osmo-bsc/doc/handover.msc

# Handover between cells, intra-BSC
msc {
	hscale=3;
	ms [label="MS via BTS"], bsc_lchan[label="BSC lchan FSM"], bsc_gscon[label="BSC conn FSM"],
	bsc_mgcp[label="BSC mgcp FSM"], mgw_msc[label="MGW/MSC"];

	ms note mgw_msc [label="intra-BSC Handover sequence"];

	bsc_gscon abox bsc_gscon [label="ST_ACTIVE"];
	bsc_gscon box bsc_gscon [label="bsc_handover_start(): init conn->ho"];
	bsc_gscon -> bsc_gscon [label="GSCON_EV_HO_START (intra-BSC)"];
	bsc_gscon abox bsc_gscon [label="ST_HANDOVER_\nWAIT_LCHAN"];
	bsc_lchan <- bsc_gscon [label="lchan_activate(lchan, FOR_HANDOVER)"];
	...;
	--- [label="on lchan FSM error or timeout"];
	bsc_lchan -> bsc_gscon [label="GSCON_EV_LCHAN_ALLOC_ERROR"];
	bsc_gscon box bsc_gscon [label="handover_end(fail)"];
	ms note bsc_gscon [label="MS happily continues on the old lchan."];
	bsc_gscon abox bsc_gscon [label="ST_ACTIVE"];
	--- [label="END: 'on error'"];
	...;
	...;

	--- [label="IF lchan FSM decides that it is an lchan for speech"];
	bsc_lchan -> bsc_gscon [label="GSCON_EV_ENSURE_MGW_ENDPOINT"];
	--- [label="IF there is an MGW endpoint for the BTS already (conn->user_plane.fi_bts)"];
	bsc_gscon box bsc_gscon [label="handover_created_mgw_endpoint = false"];
	bsc_gscon -> bsc_lchan [label="LCHAN_EV_MGW_ENDPOINT_AVAILABLE"];
	--- [label="ELSE: no MGW endpoint for the BTS side yet"];
	bsc_gscon abox bsc_gscon [label="ST_HANDOVER_\nWAIT_CRCX_BTS"];
	bsc_gscon box bsc_gscon [label="handover_created_mgw_endpoint = true"];
	bsc_gscon -> bsc_mgcp [label="mgcp_conn_create()"];
	bsc_mgcp => mgw_msc [label="CRCX (for BTS)"];
	bsc_mgcp abox bsc_mgcp [label="ST_CRCX_RESP (MGCP_MGW_TIMEOUT = 4s)"];
	bsc_gscon note bsc_mgcp [label="conn FSM relies on mgcp FSM timeout"];
	...;
	--- [label="On Timeout"];
	bsc_mgcp note bsc_mgcp [label="On timeout, the MGCP FSM will terminate, emitting the parent_term
		event set upon mgcp_conn_create():"];
	bsc_mgcp -> bsc_gscon [label="GSCON_EV_MGW_FAIL_BTS"];
	bsc_gscon note bsc_gscon [label="GSCON_EV_MGW_FAIL_BTS is handled by the conn FSM allstate
		handler. It sets conn->user_plane.fi_bts = NULL."];
	bsc_gscon -> bsc_lchan [label="LCHAN_EV_MGW_ENDPOINT_ERROR"];
	bsc_lchan note bsc_gscon [label="conn FSM error handler exits and relies on the lchan FSM
		signalling error, which should actually happen immediately:"];
	bsc_gscon <- bsc_lchan [label="GSCON_EV_LCHAN_ALLOC_ERROR"];
	bsc_gscon box bsc_gscon [label="handover_end(fail)"];
	--- [label="IF handover_created_mgw_endpoint == true"];
	bsc_gscon -> bsc_mgcp [label="mgcp_conn_delete()"];
	---;
	ms note bsc_gscon [label="MS happily continues on the old lchan."];
	bsc_gscon abox bsc_gscon [label="ST_ACTIVE"];
	--- [label="END: 'On Timeout'"];
	...;

