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.
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