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

Mouth-to-ear (M2E) latency describes the time it takes speech input in a voice communication transmit device to be output from a receiving device, and has been identifed as a key component of quality of experience (QoE) in communications. NIST's PSCR division developed a method to measure and quantify the M2E latency of any communications system transmitting audio, with specifc emphasis on push to talk (PTT) devices. This measurement method is the frst step in establishing QoE key performance indicators (KPI) for mission critical voice (MCV) and a measurement system to quantify these QoE KPIs. Additional measurement methods will be established and published in the near future.The measurement system provides a fair platform for the comparisons of M2E latency across radio communications technologies. Both single and two location measurement systems were developed. The single location measurement system is a simpler setup ideal for measurements performed in a single, controlled setting. The two location system allows for the measurement of M2E latency between devices in two distinct locations and adds the capability to see potential effects of distance and signal propagation on the latency a user experiences. Example measurements of the M2E latency of VHF and UHF land mobile radios (LMR) operating in both direct mode and in trunked mode were performed. These tests demonstrated that both the single and two location tests return consistent measurement results.i ______________________________________________________________________________________________________ This publication is available free of charge from: https://doi.org/10.6028/NIST.IR.8206

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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.