In GSM networks, network subsystems (NSS), also called switching subsystems (SSS), perform the switching tasks, which include switching the radio channels to fixed networks. The central unit that performs these node functions is the mobile switching center (MSC). The mobile switching centre supports various services for data transmission and the short message serviceSMS as well as routing from external networks to the mobile station
. Part of the NSS system are the databases for the location directory (HLR) and for the visitor directory (VLR).