diff options
Diffstat (limited to 'doc/handover.msc')
| -rw-r--r-- | doc/handover.msc | 241 |
1 files changed, 97 insertions, 144 deletions
diff --git a/doc/handover.msc b/doc/handover.msc index 8862dd97b..7529de602 100644 --- a/doc/handover.msc +++ b/doc/handover.msc @@ -1,170 +1,123 @@ # Handover between cells, intra-BSC msc { hscale=3; - ms [label="MS"], bts [label="BTS"], bsc[label="BSC"], bsc_gscon[label="BSC conn FSM"], bsc_mgcp[label="BSC mgcp FSM"], mgw[label="MGW"]; + 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 [label="intra-BSC Handover sequence"]; + ms note mgw_msc [label="intra-BSC Handover sequence"]; bsc_gscon abox bsc_gscon [label="ST_ACTIVE"]; - bsc box bsc [label="bsc_handover_start(): init conn->ho"]; - bsc -> bsc_gscon [label="GSCON_EV_HO_START"]; - bsc <- bsc_gscon [label="bsc_handover_start_gscon()"]; - - bsc box bsc [label="lchan_alloc(): pick available lchan"]; - bsc box bsc [label="rsl_chan_activate_lchan()"]; - - --- [label="is the chosen lchan on dynamic timeslot that is currently used as PDCH?"]; - bts <= bsc [label="i) RSL RF Chan Release of PDCH (Osmocom dyn TS)"]; - bts <= bsc [label="OR ii) RSL PDCH Deact (ip.access dyn TS)"]; - bsc -> bsc_gscon [label="bsc_handover_start_gscon() returns early"]; - bsc_gscon abox bsc_gscon [label="ST_WAIT_HO_COMPL (no timeout, relies on T3103 below)"]; + 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'"]; ...; - bts note bsc_gscon [linecolor="red", - label="Osmocom style dyn TS use lchan->act_timer to watch over RF Chan Release, but there - seems to be no timer watching over PDCH Deact!"]; ...; - bts => bsc [label="i) RSL RF Chan Release ACK (Osmocom dyn TS)"]; - bts => bsc [label="OR ii) RSL PDCH Deact ACK (ip.access dyn TS)"]; - bsc box bsc [label="rsl_chan_activate_lchan() re-invoked"]; - bts <= bsc [label="RSL Chan Activ"]; - --- [label="else (no dyn TS switchover)"]; - bts <= bsc [label="RSL Chan Activ"]; - bsc -> bsc_gscon [label="bsc_handover_start_gscon() returns"]; + --- [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()"]; ---; - bsc_gscon abox bsc_gscon [label="ST_WAIT_HO_COMPL (no timeout, relies on T3103 below)"]; - - ...; - bts note bsc_gscon [linecolor="red", - label="There seems to be no timer watching out for RSL Chan Activ ACK/NACK!"]; - ...; - bts => bsc [label="RSL Chan Activ ACK"]; - bsc box bsc [label="rsl_rx_chan_act_ack()"]; - bsc box bsc [label="Stop lchan->act_timer"]; - bsc box bsc [label="lchan->state = LCHAN_S_ACTIVE"]; - bsc -> bsc [label="S_LCHAN_ACTIVATE_ACK"]; - bsc box bsc [label="handover_logic.c ho_logic_sig_cb()"]; - bsc box bsc [label="ho_chan_activ_ack()"]; - bsc note bsc [label="gsm48_send_ho_cmd()"]; - ms <= bsc [label="RR Handover Command"]; - bsc abox bsc [label="start T3103"]; - --- [label="is BTS using IPA Abis? (osmo-bts, ip.access)"]; - bts <= bsc [label="IPACC CRCX"]; - bsc -> bsc [label="ho_chan_activ_ack() returns"]; - bts note bsc [linecolor="red", - label="There seems to be no timer watching over IPACC CRCX ACK/NACK! - If no response is received, we simply ignore that fact and carry on as if - everything was fine."]; - ...; - bts note bsc [label="The IPACC CRCX and MDCX ACKs may come back at any time: - before or after the Handover Detect, before or after Handover Complete."]; - bts note bsc_mgcp [linecolor="red", - label="The CRCX ACK contains vital information for routing the RTP stream. - If the CRCX ACK were very slow, we would not know which RTP/RTPC ports - to point the MGW at, below at mgcp_conn_modify()! - Even though this being unrealistic, we must make sure to receive a CRCX ACK."]; - ...; - bsc box bsc [label="osmo_bsc_audio.c"]; - bts => bsc [label="IPACC CRCX ACK"]; - bts <= bsc [label="IPACC MDCX"]; - ...; - bts note bsc [linecolor="red", - label="There seems to be no timer watching over IPACC MDCX ACK/NACK! - If no response is received, we simply ignore that fact and carry on as if - everything was fine."]; + ms note bsc_gscon [label="MS happily continues on the old lchan."]; + bsc_gscon abox bsc_gscon [label="ST_ACTIVE"]; + --- [label="END: 'On Timeout'"]; ...; - bts => bsc [label="IPACC MDCX ACK"]; - bts note bsc [label="IPACC MDCX ACK triggers no events or actions"]; - ---; + 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"]; ...; - ms => bsc [label="RR Handover Detect"]; - bsc -> bsc [label="S_LCHAN_HANDOVER_DETECT"]; - bsc