Introduction of network Please help me 8. In modern packet-switched networks, th

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Introduction of network

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8. 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 resembles the packets back into the original message. We refer to this as message segmentation. Figure 2 illustrates the end-to-end transport of a message with and without message segmentation. Consider a message that is 8x10 bits long needs to be sent from source to destination in Figure 2. Suppose the transmission rate of each link is 10 Mbps. Ignore propagation, processing and queuing delay Message a, Source Packet switch Packet switch Destination Packet b. Source Packet switch Packet switch Destination Figure 2. End-to-end message transport: (a) without message segmentation; (b) with message segmentation Consider sending the message from source to destination without message segmentation. How long does it take to move the message from the source 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? a. b. Now suppose that the message is segmented into 16,000 packets, with each packets being 500 bits long. How long does it take to move the first packet from the 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 which 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. c. d.

Explanation / Answer

(a) Time to send message from source to first packet switch = 8x106/ 10x106 sec =0.8 sec. With store-and-forward switching, the total time to move a message from source host to destination host = 0.8sec x 3 hops = 2.4 sec.

(b) Time to send first packet from source to first packet switch = 0.5x103/ 10x106 sec = 1/20 ms. Time at which second packet is received at the first switch = 2 x 1/20 ms = 1/10 ms.

(c) Time at which first packet is received at the destination host = 1/20 ms x 3 hops = 3/20 ms. After this, every 1 ms one packet will be received; thus the time at which last (16000th) packet is received = 3/20 ms + 15999? 1/10 ms = It can be seen that delay in using message segmentation is significantly less (almost 1/3 rd).

(d) Drawbacks:

i. Packets have to be put in sequence at the destination.

ii. Message segmentation results in many smaller packets. Since header size is usually the same for all packets regardless of their size, with message segmentation the total amount of header bytes is more.

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