Before delving too deep into details of DSL technologies a review of transmission technologies deployed over voice networks is in order. The telephone voice data network, commonly referred to as Public Switched Telephone Network (PSTN) operated in the frequency ranges of 300 Hz to 3.4 kHz. Voice signals are primarily analog in nature; however advances in modern technologies have led to digitization of these analog signals to digital signals. Digitization of signals usually takes place in the main backbone networks connecting Company Offices (COs).
The voice signal is carried in analog form from the user’s location to Company Offices (COs) using a copper twisted pair also referred to as a local loop. The analog signal is converted to digital form at the Company Offices (COs) by a codec (coder/decoder). This is followed by transmission of signals in digital form over backbone networks to the Company Office (CO) at the other end. Once transmitted the signal is converted back to analog form by a codec (coder/decoder).
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