diff --git a/firmware/lib/session/SessionManager.h b/firmware/lib/session/SessionManager.h
index ea974be..e9f812c 100644
--- a/firmware/lib/session/SessionManager.h
+++ b/firmware/lib/session/SessionManager.h
@@ -8,6 +8,8 @@ template <typename SessionT>
 class SessionManager
 {
 public:
+    using MeasurementType = typename SessionT::MeasurementType;
+    
     SessionManager(int scaleDoutPin, int scaleSckPin, uint8_t tareAvgCount);
 
     void begin();
diff --git a/firmware/lib/session/SimpleMeasurementSession.h b/firmware/lib/session/SimpleMeasurementSession.h
index 87081e7..8d9fbd8 100644
--- a/firmware/lib/session/SimpleMeasurementSession.h
+++ b/firmware/lib/session/SimpleMeasurementSession.h
@@ -7,7 +7,7 @@ class SimpleMeasurementSession
 {
 public:
     using ChunkT = SessionChunk<Measurement_T, MAX_SIZE>;
-
+    using MeasurementType = Measurement_T;
     // save interval in number of measurements (by default every minute)
     SimpleMeasurementSession(uint32_t saveInterval = 10 * 60)
         : chunk(nullptr), saveInterval_(saveInterval)
@@ -66,6 +66,11 @@ public:
         encoder.sendArray(chunk->getDataPointer() + startIdx, numElementsToSend);
     }
 
+    Measurement_T *getDataPointer()
+    {
+        return chunk->getDataPointer();
+    }
+
 private:
     void saveToFileSystem()
     {
diff --git a/firmware/platformio.ini b/firmware/platformio.ini
index 4542eaa..3eb5573 100644
--- a/firmware/platformio.ini
+++ b/firmware/platformio.ini
@@ -14,7 +14,7 @@ data_dir = data
 [env:esp32]
 platform = espressif32
 platform_packages = 
-    framework-arduinoespressif32 @ https://github.com/espressif/arduino-esp32.git
+    framework-arduinoespressif32 @ https://github.com/espressif/arduino-esp32
 board = esp-wrover-kit
 #platform = espressif8266
 #board = esp_wroom_02
@@ -25,10 +25,9 @@ monitor_port = /dev/ttyUSB0
 upload_port = /dev/ttyUSB0
 monitor_speed = 115200
 lib_deps =
-    https://github.com/mabau/ESPAsyncWebServer.git
-    AsyncTCP
     NTPClient
     HX711@0.7.4
+    https://github.com/gilmaimon/ArduinoWebsockets.git
 src_filter = +<*> -<native_main.cpp>
 
 [env:native]
diff --git a/firmware/src/SwimTrackerConfig.h b/firmware/src/SwimTrackerConfig.h
index 859d337..3799757 100644
--- a/firmware/src/SwimTrackerConfig.h
+++ b/firmware/src/SwimTrackerConfig.h
@@ -4,13 +4,13 @@
 
 
 // Uncomment for Version 2.0 where load cell is connected differently
-#define _HW_V_20
-//#define NEW_HEAVY_LOAD_CELL
+//#define _HW_V_20
+#define NEW_HEAVY_LOAD_CELL
 
 // ------------------------------------------ WiFi ---------------------------------------------------------------------------------
 
-//const char *CONFIG_WIFI_SSID = "WLAN";
-const char *CONFIG_WIFI_SSID = "RepeaterWZ";
+const char *CONFIG_WIFI_SSID = "WLAN";
+//const char *CONFIG_WIFI_SSID = "RepeaterWZ";
 const char *CONFIG_WIFI_PASSWORD = "Bau3rWLAN";
 const char *CONFIG_HOSTNAME = "smartswim";
 
