diff --git a/crates/wzp-video/src/controller.rs b/crates/wzp-video/src/controller.rs
index 0984cf9..3d39a0d 100644
--- a/crates/wzp-video/src/controller.rs
+++ b/crates/wzp-video/src/controller.rs
@@ -9,6 +9,8 @@ use std::sync::atomic::{AtomicU8, AtomicU32, Ordering::Relaxed};
 use wzp_proto::BandwidthEstimator;
 use wzp_proto::PriorityMode;
 
+use crate::simulcast::LayerTarget;
+
 /// Target parameters for the video encoder.
 ///
 /// A `bitrate_kbps` of `0` means video is disabled (not enough bandwidth).
@@ -277,6 +279,67 @@ impl VideoQualityController {
         *self.last_target.lock().unwrap() = smoothed;
         smoothed
     }
+
+    /// Run one simulcast controller tick.
+    ///
+    /// Returns a 3-element array of [`LayerTarget`] in order low → mid → high.
+    /// A layer is marked `active = true` when the current video budget can
+    /// sustain it (including all lower layers).
+    pub fn tick_simulcast(&self, now_ms: u32) -> [LayerTarget; 3] {
+        use crate::simulcast::SimulcastLayer;
+
+        let (_audio_budget, video_budget) = self.allocate();
+
+        let mut result = [
+            LayerTarget {
+                layer: SimulcastLayer::LOW,
+                active: false,
+            },
+            LayerTarget {
+                layer: SimulcastLayer::MID,
+                active: false,
+            },
+            LayerTarget {
+                layer: SimulcastLayer::HIGH,
+                active: false,
+            },
+        ];
+
+        // Cumulative bitrate required to sustain layers up to index i.
+        let cumulative = [
+            SimulcastLayer::LOW.bitrate_kbps,
+            SimulcastLayer::LOW.bitrate_kbps + SimulcastLayer::MID.bitrate_kbps,
+            SimulcastLayer::total_bitrate_kbps(),
+        ];
+
+        for (i, target) in result.iter_mut().enumerate() {
+            target.active = video_budget >= cumulative[i];
+        }
+
+        // Update internal smoothing state using the highest active layer's
+        // bitrate as the representative value.
+        let highest_active = result
+            .iter()
+            .rposition(|t| t.active)
+            .map(|i| cumulative[i])
+            .unwrap_or(0);
+        let raw = if highest_active > 0 {
+            self.derive_target(highest_active)
+        } else {
+            VideoTarget::DISABLED
+        };
+
+        let prev = self.last_tick_ms.swap(now_ms, Relaxed);
+        let dt_ms = if prev == 0 {
+            1000
+        } else {
+            now_ms.saturating_sub(prev)
+        };
+        let smoothed = self.smooth(raw, dt_ms);
+        *self.last_target.lock().unwrap() = smoothed;
+
+        result
+    }
 }
 
 #[cfg(test)]
@@ -432,4 +495,48 @@ mod tests {
         ctrl.set_mode(PriorityMode::Balanced);
         assert_eq!(ctrl.encoder_mode(), crate::EncoderMode::Normal);
     }
+
+    #[test]
+    fn simulcast_all_layers_at_4mbps() {
+        // 4 Mbps → ~3600 kbps video budget after audio floor.
+        let bwe = dummy_bwe(4_000_000);
+        let ctrl = VideoQualityController::new(bwe);
+        let layers = ctrl.tick_simulcast(0);
+        assert!(layers[0].active, "low should be active");
+        assert!(layers[1].active, "mid should be active");
+        assert!(layers[2].active, "high should be active");
+    }
+
+    #[test]
+    fn simulcast_low_mid_only_at_1mbps() {
+        // 1 Mbps → ~900 kbps video budget. High needs 3250 total.
+        let bwe = dummy_bwe(1_000_000);
+        let ctrl = VideoQualityController::new(bwe);
+        let layers = ctrl.tick_simulcast(0);
+        assert!(layers[0].active, "low should be active");
+        assert!(layers[1].active, "mid should be active");
+        assert!(!layers[2].active, "high should be inactive");
+    }
+
+    #[test]
+    fn simulcast_low_only_at_200kbps() {
+        // 200 kbps → ~180 kbps video budget. Mid needs 750 total.
