Digital Subscriber Line (DSL) and Cable
Today, the two most prevalent types of broadband residential access are digital Subscriber line (DSL) and cable. A residence typically obtains DSL Internet access from the same local telephone company (telco) that provides its wired local phone access. Thus, when DSL is used, a customer’s telco is also its ISP, each customer’s DSL modem uses the existing telephone line (twisted Copper line) to exchange data with a digital subscriber line access multiplexer (DSLAM) located in the telco’s local central office (CO). The home’s DSL modem takes digital data and translates it to high frequency tones for transmission over telephone wires to the CO; the analog signals from many such houses are translated back into digital format at the DSLAM.
The residential telephone line carries both data and traditional telephone signals simultaneously, which are encoded at different frequencies:
• A high-speed downstream channel, in the 50 kHz to 1 MHz band
• A medium-speed upstream channel, in the 4 kHz to 50 kHz band
• An ordinary two-way telephone channel, in the 0 to 4 kHz band