ransaction over the chain by calling SE23. The level 2 integrator (Int2) in SE10 knows that it belongs to the master, and that SE11 is the service element on the downlink. Thus it sends the event data to SE11. The level 1 integrator (Int1) in SE10 knows that SE11 exists in the logical node with SAPI Y. The Int1 contains some kind of association function. It establishes an association to SE11 in Y and sends the data from Int2 to SE11. The Int2 in SE11 receives a downlink message and therefore knows that it comes from the uplink association. It does not recognise the identity of SE2 and decides to relay the data on the downlink side. The Int2 in SE11 knows that the SE1 element on its downlink side is SE12. The association and sending procedure repeats between the two logical nodes to the right, and the event data from the master are finally terminated and acted upon by SE23. At this point in the scenario, a chain exists between SE10 through SE12. The associations between these elements will be retained until the transaction is com- pletely terminated. The SE23 can respond to the master over the existing associa- tions, and the master can continue sending events to any of SE21 through SE23 over existing associations. If there exists another element (SE13) on the downlink side of SE21, an association to that element will be initiated when SE12 relays the first message to an SE2 element in SE13. By this scenario, only those SE1 ele- ments that are actually used by a transaction will exist in the chain. However, once an element has entered the chain it remains there until the whole chain is released, even if it does nothing more than relaying. This is one of the drawbacks of the method. For efficiency reasons, it is therefore important to construct the chain SO that SE1 elements that are most frequently accessed by the master are allocated close to the master. Note also that the scenario works in a similar way, though in the other direction, when a SE2 element initiates spontaneous events to the master (sometimes calle