CMPSCI 653: Computer Networks
Homework 1: Physical Layer and Data Link Layer
Web posted: Feb 15, 18:00
Due: Feb 22 in class

VIP/NTU Students: Due one week from when you receive the assignment or view Lecture 6 whichever is later.
  1. Circuit Switching versus Packet Switching
    Compare the propagation delay in sending an x-bit message over a k-hop circuit switched network and in a lightly loaded packet switched network. The circuit setup time is i seconds, the propagation delay is d seconds per hop, the packet size is p bits and the data rate is b bps. Under what conditions does the packet switched network have a lower delay?

  2. Slotted Aloha
    Measurements of a slotted Aloha channel with an infinite number of users show that 10 percent of the slots are idle. What is the channel load, G? What is the throughput S? Is the channel underloaded or overloaded?

  3. CSMA/CD
    Two CSMA/CD stations are each trying to transmit long (multi-frame) files. After each frame is sent, they contend for the channel using the binary exponential backoff algorithm. What is the probability that contention ends on round k, and what is the mean number of rounds per contention period?

  4. Ethernet
    Ethernet frames must be at least 64 bytes long to ensure than the transmitter is still going in the event of a collision at the far end of the cable. Fast Ethernet (100 Mb/s) 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 frame size?

  5. Transparent bridges
    A university computer science department has 3 Ethernet segments, connected by two transparent bridges into a linear network. One day the network administrator quits and is hastily replaced by someone from the computer center, which is an IBM token ring shop. The new administrator, noticing that the ends of the network are not connected, quickly orders a new transparent bridge and connects both loose ends to it, making a closed ring. What happens next. Explain your answer.
    Prashant Shenoy
    Last modified: Fri Feb 18 10:08:49 EST 2000