Effects of Push-To-Talk (PTT) delays on CSMA based capacity limited Land Mobile Radio (LMR) networks

Abstract

Land Mobile Radio (LMR) networks have been traditionally used for transmitting voice signals. Systems like Facsimile, Radio Teletype and Distributed Digital Radios (DDR) use a conventional analog FM radio to transmit data [1], [2]. Digital Radios like Packet Radio and P-25, use complex modulation schemes like Continuous Frequency Shift keying (CFSK), Continuous Phase Modulation (CPM) to transmit voice/data information [3],[4]. The Medium Access Layer (MAC) implemented on such data networks ensures fair access to the half duplex FM channel. For voice transmission the users can themselves act as a default contention resolution mechanism. For data, some variations of channel access mechanisms like Time Division Multiple Access (TDMA) / Frequency Division Multiple Access (FDMA) / Carrier Sense Multiple Access (CSMA) are used [5]. The delays due to the Radio Hardware can significantly influence the throughput of such networks. This paper describes the effects of Push-To-Talk (PTT) delays associated with Land mobile Radios (LMR) for a CSMA based MAC layer. The results show that the PTT delays along with the number of nodes in a network can affect the throughput of a CSMA based capacity limited LMR network.

Asymmetric throughput problem due to Push-To-Talk (PTT) delays in CSMA/CA based heterogeneous Land Mobile Radio (LMR) networks

“…These delays result in an increase in the size of collision window. A typical PTT delay for LMR terminals can run between tens to hundreds of milliseconds [12], [13], [14]. As the use of LMR networks was traditionally restricted to voice traffic, in which a user himself provided the mechanism for channel access, the effects of PTT delays have been greatly overlooked in literature.…”