The VLR keeps a cache of the currently attached subscribers of a cellular network. This means that in absence of the VLR-HLR link, existing / registered subscribers can still perform SMS/voice transactions, but that new registrations (of subscribers of the given unreachable HLR) cannot be processed.
One concern in some use cases is that frequent / short power outages would invalidate this inn-memory VLR state, and that in absence of the back-haul link, subscribers of the given (remote) HLR cannot register.
Please note this only affects inbound roaming subscribers, as the home subscribers are assumed to have their HLR locally next to the VLR/MSC and hence there cannot be any back-haul outage.
If we want to make sure inbound roaming subscribers can still re-register after a power outage of the visited VLR/MSC, then we have to somehow make the VLR state persistent. The same applies to the SGSN state which also contains something like a VLR.
Rather than changing the VLR/SGSN code, it is proposed to use a caching GSUP proxy between the VLR+SGSN and the various [remote] HLRs. This caching proxy would then keep a cache of all InsertSubscriberData and SendAuthInfo data, so that a subsequent re-registration of an inbound roamer can proceed.