Operating Systems
and Computer Structures G2

Tutorial and Laboratory Exercises Week 13

Tutorial Week 13

<li>  
Suppose a network is organised as a binary tree. What is the best-case communication 
delay? The worst case? How is the worst case related to the number of nodes 
in the tree?

Ans: 
Best case is when two nodes are adjacent. Worst case is when they are both 
at the bottom connected only by the root. In this case there are 2*height nodes 
to traverse. The worst case path is 2*height, but the number of nodes in a 
balanced tree is proportional to 2 to the power of height. The measn that the 
length is logarithmic in the number of nodes, which is pretty good. A square 
mesh for example is only proportional to the square root. The downside is that 
the root acts as a bottleneck as any message from the left to the right of 
the tree passes through the root.

<li>  
What imformation is contained in addresses that will allow a message to be 
routed from jan@canberra.edu.au to bill_gates@microsoft.com?

Ans: 
The message originates in Oz and is destined for the US. So it must pass through 
the gateway in Melbourne. Once in the US, it belongs in the commercial domain, 
so its location can be found from a name server keeping tracks on this domain. 
Once in the Microsoft network their own name server can locate bill_gates mail 
account.

<li>  
Because a file server may crash, the file system may be replicated on another 
server. What are the advantages and disadvantages of this?

Ans: 
Advantages: speed, reliability. Disadvantages: ensuring the two file systems 
contain the same info. Often writes would be allowed to one only, and the other 
``mirrors'' this one.

<li>  
How many IP addresses can  there be? The Internet is estimated to have about 
10 million computers attached to it, but it is running out of addresses. How 
can this be?

Ans: 
Class A networks use one-bit to signify this, so 231 addresses are available 
to these. Class B use 2 bits to signify this, so 230 addresses are available. 
Class C use 3 bits to signal class, so 229 addresses are available. Class D 
allows 228 addresses. All told - a lot! Neverhtheless, it is running out. It 
is to do with the structuring of addresses. There are 216 (65536) Class B addresses, 
and within a Class B network there are 64k possible machines. We have a Class 
B, but only 1000 (?) machines on campus. So we waste a lot. So does everyone-else. 
Moving to Class D would leave us with too few addresses. So the wastage is 
the reason.

<li>  
A large software project may have n files. To speed up compilation compilation 
could be done on multiple machines. What is the maximum number that could profitably 
be used? What is the maximum speedup that could be obtained? What could stop 
you getting this speedup?

Ans: 
Run one compile per machine, gives a speedup of a maximum of n. It may be less 
because: one compile may be huge and the others tiny, so it uses all the time 
anyway; the files may need to be sent out to the other machines, and for short 
compiles this may not be worth it; linking would have to be performed back 
on one machine.

Laboratory Week 13

<li>  
Use xcwis to get the telephone numbers of people on campus, and then at ANU.

<li>  
Use netfind to find if there is anyone at ADFA with the surname Bearman.

<li>  
Experiment with xarchie to see if you can locate interesting files.

<li>  
The anonymous ftp site rtfm.mit.edu to examine the directory /pub/usenet-by-hierarchy. 
This contains information culled from the network news. For example, comp.windows.x 
would be in comp/windows/x. Find some interesting files.

<li>  
From a PC lab, use ftp to transfer files to and from Unix.