Comparison of performance of virtual coupling schemes for haptic collaboration using real and emulated internet connections

Networked haptic virtual environments (NHVEs) are those in which multiple users collaborate and experience force feedback at the same time. The robustness of such system s needs to be tested under various network conditions that closely mirror the Internet. Previously, we had proposed three virtual coupling schemes to maintain position coherency in a NHVE, which were tested using constant and then time-varying delays using the actual Internet through UDP packet reflectors. In this paper we present the results of comparing performance of the virtual coupling schemes for a time varying delay emulated using the popular network emulator NIST Net, with delay conditions that existed during our real Internet experiment to Italy. UDP was used for haptic data communication because of the high transmission rate requirements for NHVEs. Experiments were conducted for three fixed packet transmission rates of 1000, 500 and 100 Hz, and their performance compared using an independent-samples t-test to the data obtained using the Internet. Locally, the haptic update rate was maintained at 1000 Hz during the experiments. Our results show that the NIST Net was a suitable emulator for testing with lower packet transmission rates. At the transmission rate of 1000 Hz the performance of the virtual coupling schemes were significantly different from that of the actual Internet experiment. I. I NTRODUCTION Networked Haptic Virtual Environments (NHVEs) are suited for very diverse applications —from networked games to surgical [1] and maintenance procedure training [2]— sin ce they allow multiple users to collaborate on a shared virtual space and experience force feedback at the same time. In this manner, they manage to enhance game realism in networked games, improve time management in aircraft assembly, and sharpen training skills for surgery. There are, however, several obstacles in the implementatio of NHVEs: the overall system needs to remain stable and the position coherency between copies of the virtual object mus t be maintained. The former is important especially in bilate r l systems, since they are propense to destabilize rapidly whe n faced with time delays, and the latter is needed to prevent the virtual objects from drifting apart. In addition, the ha ptic update rate needs to stay at 1000 Hz to be able to feel stiffer objects [3], a condition that can consume a consider abl portion of the network bandwidth. In order to enable haptic collaboration, NHVEs are connected to each other in distributed (peer-to-peer) or centr alized (client-server) architectures through either a dedicated network or the Internet. Therefore, testing the robustness of a NHVE system requires realistic network conditions, while a labo r tory setting allows the system to be fine-tuned for performan ce. To this end, network emulators such as NIST Net, Dummynet and Hitbox are used to locally recreate network conditions. These systems can also be helpful to test teleoperation and other systems sensitive to delays. NIST Net is a Linuxbased open source software package which allows the user to define the delay table and configurate the mean delay, standard deviation, bandwidth, packet drop percentage, pa cket duplication, etc., presenting both graphical and command l ine interfaces. The client-server structure is capable of maintaining posi tion coherency but adds round-trip delay in between the clie nts and the server, since the central server manages only one copy of the virtual object. On the other hand, in a peer-topeer architecture each peer maintains a copy of the virtual object and updates it locally, resulting in half the delay bu t increasing the possibility of position discrepancy among t he virtual objects. A. Position Coherency We define position coherency in a NHVE as the position difference between the multiple copies of a virtual object a t a single instant of global time. In our previous work we proposed virtual coupling schemes to enforce position coherency in a NHVE and tested it with constant time delays [4], compared two virtual couplin g schemes (a peer to peer architecture and a client-server one ) on a global scale Internet connection [5], and implemented network emulator capable of Internet-like features [6] for haptic data communications. B. Goals of this study Our objective for this paper is to compare the performances of the three virtual coupling schemes with a time varying network delay condition emulated using NIST Net to actual Internet network conditions at three fixed packet transmiss ion rates. The delay value and the experimental parameters are chosen so that the experimental system would result in stabl e operation.