BSC / MSC volatile state / restart handling

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Neels Hofmeyr nhofmeyr at sysmocom.de
Mon Oct 15 10:40:58 UTC 2018


On Fri, Oct 12, 2018 at 12:10:25PM +0200, Keith wrote:
> With the split setup it involves a complete loss of service, basically,
> as you say, until location update timeout.

Not complete loss of service:

* any phone that contacts the network to get something done will be told to
  re-attach. So sending things and calling out (MO) will work.

* paging (MT) will not work, so contacting a phone from the core is broken
  until periodic LU time has passed. But actually only because the VLR thinks
  the phone is not attached to any cell, and so we don't even try to page it.

An easy way out of this would be to enable a BSS-wide (MSC wide?) paging
mechanism, until the first periodic location updating time has passed -- after
osmo-msc startup, just page any subscriber everywhere, if we think it is not
attached, until the first periodic LU time has passed.  That could be enabled
by config (not on by compile time default, but on in Rhizomatica's cfg). Upon
paging response, the phone would re-attach and things should work again.

That would be considerably less effort than persisting the VLR storage.


Another (less elegant) idea I'm having is that we do have some notion of which
IMSIs were attached in the HLR database. They have the VLR number stored to
match the MSC's identification.  If we could employ some tool to graze the HLR
db, and over a period of time invoke pagings for all those subscribers, we
could blanket re-attach everyone within a short time. I think I'd make that an
external tool using the CTRL interface, with some spread over time to not
insanely load the network.  The one mild problem with this is, it doesn't look
like we notify the HLR on phones detaching. That could be resolved by also
storing a timestamp of when a subscriber was last attached in the HLR, which I
suggest makes sense anyway.


The first idea, "page on all cells until first periodic LU time has passed", is
more elegant because we don't blanket page everyone and don't need external
tools nor changes to the HLR.

~N
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