The last few years have seen a surge in the awareness of energy efficiency in many fields, including the networking community. As an answer towards such demands the IEEE has just released the 802.3az norm that provides a way for Ethernet physical interfaces to enter a low power mode when there is no traffic to send. At the same time there are starting to appear proposals for how to govern the physical interface state between active and low power modes. Among them, the burst transmission method stands out for its simplicity and good performance in several simulation studies. In this paper we provide an accurate analytical model for this method that helps to gain insight in its configuration parameters.