INT21CN Computer Networks

Tutorial #13

1. What does CSMA/CD mean? CSMA/CD is sometimes referred to as the "polite dinner table" algorithm? Can you think of a reason for this?

2. Traditional Ethernet/802.3 is a shared medium, multi-access network technology.
   1. What do these terms mean?
   2. Does this pose any potential security risks? Explain. What about a modern network based on a switching hub?

3. What is interesting about the Ethernet/802.3 "MAC address"? Discuss.

4. In the lecture, it was stated that an Ethernet/802.3 collision occurs when two stations start to transmit at the same time. Discuss the meaning of the term at the same time as used in this context.

5. The 10baseT configuration has captured the Ethernet market from thin wire. Discuss reasons why network planners and managers might prefer this technology. There are lots of reasons.

6. Switching Hubs build a table which maps destination MAC addresses to physical interface port numbers. How do they build this table?

7. An IP packet is "encapsulated" into an Ethernet frame for delivery within the network. How will this encapsulation differ in the case where the packet is sent to a router for "Internet Delivery", compared to IP "local delivery"?

8. In the lecture, it was stated that the key difference between Ethernet and IEEE 802.3 LANs is the meaning of the 16 bit "type" field (used as a "length" field in 802.3). In many LANS, Ethernet and 802.3 frames co-exist perfectly happily. How can a receiving station know whether to treat the field as a length or as a type?

9. Assume^[1] a one megabyte file must be transferred across a network. Ignoring delays caused by waiting for access and other overhead (ie, counting only the data transferred), how long would it take to send the file across an Ethernet? Across a Fast Ethernet?

10. The ARP protocol maps IP addresses to MAC addresses. Can you think of an alternative approach to providing these mappings? Why is ARP better?

11. What would you expect the destination MAC address to be in an ARP request?

12. An interesting aspect of ARP is that the ARP request (and response) is not in the same format as an IP packet -- can you think of a reason why? How do machines on the network know that an Ethernet frame contains an ARP request (or response) and not an IP packet? What would you expect to be the destination MAC address in an ARP request?

13. Engineering research question^[2]: A shared-medium (non-switched) Ethernet is generally regarded as heavily loaded (approaching overloaded, in fact) if the network utilisation goes over (approximately) 20%. This, on the face of it, seems a low value. What do you think is going on?

14. Opinion question: ATM is probably the dominant high-speed networking technology at present. What do you think is the particular attraction of ATM networks over other high-speed technologies, on the basis of the material presented in the lecture?

15. Serious Research question #1:In the lecture, it was mentioned that Gigabit Ethernet is compatible with 10/100Mbps Ethernet. What does this mean? It's OK to guess...

16. Serious Research question #2: What is the historical origin of the difference between the frame formats of Ethernet and IEEE 802.3?