| # 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 note mgw [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)"]; |
| ...; |
| 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"]; |
| ---; |
| 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."]; |
| ...; |
| bts => bsc [label="IPACC MDCX ACK"]; |
| bts note bsc [label="IPACC MDCX ACK triggers no events or actions"]; |
| ---; |
| |
| ...; |
| 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"]; |
| ...; |
| --- [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 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 -> 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_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."]; |
| ...; |
| bsc_mgcp <= mgw [label="MDCX OK"]; |
| bsc_mgcp -> bsc_gscon [label="GSCON_EV_MGW_MDCX_RESP_BTS"]; |
| bsc_gscon abox bsc_gscon [label="ST_ACTIVE"]; |
| } |