Normally, in a passive monitoring state of the packet switched control network, traffic information units on a traffic signalling layer can be used to merely monitor the presence and the movement of objects in the network of the physical layer and the packet routing in the packet switched network will correspond to the object movement in the physical layer.
Alternatively, the traffic signalling layer may also comprise traffic information units which can perform a guiding function for the objects on the physical layer. In this case, the packet switched control network can actively control the object traffic on the physical layer by beforehand simulating a packet routing in the packet switched network and by outputting corresponding control information to the traffic information units which will then output corresponding traffic guidance information to the objects on the physical layer.
Such traffic management systems are described in the European patent applications No. 00 125 249.3 and EP No. 00 125 248.5 by the same applicant.
In both aforementioned operation modes, i.e. the passive mode (monitoring) and the active mode (controlling of objects), the packet routing on the traffic control layer may not correspond to the object movement on the physical layer due to various circumstances.
Firstly, of course the packet routing in a packet switched control network between two packet control units is usually much faster than the actual object movement or on the network of the physical layer. Thus, the object may arrive later at a path point, which corresponds to the packet control unit to which the corresponding packet is routed. The same problem may occur if the packet is first stored in the transmitting packet control unit, the object movement takes place and the packet is only transmitted to the target packet control unit after the object arrives at a corresponding path point.
Secondly, even if in the passive mode (monitoring) object movements correspond to packet routings, an object on a physical layer may suddenly stop between two path Points or a new object may suddenly start moving between two path points. Also in such a situation the packet routing may not be corresponding to the object movement.
Thirdly, in the active mode (object control), traffic guidance information can be provided to the objects at path points or even along the path sections to indicate to the objects essentially the succeeding path point to which the object should move in the network. However, the object may decide for various reasons not to follow the indication by the traffic guidance information and may actually move to a succeeding path point such that the path point to which the object has moved will not correspond to the target packet control unit to which the corresponding packet has been routed from the source packet control unit.
In each of the aforementioned cases the packet routing in the packet switched control network is out of synchronization to the object movement on the physical layer. This situation is herein after called the “out of sync” state. Such out of sync states in the traffic management system should be avoided because the packet routing in the packet switched control network will not be a correct reflection of the object movement and thus no accurate monitoring and/or control of the objects can take place.