Solutions of Home work 6
If the network tends to loose packets, it is better to acknowledge each one separately, so the lost packets can be retransmitted. On the other hand, if the network is highly reliable, sending one acknowledgement at the end of the entire transfer saves the bandwidth in the normal case (but requires the entire file to be retransmitted if even a single packet is lost).
Small, fixed length cells can be routed through switches quickly and completely in hardware. Small, fixed-size cells also make it easier to build hardware that handles many cells in parallel. Also, they do not block transmission lines for very long, making it easier to provide quality-of-service guarantees.
The speed of light in coax is about 200,000 km/sec, which is about 200 meters/µsec. At 10 Mbps, it takes 0.1 µsec to transmit a bit. Thus, the bit lasts 0.1 µsec in time, during which it propagates 20 meters. Thus a bit is 20 meters long here.