Questions and Answers of Computer Communications

No mention was made of reject (REJ) frames in the stop-and-wait ARQ discussion.Why is it not necessary to have REJ0 and REJ1 for stop-and-wait ARQ?
A channel has a data rate of R bps and a propagation delay of t s/km. The distance between the sending and receiving nodes is L kilometers. Nodes exchange fixed-size frames of B bits. Find a formula
frames are generated at node A and sent to node C through node B. Determine the minimum data rate required between nodes B and C so that the buffers of node B are not flooded, based on the
In Figure
The number of bits on a transmission line that are in the process of actively being transmitted (i.e., the number of bits that have been transmitted but have not yet been received) is referred to as
Consider a half-duplex point-to-point link using a stop-and-wait scheme, in which a series of messages is sent, with each message segmented into a number of frames.Ignore errors and frame overhead.a.
What are the three frame types supported by HDLC? Describe each.
Define data transparency.
What is the purpose of the flag field?
What are the transfer modes supported by HDLC? Describe each.
What are the station types supported by HDLC? Describe each.
List and briefly define three versions of ARQ.
Describe automatic repeat request (ARQ).
List common ingredients for error control for a link control protocol.
Define error control.
What is piggybacking?
What is the advantage of sliding-window flow control compared to stop-and-wait flow control?
Describe sliding-window flow control.
What are reasons for breaking up a long data transmission into a number of frames?
Describe stop-and-wait flow control.
Define flow control.
List and briefly define some of the requirements for effective communications over a data link.
A common technique for implementing CRC is to use a table lookup algorithm. The document site at box.com/dcc10e contains several papers describing this approach.Write a short paper that summarizes
states that for a given positive integer t, if a code satisfies dmin Ú 2t + 1, then the code can correct all bit errors up to and including errors of t bits. Prove this assertion. Hint: Start by
Section
and show that p1v|w12 7 p1v|w22 if and only if d1v,w12 6 d1v,w22. This proves that the codeword w that gives the largest value of p1v|w2 is that word whose distance from v is a minimum.
discusses block error-correcting codes that make a decision on the basis of minimum distance. That is, given a code consisting of s equally likely codewords of length n, for each received sequence v,
Section
Calculate the Hamming pairwise distances among the following codewords:a. 00000, 10101, 01010b. 000000, 010101, 101010, 110110
A modified CRC procedure is commonly used in communications standards. It is defined as follows:and k is the number of bits being checked (address, control, and information fields).a. Describe in
a. In a CRC error-detecting scheme, choose P1X2 = X4 + X + 1. Encode the bits 10010011011.b. Suppose the channel introduces an error pattern 100010000000000 (i.e., a flip from 1 to 0 or from 0 to 1
A CRC is constructed to generate a 4-bit FCS for an 11-bit message. The generator polynomial is X4 + X3 + 1.a. Draw the shift-register circuit that would perform this task (see Figure 6.4).b. Encode
For P = 110011 and M = 11100011, find the CRC.
Explain in words why the shift-register implementation of CRC will result in all 0s at the receiver if there are no errors. Demonstrate by example.
Using the CRC-CCITT polynomial, generate the 16-bit CRC code for a message consisting of a 1 followed by 15 0s.a. Use long division.b. Use the shift-register mechanism shown in Figure 6.4.
What is the purpose of using modulo 2 arithmetic rather than binary arithmetic in computing an FCS?
The high-speed transport protocol XTP (Xpress Transfer Protocol) uses a 32-bit checksum function defined as the concatenation of two 16-bit functions: XOR and RXOR are illustrated in Figure 6.10. The
One nice property of the Internet checksum is that it is endian-independent. Little Endian computers store hex numbers with the least significant byte last (Intel processors, for example). Big Endian
Compute the Internet checksum for the data block E3 4F 23 96 44 27 99 F3. Then perform the verification calculation.
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?

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