In this paper, we present an analytical model for the scheduled access mechanism of IEEE 802.15.6 based Wireless Body Area Network (WBAN) taking into account the impact of non-ideal transmission channel. We provide a thorough analysis and closed form expressions for the frame delay, reliability of data transfer, throughput, energy consumption, and Energy Efficiency (EE) of the sensor nodes with medium to high data rate applications. We validate our theoretical derivations through simulation results. The results show the variation in the performance indicators with respect to different system parameters, such as beacon period, data transmission rate, traffic loads, channel Signal-to-Noise Ratio (SNR), Allocation Intervals (AIs), and Payload Sizes (PSs). Our results indicate the tradeoff between average frame delay and EE with respect to selection of AIs and PSs. Thus, through this analysis we show how a network designer can prepare a look-up table for sensor nodes enabling them to select the optimum and minimum AIs and their associated PSs based on the channel SNR to achieve the maximum EE for a particular average frame delay and reliability constraint.