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0 pointsNextgrid5y ago0 comments

> your IP address has to move across

That’s handled at a higher level though. If you manage to get the lower level handover running fast enough, the IP side of things shouldn’t be a problem (as all the traffic goes to the GGSN or whatever LTE’s equivalent of that is).

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BuildTheRobots5y ago

> the IP side of things shouldn’t be a problem (as all the traffic goes to the GGSN or whatever LTE’s equivalent of that is).

I'm not sure it's quite that simple. One of the things LTE started (and 5G continues) is to push more and more of this to the edge of the network. Traffic from the UE does get tunnelled through the Serving Gateway (S-GW), but there can be multiple SGW's on a network. Mobility management is also a big enough issue that the Mobility Management Entity (MME) is an entity unto itself on the block diagram.

It's also worth pointing out that eNodeB's (the radios) can now talk directly to each other via X2 and can actually hand over a subscriber without particularly involving the core network.

tldr: I think it's enough of a problem for it to be not entirely trivial.

https://yatebts.com/documentation/concepts/lte-concepts/

yaantc5y ago

The mobile (UE) IP anchor is not the S-GW but the P-GW in the core network. For an established PDU session the IP address is fixed, and does not change during a handover.

For information MIP is used between S-GW and P-GW (PMIP IIRC), so there can be IP addresses changes when the HO is such that the S-GW changes. Still, this is transparent to the UE: it's a change at the tunnel level, and the UE traffic flowing inside this tunnel is not affected.

BTW what "anchor" means here is that the IP address the UE gets is routed to the P-GW. From then on the traffic is tunneled to the UE across the cellular network core and radio access networks, but all this is transparent for the end device. There's a first leg from P-GW to S-GW, then from S-GW to the eNB (the base station), then the over-the-air and last leg to the mobile device. That's 3 tunnels joined together to transport data seamlessly from P-GW to UE. During a HO the radio one always changes, the S-GW to P-GW rarely changes. And the P-GW is fixed to provide IP session continuity.

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