Fast Dormancy (UMTS state switching on the air interface)Fast Dormancy is a relatively recent technology that is aimed at saving battery life on your phone as well as reducing congestion on mobile networks. Essentially when your phone stops submitting data, it waits for a message from the mobile network telling it to close the connection. On some phones you might notice this is when the H or H+ symbol becomes a 3 or 3G symbol. Essentially its a fast way of making the mobile data connection become dormant, thus saving battery. Many recent devices, including the Samsung Galaxy SIII support Fast Dormancy and in general that is a good thing.*

Traditional Fast Dormancy

This technology was introduced to 3G UMTS to save the current consumption of the device by switching between different mobile device activities states on the air interface. When transferring data, the mobile is in cell_DCH state and uses the high speed channels to transmit and receive data.

The UE sends a Signalling Connection Release Indication (SCRI) message to RNC without IE “SCRI Cause”. By doing this the UE requests a release of signalling connections and move to Cell_Idle state. Then network puts the connection in idle state in which the physical connection is removed while the IP address is kept.

While the handsets in idle state then there is a delay up to 2.5s to establish a channel to do data exchange. But in cell_PCH the downlink does not have to be observed continuously, only the paging channel must be checked every now and then to make sure incoming connections can be established. So this state is efficient as staying in idle state and the signalling connections stays in place. And the delay to establish a channel to do data exchange is only 0.9s.

Release 8 Fast Dormancy

The UE sends a SCRI message with IE “SCRI Cause – PS data session end” to RNC, on reception of this message RNC use inactivity timer to make inactivity decisions. The UE starts the inactivity timer (T323) soon as it realised that there is no active data on the phone and no user inactivity. If the inactivity timer sends from RNC is set to 0 (T323=0), then the handset can send the SCRI message with the cause immediately.

Instead of just releasing the signalling connection when it desires the mobile has to wait for the expiration of a network configured timer (T323), the after the timer expires the UE sends an SCRI message with a new parameter “UE requests PS data session end” then the network the DRX cycle length in use is equal to or longer than the shorter CN domain specific DRX cycle length for the PS domain and CS domain.