[Galaxy S 4 Q&A, Help & Troubleshooting] Jovke:
2G/3G

Things are not so simple as they seem.

2G base station belongs to separate system than 3G base station, even when they are located at same physical location.

2G base station needs to have 3G and other 2G neighboring sites defined in its configuration (and the other way around) for smooth transition of phone in both directions without losing signal.

Switchover to another base station of same or different technology (2G/3G) is initiated when signal quality of active base station detected by phone is below certain level. In terminology it is called "handover".

Handover can be initiated by phone or network if it is detected that signal of currently connected base station is approaching too low level and/or quality.

If handover to another base station with better signal is done properly, user will not detect any problem and will not lose signal. Even during phone call in progress noting will be noticed during handover.

If the parameters are not set properly, phone can lose signal before decision to switch to another tower is made.

In that case everything is disconnected (call, data connection) and phone starts searching for network just like it was just switched on. And, after few seconds it connects to best available base station.

There are many things that cause this:

- bad settings on the base stations themselves,

- too weak or too low quality signal in the whole area, there is no base station available which can provide stable service,

- compatibility of phone software with specific network in question, that is with vendor and model of base stations,

- buggy hardware radio module on phone,

- buggy phone software​

Anyway, often case is that 2G to 3G and/or reverse handover is not even defined in base stations' settings. So, when the phone has no other option but to switch from 2G base station to 3G because that is the only one available with good signal, it cannot do so because network has not been configured for that option and it does not allow the transition. The only solution for phone in that case is to drop the network completely and start searching for available networks from beginning.

---------- Post added at 02:51 PM ---------- Previous post was at 02:37 PM ----------

"Auto" means that phone will choose 2G or 3G automatically. Phone software is usually setup to use 3G as much as it can and to switch to 2G only when there's no 3G signal. Since usually 3G coverage has a lot of holes, phone will very often have 2G as it's only option. Since many mobile operators have not configured smooth transition from 3G to 2G, phone will drop the network and start searching for networks again.

This could happen even if you have a good 3G signal and you are physically not moving, being at the same spot the whole time. Reason for this is that 3G base station, compared to 2G, has variable coverage radius. 3G signal strength varies and is adjusted by the base station itself. 3G signal strength is a function of number of users connected to base station, their throughput and distance from base station.

The more users 3G base station has near itself and the more their data throughput increases, 3G base station automatically decreases signal strength, which leaves users which are farther away without coverage.

You can observe that by having let's say 50% signal and then suddenly you lose the signal completely and get "no network" message. This rarely happens if you are close to base station, you will experience drop in signal but it will not disconnect from network.

This does not happen with 2G, signal is always at maximum and constant. Signal strength depends only of how far you are away and how many obstacles you have between you and base station.

Also, most of the operators tend to have full area coverage for 2G and holes on 3G because of cost of 3G technology.