Sending packets vs. sending messages In packet switched networks, the source hos
ID: 3757003 • Letter: S
Question
Sending packets vs. sending messages In packet switched networks, the source host divides large, application-layer messages (for example, a video or music file) into smaller packets and then sends the packets individually. The receiver reassembles the packets back into the original message. The illustrations below show the end-to-end delivery of a message with and without dividing it into packets. Packets Message Consider a message with 9 Mb that is to be sent from source to destination. Suppose each link in the figure (source host to router and router to destination host) sends data at a rate of 2 Mbps Note that the router must wait until is can received an entire data unit (message or packet) before it can forward the data unit on to the next link. 2. Consider sending the message without dividing into packets. a) How long does it take to send the message from the source host to the router? b) What is the total time to send the message from source host to destination host? . Now suppose that the message is divided into packets each containing 10,000 bits. a) How many packets will be needed for the entire message? b) How long does it take to send the first packet from source host to the router? c) When the source host completes sending the first packet, it will begin sending the second one. As soon as the router receives the first packet, it will begin sending it to the destination host. How long does it take to send the entire message from source host to destination host?Explanation / Answer
2. The message of 9MB is sent without dividing from the source to the destination via a router.
a. Since the package is not divided and the data transfer is 2 Mbps.
It will take 4.5 seconds to transfer the whole message to the router.
b. Time take to send to the router + time take to send from router to destination host is the total time.
Since both the transfer rates are 2Mbps, It will take 4.5 seconds for each.
Total time takes is 9 seconds.
3. The message is divided into packets each containing 10,000 bits.
10,000 bit = 1250 byte.
9MB = 9000000 byte.
Total packets is 9000000/1250 = 7200 packets.
a. Thus the total number of packets required to transmit the entire message will be 7200 packets.
b. Time taken for the first packet to reach the router from the source:
1250/2000000 = .000625 seconds.
c. When the first packet is sent from source to router, router destination link is unused.
The same thing goes for source- router link when the last packet is sent from the router to the destination.
Thus the time taken for all the packets to reach from source to destination will be
7200*.000625 + .000625 seconds = 4.5 seconds + .000625 seconds = 4.500625 seconds.
The extra .000625 seconds is for the last packet to transfer from the router to the destination host.
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