and imaginary coordinates. Consequently, 32-CAP would result in 32 distinct equations of the type, each one representing five bits of data. CAP modulation is very suitable for use with ADSL. DMT The spectrum from 0 to 4 kHz, voice band, is designated for plain old telephone services (POTS). Downstream (ATU-C to ATU-R), the spectrum from 26 kHz to 1.1 MHz is further divided into 249 discrete channels. Upstream (ATU-R to ATU-C), the spectrum above the POTS band consists of 25 channels between 26 kHz and 138 kHz. Echo canceling between the downstream and upstream signals permits reuse of these sub-channels. With the exception of carriers used for timing, each carrier is capable of carrying data. However, only those carriers with sufficient signal to noise ratio (SNR) are allocated payload for transmission. Each transmitting carrier is allotted a bit count and transmits power, based on the characteristics of the sub-channel. This results in an optimized d...
