Questions and Answers of Computer Communications

Distinguish among TCP, UDP, and DCCP.
How can TCP be used to deal with network or internet congestion?
What are the differences between token bucket and leaky bucket?
Briefly explain the three general approaches to explicit congestion signaling.
What is the difference between backward and forward explicit congestion signaling?
Give a brief explanation of each of the congestion control techniques illustrated in Figure 20.5.
Why is it that when the load exceeds the network capacity, delay tends to infinity?
When a node experiences saturation with respect to incoming packets, what general strategies may be used?
BGP provides a list of autonomous systems on the path to the destination. However, this information cannot be considered a distance metric. Why?
BGP’s AS_PATH attribute identifies the autonomous systems through which routing information has passed. How can the AS_PATH attribute be used to detect routing information loops?
Consider a system using flooding with a hop counter. Suppose that the hop counter is originally set to the “diameter” of the network. When the hop count reaches zero, the packet is discarded
Build a centralized routing directory for the networks of Figure 19.16.
Another adaptive routing scheme is known as backward learning. As a packet is routed through the network, it carries not only the destination address but also the source address plus a running hop
With random routing, only one copy of the packet is in existence at a time.Nevertheless, it would be wise to utilize a hop count field. Why?
It was shown that flooding can be used to determine the minimum-hop route. Can it be used to determine the minimum delay route?
In Figure 19.3, node 1 sends a packet to node 6 using flooding. Counting the transmission of one packet across one link as a load of one, what is the total load generated if:a. Each node discards
Both Dijkstra’s algorithm and the Bellman–Ford algorithm find the least-cost paths from one node to all other nodes. The Floyd–Warshall algorithm finds the least-cost paths between all pairs of
Will Dijkstra’s algorithm and the Bellman–Ford algorithm always yield the same solutions? Why or why not?
Repeat Problem 19.9 using the Bellman–Ford algorithm.
Apply Dijkstra’s routing algorithm to the networks in Figure 19.16. Provide a table similar to Table 19.4a and a figure similar to Figure 19.14.
Repeat Problem 19.7 using the Bellman–Ford algorithm.
Using Dijkstra’s algorithm, generate a least-cost route to all other nodes for nodes 2 through 6 of Figure 19.1. Display the results as in Table 19.4a.
In step 3 of Dijkstra’s algorithm, the least-cost path values are only updated for nodes not yet in T. Is it possible that a lower-cost path could be found to a node already in T? If so,
In the discussion of the Bellman–Ford algorithm, it is asserted that at the iteration for which h = K, if any path of length K+1 is defined, the first K hops of that path form a path defined in the
In the discussion of Dijkstra’s algorithm in Section 19.4, it is asserted that at each iteration, a new node is added to T and that the least-cost path for that new node passes only through nodes
Dijkstra’s algorithm, for finding the least-cost path from a specified node s to a specified node t, can be expressed in the following program:for n := 1 to N do begin L[n] := ∞; final[n] :=
Consider a binary tree topology for a packet-switching network. The root node connects to two other nodes. All intermediate nodes connect to one node in the direction toward the root, and two in the
Compare the three main approaches to routing.
What is the difference between an interior router protocol and an exterior router protocol?
What is the essential difference between Dijkstra’s algorithm and the Bellman–Ford algorithm?
What is a least-cost algorithm?
What are the advantages and disadvantages of adaptive routing?
What is flooding?
What is fixed routing?
What are the key requirements for a routing function for a packet-switching network?
The token bucket scheme places a limit on the length of time at which traffic can depart at the maximum data rate. Let the token bucket be defined by a bucket size B octets and a token arrival rate
The CVSD scheme is described in Section 18.5. Assume that the current value of x(k)is 1000, the current value of xn1k - 12 is 990, and the current value of δ(k - 1) is 30.a. Assume that the last 4
that illustrate the use of multislot packets. For the first diagram, assume that the station begins with a threeslot packet followed by two 1-slot packets. For the second diagram, assume that the
Draw two time sequence diagrams similar to Figure
In Figure 18.6, the DL subframe contains both DL-MAP and UL-MAP. Why not make UL-MAP a preamble in the UL subframe?
