Sample Quiz 1 Questions/Answers
For the quiz, you may bring the class notes, the papers that were
assigned, your writing assignments, the presentation you prepared, and
the text book (assuming it arrives...)
In Ethernet: Distributed Packet Switching for Local Computer
Networks, the author describes a link-layer protocol used to send
packets around a local computer network. The physical medium for this
network is a coax cable (like the kind you get cable TV from). The
communicating computers are connected to the same cable at the same
Q1: When two computers try to inject a message into the cable
at the same time, the messages collide. What does the Ethernet
Protocol say the computers should do to try to get their respective
Each computer waits before trying again, using a Binary Exponential
Backoff algorithm. The wait time is determined by computing a random
number then multiplying it by an estimate of the network load.
Subseqent collisions lead the algorithm to exponentially increase the
Q2: You are getting bad performance on your local
Ethernet network. A little investigation shows that your nemesis, Net
Vader, is watching lots of videos - and he's sharing the same Ethernet
cable as you! To retaliate, you decide to create a rogue Ethernet
card that gives preference to your packets over NV's during a
collision. What will your card do different when a collision is
detected? Be concrete.
Any of the following gets full credit - this isn't a complete set
Q3: The paper describes the use of a packet
transceiver for connecting two coax cables into what looks like a
single cable. Describe how such a system should handle
collisions. (Hint: Consider two cases: A collision on one cable but
not the other, a collision on both cables at the same time)
For a collision that results when trying to send data, the rogue
ethernet card sets its waiting time to zero. This gives the rogue
card's data a higher probability of making it out.
Before sending a request, the rogue card can be used to disrupt any
video streams by creating collisions that cause NV's card to back off
The rogue card can create collisions
whenever it sees packets addressed to Net Vader's machine. Hopefully,
this will cause NV to give up watching videos altogether.
Collisions on any point in an Ethernet cable need to incapacitate the
whole cable. To act as a single cable, a collision on one of the two
cables must be propagated to the other cable. Collisions on both
cables at the same time need not be propagated.
Q4: Do you think the tranceiver setup described in Q3 would
scale out to hundreds of cable segments connected together? Briefly
describe why you think it would or would not.
Either of the following two gets full credit (there are other
In Disk System Architectures for High-Performance Computing,
you learned about the 5 RAID levels. Suppose you are configuring a
service like photo.net with three RAIDs. In the questions below, make
sure you explain your choice and list the key assumptions you used to
choose the RAID you think is most appropriate.
1> No. As more hosts are added to a single "virtual" cable, the steady-state
load on the cable will likely grow. If the steady-state load
surpasses the optimal load (as described in the paper), the
performance will be severely degraded.
2> No. Even if there are few hosts, the following can occur:
As more and more physical cables are connected into a virtual cable,
the propagation of collisions to the entire network can have a
devistating effect on performance.
Consider a simple example - two computers (C1, C2) at the end of one physical
cable, connected to two computers (C3, C4) at the other end of a connected
physical cable. These cables are joined into one virutal cable.
C1-C2----------------------| packet tranceiver/repeater |----------------------C3--C4
Suppose C1 and C2 are communicating, and have a collision. That
collision must be propagated to C3 and C4. Suppose C3 and C4
subsequently have a collision. That collision must be propagated back
to C1 and C2. Thus, even hosts that are physically far away can break
down local communication. Better to install packet filters and treat
the two cables as two separate subnets instead of one virtual cable.
Q5: You must first configure a RAID for photo.net's swap file.
Given your knowledge of virtual memory, what RAID level is most
appropriate? (assume photo.net has enough main memory to prevent
We're assuming there's no thrashing. Let's also assume that, as
stated in lecture, the virtual memory manager wants to read/write in
large blocks. Let's also assume that we want to minimize read errors,
as corrupted memory could bring the server down. Finally, as we heard
in lecture, let's assume there's only one virtual memory manager
(i.e., we need to do only one I/O at a time). Thus, we choose RAID
Level III. This gives us lots of bandwidth for reads and writes, plus good
error correction. While Level II would have been a good choice, Level
III will give us better performance (especially if we replace the
check disk with a solid-state device)
Q6 Now, you must configure a RAID for photo.net's JPG files.
These are the photo files that Philip or users upload. Assume there
are 50,000 JPG files on photo.net and that 250 jpg files are uploaded
each day. Also assume that photo.net must serve several requests for
distinct JPG files at any one time, all day long. What RAID level is
We're assuming that we want to access different files at the same
time, and that writes are not common (e.g., ~10/hour). We're
also not concerned if a few bit errors are not detected or corrected.
Who's going to notice? Thus, Level I would give us good performance.
Q7 Finally, you must configure the RAID for the Oracle database
files. Like the JPG case, there are many more reads than writes.
Unlike the JPG case, Oracle uses only a handful of very large files.
Most of the time, the same file is being read or written. What RAID
level is most appropriate?
Let's assume that even though there are more reads than writes, that
there will be lots more reads and writes in total than in the JPG
case. This makes sense - every time users input data to the website,
that's another database write. As we get more users, we'll get more
interaction, leading to more writes. We also need data integrity, and
improved read/write bandwidth over a single disk. Thus, Level V is