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Part II: Evaluating Internet Connection Choices for a
Small Home PC Network
We investigate application performance and capacity planning, by changing the link
speed between a home LAN and its ISP.
You can do “what if” analyses (called scenarios in IT Guru) on network designs,
just as you can on spreadsheets with financial business models. However, instead of
looking at “bottom line” financial numbers, you will be looking at how response times,
latency (delays) and other network performance measures will change under different
network design approaches.
To create a network simulation (called a project in IT Guru), you specify the nodes
(computers, switches, routers, etc.) in your network, the links between nodes, and the
applications that will be running on the nodes.
In this exercise, the initial simulation (project) has been built for you. It models a
family’s home PC network, which has three PCs connected to the Internet for game
playing, web browsing, E-mail, audio streaming, and FTP (file transfer protocol). Your
objective will be to conduct a series of what-if simulations (scenarios) to see how
performance differs if the family connects to the Internet using: 1) a slow modem
downloading at 20 kbps, 2) a fast modem downloading at 40 kbps, 3) a cable modem
or DSL line downloading at 512 kbps, or 4) a T1 line with download speed of 1.
Mbps.
For each scenario, you will set the download speed in the simulation model, run
a simulation, and view the results. You will be addressing the question of whether faster
connections are worth higher prices for the home network.
The figure above shows the simulated network.
Each PC connects to the family’s 100 Mbps Ethernet switch via a UTP connection. The switch connects to the router, also via UTP.
The cable modem is not shown; it is implicit in the WAN link connecting the home PC network to the Internet. Three Internet servers provide different services to the client PCs.
Near the top of the figure are two boxes that do not represent physical components: Applications and Profiles. The Applications node contains data about the applications used in the network, such as Web browsing. More specifically, traffic is associated with each application, so there is a difference between “light Web browsing” and “heavy Web browsing.” Internal file service and print service traffic are not shown; these would be too light to make a difference in performance because the Internet WAN connection is the weak link in this network. In the Profiles icon, different applications are associated with different PCs.
The complete topology is laid out and the attributes for all the objects are pre- configured except the link data rate between the Router and the Internet cloud, this is our WAN link.
Step 2: Configure the Link to 20 Kbps
In your first scenario, you will configure the WAN link as a 20 kbps dial-up line.
- Right-click on the WAN link select Edit Attributes.
Here we can see the different link attributes. We will be changing the data rate attribute of this link.
- Click in the Value field of the data rate attribute and select Edit…
- Enter 20000 ; press Enter and then click OK.
Step 3: Configure and Run the Simulation
To simulate this network, we will use high fidelity discrete event simulation. The model simulates client/server application packets, which represent real world network traffic.
- Click on the configure/run simulation button.
- Make sure the Simulation Duration is set to 8 hours to represent a typical day.
- Click Run , monitor the progress bar as the simulation proceeds.
- When the simulation completes, Click Close.
Step 4: View Results
We can now view various statistics including the web application Response Time experienced by the Researcher and the WAN link utilization.
Follow the instructions below to view the statistics.
- Select Show and then click Close in the View Results window.
Performance for PC2 (Researcher PC)
- Right-click on the PC2 Researcher client and select View Results to view the web Response Time and Traffic Received.
- Expand Client Http and select Page Response Time (seconds). Also make sure that the pull-down menu on the bottom right-hand corner is set to As Is.
- Click Close in the View Results window.
- You can use the hide or show all graphs button to hide/show the graphs.
Your results should be similar to the graphs above. The download link Utilization averages about 80% and the upload link Utilization about 2%. With a download link utilization of 80%, this does not give much available bandwidth for potential new applications or users. The Response Time that the Researcher experiences is in the range of 5 to 7.5 seconds, which is painfully long. This slow WAN link is badly overloaded.
Step 5: 40 kbps Scenario
Implement a fast modem connection downloading at 40 Kbps. This is realistic throughput for a modem whose download speed is rated at 56 Kbps.
- Select Scenarios Duplicate Scenario… and name the scenario as 40K_dialup_connection.
- Click OK. This creates a copy of the existing scenario.
Step 6: Configure the Link to 40 Kbps
- Right-click on the WAN link and change the data rate attribute to 40000.
Step 7: Run the 40 Kbps Simulation
- Rerun the simulation. You can refer to the steps given previously for setting the duration and running the simulation.
Step 8: View Results for 40Kbps Scenario
- Follow the same steps mentioned before to view the link utilization , and Response Time by the researcher PC.
- Notice, that the link Utilization is reduced by half.
- The web application Response Time also went down from about 6 seconds to around 2.25 seconds. This is a significant improvement, both, in Utilization and Response Time.
- Duplicate the scenario again and name it T1_connection.
- Change the data rate of the WAN link to T1 from the pull-down menu.
- Rerun the simulation.
Step 11: Compare Results
Rather than viewing the results for the T1 link alone, let us compare the results of Utilization and Response Times for all the 4 scenarios. This will give us a broader picture of the effect of changing the data rate.
- Select Results Compare Results…
- To compare the utilization statistics, choose the following statistics:
- Make sure that All Scenarios is select.
- Click Show. To compare the Response Time , unselect the previous statistics, change the filter on the right-hand bottom corner from As Is to average and then choose the following statistics:
Here are the results
- We can see from these results that as the data rate increases from 20K to 40K to 512K, the Utilization becomes better. Also the Web Application response time becomes better.
