add some document about the channel release procedure
also note how we are not doing what the spec says :)
diff --git a/openbsc/doc/channel_release.txt b/openbsc/doc/channel_release.txt
new file mode 100644
index 0000000..bacf09c
--- /dev/null
+++ b/openbsc/doc/channel_release.txt
@@ -0,0 +1,74 @@
+
+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 ==
+
+chan_alloc.c:lchan_auto_release()
+ * checks if use count still > 0 (abort)
+ * calls gsm48_send_rr_release()
+ * which calls rsl_deact_sacch()
+ * calls rsl_release_request()
+ * which sends RLL Link Release request
+
+RX of RELease INDication:
+ * call rsl_rf_chan_release() (send RF_CHAN_REL)
+
+RX of RELease CONFimem:
+ * call rsl_rf_chan_release() (send RF_CHAN_REL)
+
+* RX of RF_CHAN_REL_ACK
+ * call lchan_free()
+ * subscr_put()
+ * delete release_timer
+
+
+=== Integration with SMS ===
+
+* RX of CP_ERROR or unimplemented MT
+ * trigger trans_free() which will lchan_auto_release()
+
+* CP TC1* expired while waiting for CP-ACK
+ * trigger trans_free() which will lchan_auto_release()
+
+* RX of RP_ERROR
+ * trigger trans_free() which will lchan_auto_release()
+
+* TX of CP-ACK in MT DELIVER
+ * trigger trans_free() which will lchan_auto_release()
+
+* RX of CP-ACK in MO SUBMIT
+ * trigger trans_free() which will lchan_auto_release()
+
+* RX of RP-ACK in MT DELIVER (and no more messages)
+ * trigger rsl_release_request() for SAPI3
+
+* RX of RP-SMMA in MT DELIVER (and no more messages)
+ * trigger rsl_release_request() for SAPI3