Problem #10

[Harsh Thanki]

Many students wanted to know the solution of problem 10. Here it is.

a)      The propagation speed is 2.5*10^8 m/s. So, the propagation time to cover 10,000 km is (10,000*10^3)/(2.5*10^8) = 0.04 secs.

The file size is 1 million bytes i.e 8 Mb. (1 million = 10^6, 1 byte = 8 bits)

The transmission rate is 1Mbps hence for 8 Mb data,

Total transmission time = (8*10^6)/(1*10^6) = 8 secs.

Total time taken to send the file = Transmission time + propagation time

Total time = 8 + 0.04

Total time to transfer the file = 8.04 secs.

b)      Here the data is divided into 20 packets with each packet containing 400,000 bits.

Transmission time for 1 packet = 400,000/(1*10^6) = 0.4 secs

Therefore, the transmission time for 20 packets = 20*0.4 = 8 secs

The propagation time for 1 packet is 0.04 secs. Hence for 20 packets the propagation time is 20*0.04 = 0.8 secs

As given, each packet of host A needs to be acknowledged by host B before host A sends another packet.

So, one acknowledgement packet of host B will take 0.04 secs to reach host A.

For host A to send 20 packets of data, 20 acknowledgements are needed from host B.

Therefore, the total propagation time for 20 acknowledgement packets = 20*0.04 = 0.8 secs

Transmission time of the acknowledgement packet is given as negligible.

Total time to transfer the whole file =  [ Transmission time of host A + Propagation time from host A to B + Transmission time of host B  + Propagation time from host B to A ]

                     = 8 + 0.8 + 0 + 0.8

            Total time to transfer the file = 9.6 secs.


Thanks,
Harsh

[min zhang]

Hi Could you post the answer for problem 4( the  amazon web server and mail server)? Did not quite get that during class.

Thanks !

[Harsh Thanki]

The question asks if you can have exactly same alias for both web server and mail server.
Yes, DNS has separate Resource Records (RR) for both of them. The mail server mapping uses MX record and the web server the mapping uses A or AAAA record. These variables are set in the 'type' field of a RR. When you make a DNS query, you also mention the type of record you are looking for. (see attached capture)
So, a one-to-one mapping can be made using the same alias.

This link helped me know Resource Records better to start with.
https://support.google.com/domains/answer/3251147?hl=en

Thanks!