CSC460 Homework Set 5 Fall 2001

Due: Tuesday, November 6

  1. A link has data rate 4 kbps and propagation delay 20 milliseconds. For what range of frame sizes does Stop-and-Wait give an efficiency of at least 50% ?

  2. What fraction of data rate is wasted on headers and re-transmissions for Selective Repeat on a heavily loaded 50 kbps satellite link with 0.25 second propagation delay. Frames are fixed size containing 3960 data bits and 40 header bits, including a sequence number and a piggybacked ACK. There are no separate ACK frames, but there are separate 40-bit NAK frames. The (data) frame error rate is 1%. Neglect the NAK frame error rate. Sequence numbers have 8 bits.

  3. Suppose that in the Selective Repeat protocol of the preceding question there occurs a period of silence (absence of data to be sent) in just 1 direction. Assuming that the sender has a 1-second timeout for repeating un-acknowledged frames, how many times, on average, would each frame be sent in the other (busy) direction during a period of silence in the 1 direction? Assume that the receiver lacks a timer for detecting absence of reverse traffic.

  4. An error-free 64 kbps satellite link with 0.25 second 1-way propagation delay uses 512-byte data frames in 1 direction and tiny ACK frames in the other direction. What is the maximum effective data rate for a window of size 7?

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