PROBLEMS AND EXERCISES

Transcription

1 Departamento de Tecnología Electrónica Computer Networking Unit 1: Computer Networks and Internet PROBLEMS AND EXERCISES Introduction 1-79

2 TDM How long does it take to send a file of 640,000 bits from host A to host tb over a circuit ed it it network? all link speeds: Mbps each link uses TDM with 24 slots/sec 500 msec to establish end to end circuit Introduction 1-80

3 End to end delay Consider a packet of length L which begins at end system A and travels through three links to a destination end system. These three links are connected through two packet es. Let d i, s i and R i denote the length, propagation speed, and the transmission rate of link i, for i=1,2,3. The packet delays each packet by d proc. Assuming no queuing delays, in terms of di, si, Ri (i=1,2,3), and L, what is the total end to end delay for the packet? Suppose now the packet is 1500 bytes, the propagation speed on both links is 2.5x10 8 m/s, the transmission rate of the three links is 2 Mbps, the packet processing delay is 3 msec, the length of the first link is 5000 km, the length of the second link is 4000 km and the length of the last link is 1000 km. For these values, what is the end to end delay? Introduction 1-81

4 Queuing delay A packet receives a packet and determines the outbound link to which the packet should be forwarded. When the packet arrives, one other packet is halfway done being transmitted on this outbound link and three other packets are waiting to be transmitted. s are transmitted in order of arrival. Suppose all packets are 1500 bytes and the link rate is 2Mbps. What is the queuing delay for the packet? More generally, what is the queuing delay when all packets have length L, the transmission rate is R, x bits of the currently being transmitted packet have been transmitted, and n packets are already in the queue? Introduction 1-82

5 Encapsulation and segmentation Suppose that, in the OSI model, a data link protocol has the maximum size of E_SDU ESDUlimited it to 1000 bytes, bt and the network protocol doesn t have any maximum limitation for R_SDU to the higher level. If T_PDUs are always2000bytes and R_PCI PCIare 100 bytes, how many R_PDUs will network layer send? What is the content of each R_PDU? Introduction 1-83

6 Segmentation In modern packet ed networks, the source host segments long, application layer messages (for example, an image or a music file) into smaller packets and sends the packets into the network. The receiver then reassembles the packet back into the original message. We refer to this process a message segmentation. Figure illustrates the end to end transport of a message with or without message segmentation: without segmentation Source host Destination host with segmentation Source host Destination host Introduction 1-84

7 Segmentation Consider a message that is bits long that is to be sent from source to destination in that figure. Suppose each link in the figure is 2 Mbps. Ignore propagation, queuing and processing delays. a) Consider sending the message from source to destination without message segmentation. How long does it take to move the message from the source host to the first packet? Keeping in mind dthat t each uses store and forward packet kting, what htis the total t time to move the message from source host to destination host? b) Now suppose that the message is segmented into 4000 packets, with each packet being 2000 bits long. How long does it take to move the first packet from source host to the first? When the packet is being sent from the source host to the first, the second packet is being sent from the source host to the first. At what time will the second packet be fully received at the first? c) How long does it take to move the file from source host to destination host when message segmentation is used? Compare this result with your answer in part a) and comment. d) Discuss the drawbacks of message segmentation. Introduction 1-85