# Computer Networks – Math Problem Example

QUESTION ONE2 points: Compare and contrast the go-back-N recovery method with the selective retransmission error recovery method. The GO-BACK-N error recovery method and the selective retransmission error recovery method are methods of data transmission that uses time-outs and acknowledgement signals to achieve reliable transmission of data over un-reliable network service, commonly the internet. An acknowledgement is a message sent by the receiver to the transmitter to indicate that it has correctly received a data frame or packet. A timeout is a reasonable point of time, usually after the sender sends the frame or packet.

Both of these error recovery methods use Automatic Repeat Request(ARQ) to request for retransmission if a packet fails to arrive successfully. However, in GO-BACK-N ARQ protocol, the sender continues to send the frames without receiving the ACK signal from the receiver. The receiver on the other hand, receives the next frame following the sequence number and sends that number with every ACK signal it sends back. The receiver does not accept frames that don’t contain the SEO number it expects. After sending all the signals, the sender will detect that all the packets since the last lost packet are outstanding, and will go back and resend all the packets since the last ACK signal it received. Selective Repeat ARQ uses a different approach to correct transmission errors.

Whereas in Go-Back-N method, all packets after the last unacknowledged frame is retransmitted, selective repeat retransmits only the unacknowledged frame. This makes it more efficient than Go-Back-N method, although more complicated to implement. This method also allows packets that have been received out of order to be stored by the receiver and later on re-ordered into their correct order, which is different from the Go-Back-N method where out of order packets are treated as errors and retransmission requested. QUESTION TWOError detection is of the most crucial importance for network users.

a) 1 point: Propose a generator. b) 1 point: Calculate CRC value for the following message: 101110101. c) 1 point: Show the whole message sent by the sender. CRC-8-ATM(HEC) polynomial x8+x2+x+1.. CRC value for 101110101Step 1.Coefficient of the highest poser of x in x8+x2+x+1. And adding 1to the leftmost part results in 10000011110111010100000000message-100000111divisor00111001000000000result-00000000000111001000000000-10000011100011001111000000-10000011100001001100100000-10000011100000001101010000-00000000000000001101010000-0000000000000000001101010000-10000011100000000101011110CRC Value-10000011100000000001011001Remainder (8 bits) is 01011001, is the CRC valueThe complete message sent by the sender is the message with the CRC value, i.e 101110101 01011001QUESTION THREEDefine, compare and contrast the sliding window protocol with the stop-and-wait protocol in error correctionThe sliding window protocol operates on the principle that both the sender and receiver keep a ‘window’ of acknowledgement and frames to transmit.

The message to be sent is broken down into packets by the sender, where each packet is assigned a unique sequential number. The sender will keep the value of an expected ACK for every packet, whereas the receiver keeps a value of the expected frame to receive.

When the sender receives an ACK from the receiver for a frame sent, it advances the window (or sends the next frame in the sequence), whereas when the receiver receives the expected frame, it sends an ACK for that frame and advance the window. In this case, both the receiver and sender maintain a window for messages for which no ACK have been received. The term ‘window’ depicts the total number of packets and ACK yet to be send and received.