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Data Communications

Encoding and Modulating

     

  1. Digital-to-Digital encoding or conversion is the representation of digital information by a digital signal.

       

    • Unipolar encoding uses one polarity, assigned to one of the two binary states, usually the 1. The other state, usually the 0, is represented by zero voltage. It has two problems: a DC component and synchronization. A synchronization problem can occur whenever the data stream includes a long uninterrupted series of 1s or 0s. Lack of synchronization between the sender's and the receiver's clocks distorts the timing of the signal.

       

    • Polar encoding uses two voltage levels: one positive and one negative.

       

      • In nonreturn to zero (NRZ) encoding, the level of the signal is always either positive or negative.

         

      • In NRZ-L encoding, the level of the signal depends on the type of bit it represents. A positive voltage usually means the bit is 0, and a negative voltage means the bit is a 1 (or vice versa).

         

      • In NRZ-I an inversion of the voltage level represents a 1 bit. It is the transition between a positive and a negative voltage, not the voltages themselves, that represents a 1 bit. A 0 bit is represented by no change. It is superior to NRZ-L due to the synchronization provided by the signal change each time a 1 bit is encountered.

         

      • In return to zero (RZ) the signal changes not between bits but during each bit. Unlike NRZ-L, halfway through each bit interval, the signal returns to zero. A 1 bit is represented by positive-to-zero and a 0 bit by negative-to-zero. The main disadvantage of RZ encoding is that it requires two signal changes to encode one bit and therefore occupies more bandwidth.

         

      • With biphase the signal changes at the middle of the bit interval but does not return to zero. These midinterval transitions allow for synchronization.

         

        • Manchester encoding uses the inversion at the middle of each bit interval for both synchronization and bit representation. A negative-to-positive transition represents binary 1 and a positive-to-negative transition represents binary 0.

           

        • In Differential Manchester the inversion at the middle of the bit interval is used for synchronization, but the presence or absence of an additional transition at the beginning of the interval is used to identify the bit. A transition means a binary 0 and no transition means binary 1.

       

    • Bipolar encoding, like RZ uses three voltage levels: positive, negative, and zero. Unlike RZ, the zero level in bipolar encoding is used to represent binary 0. The 1s are represented by alternating positive and negative voltages. If the first 1 bit is represented by the positive amplitude, the second will be represented by the negative amplitude, the third by the positive amplitude, and so on.This alternation occurs even when the 1 bits are not consecutive.

       

      • Alternative mark inversion (AMI) means alternate 1 inversion. A zero voltage represents binary 0. Binary 1s are represented by alternating positive and negative voltages.

         

      • Bipolar 8-Zero Substitution (B8ZS) is the convention adopted in North America to provide synchronization of long strings of 0s. In most situations, B8ZS functions identically to bipolar AMI. It forces artificial signal changes, called violations, within the 0 string.

         

      • High-Density Bipolar 3 (HDB3) is used in Europe and Japan, and introduces changes into the bipolar AMI pattern evey time four consecutive 0s are encountered.

         

     

  2. Analog-to-Digital conversion digitizes an analog signal, and uses a converter called a codec.

     

    • Pulse Amplitude Modulation (PAM) takes an analog signal, samples it, and generates a series of pulses based on the results of the sampling.

       

    • Pulse Code Modulation (PCM) modifies the pulses created by PAM to create a completely digital signal. It first quantizes the PAM pulses. Quantization is a method of assigning integral values in a specific range to sampled instances. Each value is translated into its seven-bit binary equivalent. The binary digits are then transformed into a digital signal using one of the digital-to-digital encoding techniques. It is the sampling method used to digitize voice in T-line transmission in the North American telecommunication system.

       

    • According to the Nyquist Theorem, to ensure the accurate reproduction of an original analog signal using PAM, the sampling rate must be at least twice the highest frequency of the original signal.

     

  3. Digital-to-Analog conversion, or modulation is the process of changing one of the characteristics of an analog signal based on the information in a digital signal.

     

    • The bit rate is the number of bits transmitted in one second. Baud rate refers to the number of signal units per second that are required to represent those bits. Bit rate equals the baud rate times the number of bits represented by each signal unit. Bit rate is always greater than or equal to the baud rate.

       

    • The carrier signal or frequency is a high-frequency signal that acts as the basis for the information signal. Digital information is modulated on the carrier signal by modifying one or more of its characteristics (amplitude, frequency, phase).

       

    • In amplitude shift keying (ASK) the strength of the carrier signal is varied to represent binary 1 or 0. It is highly susceptible to noise interference. The baud rate and the bit rate are the same.

       

    • In frequency shift keying (FSK) the frequency of the carrier signal is varied to represent binary 1 or 0. The bandwidth is equal to to the baud rate of the signal plus the frequency shift.

       

    • In phase shift keying (PSK) the phase of the carrier is varied to represent binary 1 or 0. When two different phases are used it is called 2-PSK, or binary PSK. 4-PSK uses four variations where each phase shift represents two bits. PSK bit rates using the same bandwidth as ASK and PSK can be two or more times greater.

       

    • Quadrature amplitude modulation (QAM) combines ASK and PSK. Then we have x variations in phase and y variations in amplitude, giving us x times y possible variations and the corresponding number of bits per variation. It also has lower suceptibility to noise.

     

  4. Analog-to-Analog conversion is the representation of analog information by an analog signal.

     

    • In amplitude modulation (AM) the carrier signal is modulated so that its amplitude varies with the changing amplitudes of the modulating signal. The bandwidth of an AM signal is equal to twice the bandwidth of the modulating signal and covers a range centered around the carrier frequency. The bandwidth of an audio signal is usually 5 KHz, therefore each AM station needs a minimum bandwidth of 10 KHz. The Federal Communications Commission (FCC) allows 10 KHz for each AM station. AM stations are allowed carrier frequencies anywhere between 530 and 1700 KHz (1.7 MHz). Each station's carrier frequency must be separated from those on either side of it by at least 10 KHz.

       

    • In frequency modulation (FM) the frequency of the carrier signal is modulated to follow the changing voltage level of the modulating signal. The bandwidth of an FM signal is equal to 10 times the bandwidth of the modulating signal and, like AM bandwidths, covers a range centered around the carrier frequency. The bandwidth of a stereo audio signal is usually 15 KHz. Therefore, an FM station needs at least a bandwidth of 150 KHz. The FCC requires the minimum bandwidth to be at least 200 KHz.