What is a flow specification?
List and briefly define Bluetooth baseband logical channels.
How is it possible to combine frequency hopping and time division duplex?
What is the relationship between master and slave in a piconet?
List and briefly describe the three IEEE 802.16 physical layer options.
List and briefly define IEEE 802.16 service classes.
Define fixed broadband wireless access.
Consider a CDMA system in which users A and B have the Walsh codes (−1 1 −1 1 −1 1 −1 1) and (−1 −1 1 1 −1 −1 1 1), respectively.a. Show the output at the receiver if A transmits a
Demonstrate that the codes in an 8 * 8 Walsh matrix are orthogonal to each other by showing that multiplying any code by any other code produces a result of zero.
Walsh codes are the most common orthogonal codes used in CDMA applications. A set of Walsh codes of length n consists of the n rows of an n * n Walsh matrix. That is, there are n codes, each of
In any use of pseudorandom numbers, whether for encryption, simulation, or statistical design, it is dangerous to trust blindly the random number generator that happens to be available in your
We would like m to be very large so that there is the potential for producing a long series of distinct random numbers. A common criterion is that m be nearly equal to the maximum representable
By far, the most widely used technique for pseudorandom number generation is the linear congruential method. The algorithm is parameterized with four numbers, as follows:m the modulus m 7 0 a the
Figure 17.15 depicts a simplified scheme for CDMA encoding and decoding. There are seven logical channels, all using DSSS with a spreading code of 7 bits. Assume that all sources are synchronized. If
Assume we wish to transmit a 56-kbps data stream using spread spectrum.a. Find the channel bandwidth required to achieve a 56-kbps channel capacity when SNR = 0.1, 0.01, and 0.001.b. In an ordinary
What is CDMA?
What is the relationship between the bit rate of a signal before and after it has been encoded using DSSS?
What is direct sequence spread spectrum?
List three benefits of spread spectrum.
What is the relationship between the bandwidth of a signal before and after it has been encoded using spread spectrum?
Briefly define OFDM, OFDMA, and SC-FDMA
Briefly define MIMO and MU-MIMO.
Using the same assumptions that are used for Figure 16.11, plot line utilization as a function of P, the probability that a single frame is in error for the following errorcontrol techniques:a.
Consider the use of 1000-bit frames on a 1-Mbps satellite channel with a 270-ms delay. What is the maximum link utilization fora. Stop-and-wait flow control?b. Continuous flow control with a window
A channel has a data rate of 4 kbps and a propagation delay of 20 ms. For what range of frame sizes does stop-and-wait give an efficiency of at least 50%?
A LDPC code is defined by the parity-check matrix h = D 1 1 0 1 0 0 0 1 1 0 1 0 1 0 0 0 1 1 0 0 1 1 0 1 TThe valid codeword 001011 is transmitted and the invalid codeword 101011 is received. Using
Draw the Tanner graphs for each of the LDPC codes in Figure 16.6.
A (6, 3) parity-check code is defined by the parity-check matrix h = C 1 0 1 1 0 0 1 1 0 0 1 0 0 1 1 0 0 1 Sa. Determine the generator matrix G.b. Find the codeword that begins 101.
A (6, 3) parity-check code is defined by the following equations:c1 c3 c4 = 0 c1 c2 c3 c5 = 0 c1 c2 c6 = 0a. Determine the parity-check matrix H.b. Determine the generator matrix G.c. List all
A simple FEC code transmits every data bit 5 times. The receiver decides on the value of each data bit by choosing majority vote. If the uncoded bit error probability is 10-3, what is the coded bit
For the example related to Table 16.1:a. Draw the LFSR.b. Using a layout similar to Figure 6.6b, show that the check bits for the data block 1010 are 001.
