A CSMA/CD local area network consists of single server (the channel) and multiple interacting queues of message packets. The message queueing process in a buffered, P-persistent CSMA/CD system is modeled as a multidimensional semi-markov chain. An effective approximation method to compute the mean packet delay in equilibrium is developed, based on the joint probability generating function of the queue length vector at embedded Markov epochs. We also develop a simulation model to validate approximation results. To the best of our knowledge, this work is the first in the literature that enables optimization of the control parameter P for the CSMA/CD system with more than two users. (Author)