Simulation Frame

The simulation frame represents an internal clock that every Client syncs with a Replication Server. This clock runs at a 60Hz frequency which means that the resolution of a single simulation frame is ~16ms.

There are 3 different simulation frame types used within the coherence:

Simulation Frame

Accessible via CoherenceBridge.NetworkTime.

void Start()
    if (!CoherenceBridgeStore.TryGetBridge(gameObject.scene, out var bridge))
        Debug.LogWarning("Couldn't find CoherenceBridge in the scene.");
    var networkTime = bridge.NetworkTime;

    var serverFrame = networkTime.ServerSimulationFrame.Frame;
    Debug.Log("Server Simulation Frame: " + serverFrame);
    var clientFrame = networkTime.ClientSimulationFrame.Frame;
    Debug.Log("Client Simulation Frame: " + clientFrame);

Every Client tries to match the Client simulation frame with the Server simulation frame by continuously monitoring the distance between the two and adjusting the NetworkTime.NetworkTimeScale based on the distance, ping, delta time, and several other factors starting from the first simulation frame captured when the client first connects in NetworkTime.ConnectionSimulationFrame

Unity's Time.timeScale is automatically set to the value of NetworkTime.NetworkTimeScale if the CoherenceBridge.controlTimeScale is set to true (default value).

In perfect conditions, all Clients connected to a single session should have exactly the same ClientSimulationFrame value at any point in the real-world time.

The value of the ClientSimulationFrame can jump by more than 1 between two engine frames if the frame rate is low enough.

The Client simulation frame is used to timestamp any outgoing Entity changes to achieve a consistent view of the World for all players. The receiving side uses it for interpolation of the synced values.

Client fixed simulation frame

Local simulation frame that progresses in user-controlled fixed steps. Accessible via CoherenceBridge.ClientFixedSimulationFrame.

By default, the fixed step value is set to the Time.fixedDeltaTime.

Just like the basic Client simulation frame, it uses the NetworkTime.NetworkTimeScale to correct the drift. The fixed simulation frame is used as a base for the fixed-step, network-driven simulation loop that is run via CoherenceBridge.OnFixedNetworkUpdate. This loop is used internally to power the CoherenceInput and the GGPO code.

Unlike ClientSimulationFrame, the CoherenceBridge.OnFixedNetworkUpdate loop never skips frames - it is guaranteed to run for every single frame increment.

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