Answer the following questions: Each calculated expression and a brief descripti

Question

Answer the following questions: Each calculated expression and a brief description of each element in the calculation are needed. Solutions limited to final numerical values will not be considered correct. a) There is one packet switch between two hosts, the first link has a rate of 1 Mbps and the second link has a rate of 2 Mbps. Assume there is no other traffic on this network, the processing delays are negligible. If every packet has size 64 bytes, what is the time needed to transfer 40 packets between these two hosts? What is the expected queuing delay at the output buffer of the packet switch? b) Suppose there is a 10 Mbps microwave link between a base station on Earth and a geostationary satellite. Every minute the satellite takes a grayscale image, where each pixel takes 8 bits, then sends it to the base station. If the propagation speed is 2.4 times 10^8 m/s. What is the propagation delay of the link? If the image is square, what is the minimum size of this image in pixels for the link to be continuously transmitting?

Explanation / Answer

a)

packet switch between 2 hosts

1st link rate - 1 Mbps
2nd link rate - 2 Mbps

packet size - 64B

Time needed to transfer 40 packets between 2 hosts?

Assuming propagation delay = 0 ms

Assuming transfer occurs from 1st link to 2nd link
For each link, it takes 1Mbps/64B = 61 micro seconds to transmit the packet on the link.
For 40 packets the transfer time is : 61 X 40 = 2.44 milli seconds

Assuming transfer occurs from 2nd link to 1st link
For each link, it takes 2Mbps/64B = 30 micro seconds to transmit the packet on the link.
For 40 packets the transfer time is : 30 X 40 = 1.2 milli seconds

Expected queueing delay at output buffer of packet switch?

Assuming transfer occurs from 1st link to 2nd link
For each link, it takes 2Mbps/64B = 30 micro seconds to consume the packet on the link.
But the next packet will arrive after 61 micro seconds.
So the 2nd link has 30 micro seconds extra to serve the queue. Hence there won't be any queueing delay.

Assuming transfer occurs from 2nd link to 1st link
For each link, it takes 1Mbps/64B = 61 micro seconds to consume the packet on the link.
But the next packet will arrive after 30 micro seconds.
So the 1st link packet has to wait 31 micro seconds extra to be consumed.

So total queueing delay : 31 X 40 = 1.240 milli seconds

b)

10 Mbps Microwave link between Earth and Geo-stationary satellite.

Satellite takes pics every minute.
Size of each pixel = 8 bits
Propagation speed = 2.4 X 108 m/s

What is propagation delay?

Distance of a geo-stationary satellite from Earth = 35786 Km
Propagation speed = 2.4 X 108 m/s
Propagation delay is the time taken for the signal sent from the sender to reach the reciever.
Hence the Propagation delay is : Distance/Speed
=> 35786 X 1000 / (2.4 X 108) secs = 0.1491 secs ~ 0.15 secs (approx)

If the picture is square, what is the minimum size of image in pixels for the link to be continuosly streaming?

For continuous streaming the picture size has to be large enough so that the data flow is continuous.
Satellite link - 10 Mbps
For 1 minute - 10 Mb/s X 60 s = 600 MB.
So, for continuous streaming, the image size should be 600 MB.
size of each pixel - 8 bits = 1 byte

Total pixels in the image - 600 X 106 / 1 = 6 X 108

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