Private mobile radio (PMR) differs from the more familiar mobile phone system in the sense that access is restricted to those with licences. It is usually used for communication of a mobile station with its base, although in many instances two mobile stations can communicate. Other major uses of this type of radio communications are the emergency services. Local PMR is the simplest of the PMR systems and it uses a single antenna site for the location of the base station. In trunked PMR, a network of stations is set up where each station is linked generally using land lines, although optical fibres and point to point radio are also used. In this way the different base stations are able to communicate with each other. Moreover, TErrestrial Trunked RAdio (TETRA) is the newest PMR system that provides a number of advantages over trunked private mobile radio. It uses digital speech transmission. This provides a large degree of privacy against listeners, but in addition to this encryption is also supported, enabling it to be completely secure for services such as the police.