These are my lecture notes from IMT’s NET02x (4G Network Essentials) course, I thought I’d post them here as they may be useful to someone. You can find my complete notes here.

As we just saw when a terminal moves to ECC-Idle while in EMM-Registered state, it releases it’s radio resources, so what happens when the UE needs to send / receive data again?

While one option could have been to go through the full attach procedure again when the UE is triggered, the 3GPP team wanted the re-connection process to be as fast as possible.

As we saw in the last post we don’t drop the S-GW <-> P-GW tunnel, which saves time on re-establishing a connection. The S1 tunnel is also not completely released; the TEID value from the S-GW end of the tunnel is saved by the MME so it can be reused by the new tunnel when the UE reconnects, without needing to inform the S-GW.