diff --git a/xos/synchronizer/pull_steps/pull_olts.py b/xos/synchronizer/pull_steps/pull_olts.py
index 7c2cadf..8e33642 100644
--- a/xos/synchronizer/pull_steps/pull_olts.py
+++ b/xos/synchronizer/pull_steps/pull_olts.py
@@ -114,11 +114,10 @@
             try:
                 if "host_and_port" in olt:
                     model = OLTDevice.objects.filter(device_type=olt["type"], host=host, port=port)[0]
-                    log.debug("[OLT pull step] OLTDevice already exists, updating it", device_type=olt["type"], host=host, port=port)
+                    log.debug("[OLT pull step] OLTDevice already exists, updating it", device_type=olt["type"], id=model.id, host=host, port=port)
                 elif "mac_address" in olt:
                     model = OLTDevice.objects.filter(device_type=olt["type"], mac_address=mac_address)[0]
-                    log.debug("[OLT pull step] OLTDevice already exists, updating it", device_type=olt["type"], mac_address=mac_address)
-
+                    log.debug("[OLT pull step] OLTDevice already exists, updating it", device_type=olt["type"], id=model.id, mac_address=mac_address)
 
                 if model.enacted < model.updated:
                     log.debug("[OLT pull step] Skipping pull on OLTDevice %s as enacted < updated" % model.name, name=model.name, id=model.id, enacted=model.enacted, updated=model.updated)
@@ -136,6 +135,7 @@
                 if olt["type"] == "simulated_olt":
                     model.host = "172.17.0.1"
                     model.port = 50060
+                    log.debug("[OLT pull step] OLTDevice is new, creating it. Simulated")
                 elif "host_and_port" in olt:
                     [host, port] = olt["host_and_port"].split(":")
                     model.host = host
@@ -151,6 +151,8 @@
                 nni_ports = [p for p in olt_ports if "ETHERNET_NNI" in p["type"]]
                 if not nni_ports:
                     log.warning("[OLT pull step] No NNI ports, so no way to determine uplink. Skipping.", device_type=olt["type"], host=host, port=port)
+                    model.device_id = olt["id"]  # device_id must be populated for delete_olts
+                    updated_olts.append(model)
                     continue
 
                 # Exctract uplink from the first NNI port. This decision is arbitrary, we will worry about multiple
@@ -160,10 +162,44 @@
                 # Initial admin_state
                 model.admin_state = olt["admin_state"]
 
+            # Check to see if Voltha's serial_number field is populated. During Activation it's possible that
+            # Voltha's serial_number field may be blank. We want to avoid overwriting a populated data model
+            # serial number with an unpopulated Voltha serial number. IF this happened, then there would be
+            # a window of vulnerability where sadis requests will fail.
+
+            if olt['serial_number'] and model.serial_number != olt['serial_number']:
+                # Check to see if data model serial number is already populated. If the serial number was
+                # already populated, and what we just learned from Voltha differs, then this is an error
+                # that should be made visible to the operator, so the operator may correct it.
+                if model.serial_number:
+                    log.error("Serial number mismatch when pulling olt. Aborting OLT Update.",
+                              model_serial_number=model.serial_number,
+                              voltha_serial_number=olt['serial_number'],
+                              olt_id=model.id)
+                    model.backend_status = "Incorrect serial number"
+                    model.backend_code = 2
+                    model.save_changed_fields()
+                    # Have to include this in the result, or delete_olts() will delete it
+                    updated_olts.append(model)
+                    # Stop processing this OLT
+                    continue
+
+                # Preserve existing behavior.
+                # Data model serial number is unpopulated, so populate it.
+
+                # TODO(smbaker): Consider making serial_number a required field, then do away with this.
+                #                Deferred until after SEBA-2.0 release.
+
+                log.info("Pull step learned olt serial number from voltha",
+                         model_serial_number=model.serial_number,
+                         voltha_serial_number=olt['serial_number'],
+                         olt_id=model.id)
+
+                model.serial_number = olt['serial_number']
+
             # Adding feedback state to the device
             model.device_id = olt["id"]
             model.oper_status = olt["oper_status"]
-            model.serial_number = olt['serial_number']
 
