N-GEN Communications Web-based Seminars ©NGCS

Seminar - An Overview of Communications Technologies

Module 2 - Data Communications Basics and Legacy Technologies                                                Slide 14 of 45 _________________                                                                                  __________

                             Building Blocks - Analogue to Digital Conversion

                                            Voice Channel - Analogue

                                            Voice Channel - Digital

                                            Data Compression Technologies

_________________                                                                                  __________

In the 70s and 80s, analogue transmission was used for voice and data communication. 4 K Hz channel on the copper media was adequate to transmit good quality voice. At the same time 4 K Hz voice grade channel was good to transmit up to 9600 K bps data. As multiplexing technology improved push was to multiplex many data and voice streams over one channel. A voice grade channel is equivalent to 64 K bps. The compression technologies helped to reduce good quality voice conversation from 64 K bps to 8 k bps, thus transmitting 8 voice conversations simultaneously over one voice-channel.

A digital stream is converted into an analogue for transmission over an analogue channel.

Analogue to digital conversion:
        Analogue signal    - 4000 Hz
        Signal sampling rate    - 8000Hz (Nyquist Theorem)
        Quantized sample to one of the 256 values (levels)
        Encode using binary code
        Digital conversion of a 4 K Hz voice channel - 8000x8=64 K bps

Data compression technologies suppress silences and use a complex algorithm to reduce voice transmission rate.