diff --git a/model/isl-mock-channel.cc b/model/isl-mock-channel.cc
index 4fc1765..4e9b697 100644
--- a/model/isl-mock-channel.cc
+++ b/model/isl-mock-channel.cc
@@ -67,8 +67,21 @@ IslMockChannel::TransmitStart (
 
   if (dst == nullptr)
   {
-    NS_LOG_LOGIC ("destination address " << destAddr << " unknown on channel");
-    return false;
+    if (src->IsBroadcast () || src->IsMulticast ())
+      // try to deliver to every node in LOS
+      {
+        for (size_t i = 0; i < GetNDevices (); i ++)
+          {
+            if (i == srcId) continue;
+            Deliver (p, src, GetDevice (i), txTime);
+          }
+        return true;
+      }
+    else
+      {
+        NS_LOG_LOGIC ("destination address " << destAddr << " unknown on channel");
+        return false;
+      }
   }
   else
   {
diff --git a/model/leo-mock-channel.cc b/model/leo-mock-channel.cc
index 9cd71f7..004ab6c 100644
--- a/model/leo-mock-channel.cc
+++ b/model/leo-mock-channel.cc
@@ -64,7 +64,7 @@ LeoMockChannel::TransmitStart (Ptr<const Packet> p,
         }
       return Deliver (p, srcDev, it->second, txTime);
     }
-  else
+  else if (srcDev->IsBroadcast () || srcDev->IsMulticast ())
     // space to ground delivers to everything within the beam
     {
       DeviceIndex::iterator it = m_groundDevices.find (dst);