Divide f(X) = X6 + 1 by g(X) = X4 + X3 + X + 1. Verify the result by multiplying the quotient by g(X) to recover f(X).
Let m1(t) and m2(t) be message signals and let s1(t) and s2(t) be the corresponding modulated signals using a carrier frequency of fc.a. Show that if simple AM modulation is used, then m1(t) + m2(t)
Consider the angle-modulated signal s(t) = 10 cos [2p(106)t + 0.1 sin (103)pt]a. Express s(t) as a PM signal with np = 10.b. Express s(t) as an FM signal with nf = 10 π.
Consider the angle-modulated signal s(t) = 10 cos [(108)pt + 5 sin 2p(103)t]Find the maximum phase deviation and the maximum frequency deviation.
Explain the significance of the parameter a in ARQ performance.
What is the difference between a parity-check matrix code and an LDPC code?
What function is performed by the matrices H and G in a parity-check matrix code?
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?
What are the differences among angle modulation, PM, and FM?
TCP mandates that both the receiver and the sender should incorporate mechanisms to cope with SWS.a. Suggest a strategy for the receiver. Hint: Let the receiver “lie” about how much buffer space
A poor implementation of TCP’s sliding-window scheme can lead to extremely poor performance. There is a phenomenon known as the Silly Window Syndrome (SWS), which can easily cause degradation in
A host is receiving data from a remote peer by means of TCP segments with a payload of 1460 bytes. If TCP acknowledges every other segment, what is the minimum uplink bandwidth needed to achieve a
Suppose two hosts are connected with each other by means of a 100-Mbps link, and assume the round-trip time between them is 1 ms. What is the minimum TCP window size that would let TCP achieve the
Suppose the round-trip time (RTT) between two hosts is 100 ms, and that both hosts use a TCP window of 32 Kbytes. What is the maximum throughput that can be achieved by means of TCP in this scenario?
Ordinarily, the Window field in the TCP header gives a credit allocation in octets.When the Window Scale option is in use, the value in the Window field is multiplied by a 2F, where F is the value of
In discussing connection termination with reference to Figure 15.8, it was stated that in addition to receiving an acknowledgement of its FIN and sending an acknowledgement of the incoming FIN, a TCP
What happens in Figure 15.3 if a SYN comes in while the requested user is in CLOSED? Is there any way to get the attention of the user when it is not listening?
In a credit flow control scheme such as that of TCP, what provision could be made for credit allocations that are lost or misordered in transit?
Listed are four strategies that can be used to provide a transport user with the address of the destination transport user. For each one, describe an analogy with the Postal Service user.a. Know the
Is a deadlock possible using only a two-way handshake instead of a three-way handshake?Give an example or prove otherwise.
In a network that has a maximum packet size of 128 bytes, a maximum packet lifetime of 30 s, and an 8-bit packet sequence number, what is the maximum data rate per connection?
Consider a transport protocol that uses a connection-oriented network service.Suppose that the transport protocol uses a credit allocation flow control scheme, and the network protocol uses a
The discussion of retransmission strategy made reference to three problems associated with dynamic timer calculation. What modifications to the strategy would help to alleviate those problems?
Consider a connection-oriented network service that suffers a reset. How could this be dealt with by a transport protocol that assumes that the network service is reliable except for resets?
With a reliable sequencing network service, are segment sequence numbers strictly necessary? What, if any, capability is lost without them?
Draw diagrams similar to Figure 15.4 for the following (assume a reliable sequenced network service):a. Connection termination: active/passiveb. Connection termination: active/activec. Connection
Someone posting to comp.protocols.tcp-ip complained about a throughput of 122 kbps on a 256-kbps link with a 128-ms round-trip delay between the United States and Japan, and a throughput of 33 kbps
Two transport entities communicate across a reliable network. Let the normalized time to transmit a segment equal 1. Assume that the end-to-end propagation delay is 3, and that it takes a time 2 to

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