Access to PLMN services is achieved by means of an air Interface involving radio communications between mobile phones or other wireless enabled user equipment and land based radio transmitters or radio base stations
PLMNs interconnect with other PLMNs and PSTNs for telephone communications or with ISPs for data and Internet access.
Note: A PLMN may be considered as an extension of a fixed network, e.g. the public switched telephone network (PSTN) or as an integral part of the PSTN.
Source: Partially from Federal Standard 1037C