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Questions and Answers of Computer Networking

In a codeword, we add two redundant bits to each 8-bit data word. Find the number ofa. valid codewords.b. invalid codewords
In Table 10.1, the sender sends dataword 10. A 3-bit burst error corrupts the codeword. Can the receiver detect the error? Defend your answer.Table 10.1 Dataword Codeword Dataword Codeword 101 110
What is the minimum Hamming distance?
If we want to be able to detect two-bit errors, what should be the minimum Hamming distance?
Prove that the code represented by the following codewords is not linear. You need to find only one case that violates the linearity.{(00000), (01011), (10111), (11111)}
A category of error detecting (and correcting) code, called the Hamming code, is a code in which dmin = 3. This code can detect up to two errors (or correct one single error). In this code, the
What is the Hamming distance for each of the following codewords?a. d (10000, 00000)b. d (10101, 10000)c. d (00000, 11111)d. d (00000, 00000)
In CRC, if the dataword is 5 bits and the codeword is 8 bits, how many 0s need to be added to the dataword to make the dividend? What is the size of the remainder? What is the size of the divisor?
Although it can be formally proved that the code in Table 10.3 is both linear and cyclic, use only two tests to partially prove the fact:Table 10.3a. Test the cyclic property on codeword 0101100.b.
In CRC, which of the following generators (divisors) guarantees the detection of a single bit error?a. 101b. 100c. 1
Referring to the CRC-8 in Table 5.4, answer the following questions:a. Does it detect a single error? Defend your answer.b. Does it detect a burst error of size 6? Defend your answer.c. What is the
In CRC, which of the following generators (divisors) guarantees the detection of an odd number of errors?a. 10111b. 101101c. 111
Assuming even parity, find the parity bit for each of the following data units.a. 1001011b. 0001100c. 1000000d. 1110111
In CRC, we have chosen the generator 1100101. What is the probability of detecting a burst error of lengtha. 5?b. 7?c. 10?
A simple parity-check bit, which is normally added at the end of the word (changing a 7-bit ASCII character to a byte), cannot detect even numbers of errors. For example, two, four, six, or eight
Assume we are sending data items of 16-bit length. If two data items are swapped during transmission, can the traditional checksum detect this error? Explain.
Given the dataword 101001111 and the divisor 10111, show the generation of the CRC codeword at the sender site (using binary division).
Can the value of a traditional checksum be all 0s (in binary)? Defend your answer.
Apply the following operations on the corresponding polynomials:a. (x3 + x2 + x + 1) + (x4 + x2 + x + 1)b. (x3 + x2 + x + 1) − (x4 + x2 + x + 1)c. (x3 + x2) × (x4 + x2 + x + 1)d. (x3 + x2 + x + 1)
Show how the Fletcher algorithm (Figure 10.18) attaches weights to the data items when calculating the checksum.Figure 10.18 Notes Start L: Left 8-bit checksum R: Right 8-bit checksum R=L= 0 D;: Next
Answer the following questions:a. What is the polynomial representation of 101110?b. What is the result of shifting 101110 three bits to the left?c. Repeat part b using polynomials.d. What is the
Show how the Adler algorithm (Figure 10.19) attaches weights to the data items when calculating the checksum.Figure 10.19 Start Notes L: Left 16-bit checksum R: Right 16-bit checksum D;: Next 16-bit
Which of the following CRC generators guarantee the detection of a single bit error?a. x3 + x + 1b. x4 + x2c. 1d. x2 + 1
Referring to the CRC-8 polynomial in Table 10.7, answer the following questions:a. Does it detect a single error? Defend your answer.b. Does it detect a burst error of size 6? Defend your answer.c.
Referring to the CRC-32 polynomial in Table 10.4, answer the following questions:Table 10.4a. Does it detect a single error? Defend your answer.b. Does it detect a burst error of size 16? Defend your
Manually simulate the Adler algorithm (Figure 10.19) to calculate the checksum of the following words: (FBFF)16and (EFAA)16. Also show that the result is a weighted checksum.Figure 10.19 Start Notes
One of the examples of a weighted checksum is the ISBN-10 code we see printed on the back cover of some books. In ISBN-10, there are 9 decimal digits that define the country, the publisher, and the
An ISBN-13 code, a new version of ISBN-10, is another example of a weighted checksum with 13 digits, in which there are 12 decimal digits defining the book and the last digit is the checksum digit.
Assume we want to send a dataword of two bits using FEC based on the Hamming distance. Show how the following list of datawords/codewords can automatically correct up to a one-bit error in
Using the ideas in the previous two problems, we can create a general formula for correcting any number of errors (m) in a codeword of size (n). Develop such a formula. Use the combination of n
In Figure 10.22, assume we have 100 packets. We have created two sets of packets with high and low resolutions. Each high-resolution packet carries on average 700 bits. Each low-resolution packet
Alice and Bob are experimenting with CSMA using a W2Walsh table (see Figure 12.29). Alice uses the code [+1, +1] and Bob uses the code [+1, ˆ’1]. Assume that they simultaneously send a
What is the purpose of NAV in CSMA/CA?
