S-GW
Serving Gateway: the EPC user-plane node that routes and forwards user data packets, anchors the user plane during inter-eNB handovers, and manages GTP tunnels.
The Serving Gateway is a user-plane router with one special talent: it stays put while you move. As you hand over from one eNB to the next, your packets keep arriving at the same S-GW, which simply re-points the GTP tunnel toward your new eNB. The eNB underneath changes; the anchor above doesn't. That's what keeps a video call from dropping every time you cross a cell boundary.
It sits between the eNB (over S1-U) and the PDN Gateway, forwarding traffic in GTP tunnels in both directions. It also does the unglamorous-but-essential work: buffering downlink packets while the network pages an idle device back to active, and collecting per-bearer usage counts for charging. In smaller or virtualised deployments the S-GW and P-GW often run combined as a single SGW/PGW (sometimes called S/PGW). In the 5G core, both collapse into the UPF, and the control logic gets hived off to the SMF.
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