Cap

CAP is a single-carrier modulation technique that divides the available space into three bands:

■ The range from 0 to 4 kHz is allocated for POTS transmission.

■ The range of 25 kHz to 160 kHz is allocated for upstream data traffic.

■ The range of 240 kHz to 1.1 MHz is allocated for downstream data traffic.

These ranges may vary slightly based on environmental factors and implementations. Figure 4-2 illustrates the channels on the wire.

Figure 4-2 CAP Modulation

This figure effectively shows how the voice and downstream/upstream data are separated on the wire. Note that the range of frequencies available to downstream data is significantly wider than that available to upstream data or voice.

CAP is a variation of quadrature amplitude modulation (QAM). QAM conveys data by changing the amplitude of two carrier waves that are 90 degrees out of phase from one another (for example, sine vs. cosine wave forms), hence the name quadrature carriers. The changes (modulations) in the wave represent the data signal.

CAP is used only in legacy ADSL implementations. This is primarily because it offers significantly inferior performance, albeit at a reduced cost, compared to DMT. Unlike DMT, CAP is not an industry standard. CAP was, however, the de facto standard for xDSL deployments (deployed in 97 percent of xDSL installs) up until 1996; therefore, it is still commonly encountered.

With the advent of DMT, CAP is rarely, if at all, used today in ADSL service offerings.

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