**5. Acknowledgment**

These researches were supported partially by grants from the National Science Council, Taiwan, ROC, under contracts NSC 94-2215-E-035-003, NSC 94-2215-E-035-009, NSC 96- 2221-E-035-052, NSC 97-2221-E-035-016-MY3, and the Lee & MTI Center for Networking at National Chiao Tung University, Taiwan, ROC.
