diff --git a/.gitignore b/.gitignore
index 4fbb073..235a653 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,2 +1,3 @@
 /vendor/
 /composer.lock
+/.phpunit.*
diff --git a/composer.json b/composer.json
index d447569..f9c84b5 100644
--- a/composer.json
+++ b/composer.json
@@ -16,5 +16,8 @@
     ],
     "require": {
         "react/http": "^1.9.0"
+    },
+    "require-dev": {
+        "phpunit/phpunit": "^11.0"
     }
 }
diff --git a/phpunit.xml b/phpunit.xml
new file mode 100644
index 0000000..0e66c06
--- /dev/null
+++ b/phpunit.xml
@@ -0,0 +1,23 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<phpunit xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:noNamespaceSchemaLocation="https://schema.phpunit.de/11.0/phpunit.xsd"
+         bootstrap="vendor/autoload.php"
+         cacheDirectory=".phpunit.cache"
+         executionOrder="depends,defects"
+         requireCoverageMetadata="true"
+         beStrictAboutCoverageMetadata="true"
+         beStrictAboutOutputDuringTests="true"
+         failOnRisky="true"
+         failOnWarning="true">
+    <testsuites>
+        <testsuite name="default">
+            <directory>tests</directory>
+        </testsuite>
+    </testsuites>
+
+    <source restrictDeprecations="true" restrictNotices="true" restrictWarnings="true">
+        <include>
+            <directory>src</directory>
+        </include>
+    </source>
+</phpunit>
diff --git a/src/WebSocketCodec.php b/src/WebSocketCodec.php
new file mode 100644
index 0000000..de9cbcd
--- /dev/null
+++ b/src/WebSocketCodec.php
@@ -0,0 +1,189 @@
+<?php
+
+namespace NoccyLabs\React\WebSocket;
+
+use Evenement\EventEmitterTrait;
+use NoccyLabs\React\WebSocket\Group\ConnectionGroup;
+use NoccyLabs\React\WebSocket\Group\GroupManager;
+use Psr\Http\Message\ServerRequestInterface;
+use React\Http\Message\Response;
+use React\Socket\ConnectionInterface;
+use React\Stream\CompositeStream;
+use React\Stream\DuplexStreamInterface;
+use React\Stream\ReadableStreamInterface;
+use React\Stream\ThroughStream;
+use React\Stream\WritableStreamInterface;
+
+class WebSocketCodec
+{
+
+    const OP_CONTINUATION = 0x0;
+    const OP_FRAME_TEXT = 0x1;
+    const OP_FRAME_BINARY = 0x2;
+    const OP_CLOSE = 0x8;
+    const OP_PING = 0x9;
+    const OP_PONG = 0xA;
+
+    public function encode(array $frame): string
+    {
+        // Encoded frame
+        $encoded = null;
+
+        // Unpack frame with defaults
+        $frame = [
+            ...[
+                'final' => true,
+                'opcode' => null,
+                'masked' => false,
+                'mask' => null,
+                'rsv1' => false,
+                'rsv2' => false,
+                'rsv3' => false,
+                'payload' => null
+            ],
+            ...$frame
+        ];
+
+        $len = strlen($frame['payload']);
+        if ($len > 65535) {
+            $size0 = 127;
+            $size1 = ($len >> 24) & 0xFF;
+            $size2 = ($len >> 16) & 0xFF;
+            $size3 = ($len >> 8) & 0xFF;
+            $size4 = $len & 0xFF;
+        } elseif ($len > 126) {
+            $size0 = 126;
+            $size1 = ($len >> 8) & 0xFF;
+            $size2 = $len & 0xFF;
+            $size3 = null;
+        } else {
+            $size0 = $len;
+            $size1 = null;
+            $size3 = null;
+        }
+
+        $encoded .= chr(($frame['final']?0x80:0x00) 
+                    | ($frame['rsv1']?0x40:0x00) 
+                    | ($frame['rsv2']?0x20:0x00) 
+                    | ($frame['rsv3']?0x10:0x00)
+                    | ($frame['opcode'] & 0xF));
+        $encoded .= chr(($frame['masked']?0x80:0x00)
+                    | ($size0 & 0x7F));
+        if ($size1 !== null) {
+            $encoded .= chr($size1) . chr($size2);
+        }
+        if ($size3 !== null) {
+            $encoded .= chr($size3) . chr($size4);
+        }
+
+        if ($frame['masked'] === true) {
