,---------------------------------------------------------------------------,
|         Um                           A                                     |
|      Interface       A-bis       Interface                                 |
|          |         Interface         |  ,--------------------,             |
|                                         |  ,-----,  ,-----,  |             |
|          |  ,----------|----------,  |  |  | VLR |  | HLR |  |  ,-------,  |
| ,-----,     |  ,-----,            |     |  `-----'  `-----'  |  | Other |  |
| | SIM |  |  |  | BTS | | ,-----,  |  |  |  ,-----,           |  | MSCs  |  |
| `--,--'     |  `-----'---|     |  |     |  |     |--------------'-------'  |
|    |     ,  |     :    | | BSC |-----------| MSC |           |             |
| ,-----, /|  |  ,-----,---|     |  |     |  |     |--------------,------,   |
| | MS  |' | ,---| BTS | | `-----'  |  |  |  `-----'           | / PSTN / \  |
| `-----'  |/ |  `-----'            |     |  ,-----,   ,-----, | \  ISDN  /  |
|          '  `----------|----------'  |  |  | EIR |   | AC  | |  `------'   |
|                                         |  `-----'   `-----' |             |
|          |  Base Station Subsystem      `--------------------'             |
|                                           Network Subsystem                |
`----------------------------------------------------------------------------'
SIM  Subscriber Identity Module        HLR  Home Location Register
MS   Mobile Station                    VLR  Vistor Location Register
BTS  Base Transceiver Station          EIR  Equipment Identity Register
BSC  Base Station Controller           AC   Authentication Center
MSC  Mobile services Switching Center  PSTN Public Switched Telecomm Network
VLR  Visitor Location Register         ISDN Integrated Services Digital Network