There is no acknowledgment mechanism in CSMA/CD, but we need this mechanism in CSMA/CA. Explain the reason.
We have a pure ALOHA network with a data rate of 10 Mbps. What is the maximum number of 1000-bit frames that can be successfully sent by this network?
In a wireless LAN, station A is assigned IFS = 5 milliseconds and station B is assigned IFS = 7 milliseconds. Which station has a higher priority? Explain.
Although the throughput calculation of a CSMA/CD is really involved, we can calculate the maximum throughput of a slotted CSMA/CD with the specification we described in the previous problem. We found
Assume we have a slotted CSMA/CD network. Each station in this network uses a contention period, in which the station contends for access to the shared channel before being able to send a frame. We
Assume the propagation delay in a broadcast network is 12 μs and the frame transmission time is 8 μs.a. How long does it take for the first bit to reach the destination?b. How long does it take for
The random variable R (Figure 12.13) is designed to give stations different delays when a collision has occurred. To alleviate the collision, we expect that different stations generate different
Assume the propagation delay in a broadcast network is 5 μs and the frame transmission time is 10 μs.a. How long does it take for the first bit to reach the destination?b. How long does it take for
In a bus 1-persistence CSMA/CD with Tp = 50 μs and Tfr = 120 μs, there are two stations, A and B. Both stations start sending frames to each other at the same time. Since the frames collide, each
Explain why collision is an issue in random access protocols but not in channelization protocols.
To understand why we need to have a minimum frame size Tfr = 2 × Tp in a CDMA/CD network, assume we have a bus network with only two stations, A and B, in which Tfr = 40 μs and Tp = 25 μs. Station
Explain why collision is an issue in random access protocols but not in controlled access protocols.
There are only two stations, A and B, in a bus 1-persistence CSMA/CD network with Tp= 25.6 Î¼s and Tfr= 51.2 Î¼s. Station A has a frame to send to station B. The frame is
Assume the propagation delay in a broadcast network is 6 μs and the frame transmission time is 4 μs. Can the collision be detected no matter where it occurs?
Assume that there are only two stations, A and B, in a bus CSMA/CD network. The distance between the two stations is 2000 m and the propagation speed is 2 × 108 m/s. If station A starts transmitting
Assume the propagation delay in a broadcast network is 3 μs and the frame transmission time is 5 μs. Can the collision be detected no matter where it occurs?
In a bus CSMA/CD network with a data rate of 10 Mbps, a collision occurs 20 μs after the first bit of the frame leaves the sending station. What should the length of the frame be so that the sender
Assume the propagation delay in a broadcast network is 5 μs and the frame transmission time is 10 μs.a. How long does it take for the first bit to reach the destination?b. How long does it take for
We have defined the parameter a as the number of frames that can fit the medium between two stations, or a = (Tp)/(Tfr). Another way to define this parameter is a = Lb/Fb, in which Lb is the bit
Based on Figure 12.15, how do we interpret success in an Aloha network?Figure 12.15 Station has a frame to send K= 0 Legend K: Number of attempts Tg: Backoff time IFS: Interframe Space RTS: Request
Another useful parameter in a LAN is the bit length of the medium (Lb), which defines the number of bits that the medium can hold at any time. Find the bit length of a LAN if the data rate is 100
One of the useful parameters in a LAN is the number of bits that can fit in one meter of the medium (nb/m). Find the value of nb/m if the data rate is 100 Mbps and the medium propagation speed is 2
Based on Figure 12.3, how do we interpret success in an Aloha network?Figure 12.3 Station has Legend a frame to send K : Number of attempts Tp: Maximum propagation time Tr: Average transmission time
A slotted Aloha network is working with maximum throughput.a. What is the probability that a slot is empty?b. How many slots, n, on average, should pass before getting an empty slot?
To understand the uses of K in Figure 12.15, find the probability that a station can send immediately in each of the following cases:Figure 12.15a. After two failures.b. After five failures. Station
There are only three active stations in a slotted Aloha network: A, B, and C. Each station generates a frame in a time slot with the corresponding probabilities pA = 0.2, pB = 0.3, and pC = 0.4
To understand the uses of K in Figure 12.13, find the probability that a station can send immediately in each of the following cases:Figure 12.13a. After one failure.b. After four failures. Station
There are only three active stations in a slotted Aloha network: A, B, and C. Each station generates a frame in a time slot with the corresponding probabilities pA = 0.2, pB = 0.3, and pC = 0.4
To understand the uses of K in Figure 12.3, find the probability that a station can send immediately in each of the following cases:Figure 12.3a. After one failure.b. After three failures. Station
In the previous problem, we found the throughputs of a pure and a slotted Aloha network as S = Np (1−p)2(N−1) and S = Np (1−p)(N−1) respectively. In this problem we want to find the maximum
In a slotted Aloha network with G = 1/2, how is the throughput affected in each of the following cases?a. G is increased to 1.b. G is decreased to 1/4.
