Holger Hans Peter Freyther | 0a7c6a3 | 2013-02-06 11:51:56 +0100 | [diff] [blame] | 1 | |
| 2 | GSM Paging implementation in OpenBSC |
| 3 | |
| 4 | == Code structure == |
| 5 | |
| 6 | The code is implemented in the libbsc/paging.c file. The external |
| 7 | interface is documented/specified in the include/openbsc/paging.h |
| 8 | header file. The code is used by the NITB and BSC application. |
| 9 | |
| 10 | |
| 11 | == Implementation == |
| 12 | |
| 13 | Paging can be initiated in two ways. The standard way is to page by |
| 14 | LAC. Each BTS has its own list/queue of outstanding paging operation. |
| 15 | When a subscriber is paged one "struct paging_request" per BTS will |
| 16 | be allocated and added to the tail of the list. The BTS is supposed |
| 17 | to be configured to not repeat the paging. |
| 18 | |
| 19 | A paging_request will remain in the queue until a paging response or at |
| 20 | the expiry of the T3113. Every 500 milliseconds a RSL paging command is |
| 21 | send to the BTS. The 500 milliseconds is a throttling to not crash the |
| 22 | ip.access nanoBTS. Once one paging_request has been handled it will be |
| 23 | put at the end of the queue/list and the available slots for the BTS |
| 24 | will be decreased. |
| 25 | |
| 26 | The available slots will be updated based on the paging load information |
| 27 | element of the CCCH Load indication. If no paging slots are considered |
| 28 | to be available and no load indication is sent a timer is started. The |
| 29 | current timeout is 500 milliseconds and at the expiry of the timer the |
| 30 | available slots will be set to 20. |
| 31 | |
| 32 | OpenBSC has the " paging free <-1-1024>" configuration option. In case |
| 33 | there are less free channels than required no paging request will be |
| 34 | sent to the BTS. Instead it will be attempted to send the paging request |
| 35 | at the next timeout (500 milliseconds). |
| 36 | |
| 37 | == Limitation == |
| 38 | |
| 39 | The paging throughput could be higher but this has lead to crashes on the |
| 40 | ip.access nanoBTS in the past. |
| 41 | |
| 42 | == Configuration == |
| 43 | |
| 44 | === ip.access nanoBTS === |
| 45 | |
| 46 | The current CCCH Load indication threshold is 10% and the period is 1 second. |
| 47 | The code can be found inside the src/libbsc/bts_ipaccess_nanobts.c inside the |
| 48 | nanobts_attr_bts array. |