box bsc [label="ho_rsl_detect(): no action, only logging"]; - bsc note bsc_gscon [label="Handover Detect triggers no events or actions"]; - bsc note bsc_gscon [linecolor="red", - label="upon Handover Detect, we should already start re-routing the RTP! - Instead we wait for Handover Complete."]; - ...; - ms => bsc [label="RR Handover Complete"]; - bsc -> bsc [label="S_LCHAN_HANDOVER_COMPL"]; - bsc box bsc [label="handover_logic.c ho_logic_sig_cb()"]; - bsc box bsc [label="ho_gsm48_ho_compl()"]; - bsc box bsc [label="stop T3103"]; - bts note bsc_gscon [label="If anything goes wrong from this point on, we will not move back - to the old lchan: would be pointless after Handover Complete."]; - bsc note bsc [label="officially take over new lchan: conn->lchan = ho->new_lchan"]; - --- [label="Release old lchan"]; - bsc box bsc [label="_lchan_handle_release(sacch_deact=0)"]; - bsc box bsc [label="rsl_release_sapis_from(start=1)"]; - bts <= bsc [label="RSL Release Request (Local End)..."]; - bts <= bsc [label="...for each SAPI except link_id=0"]; - bsc box bsc [label="rsl_release_request(link_id=0)"]; - bts <= bsc [label="RSL Release Request (Local End) for link_id=0"]; - bsc box bsc [label="_lchan_handle_release() returns here, the remaining release is asynchronous; - see `End: 'Release old lchan'` below."]; - ...; - bts note bsc_gscon [linecolor="red", - label="There seems to be no timer watching over RSL Release Request!"]; - ...; - bts => bsc [label="RSL Release Confirm..."]; - bts => bsc [label="...for each SAPI and link_id=0"]; - bsc abox bsc [label="start T3111"]; - ...; - bsc box bsc [label="T3111 expires"]; - bsc abox bsc [label="Start lchan->act_timer with lchan_deact_tmr_cb"]; - bts <= bsc [label="RSL RF Channel Release"]; + 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 box bsc [label="lchan_deact_tmr_cb()"]; - bsc box bsc [label="rsl_lchan_mark_broken(): state=LCHAN_S_BROKEN"]; - bsc box bsc [label="lchan_free()"]; - bsc -> bsc [label="S_LCHAN_UNEXPECTED_RELEASE"]; - bsc box bsc [label="bsc_api.c handle_release()"]; - bsc box bsc [label="bsc->clear_request()"]; - bsc box bsc [label="bsc_clear_request encodes a BSSMAP Clear Request message and passes it on - to the conn FSM as data argument via:"]; - bsc -> bsc_gscon [label="GSCON_EV_TX_SCCP"]; - bsc_gscon rbox bsc_gscon [label="BSSMAP Clear Request to MSC"]; - bsc note bsc_gscon [linecolor="red", - label="During Handover, we actually release the entire conn just because we failed to - gracefully release the old lchan. That is obviously nonsense."]; - bsc note bsc [label="Stop T3101 (but was not active in this code path)"]; - bsc -> bsc [label="S_CHALLOC_FREED"]; - --- [label="End: 'On timeout'"]; - ...; - bts => bsc [label="RSL RF Channel Release Ack"]; - bsc box bsc [label="Stop lchan->act_timer"]; - bsc box bsc [label="Stop lchan->T3111"]; + 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 box bsc [label="still in ho_gsm48_ho_compl()"]; - bsc note bsc [label="handover_free(), conn->ho = NULL"]; - bsc -> bsc_gscon [label="GSCON_EV_HO_COMPL"]; - bsc note bsc_gscon [linecolor="orange", - label="Handover information is cleared before signalling the conn FSM. - That means the conn FSM cannot possibly log sensible information about exactly - which Handover has just completed."]; - - bsc_gscon abox bsc_gscon [label="ST_WAIT_MDCX_BTS_HO - (MGCP_MGW_TIMEOUT=4s with MGCP_MGW_HO_TIMEOUT_TIMER_NR)"]; - + 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 => mgw [label="MDCX (for BTS)"]; + bsc_mgcp abox bsc_mgcp [label="ST_MDCX_RESP"]; + bsc_mgcp => mgw_msc [label="MDCX (for BTS)"]; ...; - bsc_gscon note mgw [ - label="If we get no MDCX ACK, the MGCP FSM terminates, and emits GSCON_EV_MGW_FAIL_BTS. - Besides invalidating the MGCP FSM pointer, this event has no - effect in ST_WAIT_MDCX_BTS_HO, and we rely on above conn FSM - timeout instead."]; - bsc_gscon note bsc_gscon [linecolor="red", - label="A timeout of ST_WAIT_MDCX_BTS_HO simply transitions back to ST_ACTIVE! - Even though the MGW failed, we carry on as if everything were fine."]; + --- [label="Should the Handover Complete arrive early"]; + ms => bsc_gscon [label="RR Handover Complete"]; + bsc_gscon box bsc_gscon [label="handover_complete_received = true"]; + ---; ...; - bsc_mgcp <= mgw [label="MDCX OK"]; + --- [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"]; } |