Introduction
A ordable and ubiquitous Internet connections arerapidly linking together laboratories and o ces acrossnations and continents. Furthermore, Internet brows-ing programs are fundamentally changing the ways weuse computers to display and exchange data. Thesetwo factors are making Internet the ideal testingground for sophisticated new applications, such asvideo-conferencing and scienti c experiment manage-ment. Internet browsers owe their success to innova-tive data visualization and communication technolo-gies. However, these technologies cannot yet supportthe development of Internet programs for direct con-trol applications, such as haptic interfaces and teler-obotics. These protocols introduce random time de-lays between packets which would make a control sys-tem unstable if it were to close the control loop overthe Internet. Such a control system cannot even bedesigned, since there are no analysis or design toolsfor modeling the e ects of packet switched communi-cation on a digital control system.This paper summarizes the issues related toInternet-based telerobotics, presents the current ap-proaches proposed for the the design of control pro-cedures using Internet networks, and summarizes theresults of experiments to identify the performance ofInternet connections.2 Network Communication ProtocolsInternet-based control systems must rely on theavailable communication protocols to exchange realtime data between the controller and the process. To-day, most network protocols provide a transparent andreliable support for data exchange among computersby using the Transmission Control Protocol (TCP).This protocol provides a full duplex stream service,with automatic error handling, retransmission, packetre-ordering, and guarantee of safe delivery. However,from the point of view of a real-time application, thisprotocol has the drawback of having unpredictable ar-rival time of the data.This limitation can be overcome by using the UserDatagram Protocol (UDP), which does not requireany acknowledgment message between sending andreceiving processes, and therefore it is not blocking.However, UDP does not guarantee data delivery, sinceit provides no feedback from the receiver about lostdata packets.The TCP mechanisms ensuring data delivery can-not be skipped, and therefore real-time applicationscannot be implemented using the TCP protocol. UDPinstead, is a possible protocol for real-time applica-tions, since is designed for single-datagram exchangeand o ers a faster access to the network 6].UDP relies on packet switched techniques to send1
data over the network, therefore it is also a ected bydelay jitter on data arrival. Data sent at a constantrate over a packet switched network may, in fact, ar-rive to their destination with a variable inter-arrivaltime. This jitter is due to the combined e ects ofbu ering in routers and of di erent routing policies.A second possible problem of packet switched com-munication is that some packet may never reach itsdestination, when noise or high network load corruptthe data stream
hey see the full seminar report and more details
http://citeseerx.ist.psu.edu/viewdoc/dow...1&type=pdf
http://ace.ucv.ro/sintes13/SINTES13_2007...lenary.pdf