In the past when the mobile phones were not available and it was not within everybody’s means to have landlines with the STD and ISD connections. The Telephone Exchange used to offer the use of trunk calls, to where ever you needed.  This was possible with help of the Trunking systems.

The idea of trunking , in which a main telephone  switch board with many connections that use a same  sound wave length  can use instead of each one being allotted a different personal  link. A miniature form of the trunking system can be seen in offices where a telephone operator and intercom systems are used. 

With the use of the two way radio handset, called walkie talkie, nobody can complain about the communication amongst the working team or other group members. But, yes, one has to watch the area coverage by the walkie talkies. The walkie talkies were originally designed for the exclusive use of the police and security force.  Later on the commercial models of the same were sold in the market for the use in many professional fields like construction.

The PMR radios were replaced with an advanced version called DMR radio or the digital mobile radios. The DMR offers talking, information and various other facilities and aspects. The chief motto of the DMR is to offer an easy to handle, minimum price factor and inter usable feature with all the other brands.  

Instead of using many radio sets and other connecters to improve the reception range of the service repeaters are used at intervals to increase the area of reception of conversations, information and other features. Tetra or the terrestrial trunked radio is movable radio working on two frequencies. Tetra was patterned mainly for the use of various departments of the governing bodies and other SOS services.