TSIN02 Internetworking

Problems

General NetworkingN1: Show the communication at the application layer for the following simple private inter-net:

N2: Show the communication at the application layer for the following simple private inter-net:

N3: Describe three possible reasons to why a sender does not receive an ACK for a transmit-ted packet!

N4: Describe two different ways of performing channel partitioning.

N5: In the standard Ethernet (10 Mbps), if the maximum propagation time is 25.6 µs, what isthe minimum size of the frame?

N6: Describe the hidden terminal problem!

N7: Describe the exposed station problem!

N8: Ethernet uses CSMA-CD. User 1 starts transmitting at time 0, and user 2 starts to transmit1 µs later. The users are situated at a distance of 900 m from each other. Assume that thepropagation speed of the signal is the speed of light. The jamming signal lasts 1 µs.a) When does user 2 detect user 1’s signal?b) When is the first time that user 1 can start retransmitting?

N9: Why is CSMA-CD chosen for Ethernet, while CSMA-CA is chosen for Wi-Fi?

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N10: Host A sends an ICMP query message to host B, but never receives an answer. Give threepossible causes and the corresponding course of actions for host A.

N11: A home user has installed a switch to link her ADSL modem M, her two laptop comput-ers A and B, and her husband’s desktop H. M is on switch interface 1, A is on switch interface2, B is on switch interface 3, and H is on switch interface 4. How are frames forwarded by theswitch after that the following frames have been sent: A to H, B to M, M to B, and M to A? Theswitch’s forwarding table is initially empty. Assume that the computers know their destina-tion IP and MAC addresses.

N12: In the same network, assume again that the switch’s forwarding table is empty. Assumenow that the host A wants to send a message to host B, and that A knows B’s IP address, butnot B’s MAC address.a) List the steps until the first packet is sent if A would be using ARP. Also, discuss the switch’sbehavior!b) List the steps if A would not be using ARPc) Which strategy of that in (a) and that in (b) is best, and why?

N13: Which parameters does a newly added host in a network need to know in order to com-municate outside the local network?

N14: Why does a newly added host need to knowa) the IP address of a router?b) the IP address of a name server?c) its subnet?

N15: What is the range of addresses in the following blocks:a) 123.56.77.32/29 b) 200.17.21.128/27c) 17.34.16.0/23 d) 180.34.64.64/30

N16: An ISP is granted a block of addresses starting with 150.80.0.0/16.The ISP wants to distribute these blocks to customers as follows:a) The first group has 200 medium-size businesses, each needs 128 addresses.b) The second group has 400 small businesses, each needs 16 addresses.c) The third group has 2048 households, each needs 4 addresses.Design the subnets for the businesses and the households and give the slash (CIDR) notationfor each sub-block. Find out how many addresses are still available after these allocations.

N17: A large number of consecutive IP addresses are available starting at 198.16.0.0.Suppose that four organizations A, B, C, and D request 4000, 2000, 4000, 8000 addresses re-spectively, and in that order. For each of these, give the first IP address assigned, the lastaddress assigned, and the subnetwork address in the CIDR notation.

N18: A router has the following (CIDR) entries in its routing table:

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Address/mask Next hop135.46.56.0/22 Interface 0135.46.60.0/22 Interface 1192.53.40.0/23 Router 1default Router 2For each of the following IP addresses, what does the router do if a packet with that destina-tion address arrives?a) 135.46.63.10 b) 135.46.52.2 c) 135.46.57.14d) 192.53.40.7 e) 192.53.56.7

N19: For a given IP address 201.14.78.65 and the subnet mask 255.255.255.224, what is thesubnet address (in slash notation)?

N20: The network layer divides the datagrams into smaller datagrams if they are bigger thanthe MTU of a link layer over which they are to be sent.a) Write down an advantage of having a large MTU.b) Write down an advantage of having a small MTU.

N21: How does each router table look in a network with three routers A, B, and C, where allrouters connect to each other, but where the transport cost between router A and B; and Aand C are 1; while the cost between routers B and C is 3?

N22: Suppose C sends frame to I, and I responds to C. Show switch tables and packet for-warding in S1, S2, S3, S4.

N23: In the figure below, a router is connected to some (sub)networks. The figure also showsthe routing table for the router.A datagram arrives at one of the router interfaces. To which interface is the datagram for-warded when the destination address isa) 145.14.192.71? b) 135.11.80.21?

N24: A router has three interfaces, each of them connected to a subnet.a) What is the computational complexity for forwarding the packet on the right interface?What is the storage complexity associated with the sorting?b) What would the complexity be if the IP addresses were not organized in subnets? What is

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the storage complexity associated with the sorting?

N25: A DHCP packet in encapsulated in a UDP packet, which is encapsulated in an IP packet,which is encapsulated in an Ethernet frame. Find the efficiency of a DHCP packet when nooption is used. The efficiency, in this case, is measured in the number of bytes in the DHCPpacket to the total number of bytes transmitted on the physical layer.

N26: Which are the two address allocations provided by DHCP? Describe when they are used!

