sit202-a3-solutions

# sit202-a3-solutions - Solutions Task 1 Control in Networks...

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Solutions Task 1: Control in Networks 1. Compare and contrast the go-back-N recovery method with the selective retransmission error recovery method. Go-Back-N recovery method works by sending unacknowledged packets within a given window size (a pipelined operation). The receiver responds with an ack with the ack number set to the last acknowledged packet. If a packet from the sender is lost in transmission and other later packets with higher sequence number pass through, the receiver will send an acknowledgement for each received packet with the ack number always set to the last acknowledged packet (in correct sequence) back to the the sender. . If a sender packet or acknowledgement is not received within a given timeout period, all the packets that have not yet been ack’d will be resent. Selective Repeat aims to reduce the number of packets that have to be resent when packet error occurs. It does this by only sending the packets that is suspected of being received in error. A selected retransmission like this calls for the receiver to acknowledge the individual packet. Similar to the GBN protocol a window size is use to limit outstanding packets (pipelined). However, unlike GBN the receiver will acknowledge a correctly received packet even if it is out of order. These packets will be kept in a buffer while the missing lower end packets are received.

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2. Calculate CRC value for the following message: 101110101. Show the whole message sent by the sender. Note: I don’t know what value your expecting me to use for the polynomial divisor so I’m assuming the same as the one used in the textbook. G = 1001, r = 3 101001111 1001 )101110101 000 1001 101 000 1010 1001 111 000 1110 1001 1111 1001 110 0 1001 101 0 1001 11 0 000 110 R = 110 Full Message: 101110101 110
3. Consider a scenario in which Host A wants to simultaneously send using a broadcast channel messages to Host B and Host C. Suppose that the broadcast channel can independently lose and corrupt messages (e.g., a message sent by A can be received correctly by B, but not C). Design a stop-and-wait-like error-control protocol for reliably transferring a packet from A to B and C, such that A will not get new data from the upper layer until it knows that both B and C have correctly received the current packet. Give FSM description of A and C (Hint 1: the FSM for B should be essentially the same as for C). Also give a description of the packet format(s) used. (Hint 2: study protocol rdt3.0) Sender FSM

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sit202-a3-solutions - Solutions Task 1 Control in Networks...

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