Questions and Answers of Computer Communications

Show that the value of p in Figure 6.3a is the same whether it is calculated as the parity of all the data bits, the parity of all the row parity bits, or the parity of all the column parity bits.
Consider a frame consisting of two characters of four bits each. Assume that the probability of bit error is 10-3 and that it is independent for each bit.a. What is the probability that the received
Two communicating devices are using a single-bit even parity check for error detection.The transmitter sends the byte 10101010 and, because of channel noise, the receiver gets the byte 10011010. Will
Would you expect that the inclusion of a parity bit with each character would change the probability of receiving a correct message?
For the example of Figure 6.1, what is the minimum and maximum value of x that satisfies the formal definition of error burst in Section 6.1?
In an (n, k) block ECC, what do n and k represent?
Is it possible to design an ECC that will correct some double-bit errors but not all double-bit errors? Why or why not?
List three different ways in which the CRC algorithm can be described.
Why would you expect a CRC to detect more errors than a parity bit?
What is the CRC?
Are the modem and the codec functional inverses (i.e., could an inverted modem function as a codec, or vice versa)?
Why should PCM be preferable to DM for encoding analog signals that represent digital data?
Show that this arrangement does recover the two signals d1(t) and d2(t), which can be combined to recover the original input.
Figure 5.25 shows the QAM demodulator corresponding to the QAM modulator of Figure
Assume that a telephone line channel is equalized to allow bandpass data transmission over a frequency range of 600 to 3000 Hz. The available bandwidth is 2400 Hz.For r = 1, evaluate the required
An NRZ-L signal is passed through a filter with r = 0.5 and then modulated onto a carrier. The data rate is 2400 bps. Evaluate the bandwidth for ASK and FSK. For FSK assume that the two frequencies
What SNR is required to achieve a bandwidth efficiency of 1.0 for ASK, FSK, PSK, and QPSK? Assume that the required bit error rate is 10-6.
Derive an expression for baud rate D as a function of bit rate R for QPSK using the digital encoding techniques of Table 5.2.
A sine wave is to be used for two different signaling schemes: (a) PSK; (b) QPSK.The duration of a signal element is 10-5 s. If the received signal is of the following form:s1t2 = 0.005 sin12p106t +
One positive side-effect of bipolar encoding is that a bipolar violation (two consecutive+ pulses or two consecutive - pulses separated by any number of zeros)indicates to the receiver that an error
The bipolar-AMI waveform representing the binary sequence 0100101011 is transmitted over a noisy channel. The received waveform is shown in Figure 5.24; it contains a single error. Locate the
Draw the waveform for this sequence usinga. NRZ-Lb. Bipolar-AMIc. Pseudoternary
Consider a stream of binary data consisting of a long sequence of 1s followed by a zero followed by a long string of 1s, with the same assumptions as Problem
The waveform of Figure 5.23 belongs to a Manchester-encoded binary data stream.Determine the beginning and end of bit periods (i.e., extract clock information) and give the data sequence.
For the bit stream 01001110, sketch the waveforms for each of the codes of Table 5.2.Assume that: the signal level for the preceding bit for NRZI was high; the most recent preceding 1 bit (AMI) has a
Consider the following signal encoding technique. Binary data are presented as input, am, for m = 1, 2, 3, .… Two levels of processing occur. First, a new set of binary numbers are produced:b0 = 0
Develop a state diagram (finite state machine) representation of pseudoternary coding.
A modified NRZ code known as enhanced-NRZ (E-NRZ) is sometimes used for high-density magnetic tape recording. E-NRZ encoding entails separating the NRZ-L data stream into 7-bit words; inverting bits
Develop algorithms for generating each of the codes of Table 5.2 from NRZ-L.
Which of the signals of Table 5.2 use differential encoding?
What does the sampling theorem tell us concerning the rate of sampling required for an analog signal?
What is QAM?
What is the difference between QPSK and offset QPSK?
How are binary values represented in amplitude shift keying, and what is the limitation of this approach?
