msc {
  hscale="2";
  ms[label="MS,BSS"],__msc[label="MSC"],hlr[label="HLR proxy"],home[label="Home HLR"];

  ms => __msc [label="CM Service Request / Paging Response"];
  __msc => hlr [label="Send Auth Info Request"];
  hlr rbox hlr [label="Use already set up proxy path"];
  hlr abox hlr [label="there still are unsent auth tuples\nin the cache"];
  hlr rbox hlr [label="(3) Send cached, fresh tuples"];
  __msc <= hlr [label="Send Auth Info Result\ncontaining auth tuples\nfrom the proxy cache"];
  ms rbox __msc [label="Authentication"];
  ms rbox __msc [label="Continue the CM Service / Paging action"];
  hlr abox hlr [label="Note that there are no/few unused tuples in the cache, fill up again"];
  hlr rbox hlr [label="(4) Ask for more auth tuples to cache"];
  hlr => home [label="Send Auth Info Request"];
  --- [label="If the home HLR link is not working"];
  hlr abox hlr [label="no link\nor\nresponse timeout"];
  hlr rbox hlr [label="(5) Set up a timer to retry SAI\n(a few minutes?)"];
  hlr abox hlr [label="Timer triggers"];
  hlr => home [label="Send Auth Info Request"];
  --- [label="If the home HLR link is functional"];
  hlr <= home [label="Send Auth Info Result"];
  hlr rbox hlr [label="store 5 more tuples"];
}