	bsc_mgcp <= mgw_msc [label="CRCX OK (for BTS)"];
	bsc_mgcp box bsc_mgcp [label="libosmo-mgcp-client fsm_crcx_resp_cb()"];
	bsc_mgcp abox bsc_mgcp [label="ST_READY"];
	bsc_mgcp -> bsc_gscon [label="GSCON_EV_MGW_CRCX_RESP_BTS"];
	bsc_gscon abox bsc_gscon [label="ST_HANDOVER_\nWAIT_LCHAN"];
	bsc_gscon -> bsc_lchan [label="LCHAN_EV_MGW_ENDPOINT_AVAILABLE"];
	--- [label="END: lchan FSM decides that it is an lchan for speech"];
	...;
	...;

	bsc_lchan -> bsc_gscon [label="GSCON_EV_LCHAN_ACTIVE"];
	bsc_gscon abox bsc_gscon [label="ST_HANDOVER_\nWAIT_DETECT\nT3103"];
	bsc_gscon box bsc_gscon [label="gsm48_send_ho_cmd()"];
	ms <= bsc_gscon [label="RR Handover Command"];

	...;
	--- [label="On timeout"];
	bsc_lchan <- bsc_gscon [label="NEW lchan: LCHAN_EV_RELEASE"];
	ms <=> bsc_lchan [label="release procedure (async)"];
	bsc_gscon box bsc_gscon [label="handover_end(fail)"];
	--- [label="IF handover_created_mgw_endpoint == true"];
	bsc_gscon -> bsc_mgcp [label="mgcp_conn_delete()"];
	---;
	ms note bsc_gscon [label="MS happily continues on the old lchan."];
	bsc_gscon abox bsc_gscon [label="ST_ACTIVE"];
	--- [label="END: On timeout"];
	...;
	ms => bsc_gscon [label="RR Handover Detect"];

	bsc_gscon abox bsc_gscon [label="ST_HANDOVER_\nWAIT_MDCX_BTS\ncontinue T3103"];
	bsc_gscon -> bsc_lchan [label="OLD lchan: LCHAN_EV_RELEASE"];
	ms <=> bsc_lchan [label="release procedure (async)"];
	bsc_lchan note bsc_gscon [label="officially take over new lchan: conn->lchan = ho->new_lchan"];
	bsc_gscon -> bsc_mgcp [label="mgcp_conn_modify()"];
	bsc_mgcp note bsc_mgcp [label="mgcp FSM that was established for old lchan, for BTS side"];
	bsc_mgcp abox bsc_mgcp [label="ST_MDCX_RESP"];
	bsc_mgcp => mgw_msc [label="MDCX (for BTS)"];
	...;
	--- [label="Should the Handover Complete arrive early"];
	ms => bsc_gscon [label="RR Handover Complete"];
	bsc_gscon box bsc_gscon [label="handover_complete_received = true"];
	---;
	...;
	--- [label="On timeout of the mgcp FSM"];
	bsc_gscon note mgw_msc [label="MGCP FSM terminates"];
	bsc_gscon <- bsc_mgcp [label="GSCON_EV_MGW_FAIL_BTS"];
	bsc_lchan note bsc_gscon [label="The phone has already taken on the new lchan, but now we happen
		to not be able to use it. The only sensible thing is to end the conn."];
	bsc_gscon abox bsc_gscon [label="ST_WAIT_CLEAR_CMD"];
	bsc_gscon => mgw_msc [label="BSSMAP Clear Request\n(Equipment Failure)"];
	bsc_lchan <- bsc_gscon [label="NEW lchan: LCHAN_EV_RELEASE"];
	ms <=> bsc_lchan [label="release procedure (async)"];
	bsc_gscon -> bsc_mgcp [label="mgcp_conn_delete()"];
	...;
	bsc_mgcp <= mgw_msc [label="MDCX OK"];
	bsc_mgcp abox bsc_mgcp [label="ST_READY"];
	bsc_mgcp -> bsc_gscon [label="GSCON_EV_MGW_MDCX_RESP_BTS"];
	--- [label="IF !handover_complete_received"];
	bsc_gscon abox bsc_gscon [label="ST_HANDOVER_\nWAIT_COMPLETE\ncontinue T3103"];
	--- [label="ELSE"];
	bsc_gscon -> bsc_gscon [label="gscon_handover_post_complete()"];
	---;
	...;
	ms => bsc_gscon [label="RR Handover Complete"];
	bsc_gscon box bsc_gscon [label="gscon_handover_post_complete()"];
	bsc_gscon note bsc_gscon [label="handover_end(success), conn->ho = NULL"];
	bsc_gscon abox bsc_gscon [label="ST_ACTIVE"];
}