Tuesday, October 23, 2012

Comparing Wi-Fi performance using experiments

File downloading via HTTP is one of the common tasks we do in the internet. Delays in file download can be caused by factors such as propagation, processing, file size and traffic on the network. Delays caused by propagation are unavoidable, and therefore our interest is to keep the other delay factors to the minimal level possible. In this paper we design and conduct experiments from two public Wi-Fi spots to identify the effects of key factors that are perceived to be affecting the file download time over the internet. By doing so we explore if ping can be used to identify the file downloading time and the impact of location on file download time. From the results and analysis, we conclude on the comparative performance of the two Wi-Fi spots using statistical models.

INTRODUCTION

File downloading is a common task we do on internet. The factors that affect the delay for file downloading includes the traffic on the network, file size, time and other related things. Nowadays Wireless Networks are made available free of cost at public places by many private business dealers in order to improve the customer satisfaction. Such Wi-Fi hotspots are often jammed with lot of users which causes delay in file downloading.

In this paper we perform experiments on two Wi-Fi spots with a view to understand the comparative performance with regards to file downloading. HTTP is a common internet protocol used for file retrieving. We compare the HTTP file downloading performance in two separate Wi-Fi hot spots at University Mall located near Tampa. Ping request is generally used by network administrators to check the status of remote servers. By default Ping request sends a 32 byte ICMP packets to the server. The server in turn echoes back the same packet. Using the ping request thus we will be able to get an estimate of the total round trip time of the packet. In this paper we also do experiments to understand if ping can be used to predict the file download time.

The related work section gives a brief overview on the research ongoing regarding the performance of file download. It is followed by the experimental setup and results. Finally the results are analyzed and a conclusion is made.


RELATED WORK

File downloading performance has been a major field of research among network scientists in the past decade. Researchers have suggested that slow start in TCP cause delays for small file downloads. Some researchers have suggested that the DNS causes delays in file downloading. The delay in downloading is caused by numerous factors which is hard to theoretically estimate. However time of the day and file size is known to play a major role in deciding the file download time. Prefetching the file at server was suggested by some researchers to improve the performance. It has been also suggested that the performance can be increased when the server rejects requests that exceed a threshold.

EXPERIMENTS

The experiment was conducted in two the free Wi-Fi spot available at the University Mall.

Setup

A HTTP server was setup at the ENB building on USF campus. The C based HTTP web server called Weblite written by Dr. Ken Christensen at USF was used to do the experiment. A UNIX based Client was setup to request the file at the server through HTTP GET request. Two files with size 16 Kb, 484 Kb were respectively used to do the experiment. We assume these files as small and large based on some pre-experimental speed test on the location. Each experiment had 10 HTTP get requests send to the server. Once a request is finished the Client would wait for 100 milli-seconds before sending the next request. In this way ten experiments were done, with an interval of five seconds between each experiment. The experiment was done at two different Hot Spots at University mall, namely Food Court and Center Court. Experiments were conducted during peak traffic hours in noon and during less traffic hours at night. Each experiment has ten sample mean file download time. Ping time was also noted using command line tool during the experimentation and the average of random sample was used.

Results

The results of the experiments are tabulated in table one. It shows the Mean file download time for each of the eight scenarios along with the mean ping time for each experiment. The mean for each experiment is based on the sample download time during the experiment. Separate columns are shown based on the time of day and the file download size.

TABLE I. EXPERIMENT RESULTS


















Comparison

We have the sample mean file download time for the 10 experiment done in the eight different scenarios. Now we would like to compare the results using confidence interval estimate. First we determine the difference between sample means. Then we use T-scores to get the normal distribution. The result is shown in table 2.

TABLE II. COMPARATIVE RESULTS















We also checked if ping can predict the download time. As we know ping gives the total round trip time for 32 bytes of data. Therefore the one-way download time would be half of it. Using the mean ping time for random samples in table one, we can calculate the estimated download time by multiplying the one byte ping time with the file size. The result is shown in Table 3.


TABLE III. PING TIME


As we can see the ping can mostly predict the file download time for smaller files, however for larger files it’s not able to do so.

ANALYSIS

The reason for variation in the file download time over the two locations is not so obvious from the results. Traffic in the network is likely to be one issue. In the food court there were more people in afternoon when compared to the center court. Similarly at night in the center room there were more people than in the food court (since food court was closed at night). The impact of file size is still not very clear, however as the average shows the large file download was slightly faster than small file download. It could be the impact of slow start in the TCP protocol. Similarly the reason why ping is not able to predict large file download time is because of the packet loss observed during the ping experiment. It means that the link is not reliable for long period of time where random delays occur due to unknown factors.

CONCLUSION

The experiments were conducted to compare the two public Wi-Fi spots. The results showed that one of the Wi-Fi was better than the other. Similarly we showed that ping can predict the download time if the random delays during large file downloads can be avoided.


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