N27: A user Ada connects a new computer to an Ethernet LAN, opens a browser, and types ingoogle.com. Well at Google, she types in the word “guitar”. She gets a list of sites and accessesguitar.com. What happens on the different TCP/IP stack layers?

N28: Is registration required if the mobile host acts as a foreign agent? Explain your answer.

N29: A datagram arrives at router R1 in the figure below. The figure also shows the routing ta-ble for router R1. To which interface is the datagram forwarded when the destination addressisa) 201.4.16.70? b) 202.70.20.30?

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N30: Show the shortest form of the following IPv6 addresses:a) 2340:1ABC:119A:A000:0000:0000:0000:0000b) 0000:00AA:0000:0000:0000:0000:119A:A231c) 2340:0000:2222:0000:0000:119A:A001:0000d) 0000:0000:0000:2340:0000:0000:0000:0000

N31: Are both UDP and IP unreliable to the same degree? Why or why not?

N32: A client uses UDP to send data to a server. The data length is 16 bytes.a) What is the efficiency of this transmission at the transport layer, in terms of the ratio ofdata bytes to the total number of transmitted bytes?b) What is the corresponding efficiency at the network layer (assume no options for the IPdatagram header)?

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Optical NetworkingO1: What are the advantages and disadvantages of single-mode fiber versus multi-modefiber?

O2: Which transmission window is generally used for commercial wide-area optical networksand what are the reasons for using it?

O3: What is the available theoretical bandwidth for the C-band using a common value of 60dB for the signal-to-noise ratio? Note: the speed of light in a vacuum is 299792458 (m/s) asspecified in ITU-T G.694.1.

O4: An optical transmission system is constrained to have 500 GHz channel spacing. Howmany wavelength channels can be utilized in the 1536-to-1556 nm spectral band? Note: thespeed of light in a vacuum is 299792458 (m/s) as specified in ITU-T G.694.1.

O5: Why is it difficult to make use of the total bandwidth of an optical fiber by using a singlewavelength?

O6: What is the difference between an optical amplifier and regenerator?

O7: What are the advantages and disadvantages of an optical amplifier?

O8: Consider an optical fiber with an attenuation of 0.2 dB/km. The transmitter power is 1mW and the receiver sensitivity is 10−5 mW. What is the maximum length of the optical link,assuming that there are no amplifiers? (Sensitivity: min power needed to detect the signal).

O9: Consider the transmission path from point 1 to point 4 shown in Fig 1. The signal is at-tenuated by 9 dB between points 1 and 2. After getting an X-dB boost from an amplifier atpoint 3, it is again attenuated by 3 dB between points 3 and 4. The signal has a factor of 1.58gain in power in going from point 1 to point 4. Find the value of X.

Figure 1: Optical transmission path

O10: Where is optical layer located in the layer hierarchy and what functionalities does it of-fer?

O11: What are the main features of the 1st and 2nd generation optical networks?

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O12: What are the different tiers of the telecommunication network and what are the factorsthat differentiate them?

O13: What is the difference between a router and ROADM?

O14: What are the different elements of an optical node?

O15: What are the main wavelength constraints in routing and wavelength assignment (RWA)?What effects do they have on network performance?

O16: Describe the concept of routing and wavelength assignment (RWA). What are the ob-jectives of static RWA and dynamic RWA?

O17: To solve the RWA problem, why is the combined problem often divided into a routingsubproblem and a wavelength assignment subproblem?

O18: Draw the schematic diagram for a non-reconfigurable optical node with three inputfibers/ports and three output fibers/ports. Each fiber carries three wavelength/lambda chan-nels, which need to be switched from the input fiber to the output fiber in accordance to theswitching matrixes shown in Fig.2. The one’s in the (i , j ) entries of the matrixes indicate thatthe lambda in the i th input fiber should be switched to the j th output fiber. For example,lambda 1 in input fiber 1 should be switched to output fiber 1 and similarly lambda 1 in inputfiber 2 should be switched to output fiber 3.

Figure 2: Switching matrixes for the wavelength/lambda channels

O19: Consider the same node design problem of exercise 18 but with switching requirementsshown in Fig.3. Explain why it’s not feasible to design a node for the given switching require-ments?

O20: Draw a schematic diagram for a reconfigurable optical node with three input and threeoutput fibers, where each fiber has a maximum of four wavelength/lambda channels. As-

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Figure 3: Switching matrixes for the wavelength/lambda channels

sume that the node is neither adding nor dropping any lambda/channel locally.

O21: Consider an optical network shown in Fig.4. Each undirected link represents two fibers,one for the transmission in each direction. In addition, assume that there are two wave-length/lambda (λ1,λ2) channels per fiber. Suppose that a number of connection requestsarrive into the network at different time instants as shown in Fig.5. Each request needsone wavelength/lambda channel to be established between the given source and destina-tion nodes for specific time units (holding time). The established connections will departthe network once their holding time expires; releasing the channels that can be used for es-tablishing the other requests. Assume the First-fit wavelength assignment and fixed routingtechniques for RWA problem.

