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+Overview
+========
+
+Gammu SMS Daemon is a program that periodically scans GSM modem for received
+messages, stores them in defined storage and also sends messages enqueued in
+this storage.
+
+Overall schema
+--------------
+
+The interactions of SMS Daemon and related components can be seen on following
+picture.
+
+.. graphviz::
+
+   digraph smsdinteractions {
+       "gammu-smsd-inject" [shape=box];
+       "gammu-smsd" [shape=box];
+       "Run on receive" [shape=box, style=dotted];
+       "GSM modem" [shape=box, style=dashed];
+       "gammu-smsd-monitor" [shape=box];
+       "Service\nstorage" [shape=doublecircle];
+       "Kalkun" [shape=box, style=dotted];
+       "gammu-smsd" -> "GSM modem" [label="Outgoing"];
+       "GSM modem" -> "gammu-smsd" [label="Incoming"];
+       "gammu-smsd" -> "Service\nstorage" [label="Save"];
+       "Service\nstorage" -> "gammu-smsd" [label="Read"];
+       "gammu-smsd" -> "Run on receive" [label="Execute"];
+       "Service\nstorage" -> "Run on receive" [label="Read"];
+       "Kalkun" -> "Service\nstorage" [label="Inject"];
+       "Service\nstorage" -> "Kalkun" [label="Read"];
+       "gammu-smsd-inject" -> "Service\nstorage" [label="Inject"];
+       "gammu-smsd" -> "gammu-smsd-monitor" [label="Monitor"];
+   }
+
+SMSD operation
+--------------
+
+The SMSD operation consist of several steps.
+
+1. Process command line options.
+2. Configure backend service.
+3. Main loop is executed until it is signalled to be terminated.
+    1. Try to connect to phone if not connected.
+    2. Check for security code if configured (configured by ``CheckSecurity``).
+    3. Check for received messages (frequency configured by ``ReceiveFrequency``).
+    4. Check for reset of the phone if configured (frequency configured by ``ResetFrequency``).
+    5. Check for messages to send (frequency configured by ``CommTimeout``).
+    6. Check phone status (frequency configured by ``StatusFrequency``).
+    7. Sleep for defined time (``LoopSleep``).
+4. Backend service is freed.