The PDN-GW provides access from the mobile operator's network to the PS networks that host the payload contents and operator's IP services. If a user has access to more than one packet data network it is possible that this user is connected to more than just one PDN-GW. It is not possible for the same UE to simultaneously open connections to a PDN-GW and to a GGSN in the 3G PS domain, according to 3GPP standards.
The main function of the PDN-GW is to establish, maintain, and delete GTP tunnels to S-GW or SGSN in the case of inter-RAT mobility scenarios. The PDN-GW allocates the user's dynamic IP addresses and routes the user plane packets. In addition, it provides functions for lawful interception, policy/QoS control, and charging.
For policy control and charging, the PDN-GW can be connected to a Policy and Charging Rule Function (PCRF) via the Gx reference point. The PCRF provides guidance on how a particular service data flow should be treated in terms of priority, throughput, and other QoS parameters according to the user's subscription profile.
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