Cellular Network Infrastructure » Distributed GSM

h1. VlrPersistency

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.

{{graphviz_link()

digraph G {

  rankdir = LR;

  subgraph cluster_0 {

    vlr [label="MSC/VLR"];

    sgsn [label="SGSN"];

    lhlr [label="Local HLR C"];

    gsupp [label="GSUP Proxy"];

    label = "Village C";

    vlr -> gsupp;

    sgsn -> gsupp;

    gsupp -> lhlr;

    { rank=same; gsupp; lhlr; }

  }

  rhlra [label="Remote HLR A"];

  rhlrb [label="Remote HLR B"];

  gsupp -> rhlra [label="via Backhaul"];

  gsupp -> rhlrb [label="via Backhaul"];

  { rank=same; rhlra; rhlrb; }

}

}}