# ECE/CS 438: Communication Networks Machine Problem 4 solution

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This machine problem tests your understanding of the random backoff and medium access in a
wired network.
1 The Scenario
In this assignment, you will develop a toy simulator that evaluates the performance of simplified
CSMA protocols in a wired network. In this toy simulator, start with two nodes only – nodes
A and B – and assume each node always has packets to transmit to its counterpart (later you
will increase the number of nodes). To transmit a packet, a node picks an integer random
number from the range [0, R) and counts down as long as the channel is idle. If the channel gets
busy (because say node B has already started transmitting, or due to a collision with another
transmission), node A freezes its countdown and waits till the channel is idle again. Once B
finishes and the channel is idle, node A resumes countdown; node B picks a new random number
and also starts counting down at the same time. In the actual assignment we will have more
than 2 nodes and all nodes will perform the same operation.
Now let’s see what happens when a collision occurs. Only nodes that were transmitting at the
time of the collision doubles the value of R (unless specified otherwise) and also increments their
collision count. When a node’s collision count reaches a maximum value of M, the node gives
up and drops this packet. It then resets R to its minimum specified value, resets the collision
count to zero again, and attempts a fresh new transmission.
An input file will provide you the following configuration parameters: number of nodes N (not
just 2), the packet length L, the minimum value of R (and how it increments), the value of M,
and the total simulation time T. Your goal is to evaluate the network utilization, as described
later.
2 Simplifying Assumptions
To keep the code simple, here is a list of assumptions you can make:
1. This simulation entirely takes place within one program, just like mp3.
2. Assume that clocks at all nodes are perfectly synchronized, which means all the nodes
3. Assume that the transmission time of data packets are in multiples of clock ticks (e.g., a
packet might last for say 10 clock ticks); also assume that the propagation delays between
all nodes are zero.
4. Assume that collisions are the only reason for packet corruption – if there is no collision,
the packet is always received correctly; also assume no ACKs are sent by the receiver.
5. Assume that any processing (such as picking random numbers) takes zero time.
6. To fix the output and allow autograding, please use a pseudo number generator to set the
backoff time.
backoff = mod(nodeID + ticks, R) (1)
where nodeID and ticks both start from 0, and R is the backoff window.
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7. At the start of each tick, please check if there are packets being transmitted or if multiple
packets are colliding at that tick. If there is no transmission or collision (i.e., channel is
idle), then all the nodes that have packets to transmit count down by one tick.
3 Hints on How to Simulate
Now, here are hints on how you can develop such a toy simulator. Imagine that the clock is
a global integer variable which increments perhaps in a for loop. This simulates the passage
of time. Now for every increment of the clock, all events that happen at that time can be
inside the for loop. Now say two nodes are two structures that have their own backoff values,
e.g., nodeA.backoff and nodeB.backoff. The operation of counting down can be simulated
by decrementing the nodeA.backoff and nodeB.backoff variable in each iteration of the loop.
Of course, before counting down the channel needs to be sensed, which can be simulated by a
global flag, say channelOccupied. Once a node reaches zero, say A, it can begin transmission
essentially by setting the channelOccupied flag. Node B does not decrement its counter in this
iteration since the channelOccupied flag is already set.
If the packet lasts for say 10 clock ticks, then 10 iterations of the for loop occur without any
significant activity. After that, node A pretends that the packet has been transmitted, and
hence, resets the channelOccupied flag, and assigns a new random number to nodeA.backoff.
In the next iteration, both nodes can decrement their backoff values, simulating the notion of a
resumed count-down. Note that no real transmission of the packet is necessary.
Of course, the above is a sketch and some details are missing. For instance, you need to
determine how collisions can be simulated and accordingly count how many packets are getting
transmitted correctly. You also need to carefully design the order of events—in a given iteration
of the for loop, node A should not decrement its backoff value if node B is going to set the
channelOccupied flag later. Put differently, you need to check if there is any node who might
set the channelOccupied flag—if no node does so, only then can all nodes decrement their
backoff.
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4 Input Formats
Your program will be run using the following command: ./csma inputfilename
The input file will contain five items delimited by newlines. Each line starts with a character
denoting the parameter, followed by the value(s) of the parameter.
Note that we have swapped the M and R rows compared to the sample input in
the released code, so that you can know the length of R after reading M.
Sample Input:
N 4
L 2
M 6
R 4 8 16 32 64 128
T 10
This means that the number of nodes N is 4, the packet size L is 2 clock ticks, the initial
random number range is [0,R = 4) and R is increased to 8, 16, 32 and so on for each consecutive
collision. The maximum retransmission attempt M is 6, implying that after the 6th collision
the node should drop the packet and reset R to the minimum value 4. Finally, T denotes the
total time of simulation in terms of clock ticks.
Figure 2 shows the back-off values of each node at the start of each tick. Green slot denotes
a successful transmission, and orange slot indicates there is a collision. In both of these cases,
other nodes will pause the count down (marked as dark grey slots). Note that a node will set
its countdown using the pseudo number generator Eq. 1.
Node/T init 0 1 2 3 4 5 6 7 8 9
N0 0 0 0 2 1 1 1 0 7 6 6
N1 1 1 1 1 0 0 2 1 1 0 2
N2 2 2 2 2 1 1 1 0 1 0 11
N3 3 3 3 3 2 2 2 1 1 0 4
Figure 2: Back-off values at the start of each tick for every node.
For example, at the start of tick 5, node 1 will set its backoff to mod(1 + 5, 4) = 2. At the start
of tick 7, node 0 will set its backoff to mod(0 + 7, 8) = 7
5 Output Formats
Write the link utilization rate (round to 2 decimal place) to a file called “output.txt”
Sample Output:
0.40
Note that the link utilization rate is (the number of slots where a packet is transmitted without
collision) divided by T.
6 Notes
All the notes for the previous MPs still apply. We are not repeating those here for brevity.
New information:
1. Your project must include a Makefile whose default target makes executables called csma
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2. Command line format: “./csma inputfile”