Data Communications Networking Devices - 4th Ed.pdf

652 _________________________ WIDE AREA NETWORK DATA CONCENTRATION EQUIPMENTThe T-carrierT1 transmission popularity is a relatively recent phenomenon, with commercialservices available only since the mid-1980s, although the technology for this transmissionmethod dates backto the early 1960s. At that time, the T-carrier was usedexclusively by telephone companies and was based upon the time division multiplexingof digitized voice.Since the development of the T-carrier was based upon a voice digitizationmethod referred to as Pulse Code Modulation PCM), we will ®rst examine thattechnique prior to turning our attention to the composition of the T-carrier and itsEuropean equivalent known as the E-carrier.PCMPulse Code Modulation PCM) represents one of the earliest techniques developedto digitize voice, and serves as a reference for comparing the quality of othermethods against its `toll quality' voice reconstruction. Under PCM an analog signalsuch as the human voice is digitized into a 64 kbps data stream based upon a threestepprocess. Those steps include sampling, quantization, and coding.SamplingThe ®rst step in the PCM process involves sampling an analog wave to extractinformation from that continuously varying signal. The sampling rate used byPCM is based upon the Nyquist theorem that requires a rate twice the bandwidth tofaithfully reproduce a signal. Since the bandwidth of a telephone channel ranges inthe frequency spectrum from 0 to 4 kHz, this resulted in the selection of a samplingrate of 8000 samples.Figure 6.20 illustrates an example of the sampling of an analog wave. Theresulting samples form what is referred to as a Pulse Amplitude Modulation PAM)signal. That signal consists of a series of samples of the analog wave. Note that theheight of each sample can have an in®nite number of values.Figure 6.20Pulse amplitude modulation