# Computer Networks SamG
Assume that a sender wants to transmit a sequence of four frames to a receiver using the stop and wait protocol. Assume that the normal transit time for a frame between the two computers is 0.3 seconds, and that the timeout clock of the sender is set to 0.7 seconds.

a) Draw the sequence of frame exchanges between sender and receiver (similar to Figure 11.13) assuming that no errors occur. How long will it take to complete the exchange?
b) Now assume that errors are occurring in the transmissions. Assume that every third frame transmitted by the sender is in error, and every third frame transmitted by the receiver is in error. Draw the sequence of frame exchanges and calculate how long it will take to complete the exchange if you assume that the sender's first bad packet is the first it sends, but that the receiver's first bad packet is the third it sends.
c) Finally, assume that 2/3 of the sender's frames are corrupted by error and 1/3 of the receiver's frames are corrupted during transmission.  Draw the sequence of frame exchanges and calculate how long it will take to complete the exchange assuming the sender's first packet is good and the receiver's first packet is bad.

4) Assume you want to transfer a file containing 500,000 bytes from one computer to another on a bus topology local network. Assume the two computers are a distance D apart, that the bus data rate is B bps, and each frame is P bits long, where 80 of these bits are occupied by the header and trailer information. Each frame of the file must be acknowledged with a 96-bit frame (2 data bytes) before the next is sent. The propagation speed on the bus is 2.5x108 m/s. Compute the total time necessary to transfer the file for the following values:

a) D = 100m, B = 1Mbps, P = 256 bits
b) D = 1km, B = 10Mbps, P = 512 bits
c) D = 100m, B = 40Mbps, P = 1024 bits
d) D = 1km, B = 100Mbps, P = 10,000 bits

1. In a 5 bit CRC error detecting scheme, assume we choose the generating bit sequence as 11011.

a) Encode the bit stream 11001010011.
b) Assume the channel introduces two errors such that the first and fourth bits are flipped. What pattern is received? Can the error be detected?

c) Now assume three errors occur such that the first, third and fifth bits are flipped. What pattern is received and can the error be detected?
• 8 years ago
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