New approach in evaluating virtual labs safety into the frame of real-time DAQ based on Wi-Fi TCP/IP protocols.

Risteiu, Mircea ; Croitoru, Bogdan ; Boca, Loredana 等

1. INTRODUCTION

Distance learning has also several economical benefits. In fact, the classical educational model requires the usage of a large amount of resources, in terms of people, space, travel, and time for teaching. In certain disciplines, especially those related to engineering, physics and chemistry, a practical laboratory experience is required to complete the course of study and corroborate the theoretical parts with direct experience. For all this situations the network-level round trip time (RTT) experienced by highly interactive applications is becoming increasingly important. It becomes an emergent reason when the applications demand for remote interaction, or just a simple case of data acquisition. In our approach we have built a virtual lab for vibration measurement on a railway bridge. Because of local constrains, the DAQ implementation uses wireless transceiver with 802.11b/g protocol.

In this paper we are focusing on developing measurement-based virtual lab with remote data acquisition. Overall, we are analyzing protocol and data sent packets from the safety point of view.

2. RELATED WORK

They are some implementations of virtual labs with embedded controllers for optimizing the buffering process versus sampling rates (Compact RIO) through powerful FPGA processing phase. The wireless connection, for data applications such as PC-to-PC connectivity, 802.11 wireless local area networks (WLANs) have proven to be the preferred solutions. Is this idea, an example is the development of the reconfigurable analogue ASIC front-end, designed to provide self-test and auto calibration facilities (Taner, 2007).

On the other hand, as an indispensable spaces for developing practical activities (Gai, et al., 2001), the general structure used lab is based on the experience gained in designing and developing Virtual Laboratories. In some implementation (Taner, 2007), for real time constrains, the designers use a hard real-time kernel, called Shark, that can be tested on a real application to verify the sensitivity of timing constraints (such periods and deadlines) on the control performance (G'unther, 2005). The connection to the Internet is based on Ethernet TCP/IP receiver. Another class of implementation is based on CORBA (Common Object Request Broker Architecture) management of communication protocol.

3. MODEL DESIGN

A real-time simulation is a real-time system where some portion of the environment or portions of the real-time system itself, are realized by simulation models (Risteiu et al., 2008).

[FIGURE 1 OMITTED]

The scenario of the model is based on the authentication protocol (extended and lightweight) in witch real time data acquisition systems can be found. For safety evaluation reason, we are separating authentification sessions by the data sending (see figure 1).

4. IMPLEMENTATION

4.1 Managing the time and memory in FPGA modules

Our data acquisition system uses Xilinx Spartan XC3S400-FPGA Controller. IP blocks conditioning a signal for output or input or accessing VI scoped memory should be written such they can access as little or as much I/O necessary for any application. This reduces the FPGA footprint for each piece of IP by not including additional code for additional inputs, outputs, or memory spaces.

For using Single-Cycle Timed Loops function in a single-cycle Timed Loop, the developer must set the Write option in the Memory Properties dialog box to Arbitrate if Multiple Requestors Only or Never Arbitrate for the memory item, and this function cannot be used with the same memory item anywhere else in the FPGA VI because the Memory Write function takes an entire clock cycle to execute.

4.2 FPGA-based data acquisition with 802.11g controller

The built-in real-time controller is used as a bridge for remotely programming of the FPGA using high level FPGA development application. Every programmed FPGA is used as unique data acquisition or signal processing and manipulation board.

[FIGURE 2 OMITTED]

The controller support high-speed wireless networking of up to 108Mbps, while providing seamless interoperability with all existing IEEE802.11g and IEEE802.11b wireless equipment. It uses standard PCI interface, compliant with PCI 2.1, 2.2, 2.3, with fully compliant with IEEE802.11 b/g 2.4GHz. Figure 2 shows the general scheme of the DAQ used in our experiments.

