msc {
  hscale="2";
  ms [label="MS"], bts [label="BTS"], bsc [label="BSC"], user [label="User"];

  user -> bsc [label="Establish VTY connection"];
  user -> bsc [label="VTY command"];
  bsc -> bts [label="08.58 RSL SMS BROADCAST CMD"];
  bts -> ms [label="04.12 SMS BC REQ (1st block)"];
  bts -> ms [label="04.12 SMS BC REQ (2nd block)"];
  bts -> ms [label="04.12 SMS BC REQ (3rd block)"];
  bts -> ms [label="04.12 SMS BC REQ (4th block)"];
}