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 recently 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, a detail that the standard leaves unspecified so that different vendors can compete. In this paper we provide accurate analytical models for two of these sleeping algorithms, the frame transmission and the burst transmission methods, that stand out for their simplicity and good performance in several simulation studies. The proposed models will help to fairly compare these algorithms and analyze the influence of the configuration parameters and line card physical characteristics in their expected power savings.