By James H. Green
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Extra resources for Access technologies : DSL and cable
High-speed modems encode information in symbols instead of a raw bit stream. By using complex coding schemes, a single symbol can be made to represent some number of consecutive bits. As we have discussed, T1/E1 uses a simple coding scheme that encodes only 1 bit per symbol. ISDN improves on that by using 2B1Q line coding. Each pair of bits represents one quaternary signal. Higher modem speeds are achieved with QAM. With QAM, two carriers, each having the same frequency, are phase-shifted 90˚ with respect each other.
A simple filter separates voice and data. Hybrids and echo cancelers are not needed with CAP systems because the two directions of transmission are separate. The Digital Subscriber Line Access Technology 57 modems use adaptive equalization to overcome, to some degree, the effects of bridged tap, gauge changes, and splices. CAP is a proprietary protocol, but despite its lack of standard status, it has a large following. Spectral Compatibility The degree to which DSL signals can coexist in the same cable is a matter of concern to the ILECs.
To compensate, ILECs connect load coils in series with cable pairs when the cable length exceeds more than 18,000 ft (5,500 m) from the central office. A load coil is an inductor that has high attenuation to frequencies above the voice band. High-frequency services such as DSL cannot operate through load coils, so they must be removed if the cable is to support high-speed services. As discussed earlier, bridged tap results when an unterminated length of wire is connected in parallel with the talking path.