Charging method and network system thereof, packet data network gateway and charging system thereof

A charging process method, comprising:

• receiving a charging request containing a type of a second method when a method for a user equipment to access a packet data network gateway has been changed from a first method to the second method;
• and
• performing, based on the charging request, charging control over used service units in a first charging mode corresponding to the first method for the user equipment to access the packet data network gateway and reserving service units for a charging session for the second method based on a second charging mode corresponding to the second method for the user equipment to access the packet data network gateway.

A packet data network gateway, comprising:

• a detecting unit, configured to monitor if there is any change in a method for a user equipment to access the packet data network gateway;
• and
• a charging request unit, configured to send a charging request containing a type of a second method when detecting that the method for the user equipment to access the packet data network gateway has been changed from a first method to the second method.

A charging system, comprising:

• a receiving unit, configured to receive a charging request containing a type of a second method when a method for a user equipment to access a packet data network gateway has been changed from a first method to the second method;
• and
• a processing unit, configured to perform, based on the charging request received by the receiving unit, charging control over used service units in a first charging mode corresponding to the first method for the user equipment to access the packet data network gateway and reserve service units for the charging session for the second method based on a second charging mode corresponding to the second method for the user equipment to access the packet data network gateway.