What function does a modem perform?
Explain the function of scrambling in the context of digital-to-digital encoding techniques.
Define biphase encoding and describe two biphase encoding techniques.
Describe two multilevel binary digital-to-digital encoding techniques.
Explain the difference between NRZ-L and NRZI.
What is differential encoding?
List and briefly define important factors that can be used in evaluating or comparing the various digital-to-digital encoding techniques.
Suppose a ray of visible light passes from the atmosphere into water at an angle to the horizontal of 30º. What is the angle of the ray in the water? Note: At standard atmospheric conditions at the
Determine the height of an antenna for a TV station that must be able to reach customers up to 80 km away.
Section 4.3 states that with no intervening obstacles, the optical line of sight can be expressed as d = 3.571h, where d is the distance between an antenna and the horizon in kilometers and h is the
A microwave transmitter has an output of 0.1 W at 2 GHz. Assume that this transmitter is used in a microwave communication system where the transmitting and receiving antennas are parabolas, each 1.2
Suppose a transmitter produces 50 W of power.a. Express the transmit power in units of dBm and dBW.b. If the transmitter’s power is applied to a unity gain antenna with a 900 MHz carrier frequency,
It is often more convenient to express distance in km rather than m and frequency in MHz rather than Hz. Rewrite Equation (4.4) using these dimensions.
Let P(x1, y1) be a point on the parabola, and PF be the line from P to the focus. Construct the line L through P parallel to the x-axis and the line M tangent to the parabola at P. The angle between
Section 4.2 states that if a source of electromagnetic energy is placed at the focus of the paraboloid, and if the paraboloid is a reflecting surface, then the wave will bounce back in lines parallel
For radio transmission in free space, signal power is reduced in proportion to the square of the distance from the source, whereas in wire transmission, the attenuation is a fixed number of dB per
In satellite communications, different frequency bands are used for the uplink and the downlink. Discuss why this pattern occurs.
You are communicating between two satellites. The transmission obeys the free space law. The signal is too weak. Your vendor offers you two options. The vendor can use a higher frequency that is
Stories abound of people who receive radio signals in fillings in their teeth. Suppose you have one filling that is 2.5 mm (0.0025 m) long that acts as a radio antenna. That is, it is equal in length
The audio power of the human voice is concentrated at about 300 Hz. Antennas of the appropriate size for this frequency are impracticably large, so that to send voice by radio the voice signal must
It turns out that the depth in the ocean to which airborne electromagnetic signals can be detected grows with the wavelength. Therefore, the military got the idea of using very long wavelengths
Show that doubling the transmission frequency or doubling the distance between transmitting antenna and receiving antenna attenuates the power received by 6 dB.
Coaxial cable is a two-wire transmission system. What is the advantage of connecting the outer conductor to ground?
Given a 100-watt power source, what is the maximum allowable length for the following transmission media if a signal of 1 watt is to be received?a. 24-gauge (0.5 mm) twisted pair operating at 300
A telephone line is known to have a loss of 20 dB. The input signal power is measured as 0.5 W, and the output noise level is measured as 4.5 mW. Using this information, calculate the output
Suppose that data are stored on 8.54-Gbyte single-sided, double-layer DVDs that weigh 15 g each. Suppose that an Eurostar rail service train, London to Paris via Chunnel, carries 104 kg of these
What is the difference between diffraction and scattering?
What is refraction?
What is the primary cause of signal loss in satellite communications?
What factors determine antenna gain?
What is the advantage of a parabolic reflective antenna?
What is an isotropic antenna?
What two functions are performed by an antenna?
Indicate some significant differences between broadcast radio and microwave.
Why must a satellite have distinct uplink and downlink frequencies?
What is direct broadcast satellite (DBS)?
What are some major advantages and disadvantages of microwave transmission?
What is the difference between unshielded twisted pair and shielded twisted pair?
What are some major limitations of twisted-pair wire?
Why are the wires twisted in twisted-pair copper wire?
