A UE spends its life moving between a small number of states, and one picture holds the whole of it. The diagram on this page draws those states as boxes and the moves between them as arrows. What the picture leaves unsaid is which specification state each box stands for, and this note fills that in.

• The Diagram and What It Draws
• Three State Machines in One Picture
• What Each Box Is in Specification Terms
• Why the Arrows Carry Four Different Labels
• Reference

The Diagram and What It Draws

Following diagram shows a possible state machine that a UE would go through. The state transition in this post will be about (s0)->(a)-> (b1) -> (c1) -> (d1) -> (e1) -> (f1) -> (g1) -> (h1).  Most of other transition will be described in "Handover" page.

• The drawing carries two parallel columns of boxes. The left column is lettered (a1) down to (h1) and the right column (a2) down to (h2). The two columns are the same sequence in two different cells.
• (s0) Power On sits alone at the top. Two arrows leave it, both labelled CS, and one goes to each column. The UE lands in whichever cell cell selection gives it.
• The boxes come in two colours, and the colours separate two different kinds of thing. The green boxes are (b) Packet Comm with default EPS, (d) Idle and (g) Packet Comm With Dedicated EPS. Those are the states the UE rests in. The lavender boxes are RRC Setup, RRC Release and Paging, which are procedures it passes through.
• The legend at the top left expands the four arrow labels. CS is Cell Selection, CR is Cell Reselection, RD is ReDirection and HO is Handover.
• A double headed arrow labelled CR joins (d1) Idle to (d2) Idle, and an arrow labelled HO joins (g1) to (g2). Each sits on one row only, and which row that is turns out to matter.
• Two more arrows cross between the columns in the upper half. HO labels the pair at the top of the crossing and RD labels the pair below it. Each direction is drawn as its own arrow, which is why the pair crosses.
• A line runs down the outside of each column from the RRC Release box at the bottom back up to the Idle box. The sequence therefore loops to (d) rather than to (s0), and the reason for that is the subject of a section below.

Note : Intial Registration and Default EPS Bearer Setup procedure would be common to almost all LTE network. Of course, there would be a small variations but overall concept would be almost same. But the procedure after <Idle> would be quite different among Network Operators. Following would be two major variations.

• Setup RRC Connection, RRC Connection Reconfiguration without creating any dedicated EPS Bearer.(In this case, UE uses the existing Default EPS bearer for traffic).
• Setup RRC Connection, RRC Connection Reconfiguration with a dedicated EPS Bearer.(In this case, Ue uses the existing Default EPS bearer or Dedicated EPS Bearer depending on situation).

The example test sequence in this case shows the second case,

Three State Machines in One Picture

One set of boxes is doing the work of three separate state machines. They are defined in three different specifications, and they change at different moments. A box in the picture above can move one of them and leave the other two alone. Separating them is what makes the diagram readable.

RRC, which 36.331 defines

36.331 clause 4.2.1 gives the UE two states on E-UTRA with EPC. A UE is in RRC_CONNECTED once an RRC connection is established, and in RRC_IDLE when none is. There is a third state, RRC_INACTIVE. It applies only when the UE is connected to 5GC, so it plays no part in the picture above. Every RRC Setup box moves the UE one way and every RRC Release box moves it back.

ECM, which 23.401 defines

23.401 clause 4.6.3 tracks whether a NAS signalling connection exists. A UE in ECM-IDLE has no such connection, and no UE context exists for it in E-UTRAN. A UE in ECM-CONNECTED has one, and 23.401 describes it as two parts joined together: the RRC connection and the S1_MME connection. ECM therefore follows RRC closely through the drawing above. Half of an ECM connection is the RRC connection itself.

EMM, which 24.301 defines

24.301 clause 5.1.3.2.2 tracks registration rather than connection. In EMM-DEREGISTERED no EMM context exists and the MME cannot reach the UE. In EMM-REGISTERED an EMM context exists, and a default EPS bearer context has been activated. 24.301 also defines EMM-IDLE and EMM-CONNECTED as modes rather than states. It says plainly that they correspond to the ECM-IDLE and ECM-CONNECTED states of 23.401.

The table below sets the three side by side.

• Three specifications own three of the arrows in one drawing : 36.331 owns the RRC state, 23.401 owns the connection state, and 24.301 owns the registration state.
• EMM-IDLE and EMM-CONNECTED are modes rather than states : 24.301 maps them onto the ECM-IDLE and ECM-CONNECTED states of 23.401.

What Each Box Is in Specification Terms

The lettered boxes are convenient to talk about and say nothing about what the UE actually is at that moment. The table below gives each one its three states. Reading down the three state columns is more useful than reading across the rows, because each column changes at its own pace.

