When it shows 3.5g, the radio is actively transmitting and "online". When idle for awhile it drops to standby and you see it as 3g. Upgrade from transmitting data in 3.5g vs 3g depends on operator thresholds and settings. Likewise for 3.5g to 3g move. There's a delay when network decides to switch you to 3.5g, depending on operator 1 - 5s. In overloaded areas it might be closer to infinity delay, during which time no data moves


Power use in 3.5g mode is pretty high, a bit higher than having a voice call active. Anyway, since the decision to drop down to idle 3g depends on data transfers, a "ping -s 1400 google.com" in a terminal might be enough data to convince network to keep you on 3.5g. As I said before, however, you'll get less battery life doing that then the declared talk time.