diff --git a/features/cellular/README.md b/features/cellular/README.md
index 5c02c0e390..1c66505065 100644
--- a/features/cellular/README.md
+++ b/features/cellular/README.md
@@ -12,6 +12,10 @@ This is the Github repo for Mbed cellular connectivity:
         common      Common and utility sources
         targets     Vendor specific cellular module adaptations
 
+    TESTS           Cellular Greentea test
+
+    UNITTESTS       Cellular unit test
+
 ## Known limitations
 
 **Please note that this is a first release of Cellular framework and is subject to further development in future.**
@@ -24,7 +28,9 @@ You can find currently supported cellular modules in the `framework/targets/` fo
 
 ## Cellular configuration
 
-You can change cellular defaults in the `mbed_app.json` configuration file:
+You can change cellular defaults in the `mbed_lib.json` configuration file.
+
+You can also override cellular defaults in the `mbed_app.json` configuration file:
 
     "config": {
         "cellular_plmn": {
@@ -62,6 +68,14 @@ You can define the debug tracing level in the `mbed_app.json` configuration file
         }
     }
 
+## Greentea tests
+
+The `TESTS` folder contains Greentea tests for cellular specific classes. You need to give relevant configuration file with `--app-config` parameter, e.g.:
+
+    mbed test -n features-cellular-tests-* --app-config features\cellular\TESTS\socket\udp\template_mbed_app.json -vv
+
+    Note that Greentea tests use SIM PIN so you need to change that or your SIM card may get locked.
+
 ## Unit tests
 
 The `UNITTESTS` folder contains unit tests for cellular specific classes. Unit tests are based on the stubbing method.
diff --git a/features/cellular/TESTS/socket/udp/main.cpp b/features/cellular/TESTS/socket/udp/main.cpp
index 25e1d931a9..3f2860c5d6 100644
--- a/features/cellular/TESTS/socket/udp/main.cpp
+++ b/features/cellular/TESTS/socket/udp/main.cpp
@@ -213,7 +213,7 @@ static Case cases[] = {
 
 static utest::v1::status_t test_setup(const size_t number_of_cases)
 {
-    GREENTEA_SETUP(180, "default_auto");
+    GREENTEA_SETUP(10*60, "default_auto"); // network registration may take up to 180 seconds, DNS query a couple of minutes, etc.
     return verbose_test_setup_handler(number_of_cases);
 }
 
diff --git a/features/cellular/TESTS/socket/udp/template_mbed_app.json.txt b/features/cellular/TESTS/socket/udp/template_mbed_app.json.txt
index 8f4071ee96..0c7943a4ad 100644
--- a/features/cellular/TESTS/socket/udp/template_mbed_app.json.txt
+++ b/features/cellular/TESTS/socket/udp/template_mbed_app.json.txt
@@ -1,9 +1,5 @@
 {
     "config": {
-        "network-interface":{
-            "help": "Options are ETHERNET,CELLULAR",
-            "value": "CELLULAR"
-        },
         "cellular_sim_pin": {
             "help": "PIN code",
             "value": "\"1234\""
diff --git a/features/cellular/framework/AT/ATHandler.cpp b/features/cellular/framework/AT/ATHandler.cpp
index a2200ad790..3df6cb7102 100644
--- a/features/cellular/framework/AT/ATHandler.cpp
+++ b/features/cellular/framework/AT/ATHandler.cpp
@@ -1001,9 +1001,7 @@ void ATHandler::cmd_start(const char* cmd)
         } 
     } 
 
-    tr_debug("AT> %s", cmd);
-
-
+    at_debug("AT cmd %s (err %d)\n", cmd, _last_err);
 
     if (_last_err != NSAPI_ERROR_OK) {
         return;
@@ -1016,7 +1014,7 @@ void ATHandler::cmd_start(const char* cmd)
 
 void ATHandler::write_int(int32_t param)
 {
-    tr_debug("write_int: %d", param);
+    at_debug("AT int %d\n", param);
     // do common checks before sending subparameter
     if (check_cmd_send() == false) {
         return;
@@ -1033,7 +1031,7 @@ void ATHandler::write_int(int32_t param)
 
 void ATHandler::write_string(const char* param, bool useQuotations)
 {
-    tr_debug("write_string: %s, %d", param, useQuotations);
+    at_debug("AT str %s (with quotes %d)\n", param, useQuotations);
     // do common checks before sending subparameter
     if (check_cmd_send() == false) {
         return;
@@ -1054,6 +1052,7 @@ void ATHandler::write_string(const char* param, bool useQuotations)
 
 void ATHandler::cmd_stop()
 {
+    at_debug("AT stop %s (err %d)\n", _output_delimiter, _last_err);
     if (_last_err != NSAPI_ERROR_OK) {
         return;
     }
@@ -1063,28 +1062,33 @@ void ATHandler::cmd_stop()
 
 size_t ATHandler::write_bytes(const uint8_t *data, size_t len)
 {
+    at_debug("AT write bytes %d (err %d)\n", len, _last_err);
+    
     if (_last_err != NSAPI_ERROR_OK) {
         return 0;
     }
 
-    ssize_t write_len = write(data, len);
-    return write_len < 0 ? 0 : (size_t)write_len;
+    return write(data, len);
 }
 
-ssize_t ATHandler::write(const void *data, size_t len)
+size_t ATHandler::write(const void *data, size_t len)
 {
     pollfh fhs;
     fhs.fh = _fileHandle;
     fhs.events = POLLOUT;
-    ssize_t write_len = -1;
-
-    int count = poll(&fhs, 1, _at_timeout);
-    if (count > 0 && (fhs.revents & POLLOUT)) {
-        write_len = _fileHandle->write(data, len);
-    }
-
-    if (write_len < 0 || (size_t)write_len != len) {
-        set_error(NSAPI_ERROR_DEVICE_ERROR);
+    size_t write_len = 0;
+    for (; write_len < len; ) {
+        int count = poll(&fhs, 1, _at_timeout);
+        if (count <= 0 || !(fhs.revents & POLLOUT)) {
+            set_error(NSAPI_ERROR_DEVICE_ERROR);
+            return 0;
+        }
+        ssize_t ret = _fileHandle->write((uint8_t*)data + write_len, len - write_len);
+        if (ret < 0) {
+            set_error(NSAPI_ERROR_DEVICE_ERROR);
+            return 0;
+        }
+        write_len += (size_t)ret;
     }
 
     return write_len;
diff --git a/features/cellular/framework/AT/ATHandler.h b/features/cellular/framework/AT/ATHandler.h
index 82cdc43eb5..3deb41ea2e 100644
--- a/features/cellular/framework/AT/ATHandler.h
+++ b/features/cellular/framework/AT/ATHandler.h
@@ -458,7 +458,7 @@ private:
     void set_3gpp_error(int err, DeviceErrorType error_type);
 
     bool check_cmd_send();
-    ssize_t write(const void *data, size_t len);
+    size_t write(const void *data, size_t len);
 
     /** Copy content of one char buffer to another buffer and sets NULL terminator
      *