             model.volt_service = self.volt_service
             model.volt_service_id = self.volt_service.id
@@ -275,8 +311,8 @@
             if model.enacted < model.updated:
                 # DO NOT delete a model that is being processed
                 log.debug("[OLT pull step] device is not present in VOLTHA, skipping deletion as sync is in progress", device_id=o.device_id,
-                         name=o.name)
+                          name=o.name)
                 continue
 
-            log.debug("[OLT pull step] deleting device as it's not present in VOLTHA", device_id=o.device_id, name=o.name)
+            log.debug("[OLT pull step] deleting device as it's not present in VOLTHA", device_id=o.device_id, name=o.name, id=o.id)
             model.delete()
diff --git a/xos/synchronizer/pull_steps/test_pull_olts.py b/xos/synchronizer/pull_steps/test_pull_olts.py
index 5894f5e..de2b967 100644
--- a/xos/synchronizer/pull_steps/test_pull_olts.py
+++ b/xos/synchronizer/pull_steps/test_pull_olts.py
@@ -202,6 +202,7 @@
         existing_olt.admin_state = "ENABLED"
         existing_olt.enacted = 2
         existing_olt.updated = 1
+        existing_olt.serial_number = ""
 
         with patch.object(VOLTService.objects, "all") as olt_service_mock, \
                 patch.object(OLTDevice.objects, "filter") as mock_get, \
@@ -223,12 +224,79 @@
             self.assertEqual(existing_olt.device_id, "test_id")
             self.assertEqual(existing_olt.of_id, "of_id")
             self.assertEqual(existing_olt.dp_id, "of:0000000ce2314000")
+            self.assertEqual(existing_olt.serial_number, "serial_number")
 
