[Nexus 5 Q&A, Help & Troubleshooting] j'vai:
EPS - Mobility Management

"This EMM state machine only has two states. When a mobile is switched off or uses a different radio access network technology (e.g. GPRS or UMTS) it's state is EMM deregistered. That's simple. There's an optional feature referred to as Idle-mode Signaling Reducation (ISR) described in Annex J of 23.401 that changes that rule somewhat but let's ignore it for now. Once the mobile sees an LTE network it tries to register and if successful it's state is changed to EMM registered. At the same time the mobile is also assigned an IP address. As a consequence mobile devices in EMM registered state always have an IP address. But the EMM state machine does not care about that fact, it is only influenced by mobility management procedures such as Attach, Detach and Tracking Area Updates. While in EMM registered, the network knows the location of the mobile device either on a cell level or a tracking area level. Which of the two depends on the connection management state machine described right below."

EPS - Connection Management

"When a mobile device is registered (EMM state = registered) it can be in two connection management (ECM) states. While a data transfer is ongoing the device is in ECM connected state. For the mobile device this means that on the radio link a Radio Resource Control (RRC) connection is established. For the network, ECM connected means that both the Mobility Management Entity (MME) and the Serving (User Data) Gateway (SGW) have an connection to the mobile device via the S1 interface (the physical and logical link between the core network and the radio access network). in ECM connected state, the location of the mobile is known to the cell level and cell changes are controlled by handovers.

If there is no activity for some time, the network can decide that it is no longer worthwhile to keep a logical and physical connection in the radio network. The connection management state is then changed to ECM idle. Note the use of the term 'idle'. It doesn't mean the connection completely goes away. Logically, it is still there but the RRC connection to the mobile is removed as well the S1 signalling and data link. The mobile continues to be EMM registered and the IP address it has been assigned remains in place. In ECM idle state the location of the mobile is only known down to the tracking area level and cell changes are performed autonomously by the device without any signaling exchanges with the network"

Interactions With the Radio Interface

"From the base station and mobile device point of view there is a lot of room for maneuvering between ECM connected and ECM idle. While a lot of data is exchanged, the air interface can be fully activated for a device so it has to continuously listen for incoming data. In times of lower activity or even no activity at all, the base station can activate a discontinuous reception (DRX) mode so mobile devices can power down their transcievers for some time. The power down cycles range from milliseconds to seconds. In fact, the longest DRX cycle is as long as the paging interval. So from a mobile point of view the main difference between being in ECM connected state with a DRX cycle the length of a paging intervall and being in ECM idle state without a radio interface connection is how it's mobility is controlled. In ECM connected state, handovers are performed, in ECM idle state, it can change its serving cell autonomously and only has to report to the network when it leaves the current tracking area. In other words, the base station is likely to keep the mobile device in ECM connected state for as long as possible by using DRX so data transfers can be resumed very quickly before cutting the link entirely and setting the state to ECM idle."