Section: 4.4.3.2 Serving GW
Content: The Serving GW is the gateway which terminates the user plane interface towards E-UTRAN (except when user data is transported using the Control Plane CIoT EPS Optimisation). For each UE associated with the EPS, at a given point of time, there is a single Serving GW. The functions of the Serving GW, for both the GTP-based and the PMIP-based S5/S8, include: - the local Mobility Anchor point for inter-eNodeB handover (except when user data is transported using the Control Plane CIoT EPS Optimisation); - sending of one or more "end marker" to the source eNodeB, source SGSN or source RNC immediately after the Serving GW switches the path during inter-eNodeB and inter-RAT handover, especially to assist the reordering function in eNodeB. - Mobility anchoring for inter-3GPP mobility (terminating S4 and relaying the traffic between 2G/3G system and PDN GW); - ECM-IDLE mode downlink packet buffering and initiation of network triggered service request procedure and optionally Paging Policy Differentiation; - Lawful Interception; - Packet routing and forwarding; - Transport level packet marking in the uplink and the downlink, e.g. setting the DiffServ Code Point, based on the QCI, and optionally the ARP priority level, of the associated EPS bearer; - Accounting for inter-operator charging. For GTP-based S5/S8, the Serving GW generates accounting data per UE and bearer; - Interfacing OFCS according to charging principles and through reference points specified in TS 32.240[ Telecommunication management; Charging management; Charging architecture and principles ]  [51]; - Forwarding of "end marker" to the source eNodeB, source SGSN or source RNC when the "end marker" is received from PDN GW and the Serving GW has downlink user plane established. Upon reception of "end marker", the Serving GW shall not send Downlink Data Notification. Additional Serving GW functions for the PMIP-based S5/S8 are captured in TS 23.402[ Architecture enhancements for non-3GPP accesses ]  [2]. Connectivity to a GGSN is not supported.