+        let bwe = dummy_bwe(200_000);
+        let ctrl = VideoQualityController::new(bwe);
+        let layers = ctrl.tick_simulcast(0);
+        assert!(layers[0].active, "low should be active");
+        assert!(!layers[1].active, "mid should be inactive");
+        assert!(!layers[2].active, "high should be inactive");
+    }
+
+    #[test]
+    fn simulcast_no_video_at_20kbps() {
+        // 20 kbps → ~18 kbps total. Below audio floor.
+        let bwe = dummy_bwe(20_000);
+        let ctrl = VideoQualityController::new(bwe);
+        let layers = ctrl.tick_simulcast(0);
+        assert!(!layers[0].active, "low should be inactive");
+        assert!(!layers[1].active, "mid should be inactive");
+        assert!(!layers[2].active, "high should be inactive");
+    }
 }
diff --git a/crates/wzp-video/src/lib.rs b/crates/wzp-video/src/lib.rs
index 5f0f8d0..713c50e 100644
--- a/crates/wzp-video/src/lib.rs
+++ b/crates/wzp-video/src/lib.rs
@@ -12,6 +12,7 @@ pub mod encoder_mode;
 pub mod framer;
 pub mod mediacodec;
 pub mod nack;
+pub mod simulcast;
 pub mod videotoolbox;
 
 pub use controller::{VideoQualityController, VideoTarget};
@@ -22,6 +23,7 @@ pub use encoder_mode::EncoderMode;
 pub use framer::{FramedPacket, H264Framer};
 pub use mediacodec::{MediaCodecDecoder, MediaCodecEncoder, MediaCodecHevcDecoder, MediaCodecHevcEncoder};
 pub use nack::{CachedPacket, NackAction, NackReceiver, NackSender};
+pub use simulcast::{LayerPacket, LayerTarget, SimulcastEncoder, SimulcastLayer};
 pub use videotoolbox::{VideoToolboxDecoder, VideoToolboxEncoder, VideoToolboxHevcDecoder, VideoToolboxHevcEncoder};
 
 #[cfg(test)]
diff --git a/crates/wzp-video/src/simulcast.rs b/crates/wzp-video/src/simulcast.rs
new file mode 100644
index 0000000..624261a
--- /dev/null
+++ b/crates/wzp-video/src/simulcast.rs
@@ -0,0 +1,266 @@
+//! Simulcast encoder — drives 3 independent encoder layers per source.
+//!
+//! Each layer emits a separate stream tagged by `stream_id`:
+//! - 0 = low  (480×270,  150 kbps, 15 fps)
+//! - 1 = mid  (960×540,  600 kbps, 30 fps)
+//! - 2 = high (1920×1080, 2500 kbps, 30 fps)
+//!
+//! The sender activates layers based on available bandwidth.  The SFU
+//! (T5.6) selects which layer to forward to each receiver.
+
+use crate::encoder::{VideoEncoder, VideoError, VideoFrame};
+
+/// Configuration for one simulcast layer.
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub struct SimulcastLayer {
+    /// `stream_id` placed in `MediaHeader` v2.
+    pub stream_id: u8,
+    /// Target width in pixels.
+    pub width: u32,
+    /// Target height in pixels.
+    pub height: u32,
+    /// Target bitrate in kbps.
+    pub bitrate_kbps: u32,
+    /// Target frame rate.
+    pub fps: u8,
+}
+
+impl SimulcastLayer {
+    /// Low layer — 480×270 @ 150 kbps, 15 fps.
+    pub const LOW: Self = Self {
+        stream_id: 0,
+        width: 480,
+        height: 270,
+        bitrate_kbps: 150,
+        fps: 15,
+    };
+
+    /// Mid layer — 960×540 @ 600 kbps, 30 fps.