diff --git a/firmware/src/WebsocketServer.h b/firmware/src/WebsocketServer.h
new file mode 100644
index 0000000..f7ab0da
--- /dev/null
+++ b/firmware/src/WebsocketServer.h
@@ -0,0 +1,237 @@
+
+#pragma once
+#include "Dtypes.h"
+#include <ArduinoWebsockets.h>
+
+template <typename T>
+class SessionManager;
+
+static constexpr int MAX_CONNECTIONS = 3;
+static constexpr int NUM_DATA_CHUNK_SIZE = 1;
+
+template <typename SessionT>
+class WebsocketServer
+{
+public:
+    WebsocketServer(SessionManager<SessionT> &sessionManager, int port)
+        : sessionManager_(sessionManager), nextFreeClient_(0), port_(port),
+          numSentMeasurements_(0), running_(false)
+    {
+    }
+
+    void begin()
+    {
+        server_.listen(port_);
+    }
+
+    void iteration();
+
+private:
+    void reportSessionUpdate();
+
+    void sendMessageOnConnection(websockets::WebsocketsClient &client);
+    void sendSessionStartMessages();
+    void sendSessionStopMessages();
+    void sendNewDataMessages();
+
+    SessionManager<SessionT> &sessionManager_;
+    int nextFreeClient_;
+    int port_;
+
+    size_t sentMessageCount_;
+    websockets::WebsocketsServer server_;
+    websockets::WebsocketsClient clients_[MAX_CONNECTIONS];
+
+    // previous session state
+    size_t numSentMeasurements_;
+    bool running_;
+};
+
+using websockets::WebsocketsClient;
+
+// ------------------------------------- Message types & classes ---------------------------
+
+enum MessageType
+{
+    INITIAL_INFO = 1,
+    SESSION_STARTED = 2,
+    SESSION_STOPPED = 3,
+    SESSION_NEW_DATA = 4
+};
+
+#pragma pack(push, 1)
+class SessionStartedMessage
+{
+public:
+    SessionStartedMessage(uint32_t id) : messageType_(SESSION_STARTED), sessionId_(id) {}
+    void send(WebsocketsClient &c) const
+    {
+        c.sendBinary((const char *)(this), sizeof(*this));
+    }
+
+private:
+    uint8_t messageType_;
+    uint32_t sessionId_;
+};
+
+class SessionStoppedMessage
+{
+public:
+    SessionStoppedMessage() : messageType_(SESSION_STOPPED) {}
+    void send(WebsocketsClient &c) const
+    {
+        c.sendBinary((const char *)(this), sizeof(*this));
+    }
+
+private:
+    uint8_t messageType_;
+};
+
+template <typename MeasurementT>
+class SessionNewDataMessage
+{
+public:
+    // typically a message contains NUM_DATA_CHUNK_SIZE measurements
+    // if some measurements are skipped, because loop() takes too long
+    // there might actually be more measurements, to be safe there is an
+    // additional factor here
+    static constexpr size_t MAX_MEASUREMENTS = 4 * NUM_DATA_CHUNK_SIZE;
+
+    SessionNewDataMessage(MeasurementT *ptr, size_t numMeasurements)
+        : messageType_(SESSION_NEW_DATA), numMeasurements_(min(numMeasurements, MAX_MEASUREMENTS))
+    {
+        memcpy(measurements_, ptr, sizeof(MeasurementT) * numMeasurements_);
+    }
+
+    void send(WebsocketsClient &c) const
+    {
+        c.sendBinary((const char *)(this), numBytes());
+    }
+
+    size_t numMeasurements() const
+    {
+        return numMeasurements_;
+    }
+
+private:
+    size_t numBytes() const { return sizeof(uint8_t) + numMeasurements() * sizeof(MeasurementT); }
+
+    // data to be sent
+    uint8_t messageType_;
+    MeasurementT measurements_[MAX_MEASUREMENTS];
+
+    // book-keeping
+    size_t numMeasurements_;
+};
+#pragma pack(pop)
+
+// ------------------------------------- WebsocketServer members ---------------------------
+
+template <typename SessionT>
+void WebsocketServer<SessionT>::iteration()
+{
+    if (server_.poll())
+    {
