LTE and SAE are making big steps forward and the major specification documents are nearing completion. In part 1 of this mini-series, I've started taking a closer look at 3GPP TS 23.401, the main SAE (System Architecture Evolution) specification document and reported about the flexible meshed like architecture design. In this part, I'll have a look at Evolved Packet System Mobility Management (EMM) and EPS Connection Management (ECM) and their differences to Mobility- and Session Management of UMTS.
Before taking a look at the features in SAE, let's have a look at how similar things work today with UMTS as many of you will be familiar to that. Here, the SGSN at the border between the radio access network and the core network has two management tasks:
UMTS – Packet Mobility Management
The first is called Packet Mobility Management (PMM) and deals with keeping track of the whereabouts of mobile devices. There are three states: A mobile device is PMM detached when it is switched off or if it is not connected to the packet switched part of the UMTS network. That's the case for example if the device has been set to connect to the circuit switched part right at power on but not to the packet switched part unless it becomes necessary, i.e. the user wants to establish a data session. When a data session is established, the connection state changes to PMM connected. Afterwards, if the mobile is connected but hasn't exchanged data with the network for some time, the radio network controller (RNC) can ask the SGSN to release the mobility management connection. The connection then enters PMM idle state and the mobile only reports to the SGSN when it changes a routing or location area. If an IP address was assigned, it is kept. From the application layer point of view (e.g. the web browser) there is no difference between PMM connected and PMM idle.
UMTS – Session Management
The Mobility Management only deals with the whereabouts and tracking of the mobile device, so this state machine knows nothing about assigned IP addresses and contexts. This is task of Session Management (SM). Here there are only two states, either a device has a session and an IP address or it hasn't.
And now to SAE / LTE
In SAE things work a bit different and I guess that's the reason why the mechanisms had to change as well. The biggest difference in SAE is that once a mobile device is switched on it always has at least a default bearer. In other words, it always has an IP address when it is switched on. And again in other words it's not possible for a mobile device to be attached to the network and not have an IP address. Hence, session managements makes no sense in LTE/SAE. To reflect this, the following two state machines are used in LTE/SAE:
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.
Quite difficult to make a summary as Mobility Management, Connection Management and air interface DRX control are in theory independent from each other but have to be looked at in common to make sense. In a rough generalization I would say that during normal operation:
- a mobile is always in EMM registered state because it's identiy is known to the network and, implicity, an IP address has been assigned;
- a mobile transfering data is always in ECM connected state;
- a mobile not transfering data is also in ECM connected state but DRX is activated on the air interface;
- only mobile whith very long periods of inactivity are in ECM idle state while staying EMM registered.
I hope this look at EMM and ECM from different points of view have made the concepts a bit clearer. In the next part of this mini-series, I'll have a look at the different handover variants the SAE architecture supports to ensure the mobile device is always best connected. As always, comments are welcome.