Student Name
Date
DT021/4
COMMUNICATIONS NETWORKS 2
Laboratory 4 + Formal Element 1
Objective
Create queuing models to explore the performances of M/M/1, M/M/1/N and M/M/2 queues, using
OPNET and Queuing Theory calculations.
As part of this session, you should finish Lab 3 and Part 1 of this laboratory, and complete the results
sheets on the course web page to hand in your lab reports.
You will produce a report for Part 2, to be submitted as formal element no later than March 15th
(Group A) or March 16th (Group B).
Procedure
Start OPNET Modeler 14.5 (you must be logged in as netengineer).
Part 1:
To create the Part 1 model, re-use the lab 3 model.
The Part1 scenario corresponds to a Queue with a Poisson arrival with average rate 1 packet per
second, with a packet size exponentially distributed, with an average of 7200 bits. The Queue is
served at a rate of 9600 bits/second.
Calculate the average Queue Size.
- Capacity 1
Right click on the QUEUE module in the node model, and edit the attributes of the queue.
Change the packet capacity (pk capacity) parameter in the Queue module (expand Subqueue), to the
average queue size calculated above.
Use Matlab or Excel to calculate the average number of packets in the queue (Queue Size), based on
the formula for Pn in the lecture notes (M/M/1/N).
Calculate the average Queuing delay, using Little’s law (remember that only the arrival rate of packets
which are not dropped should be considered).
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Run the simulation and compare the measured and the expected result for the Queue Size.
Repeat the calculations and simulations for a Queue capacity of:
- Capacity 2: 5 packets
- Capacity 3: 10 packets
Include in your report the calculation of the expected values, and the graphs of the measured
performances for each of the 3 cases.
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Part 2:
You should compare the performances of 3 Queuing systems (with infinite buffer) in terms of:
- average number of packets in the Queue (Queue size),
- average delay per packet in the Queue (Queuing delay).
You should both:
- calculate the expected performances for the average Queue Size and the average delay, using
results of Queuing Theory,
- simulate the performances using OPNET.
Queuing Systems:
2 sources produce packets with an arrival rate characterised by a Poisson process with average
arrival rates of respectively 12 and 16 packets per second.
The packet size has an exponential distribution characterised by an average value of 4800 bits.
You want to compare 3 solutions to store and forward packets from these sources towards a remote
destination (SINK), and choose the solution providing the smallest average delay.
The 3 queuing systems are defined as follows:
Queuing System 1:
Each of the 2 sources feed a separate Queue, each characterised as a single server (M/M/1) with
service rate of 86,400 bits/sec.
Queuing System 2:
The 2 sources feed the same Queue characterised as a single server (M/M/1) with a service rate of
172,800 bits/sec.
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Queuing System 3:
The 2 sources feed the same Queue characterised as a queue with 2 servers (M/M/2), with service
rate each of 86,400 bits/sec.
Performance Calculations:
You will refer to the results contained in the lecture notes. In order to calculate the performances of
the 3rd system you will need the general formula:

 n x
n

n 1
x
(1 x)
2
Opnet Simulations:
In order to create the additional scenarios:
Duplicate the scenario Part1.
Right Click on the subnet, and select Edit Node Model.
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Save the node model with a name of your choice.
Modify the node model to reflect the characteristics of the queuing system.
Save the node model.
In the project view, right click on the subnet and select Edit Attributes (Advanced). Left click
on the “model” value to replace by the node model you have just created.
Note: when you save your project on an external drive, make sure you copy also the node
models you have created. They are not part of the project scenarios, but are additional model
files.
In order to modify a node model to reflect the characteristics of a queuing system:
- To change the arrival rate right click on the source module and Edit Attributes, to edit the
Inter-arrival time (average time between packets).
- To change the bit rate in the Queue server, right click on the queue module and Edit
Attributes, to edit the “service rate”.
- To choose an M/M/2 queue in queuing system 3, right click on the queue module select Edit
Attributes (Advanced). Left click on the “model” value to replace the acb_fifo by acb_fifo_ms
(multi-server). Change the parameter “num_servers” to 2.
Keep a single subqueue.
Include in your report the calculation of the expected values, and the graphs of the measured
performances for each of the 3 Queuing Systems.
Compare the 3 Queuing Systems. Which gives the best average delay performance?
Submit your Opnet model with the 3 scenarios, per email as a zip file attachment.
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