N-GEN Communications Web-based Seminars ©NGCS

Seminar - An Overview of Communications Technologies

Module 2 - Data Communications Basics and Legacy Technologies                                                Slide 12 of 45 _________________                                                                                  __________

                             Building Blocks - Data Modulation Techniques

                                            Amplitude

                                            Frequency

                                            Phase

                                            T-1

_________________                                                                                  __________

Modulation is a process of manipulating some characteristics of the carrier wave in accordance with the value or sample of information to be transmitted. Most common modulation techniques are Amplitude, Frequency and Phase. After modulation the transmitted signal pulses are represented by an ac signal having frequencies within the voice channel band suitable for transmission over the telephone network. The phase modulation  and combination of phase modulation and amplitude modulation is used for high speed data transmission.

Baud - a unit of signaling speed equivalent to the number of signal events per sec. A carrier carrying at 1200 Hz carrying one signaling element per Hz is equivalent to 12 bps while a carrier at 1200 Hz carrying 4 signaling elements per Hz is equivalent to 4800 bps. The number of signaling elements imposed on the carrier vary with the modulation technique used.

T-1 is a process to digitize 24 analogue voice channels into a 1.544 M bps data stream for transmission.
        Analogue Voice Signal Bandwidth  - 4000 Hz
        Signaling sampling rate                - 8000 Hz
        T-1 signal translation of 24 analogue voice channels  - 8000x(24x8+1) = 1.544 Mbps
Note: 8 is the number of bits to convert the sample voltage into a digital value
        1 is the framing bit