+            if ($frame['mask'] === null || strlen($frame['mask']) !== 4) {
+                $frame['mask'] = chr(mt_rand(0,255)).chr(mt_rand(0,255)).chr(mt_rand(0,255)).chr(mt_rand(0,255));
+            }
+            $encoded .= $frame['mask'];
+            $encoded .= $this->mask($frame['payload'], $frame['mask']);
+        } else {
+            $encoded .= $frame['payload'];
+        }
+
+        //$this->hexdump($encoded);
+
+        return $encoded;
+    }
+
+    /**
+     * Decode a websocket frame and return an array with the keys:
+     *   opcode (int) - the opcode
+     *   fin (bool) - final frame
+     *   rsv1-3 (bool) - reserved bits
+     *   masked (bool) - if the frame was masked
+     *   length (int) - length of payload
+     *   payload (string) - the payload
+     */
+    public function decode($frame): array
+    {
+        // Decoded frame
+        $decoded = [];
+
+        // Keep track of the number of bytes in the header
+        $header = 2;
+
+        // Peek at the first byte, holding flags and opcode
+        $byte0 = ord($frame[0]);
+        $decoded['final'] = !!($byte0 & 0x80);
+        $decoded['opcode'] = $byte0 & 0x0F;
+        // Peek at the second byte, holding mask bit and len
+        $byte1 = ord($frame[1]);
+        $decoded['masked'] = $masked = !!($byte1 & 0x80);
+        $decoded['rsv1'] = !!($byte1 & 0x40);
+        $decoded['rsv2'] = !!($byte1 & 0x20);
+        $decoded['rsv3'] = !!($byte1 & 0x10);
+        $len = $byte1 & 0x7F;
+
+        // Read extended length if present
+        if ($len == 126) {
+            $len = (ord($frame[$header+0]) << 8) 
+                 | (ord($frame[$header+1]));
+            $header += 2;
+        } elseif ($len == 127) {
+            $len = (ord($frame[$header+0]) << 24) 
+                 | (ord($frame[$header+1]) << 16) 
+                 | (ord($frame[$header+2]) << 8) 
+                 | (ord($frame[$header+3]));
+            $header += 4;
+        }
+
+        // Now for the masking
+        if ($masked) {
+            $mask = substr($frame, $header, 4);
+            $header += 4;
+        }
+
+        // Extract and unmask payload
+        $payload = substr($frame, $header, $len);
+        if ($masked) {
+            $payload = $this->mask($payload, $mask);
+            $decoded['mask'] = $mask;
+        }
+
+        $decoded['length'] = $len;
+        $decoded['payload'] = $payload;
+
+        return $decoded;
+    }
+
+    private function mask(string $payload, string $mask): string
+    {
+        $payloadData = array_map("ord", str_split($payload,1));
+        $maskData = array_map("ord", str_split($mask,1));
+
+        //printf("Mask: %02x %02x %02x %02x\n", ...$maskData);
+
+        $unmasked = [];
+        for ($n = 0; $n < count($payloadData); $n++) {
+            $unmasked[] = $payloadData[$n] ^ $maskData[$n % 4];
+        }
+        return join("", array_map("chr", $unmasked));
+    }
+
+    private function hexdump($data): void
+    {
+        printf("%4d .\n", strlen($data));
+        $rows = str_split($data, 16);
+        $offs = 0;
+        foreach ($rows as $row) {
+            $h = []; $a = [];
+            for ($n = 0; $n < 16; $n++) {
+                if ($n < strlen($row)) {
+                    $h[] = sprintf("%02x%s", ord($row[$n]), ($n==7)?"  ":" ");
+                    $a[] = sprintf("%s%s", (ctype_print($row[$n])?$row[$n]:"."), ($n==7)?" ":"");
+                } else {
+                    $h[] = (($n==7)?"    ":"   ");
+                    $a[] = (($n==7)?"  ":" ");
+                }
+            }
+            printf("%04x | %s | %s\n", 16 * $offs++, join("", $h), join("", $a));
+        }
+    }
+
+}
\ No newline at end of file
diff --git a/src/WebSocketConnection.php b/src/WebSocketConnection.php
index e5586dd..4490680 100644
--- a/src/WebSocketConnection.php
+++ b/src/WebSocketConnection.php
@@ -27,6 +27,8 @@ class WebSocketConnection implements WebSocketInterface
 