An amplifier has an output of 20 W. What is its output in dBW?
If an amplifier has a 30-dB voltage gain, what voltage ratio does the gain represent?
Fill in the missing elements in the following table of approximate power ratios for various dB levels. Decibels 1 2 3 4 5 Losses Gains 0.5 2 9 7 8 9 10 0.1 10
In a 1939 letter to Vannevar Bush, Claude Shannon said he was working on a theorem, which states that for any transmitter and receiver the length of an arbitrary message multiplied by its essential
If the received signal level for a particular digital system is -151 dBW and the receiver system effective noise temperature is 1500 K, what is Eb/N0 for a link transmitting 2400 bps?
The square wave of Figure 3.7c, with T = 1 ms, is passed through a lowpass filter that passes frequencies up to 8 kHz with no attenuation.a. Find the power in the output waveform.b. Assuming that at
Given a channel with an intended capacity of 20 Mbps, the bandwidth of the channel is 3 MHz. Assuming white thermal noise, what signal-to-noise ratio is required to achieve this capacity?
Consider a channel with a 1 MHz capacity and an SNR of 63.a. What is the upper limit to the data rate that the channel can carry?b. The result of part (a) is the upper limit. However, as a practical
Study the works of Shannon and Nyquist on channel capacity. Each places an upper limit on the bit rate of a channel based on two different approaches. How are the two related?
Given the narrow (usable) audio bandwidth of a telephone transmission facility, a nominal SNR of 56 dB (400,000), and a certain level of distortiona. What is the theoretical maximum channel capacity
What is the thermal noise level of a channel with a bandwidth of 10 kHz carrying 1000 watts of power operating at 50°C?
A digital signaling system is required to operate at 9600 bps.a. If a signal element encodes a 4-bit word, what is the minimum required bandwidth of the channel?b. Repeat part (a) for the case of
What is the channel capacity for a teleprinter channel with a 300-Hz bandwidth and a signal-to-noise ratio of 3 dB, where the noise is white thermal noise?
Given an amplifier with an effective noise temperature of 10,000 K and a 10-MHz bandwidth, what thermal noise level, in dBW, may we expect at its output?
a. Suppose that a digitized TV picture is to be transmitted from a source that uses a matrix of 480 * 500 picture elements (pixels), where each pixel can take on one of 32 intensity values. Assume
IRA is a 7-bit code that allows 128 characters to be defined. In the 1970s, many newspapers received stories from the wire services in a 6-bit code called TTS. This code carried upper- and lower case
Figure 3.5b shows the frequency domain function for a single square pulse. The single pulse could represent a digital 1 in a communication system. Note that an infinite number of higher frequencies
Figure 3.4 shows the effect of eliminating higher-harmonic components of a square wave and retaining only a few lower harmonic components. What would the signal look like in the opposite case—that
Consider two periodic functions f1(t) and f2(t), with periods T1 and T2, respectively. Is it always the case that the function f(t) = f1(t) + f2(t) is periodic? If so, demonstrate this fact. If not,
Find the period of the function f(t) = (10 cos t)2.
Decompose the signal (1 + 0.1 cos 5t) cos 100t into a linear combination of sinusoidal functions, and find the amplitude, frequency, and phase of each component. Hint:Use the identity for cos a cos b.
If the solid curve in Figure 3.17 represents sin(2πt), what does the dotted curve represent?That is, the dotted curve can be written in the form A sin(2pft + f); what are A,f, and f?
Sound may be modeled as sinusoidal functions. Compare the relative frequency and wavelength of musical notes. Use 330 m/s as the speed of sound and the following frequencies for the musical scale.
Express the following in their simplest form:a. sin12pft - p2 + sin12pft + p2b. sin12pft2 + sin12pft - p2
A signal has a fundamental frequency of 1000 Hz. What is its period?
a. For multipoint configuration, only one device at a time can transmit. Why?b. There are two methods of enforcing the rule that only one device can transmit. In the centralized method, one station

Showing 400 - 500 of 536