+        clients_[nextFreeClient_] = server_.accept();
+        //clients_[nextFreeClient_].onMessage(onMessage); // TODO
+        Serial.println("new websocket connection");
+        sendMessageOnConnection(clients_[nextFreeClient_]);
+        nextFreeClient_ = (nextFreeClient_ + 1) % MAX_CONNECTIONS;
+    }
+
+    for (int i = 0; i < MAX_CONNECTIONS; ++i)
+        clients_[i].poll();
+
+    reportSessionUpdate();
+}
+
+template <typename SessionT>
+void WebsocketServer<SessionT>::reportSessionUpdate()
+{
+    auto &session = sessionManager_.session();
+
+    // start/stop messages
+    if (!running_ && sessionManager_.isMeasuring())
+        sendSessionStartMessages();
+    else if (running_ && !sessionManager_.isMeasuring())
+        sendSessionStopMessages();
+
+    // new data
+    if (session.numMeasurements() - (NUM_DATA_CHUNK_SIZE - 1) > numSentMeasurements_)
+    {
+        sendNewDataMessages();
+    }
+}
+
+template <typename SessionT>
+void WebsocketServer<SessionT>::sendSessionStartMessages()
+{
+    SessionStartedMessage msg(sessionManager_.session().getStartTime());
+    for (auto &c : clients_)
+        if (c.available())
+            msg.send(c);
+    running_ = sessionManager_.isMeasuring();
+}
+
+template <typename SessionT>
+void WebsocketServer<SessionT>::sendSessionStopMessages()
+{
+    SessionStoppedMessage msg;
+    for (auto &c : clients_)
+        if (c.available())
+            msg.send(c);
+    running_ = sessionManager_.isMeasuring();
+}
+
+template <typename SessionT>
+void WebsocketServer<SessionT>::sendNewDataMessages()
+{
+    using MeasurementT = typename SessionT::MeasurementType;
+    auto &session = sessionManager_.session();
+    MeasurementT *dataToSend = session.getDataPointer() + numSentMeasurements_;
+    size_t numMeasurementsToSend = session.numMeasurements() - numSentMeasurements_;
+    SessionNewDataMessage<MeasurementT> msg(dataToSend, numMeasurementsToSend);
+
+    for (auto &c : clients_)
+        if (c.available())
+            msg.send(c);
+
+    numSentMeasurements_ += msg.numMeasurements();
+}
+
+template <typename SessionT>
+void WebsocketServer<SessionT>::sendMessageOnConnection(WebsocketsClient &client)
+{
+    using MeasurementT = typename SessionT::MeasurementType;
+
+    // Message format:
+    //  - uint8_t messageType
+    //  - uint8_t running
+    //  - uint32_t sessionId
+    //  - MeasurementT [] measurements (if running)
+
+    auto &session = sessionManager_.session();
+    const auto numMeasurements = session.numMeasurements();
+    const auto sessionId = session.getStartTime();
+
+    const size_t msgSize = sizeof(uint8_t) + sizeof(uint8_t) + sizeof(sessionId) + sizeof(MeasurementT) * numMeasurements;
+    char *msg = (char *)heap_caps_malloc(msgSize, MALLOC_CAP_SPIRAM);
+
+    char *writeHead = msg;
+
+    *writeHead = INITIAL_INFO;
+    writeHead += sizeof(uint8_t);
+
+    *writeHead = sessionManager_.isMeasuring();
+    writeHead += sizeof(uint8_t);
+
+    *((uint32_t *)writeHead) = sessionManager_.isMeasuring() ? sessionId : 0;
+    writeHead += sizeof(uint32_t);
+
+    assert(writeHead - msg == msgSize - sizeof(MeasurementT) * numMeasurements);
+
+    memcpy(writeHead, session.getDataPointer(), sizeof(MeasurementT) * numMeasurements);
+    client.sendBinary(msg, msgSize);
+
+    free(msg);
+}
diff --git a/firmware/src/firmware_main.cpp b/firmware/src/firmware_main.cpp
index 17a1c75..db22030 100644
--- a/firmware/src/firmware_main.cpp
+++ b/firmware/src/firmware_main.cpp
@@ -4,6 +4,8 @@
 #include "SwimTrackerConfig.h"
 
