With Evox Technology To Achieve The The Intercom City Telephone System Interconnection

Trunking communication system is a senior mobile communication system with strong commanding and distributing features, and it is one type of private communication systems. Currently there are two kinds of trunking communication systems: analog trunking system and digitaltrunking system. The main difference between these two types is the transmission method of voice : one is transmitted by analog signal and the other uses digital signal. The developing trend of trunking system is the digital mode because digital trunking system can make better use of limited frequency resource and implement more advanced features.LTD (Low Tier Digital) is the next generation of middle and low class digital trunking system initiated by Motorola. This platform makes use of C4FSK modulation method and TDMA mode is applied to share channels. Many new features will be implemented on LTD with very low cost, including the feature of phone interconnect. The problem occurs when implementing "phone interconnect" feature :because LTD platform only supports half-duplex working mode , the user needs to switch between "listening" and "talking" by pressing and releasing PTT key; on the other hand, PSTN system is a duplex system , so user can "listen" and "talk" at the same time. The problem is that the radio user and the PSTN user cannot operate at the same step because of different working mode. To resolve this, this paper has taken the EVOX technology into use. By transmitting channel switching message through the reverse channel defined by LTD common air interface, the operation of radio and PSTN users can be synchronized in a "pseudo duplex" working mode.In this paper, at first the basic knowledge of trunking system and characters of LTD digital trunking system is introduced. Based on the detail analysis of the LTD system' s air interface, frame structure, voice transmission method and reverse channel protocol, the reasons to use EVOX technology its advantages and implementing details is described in this section . Therefore, there is the introduction and analysis on how to implement EVOX in the LTD software architecture. At last, there is the summary of the design thoughts in this document and possible enhancement in future.