EPL324: Tutorials* on Communications and Networks Tutorial 2: Chapter 1 Review Questions

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1 EPL324: Tutorials* on Communications and Networks Tutorial 2: Chapter 1 Review Questions Pavlos Antoniou University of Cyprus Department of Computer Science * The material is taken from J.F. Kurose & K.W. Ross, Computer Networking: A Top-Down Approach Featuring the Internet, 4 th Edition.

2 Conversion formulas Data rate units (measure of link bandwidth) 1 kbps (kilobit per second) 1,000 bits per second 1 Mbps (megabit per second) 1,000,000 bits per second 1 Gbps (gigabit per second) 1,000,000,000 bits per second Data units (measure of packet length) 1 byte 8 bits 1 kb (kilobyte) 2 10 bytes 1024 bytes 8192 bits 1 MB (megabyte) 2 20 bytes bytes bits 1 GB (gigabyte) 2 30 bytes 8* 2 30 bits 2

3 Book Problems P5 (page 70) Two hosts, A and B, connected by a single link of rate R bps. Suppose that the two hosts are separated by m meters, and that the propagation speed is s m/s. Host A is to send a packet of size L bits to host B. Express the propagation delay, d prop, in terms of m and s. Determine the transmission time of the packet, d trans, in terms of L and R. Obtain an expression for the end-to-end delay (ignore queuing and processing delays). Suppose A begins to transmit the packet at time t0. At time td trans, where is the last bit of the packet? Suppose d prop is greater than d trans. At time td trans, where is the first bit of the packet? Suppose dprop is less than d trans. At time td trans, where is the first bit of the packet? Suppose s m/s, L100 bits, and R28kbps. Find the distance m so that d prop equals d trans. 3

4 Book Problems P9 (page 71) Consider two hosts, A and B, separated by three links. These three links are connected by two packet switches. Let di, si, and Ri denote the length, propagation speed, and the transmission rate of link i, for i 1, 2, 3. Each packet switch introduces a constant processing delay of dproc for each packet. Consider sending a packet of length L from A to B. a. Assuming no queueing delays, what is the total end-to-end delay for the packet, in terms of di, si, and Ri (i 1, 2, 3), L and d proc? Suppose now the packet consists of 1500 bytes, the propagation speed on all three links is 2.5 x 10 8 m/s, the transmission rate of all three links are 2 Mbps, the packet switch 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 1000km. For these values, what is the end to end delay? Assume that 1 Mbps 1000 Kbps bps. 4

5 Book Problems P24 (page 74) In modern packet-switched 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 packets back into the original message. We refer to this process as message segmentation. Figure 1.24 illustrates the end-to-end transport of a message with and without message segmentation. Consider a message that is bits long that is to be sent from source to destination in Figure Suppose each link in the figure is 1.5Mbps. Ignore propagation, queuing, and processing delays. 5

6 Book Problems P24 (page 74) 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 switch? Keeping in mind that each switch uses store-and-forward packet switching, what is the total time to move the message from source host to destination host? Now suppose that the message is segmented into 5,000 packets, with each packet being 1,500 bits long. How long does it take to move the first packet from source host to the first switch? When the first packet is being sent from the first switch to the second switch, the second packet is being sent from the source host to the first switch. At what time will the second packet be fully received at the first switch? 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. Discuss the drawbacks of message segmentation. 6

7 Example 1 Question Πόσο εύρος ζώνης (bandwidth) πρέπει να έχει η ζεύξη μεταξύ ενός κόμβου-πελάτη (client) και ενός εξυπηρετητή (server) για να κατεβάσει ο client εικόνα L5ΜΒ από τον στον server σε 2 δευτερόλεπτα? Η απόσταση μεταξύ των 2 κόμβων είναι D km. H γραμμή μεταφοράς έχει ταχύτητα διάδοσης s2.5x10 8 m/s. Υποθέστε ότι η καθυστέρηση επεξεργασίας και ουράς (processing, queueing delay) είναι αμελητέες. Solution D L End_to_end delay d + d + 2sec prop 20 L bits R 21. 4Mbps D s Client Server 5MB transm s R 7

8 Example 2 Question Client Server Πόσο χρόνο χρειάζεται ο κόμβος-πελάτης (client) για να κατεβάσει από τον εξυπηρετητή video (video server) ασυμπίεστο High Definition Video (HDV) διάρκειας 2 λεπτών? Το video έχει τα εξής χαρακτηριστικά: 30 frames per second, 1920 x 1080 pixels/frame, και 24 bits/pixel. Η απόσταση μεταξύ των 2 κόμβων είναι 5 km. H οπτική ίνα που συνδέει τους 2 κόμβους έχει ταχύτητα διάδοσης 3x10 8 m/s και εύρος ζώνης 50 Gbps. Υποθέστε ότι η καθυστέρηση επεξεργασίας και ουράς (processing, queueing delay) είναι αμελητέες. Αν αντί οπτική ίνα είχαμε καλώδιο ethernet με εύρος ζώνης 100 Mbps και ταχύτητα διάδοσης 2.5x10 8 m/s πως αλλάζει ο χρόνος? 8

9 Solution 1 frame has 1920 x 1080 pixels pixels/frame 30 frames per sec x pixels/frame pixels per second 24 bits per pixel x pixels per second bits per second 2 x 60 secs x bits 179GBits για 120 δευτερόλεπτα ασυμπίεστου HDV End_to_end delay d prop + d transm D s + L R sec 9

10 Solution Αν αντί οπτική ίνα είχαμε καλώδιο ethernet με εύρος ζώνης 100 Mbps πως αλλάζει ο χρόνος? d prop + d transm D s + L R min > η καθυστέρηση διάδοσης είναι αμελητέα 10