One row of the table needs a word of warning before you read it. The box marked (a) is drawn as RRC Setup alone. The note under the diagram above adds initial registration and default EPS bearer setup to it. The attach therefore completes somewhere between (a) and (b). The table records the state the UE has reached by the end of the box.

The EMM column changes once and then stops. It moves from EMM-DEREGISTERED to EMM-REGISTERED during the attach, and it holds that value for every remaining row. The RRC and ECM columns meanwhile change six times between them, cycling with each setup and each release.

That difference is the answer to a question the drawing raises and does not settle. The loop at the outside of each column returns to (d) Idle rather than to (s0) Power On, and the EMM column says why. Registration survives the release. The UE in (d) has lost its RRC connection and its S1 connection. The MME still holds its EMM context, and it can still page the UE.

The name of the (d) Idle box is worth a second look for the same reason. It is idle in the RRC and ECM sense only. 23.401 clause 4.6.3.1 gives a UE in ECM-IDLE a list of things it must still do. Tracking area updates and answering paging are both on it. The box is not a UE that has stopped.

• The EMM column moves once, at the attach : everything after that is RRC and ECM cycling between idle and connected.
• The loop returns to Idle rather than to Power On : the MME keeps the EMM context through the release, which is what makes paging possible at (e).

Why the Arrows Carry Four Different Labels

Four labels move the UE between the two columns, and at first they look like four names for moving to another cell. They are not interchangeable. Each one is available in one state and unavailable in the others. The row an arrow sits on tells you which.

36.331 clause 4.2.1 draws the dividing line in two short phrases. RRC_IDLE has UE controlled mobility. RRC_CONNECTED has network controlled mobility, which the same clause names as handover and cell change order. Everything else in this section follows from those two phrases.

The two labels that belong to idle

CS and CR both sit where the UE holds no RRC connection. Cell selection runs at (s0), before any connection exists. That is why two CS arrows leave the power on box, one to each column. Cell reselection joins (d1) to (d2) and appears on no other row. The UE decides both of them. 23.401 clause 4.6.3.1 has a UE in ECM-IDLE perform cell selection and reselection on its own.

The label that belongs to connected

HO sits on the rows where packet communication is running. It is drawn between (g1) and (g2), and again across the upper half of the drawing. The network decides it. 23.401 clause 4.6.3.2 hands the mobility of a UE in ECM-CONNECTED to the handover procedure. It is the only cross column move available while a connection is up.

The label that is neither

RD is the interesting one, because redirection is not a mobility procedure in its own right. 36.331 clause 4.2.1 describes connection release with redirection information, so the redirection travels inside the message that ends the connection. The network names a target while it still has a connection to speak over. The UE then acts on that name once the connection has gone. That is why the RD arrows leave the RRC Release boxes rather than the packet communication boxes.

• The row an arrow sits on tells you who chose the move : an idle row is the UE's decision and a connected row is the network's.
• Redirection is not a fourth mobility procedure : it travels inside the connection release, which is why it leaves the release box.

Reference

Three specifications define the states named on this page, and a fourth defines the cell selection rules referred to above. The clause numbers sit beside each claim so the wording can be checked rather than taken on trust.

• 36.331 - Evolved Universal Terrestrial Radio Access (E-UTRA); Radio Resource Control (RRC); Protocol specification, v19.3.0. Clause 4.2.1 defines RRC_IDLE and RRC_CONNECTED. It gives UE controlled mobility to the first and network controlled mobility to the second. It also describes connection release with redirection information.
• 23.401 - General Packet Radio Service (GPRS) enhancements for E-UTRAN access, v20.0.0. Clause 4.6.3.1 is ECM-IDLE and lists what a UE must still do in it. Clause 4.6.3.2 is ECM-CONNECTED, and it describes the signalling connection as an RRC connection plus an S1_MME connection.
• 24.301 - Non-Access-Stratum (NAS) protocol for Evolved Packet System (EPS); Stage 3, v20.0.0. Clause 5.1.3.2.2 defines the EMM main states in the UE, including EMM-DEREGISTERED and EMM-REGISTERED. Clause 3.1 defines EMM-IDLE and EMM-CONNECTED mode and maps them onto the ECM states of 23.401.
• 36.304 - Evolved Universal Terrestrial Radio Access (E-UTRA); User Equipment (UE) procedures in idle mode. This is the specification 23.401 clause 4.6.3.1 points at for the cell selection and reselection rules behind the CS and CR arrows. It was not read for this note beyond that reference.