We noted that communication performance of a Bluetooth Low Energy (BLE) connection is heavily affected by the radio interference from other connections or networks. Reliability is becoming a key requirement in BLE for its use in various Internet of Things applications. Hence, there is a widely recognized need for an in-depth study to reveal the parameters impacting BLE reliability under such radio interference. In this article, we investigate how transmission parameters, e.g., number of packets and packet transmission time, influence reliability of the BLE protocol. Specifically, a mathematical model is presented to explore the impact of the transmission parameters on the reliability of a BLE pair under interference caused by other pairs. This mathematical model is able to show the reliability issues from both the side of the BLE connection under interference and the side of the interference itself. The model is validated and novel insights on the common usage of BLE parameters by a wide range of experimental evaluations are provided. Experimental results highlight the correctness of the mathematical model, thus quantify the interplay between transmission parameters and coexistence, also the influence of other related parameters. This research provides a design-level or system-level insight in BLE usage and deployment