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

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Call routing in OpenBSC
Flow of events:
# MO call initiated by MS, CHANNEL RQD, IMMEDIATE ASSIGN
# MS sends CC SETUP message, we assume already on TCH/H FACCH
# OpenBSC does a subscriber lookup based on the target extension
* If a subscriber is found:
# send CALL PROCEEDING message to MO
# page the MT subscriber and ask itI to ask for TCH/H
# once paging completes, we have the TCH/H for the MT end
# send SETUP to MT
# receive CALL CONFIRMED from MT
# set-up the TRAU mux mapping between the E1 subslots for both TCH/H
# receive ALERTING from MT, route ALERTING to MO
# receive CONNECT from MT, confirm to MT with CONNECT_ACK
# send a CONNECT message to MO, receive CONNECT_ACK from MO
* If subscriber is not found:
# send RELEASE COMPLETE with apropriate cause to MO (1: unalloacated 3: no route)
Thoughts about RR/MM:
* we allocate RR/MM entities on demand, when we need them