diff --git a/main.py b/main.py
index 47f388f..3ee96dd 100644
--- a/main.py
+++ b/main.py
@@ -22,7 +22,8 @@ class TestTCP(TestCase):
         self._wait_for_ok()
 
     def test_json(self) -> None:
-        raise NotImplemented
+        self.sock.sendall(device.json_req_data().encode())
+        self._wait_for_ok()
 
     def _wait_for_ok(self) -> None:
         data = self.sock.recv(1024)
@@ -43,7 +44,10 @@ class TestHTTP(TestCase):
         ))
 
     def test_json(self) -> None:
-        raise NotImplemented
+        self._send_and_check(Request(
+            HTTP_URL + '/json',
+            data=device.json_req_data().encode(),
+        ))
 
     def _send_and_check(self, req: Request) -> None:
         with urlopen(req) as resp:
diff --git a/models.py b/models.py
index c0a0935..d546476 100644
--- a/models.py
+++ b/models.py
@@ -1,7 +1,8 @@
+import json
 from urllib.parse import quote
 
 from dataclasses import dataclass
-from typing import Collection
+from typing import Collection, Any
 
 
 type Numeric = int | float
@@ -89,4 +90,35 @@ class Device:
         return res
 
     def json_req_data(self) -> str:
-        raise NotImplemented
+        '''Generate a JSON format request body'''
+
+        obj = {
+            'devices': [{
+                'mac': self.mac,
+                'sensors': [],
+            }]
+        }
+
+        dev = obj['devices'][0]
+        if self.name is not None:
+            dev['name'] = self.name
+        if self.owner is not None:
+            dev['owner'] = self.owner
+        if self.geopos is not None:
+            dev['lat'] = self.geopos.lat
+            dev['lon'] = self.geopos.lon
+            dev['alt'] = self.geopos.alt
+
+        sl: list[dict[str, Any]] = dev['sensors']
+        for s in self.sensors:
+            sobj = {
+                'id': s.mac,
+                'value': s.value,
+            }
+            if s.time is not None:
+                sobj['time'] = s.time
+            if s.name is not None:
+                sobj['name'] = s.name
+            sl.append(sobj)
+
+        return json.dumps(obj)