5. In a packet switched network, a source host transmits two packets of 1500 byt

Question

5. In a packet switched network, a source host transmits two packets of 1500 bytes each, which travels over one link to a router then from the router over a second link to a destination end system. The propagation speed on both links is 2 ? 108 m/s. The transmission rate of the first link and the second link is 10 Mbps and 5Mbps respectively. Packet processing on the router consumes 0.5 nanoseconds per byte. The length of the first link is 1,000 km, and the length of the second link is 2,000 km. Assume no other packets exist in the network. The processing and queueing delay at hosts are ignorable.

a) What is the total delay from the source to the switch of the first packet?

b) What is the end-to-end (from the source to the destination) delay of the first packet?

c) What is the end-to-end delay of the second packet?

6. A router has five links. One link has the bandwidth of 16 Mbps, the other four has the bandwidth of 4 Mbps each. Every millisecond, a packet of 500 bytes is received by each of the 4-Mbps links. All these packets are then immediately moved to the queue of the 16-Mbps link and transmitted.

a) What is the queue length (in terms of bytes) of the 16-Mbps link at the end of the second millisecond? Note that at the first millisecond, packets are arriving at the 4-Mbps links and no packets are ready for transmission yet. Assume the queue is empty at the beginning.

b) What is the queue length of the 16-Mbps link at the end of the third millisecond?

c) When will the queue length of the 16-Mbps links exceed 3000 bytes?

Explanation / Answer

Instructions: You are required to work on the homework on your own. Please be legible and state all assumptions clearly. Show all work in order to receive partial credit. 1 (a). Consider an application which transmits data at a steady rate (e.g., the sender generates a N bit unit of data every k time units, where k is small and fixed). Also, when such an application starts, it will stay on for relatively long period of time. Answer the following questions, briefly justifying your answer: - Would a packet-switched network or a circuit-switched network be more appropriate for this application? Why? - Suppose that a packet-switching network is used and the only traffic in this network comes from such applications as described above. Furthermore, assume that the sum of the application data rates is less that the capacities of each and every link. Is some form of congestion control needed? Why? 1.(b) Consider a circuit-switched network in the given figure. Recall that there are n circuits in each link. Homework 2 - What is the maximum number of simultaneous connections that can be in progress at any one time in this network? - Suppose that all connections are between the switch in the upper-left-hand corner. What is the maximum of simultaneous connections that can be in progress? 2 (a). Consider sending a file of F = M *L bits over a path of Q links. Each link transmits at R bps. The network is lightly loaded so that there are no queueing delays. When a form of packet switching is used, the M * L bits are broken up into M packets, each packet with L bits. Propagation delay is negligible. - Suppose the network is a packet-switched virtual-circuit network. Denote the VC setup time by ts seconds. Suppose to each packet the sending layers add a total of hbits of header. How long does it take to send the file from source to destination? - Suppose the network is a packet-switched datagram network, and a connectionless service is used. Now suppose each packet has 2h bits of header. How long does it take to send the file? - Repeat (b), but assume message switching is used (i.e., 2hbits are added to the message, and the message is not segmented). - Finally, suppose that the network is a circuit switched network. Further suppose that the transmission rate of the circuit between source and destination is Rbps. Assuming tsset-up time and hbits of header appended to the entire file, how long does it take to send the file? 2 (b). This elementary problem begins to explore propagation delay and transmission delay, two central concepts in data networking. Consider two hosts, Hosts A and B, connected by a single link of rate R bps. Suppose that the two hosts are separated by m meters, and suppose the propagation speed along the link is s meters/sec. Host A is to send a packet of size L bits to Host B. - Express the propagation delay, dprop in terms of mand s. - Determine the transmission time of the packet, dtrans in terms of Land R. - Ignoring processing and queing delays, obtain an expression for the end-to-end delay. - Suppose Host A begins to transmit the packet at time t=0. At time t=dtrans, where is the last bit of the packet? - Suppose dpropis greater than dtrans . At time t=dtrans, where is the first bit of the packet? - Suppose dpropis less than dtrans . At time t=dtrans, where is the first bit of the packet? - Suppose s=2.5*108, L=100bits and R=28 kbps. Find the distance mso that dprop equals dtrans.

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