Week 2 Data Communications Fundamentals PDF

WEEK 2 FUNDAMENTALS OF DATA AND SIGNALS Let us begin our study of analog and digital signals by examining their three basic components: amplitude, frequency, and phase. A sine wave is used to represent an analog signal, as shown in the Figure. The amplitude of a signal is the height o...

WEEK 2 FUNDAMENTALS OF DATA AND SIGNALS Let us begin our study of analog and digital signals by examining their three basic components: amplitude, frequency, and phase. A sine wave is used to represent an analog signal, as shown in the Figure. The amplitude of a signal is the height of the wave above (or below) a given reference point. This height often denotes the voltage level of the signal (measured in volts), but it also can denote the current level of the signal (measured in amps) or the power level of the signal (measured in watts). That is, the amplitude of a signal can be expressed as volts, amps, or watts. Note that a signal can change amplitude as time progresses. FUNDAMENTALS OF DATA AND SIGNALS The frequency of a signal is the number of times a signal makes a complete cycle within a given time frame. The length, or time interval, of one cycle is called its period. The period can be calculated by taking the reciprocal of the frequency (1/frequency). The range of frequencies that a signal spans from minimum to maximum is called the spectrum. The spectrum of our telephone example is simply 300 Hz to 3400 Hz. The bandwidth of a signal is the absolute value of the difference between the lowest and highest frequencies. The bandwidth of a telephone system that transmits a single voice in the range of 300 Hz to 3400 Hz is 3100 Hz. FUNDAMENTALS OF DATA AND SIGNALS Because extraneous noise degrades original signals, an electronic device usually has an effective bandwidth that is less than its bandwidth. When making communication decisions, many professionals rely more on the effective bandwidth than the bandwidth because most situations must deal with the real world problems of noise and interference. FUNDAMENTALS OF DATA AND SIGNALS The phase of a signal is the position of the waveform relative to a given moment of time, or relative to time zero. FUNDAMENTALS OF DATA AND SIGNALS When traveling through any type of Amplification is the opposite of medium, a signal always experiences attenuation. When a signal is amplified by some loss of its power. This loss of power, an amplifier, the signal gains in decibels. or loss of signal strength, is called Because attenuation is a logarithmic loss attenuation. Attenuation in a medium and the decibel is a logarithmic value, such as copper wire is a logarithmic loss calculating the overall loss or gain of a (in which a value decrease of 1 represents system involves adding all the individual a tenfold decrease) and is a function of decibel losses and gains. distance and the resistance within the wire. Knowing the amount of attenuation in a signal (how much power the signal lost) allows you to determine the signal strength. Decibel (dB) is a relative measure of signal loss or gain and is used to measure the logarithmic loss or gain of a signal. FUNDAMENTALS OF DATA AND SIGNALS The decibel is a relative measure of signal loss or gain and is expressed as: dB = 10 Xlog (P2/P1) 10 in which P and P are the ending and beginning power levels, 2 1 respectively, of the signal expressed in watts. If a signal starts at a transmitter with 10 watts of power and arrives at a receiver with 5 watts of power, the signal loss in dB is calculated as follows: dB= 10 log (5/10) X 10 = 10 log (0.5) X 10 = 10 X (-0.3) =-3dB FUNDAMENTALS OF DATA AND SIGNALS Modulation is the process of sending data over a signal by varying its amplitude, frequency, or phase. FUNDAMENTALS OF DATA AND SIGNALS TRANSMITTING DATA WITH SQUARE WAVE SIGNALS We will examine six digital encoding schemes that are representative of most digital encoding schemes: NRZ-L, NRZI, Manchester, differential Manchester, bipolar-AMI, and 4B/5B. FUNDAMENTALS OF DATA AND SIGNALS Non-retum to Zero Digital Encoding Schemes The non-return to zero-level (NRZ-L) digital encoding scheme transmits 1s as zero voltages and 0s as positive voltages. The NRZ-L encoding scheme is simple to generate and inexpensive to implement in hardware. FUNDAMENTALS OF DATA AND SIGNALS The second digital encoding scheme is non-return to zero inverted (NRZI). This encoding scheme has a voltage change at the beginning of a 1 and no voltage change at the beginning of a 0. A fundamental difference exists between NRZ-L and NRZI. With NRZ-L, the receiver must check the voltage level for each bit to determine whether the bit is a 0 or a 1. With NRZI, the receiver must check whether there is a change at the beginning of the bit to determine if it is a 0 or a 1. FUNDAMENTALS OF DATA AND SIGNALS Manchester Digital Encoding Schemes The Manchester class of digital encoding schemes ensures that each bit has some type of signal change, and thus solves the synchronization