Layered application and inter layer communication

I am designing a service which has multiple layers.(To be precise 5 layers).

Design rules:

Each layer can only talk to the immediate layer beneath it.

Communication between two of them would be asynchronous.

Data Flow would be bottom up, that is when data arrives at the lower layers,the upper layers get notifications.

Subject-Observer pattern comes to my mind but I feel this tightly couples the two layers.

Is there anything that I can do with signal/slots mechanism .I havent worked on it but boost does give something similar, I guess.

Has anybody come across this problem/are there any better design solutions??