CAPÍTULO II : MARCO TEÓRICO
2.1. RED GLOBAL DE BANDA ANCHA (BGAN)
The performance evaluation of the E-training system was focused on the quality of the transmitted tracking data by measuring latency, data loss and bit rate in the transmission. The quality and rendering speed of the 2D ultrasound images for an individual simulator have been detailed in Chapter 7. In this section, we first explain the experimental conditions, then we present the results of transmission latency, data loss and bit rate under a set of defined test conditions.
8.2.1 Experiment design of the E-training system
The E-training system is intended to work in two major types of networks, i.e., cellular networks or 802.11 wireless networks. Currently, major wireless carriers in the United States have upgraded their cellular networks to 3G/4G. Accordingly, we have only been able to test our system in 3G/4G networks. The carrier’s channel access technology was not considered in our evaluation. For 802.11 wireless networks, the most common scenario is that an end-user accesses the internet through a router at his/her home, clinic or office; hence, we have only tested the system in a router-based wireless network. The current E-training system was designed to support a limited number of users in a given training session, and we tested it with the minimum number of participants, specifically three simulators (one instructor and two learner simulators), under the following three conditions.
A. All simulators in wireless networks. B. All simulators in cellular networks.
C. Same condition as A, except that the data from the operator simulator were routed via a laptop computer located in China.
The above three conditions covered most of cases where the system would be operating. Condition C was intended to simulate the case where international learners participate in the training. The test in each condition lasted 3 to 5 minutes. The hardware configurations of the three computers running the three simulators are described in Table 8-1. All three computers had 64-bit Windows 7 and Intel HD graphic cards installed.
145
Table 8-1. The summary of the three computers used in the experiment.
Computer Identity CPU Memory
0 Instructor Intel i7 2.9 GHz 8 GB
1 Learner Intel i7 2.4GHz 8 GB
2 Learner Intel Xeon 3.2GHz 16 GB
The test matrix includes three performance parameters:
(1) Bit rate: The operator simulator updates tracking data approximately 25 times per second to guarantee a smooth visual experience. Each update contains less than 100 bytes of tracking data. This is a very low bit rate so that we tested both the peak bit rate and average bit rate.
(2) Data loss: The E-training system uses the UDP protocol for transmission of tracking data. A significant loss of tracking data not only makes 2D images display on the simulators loose synchronization, but also degrades the quality of an image stream and the diagnostic utility (as would be encountered with skipped frames).
(3) Latency: This is an important factor that affects the degree to which the simulated 2D image rendering is synchronized between the operator simulator and any of the observer simulators. Given that we were not able to synchronize the system clocks of the three laptops to millisecond level, we measured two-way transmission latency instead of one-way latency.
8.2.2 Experiment results and analysis
The test results are summarized in Table 8-2. The average bit rate under all three conditions was approximately 3-4 kB/s. The data loss was less than 1% and no frameskip was detected in any of our experiments. The results show that the tracking data from the operator simulator usually reached the observer simulators in less than 100 ms so that the transmission latency did not negatively impact the quality of the image stream. In other words, the 2D images on all simulators could be considered to be synchronous.
146
Table 8-2. The summary of the experiment results.
Condition Bit rate Data loss Two-way Latency
A 3 – 4 kB/s < 1 % 50 -150 ms
B 3 – 4 kB/s < 1 % 100 -200 ms
C 3 – 4 kB/s < 1 % 200 -400 ms
Fig. 8-3 shows the bit rates over time under Condition A. The red line represents the upload (transmitted) bit rate of the operator simulator while the blue line represents the download bit rate of the operator simulator. In our experiments, the bit rate remained nearly constant, between 3-4 kB/s, while the user was performing the scan on the operator simulator; the bit rate was less than 1kB/s when no scan was performed. The spikes in the blue and red lines in Fig. 8-3 resulted from other Windows back-end programs, rather than the tracking data from the sham transducer. An approximate constant bit rate is important to ensure that the 2D images remain synchronized even in low speed networks because bit rate spikes are one of major causes making the system out-of-sync. Given that all simulators in the E-training system operate exactly in the same way irrespective of which of the three network conditions was selected, we did not measure the bit rate over time in Conditions B and C.
Fig.8-3. The E-training system bit rates (the red and blue lines represent the total upload and download bit rate, respectively, on the operator simulator).
Our experiment showed negligible data loss (less than 1%) and smooth 2D image display under all three conditions. We were unable to evaluate the data loss in a 2G cellular network or dial-up internet because of test limitations. Instead, we designed an
147
additional experiment to determine the maximum data loss that does not impact the visual smoothness of an image stream, by using a normal distribution function to determine whether a given tracking data packet would be discarded or not during the transmission. Our experiment showed that there was no observable frameskip if the tracking data loss was less than 35%. This evaluation was performed under Condition A.
The latencies under the three conditions were not exactly identical, but they met our requirement that the E-training system was operationally synchronous, meaning that human observers, looking simultaneously at the screens of the operator simulator and an observer simulator, could not detect any difference between these two displays. The two- way latencies for the three test conditions are shown in Fig. 8-4, where the left and right columns are the packets’ two-way latencies of Computers 1 and 2, respectively. It can be seen that the one-way latency is less than 100 ms for 90 % of packets under Conditions A and B. A latency of 100 ms has been widely accepted as the threshold to distinguish between detectable and indiscernible latency. In other words, we can consider the E- training system to be synchronous. In Condition C, the one-way latency mostly ranges from 100 – 200 ms. Although it is larger than the 100 ms threshold, we did not observe 2D images to be out-of-sync in our experiments.
148
149