problem. the Manchester encoding scheme has the following properties: To transmit a 1, the signal changes from low to high in the middle of the interval, and to transmit a 0, the signal changes from high to low in the middle of the interval. Note that the transition is always in the middle, a 1 is a low-to-high transition, and a 0 is a high- to-low transition. Manchester encoding is used in lower speed local area networks for transmitting digital data over a local area network cable. FUNDAMENTALS OF DATA AND SIGNALS The differential Manchester digital encoding Manchester schemes, there is always a scheme was used in a now extinct form of transition in the middle of a bit. Thus, the local area network (token ring) but still exists receiver can expect a signal change at regular in a number of unique applications. It is intervals and can synchronize itself with the similar to the Manchester scheme in that incoming bit stream. The Manchester there is always a transition in the middle of encoding schemes are called self-clocking the interval. But unlike the Manchester code, because the occurrence of a regular transition the direction of this transition in the middle is similar to seconds ticking on a dock. does not differentiate between a 0 or a 1. Instead, if there is a transition at the beginning of the interval, then a 0 is being transmitted. If there is no transition at the beginning of the interval, then a 1 is being transmitted. Because the receiver must watch the beginning of the interval to determine the value of the bit. The Manchester schemes have an advantage over the NRZ schemes: In the FUNDAMENTALS OF DATA AND SIGNALS Bipolar-AMI Encoding Scheme The bipolar-AMI encoding scheme is unique among all the encoding schemes seen thus far because it uses three voltage levels. When a device transmits a binary 0, a zero voltage is transmitted. When the device transmits a binary 1, either a positive voltage or a negative voltage is transmitted. Which of these is transmitted depends on the binary 1 value that was last transmitted. For example, if the last binary 1 transmitted a positive voltage, then the next binary 1 will transmit a negative voltage. Likewise, if the last binary 1 transmitted a negative voltage, then the next binary 1 will transmit a positive voltage FUNDAMENTALS OF DATA AND SIGNALS TRANSMITTING DIGITAL DATA WITH ANALOG SIGNAL 1. AMPLITUDE SHIFT KEYING A data value of 1 and a data value of 0 are represented by two different amplitudes of a signal. Amplitude shift keying has a weakness: It is susceptible to sudden noise impulses. FUNDAMENTALS OF DATA AND SIGNALS TRANSMITTING DIGITAL DATA WITH ANALOG SIGNAL 2. FREQUENCY SHIFT KEYING : It uses two different frequency ranges to represent data values of 0 and 1, as shown in the figure below. Frequency shift keying is not perfect. It is subject to intermodulation distortion, a phenomenon that occurs when the frequencies of two or more signals mix together and create new frequencies. FUNDAMENTALS OF DATA AND SIGNALS TRANSMITTING DIGITAL DATA WITH ANALOG SIGNAL 3. PHASE SHIFT KEYING : Phase shift keying represents 0s and 1s by different changes in the phase of a waveform. For example, a 0 could be no phase change, while a 1 could be a phase change of 180 degrees, as shown in figure below. Phase changes are not affected by amplitude changes, nor are they affected by intermodulation distortions. Thus, phase shift keying is less susceptible to noise and can be used at higher frequencies. FUNDAMENTALS OF DATA AND SIGNALS TRANSMITTING ANALOG DATA WITH DIGITAL SIGNAL Pulse Code Modulation One encoding technique that converts analog data to a digital signal is pulse code modulation (PCM). Hardware-specifically, a codec-- converts the analog data to a digital signal by tracking the analog waveform and taking "snapshots“ of the analog data at fixed intervals. Taking a snapshot involves calculating the height, or voltage, of the analog waveform above a given threshold. This height, which is an analog value, is converted to an equivalent fixed-sized binary value. FUNDAMENTALS OF DATA AND SIGNALS TRANSMITTING ANALOG DATA WITH DIGITAL SIGNAL Delta Modulation A second method of analog data-to-digital signal conversion is delta modulation. With delta modulation, a codec tracks the incoming analog data by assessing up or down "steps." During each time period, the codec determines whether the waveform has risen one delta step or dropped one delta step. If the waveform rises one delta step, a 1 is transmitted. If the waveform drops one delta step, a 0 is transmitted. Two problems are inherent with delta modulation. If the analog waveform rises or drops too quickly, the codec may not be able to keep up with the change, and slope overload noise results. What if a device is trying to digitize a voice or music that maintains a constant frequency and amplitude, like one person singing one note at a steady volume? Analog waveforms that do not change at all present the other problem for delta modulation.