     private ?string $groupName = null;
 
+    private WebSocketCodec $codec;
+
     private ?ConnectionGroup $group = null;
 
     private GroupManager $groupManager;
@@ -43,6 +45,9 @@ class WebSocketConnection implements WebSocketInterface
 
     public function __construct(ServerRequestInterface $request, ReadableStreamInterface $inStream, WritableStreamInterface $outStream, GroupManager $groupManager)
     {
+        // The codec is used to encode and decode frames
+        $this->codec = new WebSocketCodec();
+        
         $this->request = $request;
         $this->inStream = $inStream;
         $this->outStream = $outStream;
@@ -54,42 +59,10 @@ class WebSocketConnection implements WebSocketInterface
     private function onWebSocketData($data)
     {
 
-        // Keep track of the number of bytes in the header
-        $header = 2;
-
-        // Peek at the first byte, holding flags and opcode
-        $byte0 = ord($data[0]);
-        $final = !!($byte0 & 0x80);
-        $opcode = $byte0 & 0x0F;
-        // Peek at the second byte, holding mask bit and len
-        $byte1 = ord($data[1]);
-        $masked = !!($byte1 & 0x80);
-        $len = $byte1 & 0x7F;
-
-        // Read extended length if present
-        if ($len == 126) {
-            $len = (ord($data[$header+0]) << 8) 
-                 | (ord($data[$header+1]));
-            $header += 2;
-        } elseif ($len == 127) {
-            $len = (ord($data[$header+0]) << 24) 
-                 | (ord($data[$header+1]) << 16) 
-                 | (ord($data[$header+2]) << 8) 
-                 | (ord($data[$header+3]));
-            $header += 4;
-        }
-
-        // Now for the masking
-        if ($masked) {
-            $mask = substr($data, $header, 4);
-            $header += 4;
-        }
-
-        // Extract and unmask payload
-        $payload = substr($data, $header, $len);
-        if ($masked) {
-            $payload = $this->unmask($payload, $mask);
-        }
+        $decoded = $this->codec->decode($data);
+        $opcode = $decoded['opcode'];
+        $final = $decoded['final'];
+        $payload = $decoded['payload'];
         
         if (!$final) {
             if ($this->bufferedOp === null) {
@@ -111,7 +84,12 @@ class WebSocketConnection implements WebSocketInterface
 
         switch ($opcode) {
             case self::OP_PING:
-                $this->sendPong();
+                $this->sendPong($payload);
+                return;
+            case self::OP_PONG:
+                return;
+            case self::OP_CLOSE:
+                // TODO implement
                 return;
             case self::OP_CONTINUATION:
                 $this->buffer .= $payload;
@@ -125,24 +103,9 @@ class WebSocketConnection implements WebSocketInterface
         }
     }
 
-    private function unmask(string $payload, string $mask): string
+    private function sendPong(string $data): void
     {
-        $payloadData = array_map("ord", str_split($payload,1));
-        $maskData = array_map("ord", str_split($mask,1));
-
-        //printf("Mask: %02x %02x %02x %02x\n", ...$maskData);
-
-        $unmasked = [];
-        for ($n = 0; $n < count($payloadData); $n++) {
-            $unmasked[] = $payloadData[$n] ^ $maskData[$n % 4];
-            //printf("%02x ^ %02x = %02x %s\n", $payloadData[$n], $maskData[$n%4], $payloadData[$n]^$maskData[$n%4], chr($payloadData[$n]^$maskData[$n%4]));
-        }
-        return join("", array_map("chr", $unmasked));
-    }
-
-    private function sendPong(): void
-    {
-
+        $this->send(self::OP_PONG, $data, true);
     }
 
     public function setGroup(?string $name): void
@@ -221,17 +184,15 @@ class WebSocketConnection implements WebSocketInterface
     /**
      * 
      */
-    public function send(int $opcode, string $data, bool $final = true)
+    public function send(int $opcode, string $data, bool $final = true, bool $masked = false)
     {
-        $frame = chr(($final?0x80:0x00) | ($opcode & 0xF));
-        
-        $len = strlen($data);
-        if ($len > 126) {
-            $frame .= chr(0x7E) . chr(($len >> 8) & 0xFF) . chr($len & 0xFF);
-        } else {
-            $frame .= chr($len);
-        }
-        $frame .= $data;
+
+        $frame = $this->codec->encode([
+            'opcode' => $opcode,
+            'payload' => $data,
+            'final' => $final,
+            'masked' => $masked
+        ]);
 
