The document describes the architecture of a GSM network. It is divided into three main subsystems: the Network Switching Subsystem (NSS) which handles call connectivity and mobility management; the Base Station Subsystem (BSS) which handles radio communication; and the Network Management Subsystem (NMS) which handles operations and maintenance. The key components of the NSS are the Home Location Register (HLR), Mobile Switching Center (MSC), Visitor Location Register (VLR), and Authentication Center (AUC). The BSS contains the Base Transceiver Station (BTS) and Base Station Controller (BSC). The NMS contains components like the Operation and Support System.