Osmocom's Base Station Controller for 2G mobile networks https://osmocom.org/projects/osmobsc
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osmo-bsc/doc/channel_release.txt

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GSM 04.08 7.1.7 / 9.1.7 RR CHANNEL RELESE
RSL 08.58 3.4 / ? RLL Link Release Request
RSL 08.58 4.6 / 8.4.5 DEACTivate SACCH
* Deactivate SACCH according to Channel Release Proc 04.08
* to be sent after RR CHANNEL RELEASE is sent to MS
RSL 08.58 4.7 / 8.4.14 RF CHANnel RELease
* tells the BTS to release a radio channel
* "when an activated radio channel is no longer needed"
* BTS responds with RF CHANnel RELease ACKnowledge
GSM 04.08 3.4.13: RR connection release procedure
* network sends RR CHANNEL RELEASE to MS on the DCCH
* start T3109
* deactivate SACCH
* MS disconnects main signalling link (by sending DISC)
* all other data links are disconnected by local end link release
* network receives DISC (BTS sends RLL REL IND to BSC)
* stop T3109
* start T3111
* when T3111 times out, the network can reuse the channls
* if T3109 times out, the network deactivates the channels
and can reuse them
* this probably means simply RF CHANnel RELease
== Implementation in OpenBSC ==
There are two possible reasons a gsm_subscriber_connection
will be released. One is a network failure, the other is
the completion of an operation/transaction.
=== Failure ===
The BSC API will call the gsm_04_08.c:gsm0408_clear_request callback
and the MSC part will release all transactions, operations and such
and the channels will be released as error case.
=== Success ===
Every time an 'operation' or 'transaction' is finished msc_release_connection
will be called and it will determine if the gsm_subscriber_connection can
be released.
In case it can be released bsc_api.c:gsm0808_clear will be called
which will release all lchan's associated with the connection. For the
primary channel a SACH Deactivate will be send with the release
reason NORMAL RELEASE.
bsc_api.c:gsm0808_clear
* Release a channel used for handover
* Release the primary lchan with normal release, SACH deactivate
chan_alloc.c:lchan_release(chan, sacch_deactivate, reason)
* Start the release procedure. It is working in steps with callbacks
coming from the abis_rsl.c code.
* Release all SAPI's > 0 as local end (The BTS should send a
REL_CONF a message)
* Send SACH Deactivate on SAPI=0 if required.
* Start T3109 (stop it when the main signalling link is disconnected)
or when the channel released. On timeout start the error handling.
* abis_rsl.c schedules the RSL_MT_RF_CHAN_REL once all SAPI's are
released and after T3111 has timed out or there is an error.
RX of RELease INDication:
* Calls internal rsl_handle_release which might release the RF.
RX of RELease CONFirmation:
* Calls internal rsl_handle_release which might release the RF.
* RX of RF_CHAN_REL_ACK
* call lchan_free()
=== Integration with SMS ===
* RX of CP_ERROR or unimplemented MT
* trigger trans_free() which will msc_release_connection()
* CP TC1* expired while waiting for CP-ACK
* trigger trans_free() which will msc_release_connection()
* RX of RP_ERROR
* trigger trans_free() which will msc_release_connection()
* TX of CP-ACK in MT DELIVER
* trigger trans_free() which will msc_release_connection()
* RX of CP-ACK in MO SUBMIT
* trigger trans_free() which will msc_release_connection()