TDDC22 Mobila trådlösa nätverk: system och tillämpningar

Laboration 1

Effect of MAC on the transmission performance: hidden and exposed terminal problems

Introduction to mobile networks simulations using Glomosim

Glomosim is a simulator for mobile networks developed at UCLA (University of California at Los Angeles). The software is freely available for universities and for research purpose and can be obtained if you have a university e-mail. More information can be obtained from the simulator web site: http://pcl.cs.ucla.edu/projects/glomosim/. Different links from this site lead to description of simulator as well as papers that describe the utilization of the simulator when testing different protocols and evaluate performance of mobile networks.
The simulator is written in Parsec C that is a programming language similar with C. Since a complete documentation about the functions used in the simulator was not available at this time the lab series will not implies developing code for the simulator.
The documents included with the code distribution contain most of the information that you need in order to complete the labs and understand how the simulator works. For a faster and easier understanding, I compiled some of this information and you can find it in the resources files below. You have to be aware that since the Glomosim code is in continuos development, some of the information in the files may be outdated or may not be included. However, all the information that you need for completing the labs are present in the documentation or in the associated text files from the Glomosim directory. A study of the simulator and its component files may be needed in order to have the right level of understanding.
The simulator also comes with a visualization tool that allows the user to visualize the experiments and is very useful in analyzing the behavior of mobile networks. The tool is called VT, is written in Java and connects with the Glomosim code. You will not need to modify the tool for working with the simulator but you may do so if consider that the modifications will add clarification and will enhance its functionality.
As soon as understood, both the simulator code and visualization tool code can be easily modified. However, the scope of the lab is to understand and evaluate the functionality of several scenatios for wireless mobile networks and the parameters that affect their functionality.

You are advised to read the documentation beforehand!

Resources files:

• A Comprehensible GloMoSim Tutorial - en sammanställning från flera källor om Glomosim, av Jorge Nuevo, INRS
• Glomosim short manual
• Description paper about Glomosim
• Slides about Glomosim
• Configuration file example
• Application setup file example
• Visualization Tool Short Description
• Nodes positioning file example
• Nodes mobility setup file example
• Example of static routing setup
• Example of setup a wired network
• Coding guidelines

Running the programs :
1. The Glomosim simulator is situated on each computer in "Program Files/Glomosim/glomosim". The files that you need to modify are in the "bin" directory.
First thing that you have to do is to make a copy of this directory ("bin") on your local drive in order to save the original settings. After finishing the lab, you are require to restore the "bin" directory as it was at the beginning of the lab!
For running the simulator, just open a command shell, go in the "bin" directory of the "glomosim" directory and type "glomosim config.in", where the "config.in" is the default configuration file of the simulator. All this issues are also presented in  Glomosim short manual.However, you did not need to compile the files! They are already compiled! You can also have different configuration files as long as they respect the structure of a configuration file.
2. The Visual Tool  (VT) used with Glomosim is situated in the "java_gui" directory of the "glomosim" directory. As presented in the description of tool, for starting the tool you have to start a command shell, go in the "java_gui" directory and type "java GlomoMain&". Again, the files were already compiled for you. For visualize the simulation from the VT, I advice you to use the option "Play Back" of the VT. There you need to specify the trace file of the simulator. The trace file is basically what initially the glomosim simulator output on the screen. Simply set a trace file name when running the simulator (e.g. type this command in the command shell from the bin directory: "glomosim config.in >my_trace"). Then use this file when visualize the simulation.

Effect of MAC on the transmission performance. Hidden and exposed terminal problems

Your assignment will use your knowledge about Glomosim in analyzing the effect of MAC schemes on the transmission. Two very important problems related with the utilization of different MAC schemes in mobile networks, the hidden terminal and the exposed terminal problems will be studied.
You have to complete a series of experiments and answer a series of questions.
Then, you have to write a report about the network behavior when changing different parameters and using different applications. The reports must contain the description of your experiments and the resulted communication characteristics such as the number of lost packets, the throughput and transmission delay (i.e. time difference between sending moment and receiving moment).
In all tests use the VT to visualize the transmission patterns. Note that VT represents the transmission of all packets, that is both data and control packets!

a) Check the literature related to the hidden terminal problem. Then, set up a hidden terminal environment with 3 nodes (0,1,2) with node 1 as the central node. Please observe that the transmission distance of a node depends on the type of propagation model selected. For example, if the propagation option is "PROPAGATION-PATHLOSS FREE-SPACE" a distance of more than 630 meters between two nodes is necessary for avoiding a node receiving from the other. (If you are not sure about the needed distance use a simple CBR scenario with two nodes for testing the transmission distance.) The exact radio range of the nodes can be calculated using the program "radio_range.exe", which can be found in the "bin" directory. Similar with running the simulator you need to specify the configuration file when runing the program, e.g. use "radio_range config.in".

Further, use static routing for modeling the transmission of the nodes and CBR as the application and simulate the hidden terminal problem. Static routing is used to avoid other routing packets to be transmitted and to get familiar with this option of the simulator. With static routing you set the transmitting and the receiving node manually. No routing of messages between nodes is then performed. (Examples of static routing are provided in the Glomosim files.) Note that when using static routing one specifies which nodes are allowed to communicate what other nodesand does not model "real" routing. (Thus, writing 1->2 and 2-> 3 does not mean that a message from 1 will eventually go to 3!)

Then, use CSMA as the MAC scheme and simulate the messages exchange in a hidden terminal scenario by sending 500 messages of 200 bytes from/to the corresponding nodes. Send first the data with the same moment as the start moment and then delay the transmission of a sender with 10 seconds to make more visible the problem. Monitor and record the throughput and the eventual delay. What are the major conclusions?
Try to modify the application parameters to obtain a better behavior of the network (e.g. use different transmission intervals for sender nodes).

Change the MAC to MACA and then to 802.11, repeat the experiments and observe the differences. Pay attention to the fact that control packets are sent in these cases! Represent graphically the use of the 3 MAC schemes. Which one performs better in terms of delay and throughput? Can you explain why? Which of the MAC schemes provide the greatest fairness for the communication (e.g. does not allow a sender node to entirely "capture" the channel and not allow other senders to transmit)

b) Check the literature related with the exposed terminal problem. Set up an exposed terminal environment with 4 nodes. Perform the same tests as in case (a) and respond to the same questions.

Hint: You can find descriptions of CSMA, MACA and 802.11b MAC schemes as well as about the hidden and exposed terminal problems in the books given in the course literature such as Tanenbaum, Stallings and Tok. Resources on the web may also be easily found using any search engine. An example are slides from the PARSEC workshop such as http://pcl.cs.ucla.edu/slides/workshop99/Ken-pw99/