AcknowledgmentsThe authors would like to acknowledge Michael Frey, from the NIST Information Technology Laboratory, for his willingness to discuss the concept of the measurement system and his assistance in the uncertainty calculations for the measurement system. AbstractAccess time generally describes the time associated with the establishment of a talk path upon user request to speak and has been identifed as a key component of quality of experience (QoE) in communications. NIST's Public Safety Communications Research (PSCR) Division developed a method to measure and quantify the access time of any push-to-talk (PTT) communication system. This measurement method is a follow-on development to the mouth-to-ear (M2E) latency measurement system presented in Ref. [1]. Here, a broad defnition of access time is created that is applicable across multiple PTT technologies.In this paper, a speech intelligibility-based access delay measurement system is introduced. This system measures the Modifed Rhyme Test (MRT) intelligibility of a target word based on when PTT was pushed within a predefned message. It relies only on speech going into and coming out of a voice communications system and PTT timing, so it functions as a fair platform to compare different technologies. Example measurements were performed across the following land mobile radio (LMR) technologies: analog direct and conventional modes, and digital Project 25 (P25) direct, trunked Phase 1, and trunked Phase 2 modes.QoS quality of service. i, 1, 2, 6, 7 RMSE root mean square error. 21SUT system under test. 1, 9, 16, 21, 30 TIA Telecommunications Industry Association. 6, 7, 10 UE user equipment. 3, 5-7 iv ______________________________________________________________________________________________________ This publication is available free of charge from: https://doi.org/10.6028/NIST.IR.8275 Symbolsα Intelligibility scaling factor. 10, 14, 15, 20, 23 I 0 Asymptotic intelligibility. 10,[13][14][15] 23, 24, 29 λ Logistic parameter, intelligibility curve steepness. 15, 20,[23][24][25] 27, 29 L w Word length. 13, 14 P 1 First utterance of MRT keyword. 12-14, 21, 22, 29 P 2 Second utterance of MRT keyword. 12-14, 21, 22, 27, 29 T Time preceding P 1 and P 2 in audio clips. 11-14, 18 t Word invariant time. 14 t 0 Logistic parameter, intelligibility curve midpoint. 15, 23-25, 27, 29 τ A Access delay, function of α. 14, 15, 25, 27 T ptt Time PTT pressed within an audio clip. 14, 15

of the National Institute of Standards and Technology's (NIST) Public Safety Communications Research (PSCR) Division for his leadership and direction in developing measurement systems and properly communicating test results to the public. Finally, the authors would like to acknowledge the mathematical contributions and helpful revisions provided by Cara O'Malley and William Magrogan of NIST PSCR.

Steve Voran for his wealth of knowledge on audio quality and willingness to discuss new ideas and pitfalls. The team would also like to thank Don Bradshaw for his vocoder and audio signal insight.