Introduction

Land mobile radio (LMR) is different from Cellular and Personal Communications Services (PCS) in that it has its roots in push-to-talk radio systems. Push-to-talk can also be referred to as half-duplex (or simplex), whereas the half means only half of a conversation can happen at one time. This is sometimes experienced when using a household speakerphone. Full duplex, on the other hand, allows two simultaneous conversations, similar to a classic argument between a coach and a sports official. Push-to-talk communications range from a simple pair of hand-held citizen band (CB) walkie-talkies to a new breed of enhanced, full-duplex cellular-like consumer products. Licensed by the Federal Communication Communications (FCC) as specialized mobile radio (SMR), this radio service is primarily used by the public-safety sector, the industrial & construction sector and the dispatch sector like taxi-dispatch services. Though classified by the FCC as a commercial mobile radio service (CMRS), SMR is not currently classified like cellular and PCS as a common carrier telephone system.

Connection to the public switched telephone network (PSTN) by a two-way radio is usually done by incoming calls routed to a radio interface adapter. During this process, a simplex SMR radio channel is connected to a two-way regular telephone transmission line. The wired portion of the call extends the effective distance of the radio communication link. Even though a two-way wired connection is made, the communication itself remains a half-duplex.