The Optimal Packet Length (OPL) in packet-switched communication systems has been studied in the literature from various aspects. In this paper, we consider the trade-off between packet length and data transmission delay in a highspeed communication system. To simplify the analysis of the mean data transmission delay, the model is limited to a point-to-point communication system, in which each node complies with the OSI reference model. In order to study the relationship between the OPL and the number of modules performing each protocol, two model communication systems are discussed. In one each node contains two layered protocol modules, and in the other three. Moreover. for both models, the mean data transmission delay is analyzed for two cases depending on whether or not the DLC layer or the network layer performs retransmissions. After studying the OPL which minimizes the mean data transmission delay in each case, we discuss the relationships between the OPLs and the various protocol parameters.