[Galaxy S III General] d4fseeker:
Your phone has to run a whole setup, authentication and connection procedure with the antenna each time data network is cut. However when the data network is open (especially on HSPA frequencies) it drains the battery.

So FD is a 3G feature (HSPA-Versions being different extension levels) which basically tells the antenna that your phone will now disconnect the data connection but the antenna should remember him and the state of all TCP connections.

Periodically your phone will power up the data network through a very short and speedy handshake procedure which resumes the old data connection and ask the network whether any new packets have arrived. If so, it will accept and process them and keep the data connection open for new incoming/outgoing packets until it goes back to Fast Dormancy (IDLE) mode.

The network/antenna of course has to store any incoming data packets until the phone polls it so there is a measurable multi-second delay until the data is effectively received by your phone.

The exact polling frequency is defined by your phone's internal database of network operators and is e.g. for all luxembourgish network operators 5 seconds. Therere is no single 'good' value since:

- the network has to store the data. If your phone polls too slowly it might drop the data packets or even close the connection, forcing the phone to run through the full procedure again which causes huge battery drain. (This is also true if the network _SHOULD_ support FD but in reality does not)

- the incoming data packets are delayed by up to X seconds, X being the polling frequency. So you might e.g. only see incoming chat messages several seconds after them having been sent if the phone is in IDLE mode.

- too high polling frequencies put a high strain on the network (to the extend that they might refuse your mobile to reconnect for a certain timespan) and kills your battery fairly quickly.

Furthermore FD only works for incoming data packets, not outgoing ones.

If your phone sends a message, it will directly go to fully-awake 3G network until fast dormancy kicks in again after a certain idle period.

Now, basically FD is a very good solution to 3G's battery drain, however it only works if your phone does not send data and does not constanctly receive data. (Additionally of course, the FD network setup must be correctly configured with sane values... I've seen carrier-provided setups of 1 second Fast-Dormancy interval)

If you have apps which keep data connections open and constantly send/receive small amounts of data (e.g. Skype, ICQ, Msn, Facebook, ...) FD is more or less worthless for your setup and might only cause a huge battery drain.

Furthermore at least my provider (Tango, Luxembourg) sometimes shows bad cases of antenna hopping when waking up from FD which drains the battery even more.

(Antenna hopping is if the antenna tells your mobile phone to connect to an other antenna because it's overloaded or it knows that the other one has significantly more capacity available. However if not properly managed by a supervising instance, this may cause several antennas to play ping-pong with you and keep moving you to other antennas)

2G on the other hand doesn't know what FD is for a very simple reason; it uses the same frequencies for data network and voice. The latter has to be connected at all times anyway, so keeping the network connection open only causes insignificant battery drain. (As long as the device is actively sending/receiving data the battery usage will of course get higher)

So if you want your phone to be connected at all times (Chat, Push notifications, Emails, ...) you'd keep it in 2G. But due to 3G having a better KB/W ratio and being MUCH faster, you should switch when browsing the net.

There is a (paid) app for automated 3G/2G switching when the phone is idle which additionally can enable/disable Wifi when you're in range of a configured network based on it learning your exact location from signal changes of the coarse network-based location.

That app is called Juice Defender Beta (you need the 'Ultimate' donation key to unlock all the features) and so far works flawlessly on my phone.

(Additionally it can automatically dim the display below the minimum-brightness which is cool at night :D )

If the phone has a very high network drain in 2G too you most likely are either constantly receiving/sending data (e.g. Skype, I recommend using IM+ Pro and configuring it with Push notifications when in the background) or using an Exchange account which seems to have a battery-drain issue in Samsung stock firmwares.