+    pub const MID: Self = Self {
+        stream_id: 1,
+        width: 960,
+        height: 540,
+        bitrate_kbps: 600,
+        fps: 30,
+    };
+
+    /// High layer — 1920×1080 @ 2500 kbps, 30 fps.
+    pub const HIGH: Self = Self {
+        stream_id: 2,
+        width: 1920,
+        height: 1080,
+        bitrate_kbps: 2500,
+        fps: 30,
+    };
+
+    /// All three layers in ascending order.
+    pub const ALL: [Self; 3] = [Self::LOW, Self::MID, Self::HIGH];
+
+    /// Total bitrate of all layers in kbps.
+    pub const fn total_bitrate_kbps() -> u32 {
+        Self::LOW.bitrate_kbps + Self::MID.bitrate_kbps + Self::HIGH.bitrate_kbps
+    }
+}
+
+/// Active target for one layer.
+#[derive(Clone, Debug, PartialEq, Eq)]
+pub struct LayerTarget {
+    pub layer: SimulcastLayer,
+    pub active: bool,
+}
+
+/// Result of one simulcast encode call.
+#[derive(Clone, Debug, PartialEq, Eq)]
+pub struct LayerPacket {
+    pub stream_id: u8,
+    pub data: Vec<u8>,
+}
+
+/// Simulcast encoder manager.
+///
+/// Holds up to three [`VideoEncoder`] instances (one per layer).  On each
+/// incoming frame it feeds the frame to every active encoder and collects
+/// the resulting access units tagged by `stream_id`.
+pub struct SimulcastEncoder {
+    layers: Vec<LayerState>,
+}
+
+struct LayerState {
+    config: SimulcastLayer,
+    encoder: Box<dyn VideoEncoder>,
+    active: bool,
+}
+
+impl SimulcastEncoder {
+    /// Create a new simulcast encoder from a factory function.
+    ///
+    /// `factory` is called once per layer with `(width, height, bitrate_bps)`.
+    /// On failure for any layer the whole constructor fails.
+    pub fn new<F>(mut factory: F) -> Result<Self, VideoError>
+    where
+        F: FnMut(u32, u32, u32) -> Result<Box<dyn VideoEncoder>, VideoError>,
+    {
+        let mut layers = Vec::with_capacity(3);
+        for cfg in SimulcastLayer::ALL {
+            let encoder = factory(cfg.width, cfg.height, cfg.bitrate_kbps * 1000)?;
+            layers.push(LayerState {
+                config: cfg,
+                encoder,
+                active: true,
+            });
+        }
+        Ok(Self { layers })
+    }
+
+    /// Encode one raw frame on all active layers.
+    ///
+    /// Returns a vector of `(stream_id, access_unit)` pairs, one per active
+    /// layer that produced output.
+    pub fn encode(&mut self, frame: &VideoFrame) -> Result<Vec<LayerPacket>, VideoError> {
+        let mut out = Vec::with_capacity(self.layers.len());
+        for layer in &mut self.layers {
+            if !layer.active {
+                continue;
+            }
+            let data = layer.encoder.encode(frame)?;
+            if !data.is_empty() {
+                out.push(LayerPacket {
+                    stream_id: layer.config.stream_id,
+                    data,
+                });
+            }
+        }
+        Ok(out)
+    }
+
+    /// Request a keyframe on all active layers.
+    pub fn request_keyframe(&mut self) {
+        for layer in &mut self.layers {
+            if layer.active {
+                layer.encoder.request_keyframe();
+            }
+        }
+    }
+
+    /// Enable or disable individual layers.
+    ///
+    /// `mask` is a 3-bit mask where bit *i* controls layer *i*.
+    ///   bit 0 → low layer
+    ///   bit 1 → mid layer
+    ///   bit 2 → high layer
+    pub fn set_layer_mask(&mut self, mask: u8) {
+        for (idx, layer) in self.layers.iter_mut().enumerate() {
+            layer.active = (mask >> idx) & 1 != 0;
+        }
+    }
+
+    /// Current layer mask (3-bit).