             # mock_olt_save.assert_called()
             mock_pon_save.assert_called()
             mock_nni_save.assert_called()
 
     @requests_mock.Mocker()
+    def test_pull_existing_empty_voltha_serial(self, m):
+
+        existing_olt = Mock()
+        existing_olt.admin_state = "ENABLED"
+        existing_olt.enacted = 2
+        existing_olt.updated = 1
+        existing_olt.serial_number = "orig_serial"
+
+        self.devices["items"][0]["serial_number"] = ""
+
+        with patch.object(VOLTService.objects, "all") as olt_service_mock, \
+                patch.object(OLTDevice.objects, "filter") as mock_get, \
+                patch.object(PONPort, "save") as mock_pon_save, \
+                patch.object(NNIPort, "save") as mock_nni_save, \
+                patch.object(existing_olt, "save") as  mock_olt_save:
+            olt_service_mock.return_value = [self.volt_service]
+            mock_get.return_value = [existing_olt]
+
+            m.get("http://voltha_url:1234/api/v1/devices", status_code=200, json=self.devices)
+            m.get("http://voltha_url:1234/api/v1/devices/test_id/ports", status_code=200, json=self.ports)
+            m.get("http://voltha_url:1234/api/v1/logical_devices", status_code=200, json=self.logical_devices)
+
+            self.sync_step(model_accessor=self.model_accessor).pull_records()
+
+            self.assertEqual(existing_olt.admin_state, "ENABLED")
+            self.assertEqual(existing_olt.oper_status, "ACTIVE")
+            self.assertEqual(existing_olt.volt_service_id, "volt_service_id")
+            self.assertEqual(existing_olt.device_id, "test_id")
+            self.assertEqual(existing_olt.of_id, "of_id")
+            self.assertEqual(existing_olt.dp_id, "of:0000000ce2314000")
+            self.assertEqual(existing_olt.serial_number, "orig_serial")
+
+            # mock_olt_save.assert_called()
+            mock_pon_save.assert_called()
+            mock_nni_save.assert_called()
+
+    @requests_mock.Mocker()
+    def test_pull_existing_incorrect_voltha_serial(self, m):
+
+        existing_olt = Mock()
+        existing_olt.admin_state = "ENABLED"
+        existing_olt.enacted = 2
+        existing_olt.updated = 1
+        existing_olt.serial_number = "orig_serial"
+
+        self.devices["items"][0]["serial_number"] = "wrong_serial"
+
+        with patch.object(VOLTService.objects, "all") as olt_service_mock, \
+                patch.object(OLTDevice.objects, "filter") as mock_get, \
+                patch.object(PONPort, "save") as mock_pon_save, \
+                patch.object(NNIPort, "save") as mock_nni_save, \
+                patch.object(existing_olt, "save") as  mock_olt_save:
+            olt_service_mock.return_value = [self.volt_service]
+            mock_get.return_value = [existing_olt]
+
+            m.get("http://voltha_url:1234/api/v1/devices", status_code=200, json=self.devices)
+            m.get("http://voltha_url:1234/api/v1/devices/test_id/ports", status_code=200, json=self.ports)
+            m.get("http://voltha_url:1234/api/v1/logical_devices", status_code=200, json=self.logical_devices)
+
+            self.sync_step(model_accessor=self.model_accessor).pull_records()
+
+            self.assertEqual(existing_olt.backend_code, 2)
+            self.assertEqual(existing_olt.backend_status, "Incorrect serial number")
+            self.assertEqual(existing_olt.serial_number, "orig_serial")
+
+    @requests_mock.Mocker()
     def test_pull_existing_do_not_sync(self, m):
         existing_olt = Mock()
         existing_olt.enacted = 1
diff --git a/xos/synchronizer/steps/sync_olt_device.py b/xos/synchronizer/steps/sync_olt_device.py
index 469e879..6d4dae1 100644
--- a/xos/synchronizer/steps/sync_olt_device.py
+++ b/xos/synchronizer/steps/sync_olt_device.py
@@ -89,7 +89,16 @@
                 'VOLTHA Device Id is empty. This probably means that the OLT device is already provisioned in VOLTHA')
         else:
             model.device_id = res['id']
-            model.serial_number = res['serial_number']
+
+            # Only update the serial number if it is not already populated. See comments in similar code in the
+            # pull step. Let the pull step handle emitting any error message if the serial numbers differ.
+            if res['serial_number'] and (not model.serial_number):
+                log.info("Sync step learned olt serial number from voltha",
+                         model_serial_number=model.serial_number,
+                         voltha_serial_number=res['serial_number'],
+                         olt_id=model.id)
+                model.serial_number = res['serial_number']
+
             model.save_changed_fields()
 
     def activate_olt(self, model):
@@ -116,7 +125,15 @@
             attempted = attempted + 1
 
         model.oper_status = request['oper_status']
-        model.serial_number = request['serial_number']
+
+        # Only update the serial number if it is not already populated. See comments in similar code in the
+        # pull step. Let the pull step handle emitting any error message if the serial numbers differ.
+        if request['serial_number'] and (not model.serial_number):
+            log.info("Sync step learned olt serial number from voltha",
+                     model_serial_number=model.serial_number,
+                     voltha_serial_number=request['serial_number'],
+                     olt_id=model.id)
+            model.serial_number = request['serial_number']
 
         if model.oper_status != "ACTIVE":
             raise Exception("It was not possible to activate OLTDevice with id %s" % model.id)
diff --git a/xos/synchronizer/steps/test_sync_olt_device.py b/xos/synchronizer/steps/test_sync_olt_device.py
index 57330d2..40f13d8 100644
--- a/xos/synchronizer/steps/test_sync_olt_device.py
+++ b/xos/synchronizer/steps/test_sync_olt_device.py
@@ -88,6 +88,7 @@
         # feedback state
         o.device_id = None
         o.oper_status = None
+        o.serial_number= None
         o.of_id = None
         o.id = 1