3.2.1 Communication System Fundamentals

1. Ethernet uses a carrier sense multiple access with collision detect scheme. Explain the terms in italics.

   Even using CSMA/CD, There is still a finite probability of collision between packets. Explain why.

   • Because there it takes a finite time for the signal to propagate along the cable. If at time t, a station transmits, then it won't be heard till t + $\delta$t at the other stations. If they check and transmit before t + $\delta$t then a collision will occur. The longer the wire, the longer the latency, and so the greater possibility of collision. This is also why there is a minimum length of packet, so that the jamming signal can be detected.
2. Assume that two computers are sending 1000 byte packets over a shared channel that operates at 64000 bits per second. After each packet, the receiving machine sends an acknowledgement packet of 10 bytes. How long will it take to send a 1 MByte file between the two machines? State your assumptions.
   • Data Packet takes ${\frac{{1000 \times 8}}{{64000}}}$ seconds = 125 milliseconds. Acknowledgement packet takes ${\frac{{10 \times 8}}{{64000}}}$ seconds = 1.25 milliseconds. 1 MByte contains approx 1000 packets. Machines alternate in sending files, so packet is 125 milliseconds + 1.25 milliseconds = 126.25 ms. So time is approximately 125 s, assuming negligible latency, and no loss.
3. Switched networks are taking over from shared media networks as Local Area Networks. Why?
   • Matches actual wiring laid out in buildings - most wiring is a star laid out from a dry riser.
   • More secure.
   • More fault tolerant - someone inadvertently unplugging the wire doesn't break the whole network.
   • Each machine can get full bandwidth of wire
   • Can scale the switch hub easier