What roles do the S-GW and P-GW play on the LTE data path?

S-GW and P-GW sit on the data plane between phone and internet: the P-GW is the internet gateway router providing NAT services, with traffic tunneled extensively between the gateways.

Their place in the network: Both gateways lie on the data plane — the actual path user packets travel from the phone to/from the internet. Signaling (control plane) goes to the MME instead.

P-GW (PDN Gateway):

• The gateway of the mobile network to the outside world
• Acts as the internet gateway router for the mobile device
• Provides NAT services — your phone typically gets a private IP address, and the P-GW translates it for the public internet
• Makes extensive use of tunneling toward the S-GW

S-GW (Serving Gateway):

• The local anchor point for user data within the EPC
• Forwards tunneled traffic between eNodeB and P-GW

Why tunneling? Your phone keeps its IP address while moving between cells. That's only possible because the IP path is virtualized: packets are wrapped in tunnels (GTP), and when you move, only the tunnel endpoints change — the inner IP connection survives untouched.

Security observation: The NAT at the P-GW means mobile devices are normally not directly reachable from the internet — an accidental but useful security property, similar to a home router.

Go deeper:

• System Architecture Evolution (Wikipedia) — the S-GW as inter-eNodeB mobility anchor and the P-GW as the PDN/internet gateway, with the S5/S8 and S1-U interfaces.