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# Handover between cells, intra-BSC
msc {
hscale=2;
ms [label="MS via BTS"], lchan[label="BSC lchan FSM"], ho[label="BSC Handover FSM"],
gscon[label="BSC conn FSM"], msc_[label="MSC"];
ms note msc_ [label="intra-BSC Handover"];
gscon abox gscon [label="ST_ACTIVE"];
ms => ho [label="Measurement Report"];
ho box ho [label="Handover Decision"];
ho box ho [label="handover_request\n(struct handover_out_req)"];
ho note gscon [label="To make sure the conn FSM permits a handover, trigger an event:"];
ho -> gscon [label="GSCON_EV_HANDOVER_START\ndata=handover_out_req"];
gscon abox gscon [label="ST_HANDOVER"];
ho <- gscon [label="handover_start\n(handover_out_req)"];
ho box ho [label="handover_start_intra_bsc()"];
ho abox ho [label="allocate\nHO_ST_NOT_STARTED"];
...;
...;
--- [label="On any error or timeout"];
ho box ho [label="handover_end(fail)"];
ho -> gscon [label="GSCON_EV_HANDOVER_END"];
gscon abox gscon [label="ST_ACTIVE"];
ms note gscon [label="MS happily continues on the old lchan."];
--- [label="END: 'On any error or timeout'"];
...;
...;
ho box ho [label="lchan_select_by_type()"];
ho abox ho [label="HO_ST_WAIT_\nLCHAN_ACTIVE"];
lchan <- ho [label="lchan_activate(FOR_HANDOVER)"];
lchan rbox lchan [label="(most details omitted, see lchan_fsm diagrams)"];
...;
...;
--- [label="On lchan error or timeout"];
lchan -> ho [label="HO_EV_LCHAN_ERROR"];
ho rbox gscon [label="same as above"];
--- [label="END: 'On lchan error or timeout'"];
...;
...;
lchan abox lchan [label="LCHAN_ST_WAIT_ACTIV_ACK"];
ms <= lchan [label="RSL Chan Activ"];
...;
ms => lchan [label="RSL Chan Activ ACK"];
lchan -> ho [label="HO_EV_LCHAN_ACTIVE"];
ho abox ho [label="HO_ST_WAIT_\nRR_HO_DETECT"];
...;
ms => ho [label="RR Handover Detect\nHO_EV_RR_HO_DETECT"];
lchan note ho [label="At this point we should start to switch the MGW over to the new lchan.
But this is not implemented yet, as was not before introducing these FSMs."];
ho abox ho [label="HO_ST_WAIT_\nRR_HO_COMPLETE"];
...;
lchan note ho [label="The lchan FSM will continue with RSL and RTP while the HO FSM waits.
HO_EV_LCHAN_ESTABLISHED means that both RSL and RTP are established.
Usually, RTP will be done first, and the HO_EV_LCHAN_ESTABLISHED may be
received even before HO_EV_RR_HO_COMPLETE.
ho_fsm_wait_lchan_established_onenter() decides whether to wait or not."];
...;
ms => lchan [label="RSL EST IND"];
lchan -> ho [label="HO_EV_LCHAN_ESTABLISHED",ID="(may come as early as this, or...)"];
ms => ho [label="RR Handover Complete (from EST IND)\n HO_EV_RR_HO_COMPLETE"];
ho abox ho [label="HO_ST_WAIT_\nLCHAN_ESTABLISHED"];
...;
lchan rbox lchan [label="when lchan FSM is done with setting up RTP"];
lchan -> ho [label="HO_EV_LCHAN_ESTABLISHED",ID="(...may come only now)"];
ho abox ho [label="HO_ST_WAIT_\nMGW_ENDPOINT_TO_MSC"];
ho -> gscon [label="gscon_connect_mgw_to_msc()"];
...;
ho <- gscon [label="HO_EV_MSC_MGW_OK"];
ho box ho [label="handover_end(OK)"];
ho -> gscon [label="gscon_change_primary_lchan()"];
lchan <- gscon [label="LCHAN_RTP_EV_ESTABLISHED"];
ho -> gscon [label="GSCON_EV_HANDOVER_END"];
gscon abox gscon [label="ST_ACTIVE"];
ho box ho [label="detach from parent to not fire another meaningless GSCON_EV_HANDOVER_END"];
ho abox ho [label="terminate"];
}
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