By James H. Green
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Extra resources for Access technologies : DSL and cable
Voice can be compressed to 8 kbps with only a small loss of intelligibility. The quality is equivalent to a solid cell phone connection. The protocols add some overhead, so the number of channels is less than a straight multiple of the compressor output, but the voicecarrying capacity is substantial. Note, however, that faxes and modems require an uncompressed channel. The amount of bandwidth the voice channel occupies depends on the protocol used. ATM is the most common protocol. With 5 bytes of header per cell, the overhead is approximately 10 percent.
Telephone Electrical Concepts To understand why access technologies behave as they do, it is necessary to have some appreciation of electrical terms. Any circuit, such as a twisted-wire cable pair or a coaxial cable, has inherent electrical characteristics such as resistance and impedance. Direct current flows in a predictable manner in response to Ohm’s law, which states that the amount of Telephone Local Loop Characteristics 41 current flow is directly proportional to the voltage or electrical pressure and inversely proportional to the resistance.
Any UHF channels in the area were applied to a vacant VHF channel on the cable. Gradually, however, cable systems were expanded to the Cable Access Technology 65 point that the typical system today has a bandwidth of 1 GHz, of which the top 250 MHz is reserved for future applications, leaving room for about 100 channels. An analog cable system could amplify signals to extend the range almost indefinitely, except for the problems of noise and distortion, which limit the serving radius of 80channel CATV to about 8 km from the headend.