10 Replies Latest reply: Mar 25, 2014 3:34 PM by BRenES RSS

    Certification Guide book question; PCM vs PAM


      There is a question from the CCNA Voice Official Certification guide that I have problems answering and would appreciate advise. This is the question;


      Which of the following processes samples an analog waveform at timed intervals to obtain a digital mesurement of the electrical signal?


      A. Quantization

      B. PAM

      C. PCM

      D. Codec


      The book says the correct answer is B. I checked with PeasonView and they contacted the author and the author says B. I think it is C.

      The following is from Wikepedia for PCM;


      In telephony, a standard audio signal for a single phone call is encoded as 8,000 analog samples per second, of 8 bits each, giving a 64 kbit/s digital signal known as DS0. The default signal compression encoding on a DS0 is either μ-law (mu-law) PCM (North America and Japan) or A-law PCM (Europe and most of the rest of the world). These are logarithmic compression systems where a 12 or 13-bit linear PCM sample number is mapped into an 8-bit value. This system is described by international standard G.711.


      Could someone explain why the answer is pulse amplitude modulation instead of pulse code modulation? Aren't voice systems using PCM?