(a) Identify what happens to each request (i.e., put on λ1, put on λ2, or blocked).(b) What is the blocking probability of the network? Note: blocking probability is estimatedas the number of blocked requests divided by the number of total requests.(c) How many established connection will have departed from the network when the last re-quest arrives into the network?(d) What is the blocking probability of the network if each fiber has one wavelength/lambda(λ1) channel?

Note: The fixed routes for the requests are 5 → 8 → 7, 8 → 6 → 3, 1 → 3 → 6, 4 → 1 → 2,2 → 5 → 8, 3 → 6 → 5, 6 → 3 → 1 → 4, 7 → 4 → 1.

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Figure 4: Optical Network

Figure 5: Connection requests with different arrival times

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Source CodingC1: Ada rents an allotment garden which is not located near her house. She wonders whenher carrots need watering. She, therefore, installs a moist sensor system with a Wi-Fi trans-mitter at the allotment to measure and transmit moisture levels. The Wi-Fi transmitter sendsto a nearby access point, which is connected to the Internet, and the data is delivered to Ada’scomputer at home. The sensor readings take on four different values

A: very moistB: moistC: dryD: very dry

The values appear with probabilities: P(A)=0.7, P(B)=0.1, P(C)=0.1, P(D)=0.1We assume i.i.d. samples.a) Ada wants to compress the data before transmitting it. Find a good code!

b) What is the mean length of the code?

c) How much do you gain compared to when you do not use variable length coding?

d) What is the theoretical limit for compression in this case? How does it compare to themean length calculated in (b)?

e) Assume that the channel is a packet loss channel with i.i.d. losses. Are there source cod-ing and error control strategies that allow Ada to guarantee that some amount of data can besent without errors? If Ada considers all possible strategies, does she need ARQ to achievethis performance?

f) What rate can she achieve if the packet loss probability is 0.02? Ignore the packet headeroverhead!

g) Assume a simple short repetition code with interleaving repeats the two bits describing thereading in 2 packets. If the probability of a lost packet is 0.02, what is the probability of a lostreading?

h) Would it be possible to transmit 2000 readings per packet, with 4000 bits per packet, overthe connection, with channel coding and source coding? Ignore the packet header overhead!

i) Would it be possible to transmit 2000 readings per packet, with 4000 bits per packet, overthe connection, with channel coding and source coding if the values were equiprobable? Ig-nore the packet header overhead!

j) Imagine that Ada’s sensors deliver analog readings, but she only wants her four-bit readingsabove, in order to not transmit too much data. What process will she involve to achieve this?

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C2: We want to encode speech for a new voice mail application with high quality, i.e., we canignore quantization distortion. We model the speech by an AR process with zero mean noisewith variance 1 and time correlation parameter 0.9:

xn+1 = 0.9xn + un

We put thousands of speech samples in each packet.

a) What is the average distortion/lost sample in case of a lost packet if we do not employ in-terleaving?

b) What is the average distortion/lost sample in case of one lost packet if we employ errorconcealment with interleaving of many 4-speech-sample-groups in two packets (the otherpacket was not lost)?

C3: A certain network link consists of a radio channel and a receiving router. The link has ameasured packet loss probability of P = 0.05. The bit error probability for the radio channelhas simultaneously been estimated to p = 10−5.

If packets are 1000 bits long, show that 20% of the lost packets are due to bit errors on theradio channel.

C4: Consider the radio channel in the previous problem.

a) Assume that FEC is used on the radio channel. In theory, the bit error rate drops to zeroat a cost of fewer received information bits. What will be the proportion of usable bits (infor-mation) compared to the total transmitted number of bits?

b) From the perspective of number of transmitted packets, show that the FEC solution for theradio channel is preferable compared to using ARQ also for the radio channel.

Consider the whole link in the previous problem.

c) Would it still be more beneficial to use FEC than ARQ if we now consider the whole link(including the router)? Assume that interleaving is used to spread out the packet losses “ran-domly” in the FEC case so that a binary symmetric channel (BSC) model can be assumed.

d) Discuss the possible “non-intuitive” result!

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Streaming over InternetS1: Why is TCP not suitable for real-time traffic?

S2: Can you think of reasons why TCP should be used for streaming anyway?

S3: Can you come up with some other media-types than speech, audio, and video that mightbe subject for real-time transport over Internet?

S4: Design a reasonable jitter buffer for a streaming system that can have the two behaviorsbelow!

S5: Assume that we are sending audio data using an 8000 Hz payload type (8000 samples persecond). In each packet we send 800 consecutive samples and those packets are sent with100 ms intervals between the packets.Assume that the time between arriving packets always is between 80 and 140 ms.Is this information enough to deduce the minimum playback buffer size, which will smoothout the jitter? If so, how small (in samples) can this buffer be? If not, illustrate playback de-lays, optimal arrival time, and problematic delays.

S6: Compare and contrast TCP with RTP. Are both doing the same thing?

S7: Why does RTP need the service of another protocol, i.e., RTCP, but TCP does not?

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S8: How much time it takes to distribute a file from one server to N peers in the client-servermodel?

S9: How much time it takes to distribute a file from one server to N peers in the peer-to-peermodel?

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