+    pub fn layer_mask(&self) -> u8 {
+        let mut mask = 0u8;
+        for (idx, layer) in self.layers.iter().enumerate() {
+            if layer.active {
+                mask |= 1 << idx;
+            }
+        }
+        mask
+    }
+
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::encoder::{VideoEncoder, VideoError, VideoFrame};
+
+    struct DummyEncoder {
+        stream_id: u8,
+        force_keyframe: bool,
+    }
+
+    impl VideoEncoder for DummyEncoder {
+        fn encode(&mut self, frame: &VideoFrame) -> Result<Vec<u8>, VideoError> {
+            let mut out = vec![self.stream_id];
+            out.extend_from_slice(&frame.data);
+            Ok(out)
+        }
+
+        fn request_keyframe(&mut self) {
+            self.force_keyframe = true;
+        }
+
+        fn is_keyframe(&self, _packet: &[u8]) -> bool {
+            false
+        }
+    }
+
+    fn dummy_factory(stream_counter: &mut u8) -> impl FnMut(u32, u32, u32) -> Result<Box<dyn VideoEncoder>, VideoError> + '_ {
+        move |_w, _h, _br| {
+            let enc = DummyEncoder {
+                stream_id: *stream_counter,
+                force_keyframe: false,
+            };
+            *stream_counter += 1;
+            Ok(Box::new(enc))
+        }
+    }
+
+    #[test]
+    fn simulcast_encoder_creates_three_layers() {
+        let mut counter = 0u8;
+        let enc = SimulcastEncoder::new(dummy_factory(&mut counter));
+        assert!(enc.is_ok());
+        let enc = enc.unwrap();
+        assert_eq!(enc.layer_mask(), 0b111);
+    }
+
+    #[test]
+    fn simulcast_encode_produces_three_packets() {
+        let mut counter = 0u8;
+        let mut enc = SimulcastEncoder::new(dummy_factory(&mut counter)).unwrap();
+        let frame = VideoFrame::new(1920, 1080, vec![0xAB; 100], 0);
+        let packets = enc.encode(&frame).unwrap();
+        assert_eq!(packets.len(), 3);
+        assert_eq!(packets[0].stream_id, 0);
+        assert_eq!(packets[1].stream_id, 1);
+        assert_eq!(packets[2].stream_id, 2);
+    }
+
+    #[test]
+    fn simulcast_layer_mask_disables_layers() {
+        let mut counter = 0u8;
+        let mut enc = SimulcastEncoder::new(dummy_factory(&mut counter)).unwrap();
+        enc.set_layer_mask(0b101); // low + high, no mid
+        assert_eq!(enc.layer_mask(), 0b101);
+
+        let frame = VideoFrame::new(1920, 1080, vec![0xCD; 100], 0);
+        let packets = enc.encode(&frame).unwrap();
+        assert_eq!(packets.len(), 2);
+        assert_eq!(packets[0].stream_id, 0);
+        assert_eq!(packets[1].stream_id, 2);
+    }
+
+    #[test]
+    fn simulcast_request_keyframe_propagates() {
+        let mut counter = 0u8;
+        let mut enc = SimulcastEncoder::new(dummy_factory(&mut counter)).unwrap();
+        enc.request_keyframe();
+        // DummyEncoder sets force_keyframe flag; we can't inspect it directly
+        // because it's inside the Box, but the call should not panic.
+    }
+
+    #[test]
+    fn simulcast_layer_total_bitrate() {
+        assert_eq!(SimulcastLayer::total_bitrate_kbps(), 150 + 600 + 2500);
+    }
+
+    #[test]
+    fn simulcast_all_layers_ordered() {
+        let all = SimulcastLayer::ALL;
+        assert_eq!(all[0].stream_id, 0);
+        assert_eq!(all[1].stream_id, 1);
+        assert_eq!(all[2].stream_id, 2);
+        assert_eq!(all[0].width, 480);
+        assert_eq!(all[1].width, 960);
+        assert_eq!(all[2].width, 1920);
+    }
+}