Homework 4 - Chapter 4

1. Draw the Manchester and differential Manchester encodings for the bit sequence 01100011. Assume that differential Manchester starts high.
2. Two half-duplex Ethernet stations are contending for the channel. What is the mean number of contention slots for these two stations?
3. Explain why Manchester encoding cannot be used for Gigabit and Fast Ethernet. What encoding methods are used on twisted pair for Fast Ethernet and for Gigabit Ethernet instead of Manchester encoding?
4. Under what circumstances might two or more wireless stations using MACA contention resolution transmit at the same time?
5. Would it be possible for any of the 802.11 protocols to use CSMA/CD (like Ethernet) instead of CSMA/CA for channel contention? Explain why or why not.
6. How does 802.11 allow both PCF and DCF nodes to exist within range of one another? Describe how the two modes coordinate in your answer.
7. Assume an 802.11g is transmitting 128 byte frames back-to-back at its maximum rate of 54Mbps. If each bit is flipped with a probability of 1x10^-6, how many frames per second will be damaged on average?
8. A common "WiFi" setup is to have an 802.11 access point plugged in to an Ethernet LAN. Explain what the access point has to do to handle the transition between the Ethernet and 802.11 halves of the LAN.
9. When using a spanning tree algorithm to determine the topology for bridges, why might one leave bridges in place that are not used in the current spanning tree?
10. What is the purpose of virtual LANs and how does it affect the level 2 routing decisions made by a switch?