diff --git a/apps/optical-model/src/main/java/org/onosproject/net/optical/OpticalAnnotations.java b/apps/optical-model/src/main/java/org/onosproject/net/optical/OpticalAnnotations.java
index 9e9fd25..b53b3d2 100644
--- a/apps/optical-model/src/main/java/org/onosproject/net/optical/OpticalAnnotations.java
+++ b/apps/optical-model/src/main/java/org/onosproject/net/optical/OpticalAnnotations.java
@@ -26,12 +26,32 @@
     private OpticalAnnotations() {}
 
     /**
+     * Annotation key for mininum frequency in Hz.
+     * Value is expected to be an integer.
+     */
+    public static final String MIN_FREQ_HZ = "minFrequency";
+
+    /**
+     * Annotation key for maximum frequency in Hz.
+     * Value is expected be an integer.
+     */
+    public static final String MAX_FREQ_HZ = "maxFrequency";
+
+    /**
+     * Annotation key for grid in Hz.
+     * Value is expected to be an integer.
+     */
+    public static final String GRID_HZ = "grid";
+
+    /**
      * Annotation key for optical port's target power.
+     * Value is expected to be an integer.
      */
     public static final String TARGET_POWER = "targetPower";
 
     /**
      * Annotation key for optical port's current power.
+     * Value is expected to be an integer.
      */
     public static final String CURRENT_POWER = "currentPower";
 
@@ -42,6 +62,7 @@
 
     /**
      * Annotation key for optical port's neighbor's PortNumber#toString().
+     * Value is expected to be an integer.
      */
     public static final String NEIGHBOR_PORT = "neighborPort";
 }
diff --git a/apps/optical-model/src/main/java/org/onosproject/net/optical/device/OmsPortHelper.java b/apps/optical-model/src/main/java/org/onosproject/net/optical/device/OmsPortHelper.java
index 2d83aa2..109b2ed 100644
--- a/apps/optical-model/src/main/java/org/onosproject/net/optical/device/OmsPortHelper.java
+++ b/apps/optical-model/src/main/java/org/onosproject/net/optical/device/OmsPortHelper.java
@@ -29,6 +29,7 @@
 import org.onosproject.net.device.DefaultPortDescription;
 import org.onosproject.net.device.PortDescription;
 import org.onosproject.net.optical.OmsPort;
+import org.onosproject.net.optical.OpticalAnnotations;
 import org.onosproject.net.optical.impl.DefaultOmsPort;
 import org.slf4j.Logger;
 
@@ -43,20 +44,6 @@
 
     private static final Logger log = getLogger(OmsPortHelper.class);
 
-    // Annotation keys
-    /**
-     * minFrequency in Hz.
-     */
-    private static final String MIN_FREQ_HZ = "minFrequency";
-    /**
-     * maxFrequency in Hz.
-     */
-    private static final String MAX_FREQ_HZ = "maxFrequency";
-    /**
-     * grid in Hz.
-     */
-    private static final String GRID_HZ = "grid";
-
     /**
      * Creates OMS port description based on the supplied information.
      *
@@ -78,9 +65,9 @@
         Builder builder = DefaultAnnotations.builder();
         builder.putAll(annotations);
 
-        builder.set(MIN_FREQ_HZ, String.valueOf(minFrequency.asHz()));
-        builder.set(MAX_FREQ_HZ, String.valueOf(maxFrequency.asHz()));
-        builder.set(GRID_HZ, String.valueOf(grid.asHz()));
+        builder.set(OpticalAnnotations.MIN_FREQ_HZ, String.valueOf(minFrequency.asHz()));
+        builder.set(OpticalAnnotations.MAX_FREQ_HZ, String.valueOf(maxFrequency.asHz()));
+        builder.set(OpticalAnnotations.GRID_HZ, String.valueOf(grid.asHz()));
 
         long portSpeed = 0;
         return new DefaultPortDescription(number, isEnabled, Port.Type.OMS, portSpeed, builder.build());
@@ -133,9 +120,9 @@
         try {
             Annotations an = port.annotations();
 
-            Frequency minFrequency = Frequency.ofHz(Long.parseLong(an.value(MIN_FREQ_HZ)));
-            Frequency maxFrequency = Frequency.ofHz(Long.parseLong(an.value(MAX_FREQ_HZ)));
-            Frequency grid = Frequency.ofHz(Long.parseLong(an.value(GRID_HZ)));
+            Frequency minFrequency = Frequency.ofHz(Long.parseLong(an.value(OpticalAnnotations.MIN_FREQ_HZ)));
+            Frequency maxFrequency = Frequency.ofHz(Long.parseLong(an.value(OpticalAnnotations.MAX_FREQ_HZ)));
+            Frequency grid = Frequency.ofHz(Long.parseLong(an.value(OpticalAnnotations.GRID_HZ)));
 
             return Optional.of(new DefaultOmsPort(port, minFrequency, maxFrequency, grid));
 
@@ -153,7 +140,10 @@
      * @return filtered view of given {@link Annotations}
      */
     public static Annotations stripHandledAnnotations(Annotations input) {
-        return new FilteredAnnotation(input, ImmutableSet.of(MIN_FREQ_HZ, MAX_FREQ_HZ, GRID_HZ));
+        return new FilteredAnnotation(input, ImmutableSet.of(
+                OpticalAnnotations.MIN_FREQ_HZ,
+                OpticalAnnotations.MAX_FREQ_HZ,
+                OpticalAnnotations.GRID_HZ));
     }
 
     // not meant to be instantiated