| |
| GSM Paging implementation in OpenBSC |
| |
| == Code structure == |
| |
| The code is implemented in the libbsc/paging.c file. The external |
| interface is documented/specified in the include/openbsc/paging.h |
| header file. The code is used by the NITB and BSC application. |
| |
| |
| == Implementation == |
| |
| Paging can be initiated in two ways. The standard way is to page by |
| LAC. Each BTS has its own list/queue of outstanding paging operation. |
| When a subscriber is paged one "struct paging_request" per BTS will |
| be allocated and added to the tail of the list. The BTS is supposed |
| to be configured to not repeat the paging. |
| |
| A paging_request will remain in the queue until a paging response or at |
| the expiry of the T3113. Every 500 milliseconds a RSL paging command is |
| send to the BTS. The 500 milliseconds is a throttling to not crash the |
| ip.access nanoBTS. Once one paging_request has been handled it will be |
| put at the end of the queue/list and the available slots for the BTS |
| will be decreased. |
| |
| The available slots will be updated based on the paging load information |
| element of the CCCH Load indication. If no paging slots are considered |
| to be available and no load indication is sent a timer is started. The |
| current timeout is 500 milliseconds and at the expiry of the timer the |
| available slots will be set to 20. |
| |
| OpenBSC has the " paging free <-1-1024>" configuration option. In case |
| there are less free channels than required no paging request will be |
| sent to the BTS. Instead it will be attempted to send the paging request |
| at the next timeout (500 milliseconds). |
| |
| == Limitation == |
| |
| The paging throughput could be higher but this has lead to crashes on the |
| ip.access nanoBTS in the past. |
| |
| == Configuration == |
| |
| === ip.access nanoBTS === |
| |
| The current CCCH Load indication threshold is 10% and the period is 1 second. |
| The code can be found inside the src/libbsc/bts_ipaccess_nanobts.c inside the |
| nanobts_attr_bts array. |