The ultra wide band transmission present the effective solution for the precise ranging and high data rates applications. Standard IEEE 802.15.4 defining PHY and MAC layers for the low power wireless devices was in 2007 extended by amendments aiming to include the ultra wide band frequency bands in order to react on the demands for the ranging and high data rates low power applications. This standard is referred to as IEEE 802.15.4a. We have proposed the mathematical model of the transmission duration estimation. The mathematical model was applied at the set of configuration parameters such as length of the synchronization preamble, pulse repetition frequency and payload size in order to comprehensively analyze the system throughput at the physical layer.