Queuing theory and its applications to Telecommunication system. Queueing theory includes retrial queue, QBD method and many other queues related to elecommunication. Performance analysis of telecommunication system includes
characteristic of various multimedia traffics, delay and loss probability of networks and schemes to improve the performance of networks.

1972~1983 Fulltime Instructor, Assistant Prof., Kyungpook National Univ.
1983~1999 Professor, KAIST
1989~1990 Visiting Prof., North Carolina University
1995~1998 Director, Center for Applied Math., KAIST
1991~1992 Editor-in-chief, Korean Math. Society
1993~1994 Secretary in general, Korean Math. Society
1996~ Adjunct Prof., Yeonbeun Univ. China
1996~ Representative, Asia-Pacific Bernoulli Society
1998 Chairman, International Conference on Probability
1999~ Prof., Korea University
1999~ Chairman of Probability Committee, Korean Math. Society
2001~ Director, Telecommunication Math. Research Center(ITRC), Korea Univ.

1997 Academic Award, Korean Math. Society
2000 Best paper Award, IEE, United Kingdom
2001 Seoul Culture Award(Basic science)