diff --git a/pdu/submit_test.go b/pdu/submit_test.go
index 1ae1cd6..48a2661 100644
--- a/pdu/submit_test.go
+++ b/pdu/submit_test.go
@@ -76,8 +76,26 @@ func TestCalculateSizeTypicalInstance(t *testing.T) {
 		schedule_delivery_time: "",
 		validity_period:        "",
 		short_message:          "Hello, World!",
+		data_coding:            1,
 	}
-	expectedSize := 16 + len(p.service_type) + 1 + 1 + len(p.source_addr) + 1 + 1 + len(p.destination_addr) + 1 + 1 + 1 + len(p.schedule_delivery_time) + len(p.validity_period) + 1 + 1 + 1 + 1 + 1 + len(p.short_message)
+	expectedSize := 68
+	actualSize := p.Size()
+	if actualSize != expectedSize {
+		t.Errorf("Expected size %d, but got %d", expectedSize, actualSize)
+	}
+}
+
+func TestCalculateSizeTypicalGSM7Instance(t *testing.T) {
+	p := &SUBMIT_SM{
+		service_type:           "test_service",
+		source_addr:            "12345",
+		destination_addr:       "67890",
+		schedule_delivery_time: "",
+		validity_period:        "",
+		short_message:          "Hello, World!",
+		data_coding:            0,
+	}
+	expectedSize := 67
 	actualSize := p.Size()
 	if actualSize != expectedSize {
 		t.Errorf("Expected size %d, but got %d", expectedSize, actualSize)
@@ -93,8 +111,9 @@ func TestCalculateSizeMaxLengths(t *testing.T) {
 		schedule_delivery_time: string(make([]byte, maxLen)),
 		validity_period:        string(make([]byte, maxLen)),
 		short_message:          string(make([]byte, maxLen)),
+		data_coding:            1,
 	}
-	expectedSize := 16 + maxLen + 1 + 1 + maxLen + 1 + 1 + maxLen + 1 + 1 + 1 + maxLen + maxLen + 1 + 1 + 1 + 1 + 1 + maxLen
+	expectedSize := 1563
 	actualSize := p.Size()
 	if actualSize != expectedSize {
 		t.Errorf("Expected size %d, but got %d", expectedSize, actualSize)
@@ -110,7 +129,7 @@ func TestHandlesEmptyStringsForAllStringFields(t *testing.T) {
 		validity_period:        "",
 		short_message:          "",
 	}
-	expectedSize := 16 + len(p.service_type) + 1 + 1 + len(p.source_addr) + 1 + 1 + len(p.destination_addr) + 1 + 1 + 1 + len(p.schedule_delivery_time) + len(p.validity_period) + 1 + 1 + 1 + 1 + 1 + len(p.short_message)
+	expectedSize := 33
 	actualSize := p.Size()
 	if actualSize != expectedSize {
 		t.Errorf("Expected size %d, but got %d", expectedSize, actualSize)