5. VIRTUAL LAB CONTROL MEASUREMENTS

Running the network monitoring software, data is sent via 802.11b/g two kinds of protocol data: TCP (Transport Control Protocol) and UDP (User Datagram Protocol). For information that needs reliability, sequence transmission and data integrity-TCP is the transport protocol to use. For data that require realtime transmission with low overhead and less processing-UDP is the right choice (Giuseppe & Giorgio, 2003).

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

Here they are records of running experiments in two different situations: a) only data acquisition (from DAQ to server come TCP packets), b) buffer size control (from server to DAQ leaves UDP packets). Figure 3 shows the Front Panel of the VI used to control DAQ parameters, parameters wich have great influence on network monitoring software as it is shown in figure 4. During the user controlling action they are many UDP frames (b), and only few TCP frames (a). In this case, the user is acting for programming the buffer size of remote FPGA. The action consists of a number of bytes (less than 2000 bytes, but in many frames). The TCP frames are used to transport measured data from remote device to server-main application from PC (Young, 2003).

For this purpose we are filtering only UDP frames in network monitoring software because we want to evaluate unsafe captured packets. Each time, the virtual lab application starts, some UDP frames are sent to remote device (for DAQ only situation). Each time, the user commands the remote device to modify its buffer size, other UDP frames are sent. What is happening when emerging water pumps is controlled to start? Because of 802.11b/g standard, for pump starting command is used UDP protocol. It means that we have not confirmations that pump starts. Now we proceed with a deep analyze of virtual laboratory working. For instance, virtual lab runs continuously in different loops situations. In our experiment we have established 10, 20 loops (figure 4).

6. CONCLUSION

Every time a VI is running two UDP frames (packets) are sent from the server application to the remote DAQ device, representing some initializations. These UDP packets are blind sent, that means we are receiving no confirmation of packets arrival. Because we are receiving no arrival confirmation for packets, we don't know if the buffer size has been changed or not. On the opposite side, DAQ device is receiving commands and requests to begin data acquisition. All recorded data are sent to computer as TCP packets with arrival confirmation.

As far, the only TCP is safe network protocol, even only 1/7 part of TCP packet is data, it is recommended to use it when the situation requires safe transmission.

If the implementation requires confirmation of controlling sequence, we propose to implement a watchdog or a complex application to confirm arrival of remotelly sent commands.

The next approach is guided to object oriented programming, to the service-based programming, in order to analyze the behavior of the situation when a parent application asks a child application, called as SubVI.

7. REFERENCES

A.H. Taner & J.E. Brignell, (2007), Virtual instrumentation and intelligent sensors, Sensors and Actuators A: Physical Volume 61, Issues 1-3, June 1997, Pages 427-430, Proceedings of EUROSENSORS X

Andr'e G'unther & Christian Hoene, (2005), Measuring Round Trip Times to Determine the Distance between WLAN Nodes, Proc. of Networking, Waterloo, Canada, May 2005, Pages 827-835

Giuseppe C. & Giorgio B., (2003), A Virtual Laboratory Environment For Real-Time Experiments, Proceedings of the 5th IFAC International Symposium on Intelligent Components and Instruments for Control Applications (SICICA 2003), Aveiro, Portugal, July 9-11, pp. 39-44

M.Risteiu, I. Ileana & A. Tulbure, (2008), Practical approach of using embedded controllers for virtual instruments over TCP/IP and 802.11b/g protocols, Annals of DAAAM for 2008& Proceedings of 19th International Symposium ISSN 1726-9679, Vienna 2008, pages 1179-1181

P. Gai, L. Abeni, M. Giorgi & G. Buttazzo (2001), A new kernel approach for modular real-time system development, Real-Time Systems, 13th Euromicro Conference on, 2001, Volume, Issue, 2001, pages:199-206, Digital Object Identifier 10.1109/EMRTS.2001.934032

Young K. Cho., Xiaolin Hu & Bernard P. Zeigler, (2003), The RTDEVS/CORBA Environment for Simulation-Based Design of Distributed Real-Time Systems, CiteSeer/x/beta