         $this->outStream->write($frame);
 
diff --git a/tests/WebSocketCodecTest.php b/tests/WebSocketCodecTest.php
new file mode 100644
index 0000000..3a433d2
--- /dev/null
+++ b/tests/WebSocketCodecTest.php
@@ -0,0 +1,100 @@
+<?php
+
+namespace NoccyLabs\React\WebSocket;
+
+use PHPUnit\Framework\Attributes\CoversClass;
+
+#[CoversClass(WebSocketCodec::class)]
+class WebSocketCodecTest extends \PHPUnit\Framework\TestCase
+{
+
+    public function testEncodingFrames()
+    {
+        $codec = new WebSocketCodec();
+
+        $msg = $codec->encode([
+            'opcode'=>WebSocketCodec::OP_PING,
+            'payload'=>"ping"
+        ]);
+        $this->assertEquals("\x89\x04ping", $msg);
+
+        $msg = $codec->encode([
+            'opcode'=>WebSocketCodec::OP_FRAME_TEXT,
+            'payload'=>"abcdefgh"]);
+        $this->assertEquals("\x81\x08abcdefgh", $msg);
+
+        $msg = $codec->encode([
+            'opcode'=>WebSocketCodec::OP_FRAME_TEXT,
+            'payload'=>"abcdefgh",
+            'masked'=>true,
+            'mask'=>"\x00\x00\x00\x00"
+        ]);
+        $this->assertEquals("\x81\x88\x00\x00\x00\x00abcdefgh", $msg);
+
+        $msg = $codec->encode([
+            'opcode'=>WebSocketCodec::OP_FRAME_TEXT,
+            'payload'=>"abcdefgh",
+            'masked'=>true,
+            'mask'=>"\x00\xFF\x00\xFF"
+        ]);
+        $this->assertEquals("\x81\x88\x00\xFF\x00\xFFa\x9dc\x9be\x99g\x97", $msg);
+
+    }
+
+    public function testDecodingFrames()
+    {
+        $codec = new WebSocketCodec();
+
+        $msg = $codec->decode("\x89\x04ping");
+        $this->assertEquals([
+            'opcode'=>WebSocketCodec::OP_PING,
+            'payload'=>"ping",
+            'final'=>true,
+            'rsv1'=>false,
+            'rsv2'=>false,
+            'rsv3'=>false,
+            'length'=>4,
+            'masked'=>false
+        ], $msg);
+
+        $msg = $codec->decode("\x81\x08abcdefgh");
+        $this->assertEquals([
+            'opcode'=>WebSocketCodec::OP_FRAME_TEXT,
+            'payload'=>"abcdefgh",
+            'final'=>true,
+            'rsv1'=>false,
+            'rsv2'=>false,
+            'rsv3'=>false,
+            'length'=>8,
+            'masked'=>false
+        ], $msg);
+
+        $msg = $codec->decode("\x81\x88\x00\x00\x00\x00abcdefgh");
+        $this->assertEquals([
+            'opcode'=>WebSocketCodec::OP_FRAME_TEXT,
+            'payload'=>"abcdefgh",
+            'final'=>true,
+            'rsv1'=>false,
+            'rsv2'=>false,
+            'rsv3'=>false,
+            'length'=>8,
+            'masked'=>true,
+            'mask'=>"\x00\x00\x00\x00"
+        ], $msg);
+
+        $msg = $codec->decode("\x81\x88\x00\xFF\x00\xFFa\x9dc\x9be\x99g\x97");
+        $this->assertEquals([
+            'opcode'=>WebSocketCodec::OP_FRAME_TEXT,
+            'payload'=>"abcdefgh",
+            'final'=>true,
+            'rsv1'=>false,
+            'rsv2'=>false,
+            'rsv3'=>false,
+            'length'=>8,
+            'masked'=>true,
+            'mask'=>"\x00\xFF\x00\xFF"
+        ], $msg);
+
+    }
+
+}
\ No newline at end of file