diff --git a/etlers/WifiToolkit/src/main/scala/cn/edu/sjtu/omnilab/odh/spark/MergeWifiSession.scala b/etlers/WifiToolkit/src/main/scala/cn/edu/sjtu/omnilab/odh/spark/MergeWifiSession.scala
index aa38b06..3d7c954 100644
--- a/etlers/WifiToolkit/src/main/scala/cn/edu/sjtu/omnilab/odh/spark/MergeWifiSession.scala
+++ b/etlers/WifiToolkit/src/main/scala/cn/edu/sjtu/omnilab/odh/spark/MergeWifiSession.scala
@@ -191,6 +191,7 @@ object MergeWifiSession {
           .format(wsession.MAC, wsession.stime, wsession.etime, wsession.AP,
             bname, btype, bschool, blat, blon, ipaddr, account)
     }
+      .sortBy(m => m)
       .saveAsTextFile(output)
 
   }
@@ -206,11 +207,13 @@ object MergeWifiSession {
      */
     var sessions = new Array[WIFISession](0)
     var APMap = new HashMap[String, List[Long]]
+    var preAP: String = null
+    var mac: String = null
 
     logs.toSeq.sortBy(_.time)
       .foreach( log => {
 
-        val mac = log.MAC
+        mac = log.MAC
         val time = log.time
         val code = log.code
         val ap = log.payload.split(',')(0)
@@ -223,8 +226,15 @@ object MergeWifiSession {
             APMap.put(ap, List(time, time))
           else {
             val value = APMap.get(ap).get
-            if ( time >= value(1) ) {
-              APMap.put(ap, List(value(0), time))
+            if (preAP == ap) {
+              // update ending time
+              if ( time >= value(1) ) {
+                APMap.put(ap, List(value(0), time))
+              }
+            } else {
+              // create new session
+              sessions = sessions :+ WIFISession(mac, value(0), value(1), ap)
+              APMap.put(ap, List(time, time))
             }
           }
         } else if (code == WIFICode.Deauth || code == WIFICode.Disassoc) {
@@ -235,19 +245,23 @@ object MergeWifiSession {
           }
         }
 
-        APMap.foreach { case (key, value) => {
-          sessions = sessions :+ WIFISession(mac, value(0), value(1), key)
-        }}
+        preAP = ap
 
     })
 
+    if ( mac != null ){
+      APMap.foreach { case (key, value) => {
+        sessions = sessions :+ WIFISession(mac, value(0), value(1), key)
+      }}
+    }
+
     // adjust session timestamps
     var preSession: WIFISession = null
     var ajustedSessions = new Array[WIFISession](0)
 
     sessions.sortBy(m => m.stime).foreach { m => {
       if ( preSession != null ) {
-        val tdiff = m.stime - m.etime
+        val tdiff = m.stime - preSession.etime
         if (tdiff < 0)
           preSession = preSession.copy(etime = m.stime)
 
@@ -338,7 +352,7 @@ object MergeWifiSession {
     sessions.sortBy(m => m.stime).foreach { m => {
 
       if ( preSession != null ) {
-        val tdiff = m.stime - m.etime
+        val tdiff = m.stime - preSession.etime
         if (tdiff < 0)
           preSession = preSession.copy(etime = m.stime)