Comparing LTE and 3G Energy Consumption [cited from AT&T]-优快云博客

本文详细介绍了3G和LTE网络中设备的状态转换过程。3G网络中有IDLE、DCH、FACH等状态，而LTE则简化为IDLE和CONNECTED两种状态，并通过DRX机制节省电力。

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https://developer.att.com/developer/forward.jsp?passedItemId=11900006

Figure 1: The 3G state machine.

Starting in the lower left of figure 1 and proceeding clockwise, we begin with the IDLE state and follow the four black arrows indicating state changes:

In the IDLE state, the radio is off. When data is to be sent, there is a 2 second delay (due to radio signaling) before reaching the DCH (CELL_DCH) state.
DCH is the high throughput state where packets are transmitted. After packet transmission has stopped, the device remains in DCH for 5 seconds in case more packets arrive. This idle time of 5 seconds is called the DCH tail.
If no more data arrives, the radio switches to a shared channel called FACH (CELL_FACH). The FACH state can handle signaling packets, but if more data comes in, the radio switches back to the DCH state.
If no packets arrive after 12 seconds in the FACH state, the radio returns to the IDLE state and turns off.

Figure 2: The LTE state machine.

In the LTE state machine, there are two basic states: CONNECTED and IDLE (shown in the boxes marked RRC_CONNECTED and RRC_IDLE).
In the CONNECTED state, the radio is on and is using high power (1000 — 3500mW) while it either transmits data or waits for data. In the IDLE state, the radio is off and uses low power (<15mW).

While in the CONNECTED state the radio moves from Continuous Reception, when it is transferring data, to the Short DRX tail state, and then the Long DRX tail state when it is waiting for data.

The Continuous Reception state is the high-bandwidth CONNECTED state that the radio uses when it is actively transferring data. This high power and high bandwidth state is what gives LTE its speed.

When data transmission is completed, the radio moves to the Short DRX state, but is still using high power while it waits for more data. If more data arrives, the radio returns to the Continuous Transmission state, and if it does not, it progresses to the Long DRX state.

In the Long DRX state, the radio prepares to switch to the IDLE state, but is still using high power and waiting for data. If more data arrives, the radio returns to the Continuous Transmission state, and if it does not, it switches to the low power (< 15 mW) IDLE state and turns off.