Synchronising churning parameter updates in an optical network

An optical network unit (ONU) 10 has a memory 11 for storing "churning parameters" (or encryption parameters) indicating which virtual paths (VP) are "churned" (encrypted) and means 12 for "dechurning" (decrypting) data streams received from an optical line terminal (OLT) using a key. In one embodiment, the memory has a first memory bank M11a for storing parameters currently used and a second memory bank M11b for storing updates to the parameters. When a key updating time point is reached, the new updates are activated and applied to subsequent frames. In an alternative embodiment (figure 12), the memory of the ONU has a single storage area and messages specifying churning parameter updates are immediately applied to subsequent frames. The optical network is preferably an ATM based Passive Optical Network (PON). An optical line terminal for transmitting churning parameters based on the status of flags is also disclosed.