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Petri Net assignment
General Information
- The due date is Wednesday 5 November 2003, before
23:55.
- Submissions must be done via WebCT.
Beware that WebCT's clock may differ slightly from yours.
As described on the Assignments page, all results
must be uploaded to WebCT and accessible from links in the
index.html file. There is no need to upload AToM3.
- The assignment must be made in groups of 2 people.
It is understood that all partners will understand the
complete assignment (and will be able to answer questions
about it).
- Grading will be done based on correctness and completeness
of the solution. Do not forget to document your requirements,
assumptions, design, implementation and modelling and
simulation results in detail !
- Extensions, if given, will involve extending not only the
alotted time, but also the assignment !
The assignment
You will use the meta-modelling environment AToM3 (which
you also used for the first assignment), but now load the Petri Net
formalism to interactively construct a Petri Net model.
The system to model consists of a simple traffic sytem depicted
below.
Initially, there are INIT cars in the cars part of the
system. All other parts of the system are empty. The cars
part actually spans the top loop (above the top intersection)
as well as the bottom loop (below the bot intersection) and
can hold an arbitrary number of cars. The merging of cars coming from
top and from bot should not be explictly modelled.
Cars enter the centre loop of the system from the cars road segment
and proceed through the system as indicated by the arrows.
Intersections bot and top can only hold one car at a time
(more than one car would mean a collision).
Road segments turn1 and turn2 also have capacity 1.
In intersection top, there is a choice between going straight
(joining the cars section) or going back to intersection
bot via turn2. The car/driver actually makes the
decision whether to go straight or turn left while in
turn1.
The assignment consists of three parts.
- Using AToM3, build and document a Petri Net model of the
behaviour of this system. Documentation must describe how
you've addressed the different constraints given above.
Include an image of this model in your solution.
- Perform a few simulation steps of this process if the
number of cars in the system INIT=2, and comment.
- For INIT=2,
- Is the system bounded ?
- Does deadlock occur ?
- Is the system live ?
- Which quantities are conserved ?
Answer these questions by means of the reachability graph for
this Petri Net.
Include an image of this reachability graph in your solution.
- From which number of cars INIT onwards do you expect
deadlock to occur ?
For that INIT, using AToM3, build the reachability graph.
Include an image of this reachability graph in your solution.
Show occurrence of deadlock on this graph.
Constructing Petri Net models in AToM3
Download AToM3-2.2.tgz archive.
Note how this is a new archive (not the same as the one for
the first assignment). In particular, it contains the necessary
directories for dealing with Petri Nets as well as Reachability
Graphs.
Expand the archive locally (for example with the command
tar -ungzip -xvof AToM3-2.2.tgz if you're working on a
UNIX machine.
This will create a directory AToM3-2.2.
To start the AToM3 environment, python ATOM3.py.
python should be at least version 2.2 of Python
(this is installed in the SOCS labs). On UNIX, the script
atom3 is a shortcut for the above command.
On windows, you can just click on the ATOM3 icon to launch it.
AToM3 will be started with the PetriNets formalism
loaded. You can either build your own model (and
File/Save it) or File/open an existing one.
In the PNModels/models directory, you will find some
examples.
File translated from
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version 3.40. On 27 Oct 2003, 22:24.
