Example Questions for Final Exam CS 162 Spring 2011 I Stoica Store and Forward 1 Hosts A and B are connected to each other via router R R is a store and forward router The bandwidth from A to R is 10Mbps and the bandwidth from R to B is 5Mbps The one way latency of each link is 22ms Assume host A sends a 30KB file to host B a Assume the file is divided into two packets p1 and p2 where p1 has a length of 10KB and assume the packets are sent back to back What is the difference between the arrival times of the first and the second packet at host B b What is the effective throughput between A and B in part a The transmission time is the time interval from the time the first bit is sent at A until the final bit is received at B c Does the throughput increase or decrease if we divide the file into smaller packets Why d Now assume each packet is acknowledged The file is divided into 6 packets of the same size How long would it take to send the entire file assuming that the sender cannot send a new packet before it receives an acknowledgment for the previous packet The transfer time is the time interval measure at source A from the time the first segment is sent until the acknowledgement of the last segment is received Ignore the transmission time of the acknowledgements Solution a The difference is the transmission time of packet B over the second link time size bw 8 20 5000 160 5000 32ms b Total transfer time calculate how long the second packet takes and add the wait for the first packet over link A R A R wait is 8 10 10000 8ms Now time to send B is 8 20 10000 8 20 5000 44ms 16ms 32ms 44ms 92ms Total time 100ms THROUGHPUT 30k 100ms 0 3MBps or 2 4 Mbits s c The throughput will increase because a smaller packet is received at R and can be forwarded immediately don t have to wait till the entire original packet is transmitted over the first link to start transmitting over the slower second link d Size of 1 packet 5KB transmit time packet 44 44ms 5 8 10000 5 8 5000 44ms 44ms 4ms 8ms 100ms so for 6 600ms Page 1 Store and Forwarding 2 Consider two packets that are sent back to back i e one right after another along the path A B C D where A B C and D are store and forward routers Assume the capacity of link A B is 10Mbps the capacity of B C is 1Mbps and the capacity of C D is 100Mbps The propagation delay along each link is 10 ms Assume the size of the first packet is 1000 bits and that the size of the second packet is 500 bits What is the inter arrival time between the two packets at node D The inter arrival time is equal to the arrival time of the second packet minus the arrival time of the first packet at node D Notes Assume there is no cross traffic the two packets are the only ones in the network The arrival time of a packet is the time when the last bit of the packet was received With a store and forward router a packet is forwarded only after the last bit of the packet was received Answer See the time diagram below bellow l1 1000b l2 500b C1 10Mbps C2 1Mbps C3 100Mbps The inter arrival time between the two packets at D is l2 C2 l1 C3 l2 C3 500b 106Mbps 1000b 108Mbps 500b 108Mbps 0 495 ms A B C D l1 C1 l2 C1 l1 C2 l2 C2 l1 C3 l2 C2 l1 C3 l2 C3 l2 C3 Page 2 Store and Forwarding 3 Consider the network in the figure below where two source nodes A and B are connected to a destination node D through a router C Assume that node A starts to send a 600 bit packet at time 0 and node B start to send a 1000 bit packet at time T see figure below Plot the inter arrival time denoted I between the two packets at node D versus the starting time of B s packet T for 0 T 5 Notes Ignore the processing time at C The arrival time of a packet is the time when the last bit of the packet has arrived at node D The inter arrival time is I arrival time of packet sent by B at D arrival time of the packet sent by A at D 600 bits A time 0 100Kbps 2ms time T 1000 bits 500Kbp s C D 200Kbps 1ms B Hint Based on the value of T there are three cases you might want to consider The diagram in Figure a shows the first case and depicts the messages sent by A and B arriving at C You can use these diagrams to solve the problem i e finish diagram a and fill in the other two diagrams b and c Solution a T 6 8 0 T 6 T 5 b 8 T 6 9 2 0 2 T 1 T 6 8 T 8 T 9 2 T 6 T 1 6 T 5 T 6 d I T 8 d T 9 2 d 1 2ms 0 2 T T 8 d T 9 2 d c 9 2 T 6 8 T 6 9 2 8 d T 5 9 2 d 11 2 d I 11 2 d 9 2 d 2ms 2 T 1 8 T 6 8 d9 2 d T 8 d I T 8 d 9 2 d T 1 2 ms Use the following coordinates to plot the inter arrival time between the two packets at D I versus the starting time of B s packet T for 0 T 5 Page 3 I ms 5 4 T 1 2 3 2 1 0 1 1 2 1 2 3 2 4 5 T ms 2 Page 4 Store and Forward 4 Measuring link capacities Consider the network below where three hosts A B and C respectively are connected to router R The capacity of the links are x Kbps y Kbps and z Kbps respectively Assume the users of hosts A B and C respectively want to measure the capacities of the three links To achieve this they send the following messages From A to B Send two back to back 1Kbit packets the interval between receiving the first and the second packet at B is 100ms the time it takes B to receive each packet i e the interval between receiving the first bit of the packet and the last bit of the packet is 100ms From B to C Send two back to back 1Kbit packets the interval between receiving the first and the second packet at C is 200ms the time it takes B to receive each packet is 200ms From C …
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