Digital Telecommunications and Networks Essay Example | Topics and Well Written Essays - 2500 words

Digital Telecommunications and Networks - Essay Example In the signal amplification of analog systems, noise is amplified with the analog signal. Thus, the signal-to-noise ratio deteriorates for each time an analog signal is amplified. Digital regenerators, on the other hand, reproduce an entirely new digital signal from a sample noisy digital signal. The signal-to-noise ratio of this new digital signal has the same signal-to-noise ratio as the original signal. Thus, digital signals can be transmitted over longer distances than analog signals. Pulse Code Modulation is simply digitally coding analog signals. It consists of sampling analog information signals and then converting them to a serial n-bit binary code for transmission over a physical medium. With PCM, each code has the same number of bits and requires the same length of time for transmission. The presence or absence of a pulse within a specific time slot indicates either a logic condition of 1 or 0 respectively. Wayne Tomasi, in his book Electronic Communications Systems, outlined how a simplex PCM system is integrated in telephony: An analog input signal passes a band pass filter which limits it to the standard voice-band frequency range of 300Hz to 3000Hz. The sample and hold circuit periodically converts samples of the analog input signal to a multilevel Pulse Amplitude Modulation (PAM) signal. The analog-to-digital converter (ADC) converts these PAM signals to parallel PCM codes. These parallel PCM codes are then converted to serial binary data in the parallel-to-signal converter. Finally, the serial binary data are then outputted onto the transmission medium as serial pulses. At the receiver's end of the transmission medium, the serial pulses pass through the serial-to-parallel converter. The digital-to-analog converter then converts parallel PCM codes to parallel PAM signals. Finally, the hold circuit which is a low pass filter, converts parallel PAM signals to the original analog sig nal (408). Time Division Multiplexing is the transmission of information from multiple sources to one or more destinations using the same facility but at different transmission times. The following are the fundamentals of how Time Division Multiplexing is integrated in telephony as summarized by Wayne Tomasi: In a 2-channel PCM-TDM system, each channel's input is sampled and then converted to an eight-bit PCM code. While the PCM code of channel 1 is transmitted over the transmission medium, channel 2 analog input signals is sampled and converted to a PCM code. And when the PCM code of channel 2 is transmitted, it is channel 1 that then undergoes analog input sampling and conversion (453). C.) Describe the main operating principles of CDMA (Code Division Multiple Access) technology and GSM (Global System Mobile) technology in mobile communication. Code-Division Multiple Accessing is a cellular telephone system based on spread-spectrum technology. It uses a unique code rather than a frequency or time assignment to differentiate users from one another. With CDMA, the base station uses