In the previous problem, we found the probability of success for a station to send a frame successfully during the vulnerable time. The throughput of a network with a limited number of stations is
In a pure Aloha network with G = 1/2, how is the throughput affected in each of the following cases?a. G is increased to 1.b. G is decreased to 1/4.
A multiple access network with a large number of stations can be analyzed using the Poisson distribution. When there is a limited number of stations in a network, we need to use another approach for
Stations in an slotted Aloha network send frames of size 1000 bits at the rate of 1 Mbps. What is the vulnerable time for this network?
In the previous problem, we showed that the throughput is S = Ge−2G for a pure Aloha network and S = Ge−G for a slotted Aloha network. In this problem, we want to find the value of G in each
Stations in a pure Aloha network send frames of size 1000 bits at the rate of 1 Mbps. What is the vulnerable time for this network?
In the previous problem, we found that the probability of a station (in a G-station network) successfully sending a frame in a vulnerable time is P = e−2G for a pure Aloha and P = e−G for a
Which of the following is a channelization protocol?a. ALOHAb. Token-passingc. CDMA
In the previous problem, we used the Poisson distribution to find the probability of generating x number of frames, in a certain period of time, in a pure or slotted Aloha network as p[x] = (e−λ
Which of the following is a controlled-access protocol?a. Token-passingb. Pollingc. FDMA
To formulate the performance of a multiple-access network, we need a mathematical model. When the number of stations in a network is very large, the Poisson distribution, p[x] = (e−λ × λx)/(x!),
Which of the following is a random-access protocol?a. CSMA/CDb. Pollingc. TDMA
In Figure 11.20, explain why we need only one address field. Explain why the address is set to the predefined value of (11111111)2.Figure 11.20 (11111111)2 (00000011)2 Flag Address Control Protocol 1
Compare the flag byte and the escape byte in PPP. Are they are the same? Explain.
Compare and contrast HDLC with PPP.
In PPP, we normally talk about user and system instead of sending and receiving nodes; explain the reason.
Compare Figure 11.6 and Figure 11.21. If both are FSMs, why are there no event/action pairs in the second?Figure 11.6Figure 11.21 Event 1 Note: Action 1. The colored Action 2. arrow shows the
In Figure 11.16, which frame type can be used for acknowledgment?Figure 11.16 User Flag Address Control FCS Flag I-frame information Flag Address Control FCS Flag S-frame Management information FCS
Assume the only computer in the residence uses PPP to communicate with the ISP. If the user sends 10 network-layer packets to ISP, how many frames are exchanged in each of the following cases:a.
Define piggybacking and its benefit.
Assume PPP is in the authentication phase, show payload exchanged between the nodes if PPP is usinga. PAPb. CHAP
Does the duplex communication in Figure 11.10 necessarily mean we need two separate media between the two nodes? Explain.Figure 11.10 Receiving node Sending node Frame ACK [CRC Network [CRC Network
Redraw Figure 11.21 with the system not using authentication.Figure 11.21 Carrier detection failed Start Dead Carrier detected Establish Carrier dropped Authentication needed Authentication failed
In Figure 11.12, explain why we need a timer at the sending site, but none at the receiving site.Figure 11.12 Receiving node Network Sending node Network Data-link Data-link Packet Frame Legend
Assume PPP is in the established phase; show payload encapsulated in the frame.
In Figure 11.9, we show the packet path as a horizontal line, but the frame path as a diagonal line. Can you explain the reason?Figure 11.9 Sending node Receiving node Network Network Data-link
Redraw Figure 11.10 using piggybacking.Figure 11.10 Receiving node Frame Sending node ACK Network Network LCRC [CRC Data-link Data-link Logical link (duplex) Timer
Explain why there is no need for CRC in the Simple Protocol.
In the traditional Ethernet protocol (Chapter 13), the frames are sent with the CRC. If the frame is corrupted, the receiving node just discards it. Is this an example of a Simple Protocol or the
In Example 11.4 (Figure 11.13) how many frames are in transit at the same time?Figure 11.13 Sending node Receiving node Network Network Data-link Data-link Legend Packet Frame 0 Start the timer.
In Figure 11.11, show what happens in each of the following cases:Figure 11.11a. The receiver is in the ready state and a packet comes from the network layer.b. The receiver is in the ready state and
In Example 11.3 (Figure 11.12) how many frames are in transit at the same time?Figure 11.12 Receiving node Network Sending node Network Data-link Data-link Packet Frame Legend Packet ACK Start the
In Figure 11.11, show what happens in each of the following cases:Figure 11.11a. The sender is at the ready state and an error-free ACK arrives.b. The sender is at the blocking state and a time-out
In a bit-oriented protocol, should we first unstuff the extra bits and then remove the flags or reverse the process?
In Example 11.4 (Figure 11.13), assume the round trip time for a frame is 40 milliseconds. Explain what will happen if we set the time-out in each of the following cases.Figure 11.13a. 35
In a byte-oriented protocol, should we first unstuff the extra bytes and then remove the flags or reverse the process?

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