EECS 489 Homework #3

1. (15 points) Discuss why we even have a Data Link layer. What are the responsibilities of the Data Link layer? Be sure to discuss at least 4 responsibilities. As a hint, consider what services are provided to the Data Link layer by the physical layer and what services the Data Link layer provides to the Network layer. ( This question is purposely a bit open ended to encourage discussion. Grading will be lenient. )
   
   
2. (10 points) Suppose you plug a new Ethernet card into a host, say host H, and a network connected to the Internet. Assume you are also given an IP address for H and the local DNS server. Now you want to send an UDP packet U to coconuts.kaeleku.oahu.hi. Explain how this might be done by accounting for each frame of data coming in and going out of your computer through the network card. The level of detail should be enough to (1) tell what type of packet/frame is being sent. At a minimum you should see DNS, ARP &UDP packets. You don't have to walk through the TCP handshake or ACKs between hosts (yet!) (2) to give an explanation as to why the packet is being sent or received and where it's going or coming from and (3) note what significant information each frame contains. ( State any assumptions you care to make. )
   
   

Short answer section

1. (10 points) We can emulate a multipoint-to-multipoint multicast with a set of point-to-multipoint multicasts. However, this has two drawbacks.
   • (a) What is the state needed at each router for point-to-multipoint tree ?
   • (b) What happens in the point-to-multipoint case when a new sender wants to send packets to all the receivers ? What happens in the multipoint-to-multipoint case ?
     
     
2. (10 points) Find a lower bound on the time it takes to transfer a 5 megabyte file across a network that operates at: 9600 bps, 56 Kbps, 100 Mbps, and 2.4 Gbps (Comer 2.7).
   
   
3. (10 points) When bit stuffing is used, is it possible for the loss, insertion, or modification of a single bit to cause an error not detected by the checksum ? If not, why not ? If so, how? Hoes the checksum length play a role here ?
   
   
4. (5 points) Consider building a CSMA/CD network running at 1 Gbps over a 1-km cable with no repeaters. The signal speed in the cable is 2x(10^5) km/s. What is the minimum frame size ?
   
   
5. (10 points) Ethernet frames must be at least 64 bytes long to ensure that the transmitter is still going in the event of a collision at the far end of the cable. Fast Ethernet has the same 64 byte minimum frame size, but can get the bits out ten times faster. How is it possible to maintain the same minimum frame size?
   
   
6. (10 points) Ethernet is sometimes is said to be unsuitable for real-time applications because the worst-case retransmission interval is not bounded. Under what circumstances can the same argument be leveled at the token ring? Under what circumstances does the token ring have a known worst case? Assume the number of stations on the token ring is fixed and known.
   
   
7. (10 points) Frames of 1000 bits are sent over a 1-Mbps satellite channel. Acknowledgments are always piggybacked onto data frames. The headers are very short. Three bit sequence numbers are used. What is the maximum achievable channel utilization for the stop-and-wait protocol ?
   
   
8. (5 points) Imagine two LAN bridges, both connecting a pair of IEEE 802.4 networks. The first bridge is faced with 1000 412-byte frames per second that must be forwarded. The second is faced with 200 4096-byte frames per second. Which bridge do you think will need a faster CPU and why ?
   
   
9. (5 points) One common algorithm for managing the ARP cache replaces the least recently used entry when adding a new one. Under what circumstances can this algorithm produce unnecessary network traffic ? (Comer 5.3)
   
   
10. (5 points) It is desired to send a sequence of computer screen images over a link. The screen is 1280x1024 pixels, with 24 bits per pixel. There are 60 screen images per second. What is the required bandwidth for this scenario?