How does the 2G power-on sequence translate to UMTS (3G) — what changes and what stays the same?

The sequence is identical — find network → register → establish session — but UMTS renames the radio side (NodeB/RNC instead of BTS/BSC), upgrades authentication to mutual UMTS-AKA via the USIM, and keeps the GPRS data core (SGSN/GGSN, PDP Context) almost unchanged.

What gets renamed (radio side):

• BTS → NodeB, BSC → RNC (Radio Network Controller); together they form the UTRAN
• Air interface: GSM's TDMA → W-CDMA, with radio signaling handled by the RRC protocol
• Broadcast decoding, cell selection/camping, and Location/Routing Area registration all carry over conceptually

What actually improves:

• The SIM becomes a USIM running UMTS-AKA: authentication is now mutual — the network must also prove itself to the phone, closing the fake-base-station hole that 2G's one-way authentication left open
• Encryption is joined by integrity protection of signaling as a standard feature

What stays the same (core side):

• CS registration toward the MSC/VLR and PS registration via GPRS Attach toward the SGSN
• The data session is still a PDP Context to the GGSN — same tunnel, same IP-with-NAT pattern
• HLR/AUC remain the home registers behind it all

Tip: "Same story, new radio." The 2G chain survives intact — only the radio actors are recast (NodeB/RNC, W-CDMA, RRC) and authentication gets the mutual upgrade.

Go deeper:

• Authentication and Key Agreement / UMTS-AKA (Wikipedia) — the mutual challenge–response that closes 2G's one-way-auth gap: the USIM now also verifies the network, defeating naive fake base stations.