,------------------------------------------------------------------------,
|                             Um                        A                |
|                ,---------,  |                         |  ,---------,   |
|                |   CM    |                               |   CM    |   |     
|   GSM layer 3  |- - - - -|  |                         |  |- - - - -|   |    
|                |   MM    |                               |   MM    |   |  
|                |- - - - -|  |  ,-------------------,  |  |- - - - -|   |
|                |         |     |         |  DTAP   |     |   DTAP  |   |     
|                |   RR    |  |  |   RR    | BSSMAP  |  |  |  BSSMAP |   |
|   - - - - - - -|---------|     |---------|---------|     |---------|   |
|                |         |  |  |         |  SCCP   |  |  |   SCCP  |   |
|   GSM layer 2  |  LAPDm  |     |  LAPDm  |---------|     |---------|   |
|   - - - - - - -|---------|  |  |---------|         |  |  |         |   |
|   GSM layer 1  |  TDMA   |     |  TDMA   |  MTP    |     |   MTP   |   |    
|                `---------'  |  `-------------------'  |  `---------'   |
|                                   BTS       BSC                        |
|                                                                        |
|            Mobile Station      Base Station Subsystem        MSC       |
`------------------------------------------------------------------------'