3GPP TS 23.236 specifies the "Intra-domain connection of
Radio Access Network (RAN nodes to multiple Core Network (CN) nodes".
This introduces two concepts:
* a deviation from the strict hierarchy of classic GSM architecture where a BSC is always only served by one MSC
* the introduction of "MSC pools" where a pool of multiple MSCs cover one geographic service area.
This appears to be functioning by dividing the TMSI range between different MSCs, using the higher-order TMSI bits to identify the MSC and the lower-order TMSI bits for the actual TMSI within that MSC.
Section 5.3 describes the functions required to be implemented in the BSC.
* *NAS Node Selection Function*
** investigate Initial L3 message at RR establishment
** if TMSI is present, extract NRI and route requeest to MSC for that NRI
** if only IMEI or IMSI is present, or no route for that NRI, send to 'ramdom' MSC (load distribution)
* when any MSC sends a PAGING REQUEST, store the IMSI <-> MSC relation as long as the paging is active
** any incoming response to that paging gets routed to the MSC that has issued the paging