UMTS Validation 66 

Model validation is the process of determining if a simulation model is representative of the real system. A simulation can be validated using expert intuition, real system measurements or theoretical results [37]. Comparing simulation outputs and measurements from a real system is the most reliable way of validating a simulation model. Real system measurements were not available through the course of my PhD in Brunel University, since resources were not available for prototype 3G wireless equipment in the Centre for Media Communication Research (CMCR) lab/Electronics & Computer Engineering department. Comparing simulation and theoretical results was the best option used to validate the simulation model. Theoretical analysis of the system was conducted using FTP application traffic.

To validate the video file transfer within the UMTS access network, a test was conducted with one wireless client data user running FTP application. The system consists of a wireless node operating in an infrastructure Basic Service Set (BSS), which is a set of all stations that can communicate with each other. The FTP file size was increased in order to vary the network

traffic load. The FTP download response time was measured for each increment of the FTP file size and compared with the analytical results.

Figure 2-44: Analytical Model

The block diagram in figure 2-44 was used to construct the analytical model and determine all of the delays that an FTP packet will encounter in the system. The following assumptions were made in order to make the analytical model tractable.

Server Network: From 3GPP TR 25.853, the one-way delay in a 200 km link between SGSN and the GGSN would be 800μs; therefore, a total round-trip delay of 2ms was chosen between the UMTS access network and the server.

TCP Setup, Slow-start and Window size

The delays from TCP connection setup and the slow start algorithm were assumed to be constant. By assuming an infinite TCP window size, the delays from the TCP were minimized.

Node Processing Delay

The node processing delays were considered to be small as compared to the link transmission delays. Therefore, node processing delays were assumed to be zero.

Number of Packets

The number of packets was calculated by dividing the file size by the maximum segment size (MSS). The MSS for the access network was assumed to be 1500 bytes.

_ _ / (2-3)

UMTS timing and overheads

To simplify the timing overhead calculations, the term data exchange slot was defined as a complete data exchange including 2 where SIFS means short inter frame space. UMTS Interval included ten data exchange slots. The _ _ _ was

number of data exchange slots within an Interval Repetition [39]. The term _ _ _ was defined as the ratio of the data to overhead within a data exchange slot. The following equation defines this ratio.

_ _ _ (2-4)

Where,

_ _ _

_ + _ (2-5)

Packet Delay

Using the terms defined above, the per packet delay was defined in equation below:

_ __{_} * _ _ _ _ _ _ (2-6)

FTP Response time

The FTP response time was defined in equation below:

_ _ _ _ _ (2-7)

The theoretical results were calculated in MATLAB using equation (2-7) and the parameters defined earlier. Figure 2-45 shows a comparison between the theoretical and simulation results for various file sizes. The simulation results are very close to the theoretical results. The difference between the theoretical and the simulation results can be accounted for by the inability of the simple analytical models to capture the full effects of TCP. The similarity between the analytical and simulation results validates the correct operation of the data transfer for the UMTS access network. All aspects of the model passed the verification and validation process.

Figure 2-45: Simulation vs Theoretical results

2.12 Summary

In this chapter, we have presented some background information about wireless technology, evolution from 1st to 4th generation networks. We have also presented background information on WiMAX, WiFi and UMTS technology. The thesis examines the behaviour of UMTS networks and protocols and provides basic simulation procedure for UMTS by using OPNET Modeller simulation package. We use five wireless terminals located in various location within Brunel University as shown in the simulation environment.

UMTS model verification was also carried out to determine if our simulation model functions correctly and all the results obtained therein are correct. This includes such tasks as debugging the codes, testing for errors, and testing the functionality of different modules. Each node and process in the simulation were tested to verify that it functions correctly. UMTS model validation was done by comparing simulation and theoretical results.

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