diff --git a/includes/billing.php b/includes/billing.php
index f8b2d90f09..d441d0e699 100644
--- a/includes/billing.php
+++ b/includes/billing.php
@@ -79,9 +79,9 @@ function getValue($host, $port, $id, $inout) {
 
 }//end getValue()
 
-function getLastPortCounter($port_id) {
+function getLastPortCounter($port_id, $bill_id) {
     $return = array();
-    $row    = dbFetchRow("SELECT timestamp, in_counter, in_delta, out_counter, out_delta FROM bill_port_counters WHERE `port_id` = ?", array($port_id));
+    $row    = dbFetchRow("SELECT timestamp, in_counter, in_delta, out_counter, out_delta FROM bill_port_counters WHERE `port_id` = ? AND `bill_id` = ?", array($port_id, $bill_id));
     if (!is_null($row)) {
         $return[timestamp]   = $row['timestamp'];
         $return[in_counter]  = $row['in_counter'];
diff --git a/poll-billing.php b/poll-billing.php
index f7e4b4f561..9ad32e590c 100755
--- a/poll-billing.php
+++ b/poll-billing.php
@@ -66,7 +66,7 @@ function CollectData($bill_id) {
         $port_data['in_measurement']  = getValue($port_data['hostname'], $port_data['port'], $port_data['ifIndex'], 'In');
         $port_data['out_measurement'] = getValue($port_data['hostname'], $port_data['port'], $port_data['ifIndex'], 'Out');
 
-        $last_counters = getLastPortCounter($port_id);
+        $last_counters = getLastPortCounter($port_id, $bill_id);
         if ($last_counters['state'] == 'ok') {
             $port_data['last_in_measurement']  = $last_counters[in_counter];
             $port_data['last_in_delta']        = $last_counters[in_delta];
@@ -75,7 +75,7 @@ function CollectData($bill_id) {
 
             $tmp_period = dbFetchCell("SELECT UNIX_TIMESTAMP(CURRENT_TIMESTAMP()) - UNIX_TIMESTAMP('".mres($last_counters['timestamp'])."')");
 
-            if (delta_to_bits($port_data['in_measurement'], $tmp_period)-$port_data['last_in_measurement'] > $port_data['ifSpeed']) {
+            if ((delta_to_bits($port_data['in_measurement'], $tmp_period)-delta_to_bits($port_data['last_in_measurement'], $tmp_period)) > $port_data['ifSpeed']) {
                 $port_data['in_delta'] = $port_data['last_in_delta'];
             }
             elseif ($port_data['in_measurement'] >= $port_data['last_in_measurement']) {
@@ -85,7 +85,7 @@ function CollectData($bill_id) {
                 $port_data['in_delta'] = $port_data['last_in_delta'];
             }
             
-            if (delta_to_bits($port_data['out_measurement'], $tmp_period)-$port_data['last_out_measurement'] > $port_data['ifSpeed']) {
+            if ((delta_to_bits($port_data['out_measurement'], $tmp_period)-delta_to_bits($port_data['last_out_measurement'], $tmp_period)) > $port_data['ifSpeed']) {
                 $port_data['out_delta'] = $port_data['last_out_delta'];
             }
             elseif ($port_data['out_measurement'] >= $port_data['last_out_measurement']) {
@@ -101,7 +101,8 @@ function CollectData($bill_id) {
         }
 
         $fields = array('timestamp' => $now, 'in_counter' => $port_data['in_measurement'], 'out_counter' => $port_data['out_measurement'], 'in_delta' => $port_data['in_delta'], 'out_delta' => $port_data['out_delta']);
-        if (dbUpdate($fields, 'bill_port_counters', "`port_id`='" . mres($port_id) . "'") == 0) {
+        if (dbUpdate($fields, 'bill_port_counters', "`port_id`='" . mres($port_id) . "' AND `bill_id`='$bill_id'") == 0) {
+            $fields['bill_id'] = $bill_id;
             $fields['port_id'] = $port_id;
             dbInsert($fields, 'bill_port_counters');
         }
diff --git a/sql-schema/124.sql b/sql-schema/124.sql
new file mode 100644
index 0000000000..ea7bdf31cf
--- /dev/null
+++ b/sql-schema/124.sql
@@ -0,0 +1,2 @@
+ALTER TABLE `bill_port_counters` ADD `bill_id` INT NOT NULL ;
+ALTER TABLE `bill_port_counters` DROP PRIMARY KEY, ADD PRIMARY KEY ( `port_id` , `bill_id` );