 #include <WiFi.h>
+#include <HTTPClient.h>
+
 
 // Own libs
 #include "MockScale.h"
@@ -14,11 +16,13 @@
 #include "SimpleMeasurementSession.h"
 #include "EspHttp.h"
 #include "WebDAV.h"
+#include "WebsocketServer.h"
 
 using Session_T = SimpleMeasurementSession<MeasurementT, CONFIG_SESSION_MAX_SIZE>;
 SessionManager<Session_T> sessionManager(CONFIG_SCALE_DOUT_PIN, CONFIG_SCALE_SCK_PIN, CONFIG_TARE_AVG_COUNT);
 
 EspHttp espHttpServer;
+WebsocketServer<Session_T> webSocketServer(sessionManager, 81);
 
 template <typename SessionT>
 void httpSetup(SessionManager<SessionT> *sessionManager)
@@ -175,9 +179,43 @@ void setup()
 
     // HTTP & Websocket server
     httpSetup(&sessionManager);
+
+    webSocketServer.begin();
 }
 
+int measurementsSent = 0;
+
 void loop()
 {
     sessionManager.iteration();
+    webSocketServer.iteration();
+    /*
+    if (webSocketServer.poll())
+    {
+        websocketClients[nextFreeWebsocketClient] = webSocketServer.accept();
+        websocketClients[nextFreeWebsocketClient].onMessage(onMessage);
+        Serial.println("Websocket connection");
+        nextFreeWebsocketClient = (nextFreeWebsocketClient + 1) % MAX_WEBSOCKET_CONNECTIONS;
+    }
+    for (int i = 0; i < MAX_WEBSOCKET_CONNECTIONS; ++i)
+        //if (websocketClients[i].available()) {
+            //Serial.printf("Polling client %d\n", i);
+            websocketClients[i].poll();
+        //}
+
+    auto &session = sessionManager.session();
+    if (session.numMeasurements() < measurementsSent)
+        measurementsSent = 0;
+    else if (session.numMeasurements() > measurementsSent)
+    {
+        for (int i = 0; i < MAX_WEBSOCKET_CONNECTIONS; ++i)
+            if (websocketClients[i].available())
+            {
+                auto dataToSend = (const char*)(session.getDataPointer() + measurementsSent);
+                auto numBytes = (session.numMeasurements() - measurementsSent) * sizeof(MeasurementT);
+                Serial.printf("Sent %d bytes via websocket\n", numBytes);
+                websocketClients[i].sendBinary(dataToSend, numBytes);
+                measurementsSent = session.numMeasurements();
+            }
+    }*/
 }
diff --git a/firmware/websocket_test.py b/firmware/websocket_test.py
new file mode 100644
index 0000000..d37fd32
--- /dev/null
+++ b/firmware/websocket_test.py
@@ -0,0 +1,36 @@
+import asyncio
+import websockets
+import struct
+import numpy as np
+
+INITIAL_INFO = 1
+SESSION_STARTED = 2
+SESSION_STOPPED = 3
+SESSION_NEW_DATA = 4
+
+
+async def hello():
+    uri = "ws://192.168.178.110:81"
+    async with websockets.connect(uri) as websocket:
+        while True:
+            res = await websocket.recv()
+            msg_type = struct.unpack("<B", res[:1])[0]
+            payload = res[1:]
+            if msg_type == INITIAL_INFO:
+                running = struct.unpack("<B", payload[0:1])[0]
+                session_id = struct.unpack("<I", payload[1:5])[0]
+                data = np.frombuffer(payload[5:], dtype=np.uint16)
+                print(f"Initial info: running {running} session_id {session_id} data", data)
+            elif msg_type == SESSION_STARTED:
+                id = struct.unpack("<I", payload)[0]
+                print(f"Session with id {id} started")
+            elif msg_type == SESSION_STOPPED:
+                assert len(payload) == 0
+                print("Session stopped")
+            elif msg_type == SESSION_NEW_DATA:
+                data = np.frombuffer(payload, dtype=np.uint16)
+                print("New data", data)
+            else:
+                print("Got unexpected packet of type", msg_type)
+
+asyncio.get_event_loop().run_until_complete(hello())