The P/S-GW is the anchor point for the user plane for a UE moving between eNode Bs. The S-GW is only changed when the UE move to a new S-GW pool area while the P-GW is normally kept as long as the UE is attached to the network.
Functionality of P/S-GW functionality is very similar to GGSN node in 3G(WCDMA).
Some additions are like:
Even though P/S-SGW is mainly seen as a user plane node. However it can also perform some QoS related signaling and control plane signaling related functions as listed below.
EPS Bearer Handling : P/S-GW triggers the setup of EPS bearers upon request from policy control functions
Mobility Anchor- IP Point of Presence :When UE is attached to the network, it is assigned an IP address form a P-GW which then also assumes the role of mobility anchor to the UE. While the control of a UE may be transferred to another MME or S-GW because of a Handover (HO), the UE’s IP point of Presence (PoP) will remain at the P-GW. Thus, the mobility of UEs is transparent to the fixed network.
User Plane Functions
The user plane functions associated with P/S-GW are given below.
QoS Policy Control and Enforcement: To put it in simple terms. P/S-GW act as a police for QoS in the downlink from application point of view. It increases operator’s control over its network resources and limit the potential for abuse by users and network QoS is controlled.
The policy control and enforcement functions associate user’s traffic flows with appropriate QoS classes and executes rate policing to prohibit users or flows from exceeding the QoS limits specified in users’ subscription agreement . Downlink (DL) traffic is policed in P/S-GW while UL traffic is policed in eNodeB
Charging: The charging function is responsible for charging the user for its traffic according to the rate that applies for a particular service or subscription etc.
Lawful Intercept: This function enables communications to be electronically intercepted or eavesdropped , by law enforcement agencies , should it be authorized by judicial or regulatory mandates.