The MME runs its own set of EMM timers. Most of them supervise a message that the MME sends and wait for the answer from the UE. The rest decide how long the MME keeps an idle UE reachable before it stops paging and detaches the UE.

I'll start with the network side table from 24.301. Then I'll show how each timer pairs with a UE side timer, how the reachability timers work, and what later releases have added. The UE side timers are on a separate page, Timer - EPS Mobility Management - UE Side.

• EMM Timers on the Network Side
• How do the network timers pair with the UE timers ?
• How long does the MME keep an idle UE reachable ?
• What has changed in later releases ?
• Reference

EMM Timers on the Network Side

Each row is one timer that runs in the MME. The last column is different from the UE side table. It describes the 1st to 4th expiry, because the MME normally retransmits and gives up on the fifth expiry, as NOTE 1 says.

Following table comes from 24.301 - 10.2 Timers of EPS mobility management (Table 10.2.2: EPS mobility management timers – Network side)

Let's put a number on NOTE 1. T3450 is 6 seconds, and the MME retransmits ATTACH ACCEPT on each of the first four expiries. So the MME waits about 30 seconds in total for ATTACH COMPLETE before it aborts the procedure. T3460 and T3470 follow the same pattern for AUTHENTICATION REQUEST, SECURITY MODE COMMAND and IDENTITY REQUEST.

The last two rows work differently. The mobile reachable timer and the implicit detach timer do not guard a message. They measure how long an idle UE has been silent, and the section on reachability explains them.

• A network side retransmission timer is short : 6 seconds for T3422, T3450, T3460 and T3470, because the UE is in EMM-CONNECTED mode and should answer at once.
• The MME repeats a message up to four times : on the fifth expiry, it normally aborts the procedure.
• T3413 has no fixed value : the paging supervision time is network dependent, and so is the action on its expiry.

How do the network timers pair with the UE timers ?

A NAS procedure has two ends, and each end has its own supervision timer. When you read a log, it helps to know which timer is running on the other side. The table below puts the two sides of each procedure next to each other, based on 24.301 v20.0.0.

The attach shows how the two sides work together. The UE sends ATTACH REQUEST and starts T3410. The MME answers with ATTACH ACCEPT, and at that moment T3410 stops on the UE side and T3450 starts on the network side. T3450 stops only when ATTACH COMPLETE arrives. So a lost ATTACH COMPLETE shows up in the log as a repeated ATTACH ACCEPT, not as a new ATTACH REQUEST.

Detach is the one procedure with a timer of the same kind on both sides. T3421 supervises a DETACH REQUEST from the UE, and T3422 supervises a DETACH REQUEST from the MME. Which timer runs tells you which side started the detach.

• A network timer usually starts when the UE timer stops : the same message, for example ATTACH ACCEPT, ends one wait and begins the next.
• A repeated ACCEPT in a log points to the network timer : it means the MME did not receive the COMPLETE message before T3450 expired.
• T3470 has no UE side partner : the UE answers IDENTITY REQUEST directly, without a timer of its own.

How long does the MME keep an idle UE reachable ?

An idle UE sends nothing until its next periodic TAU, so the MME cannot see whether the UE is still in coverage. The MME solves this with two timers that run while the UE is in EMM-IDLE mode. Their values are linked to the UE timers T3412 and T3423.

The mobile reachable timer starts when the UE enters EMM-IDLE mode. Its default value is 4 minutes greater than T3412, so the UE has a margin of 4 minutes for its periodic TAU. With the default T3412 of 54 minutes, the mobile reachable timer runs for 58 minutes. If it expires, the network typically stops paging the UE.

The implicit detach timer starts when the mobile reachable timer expires while the network is in EMM-IDLE mode. Its value is network dependent. When ISR is activated, its default value is 4 minutes greater than T3423. On its first expiry, the MME implicitly detaches the UE, without sending any message.

Both timers stop when a NAS signalling connection is established. So a periodic TAU, a service request or a paging response resets the whole chain. A UE that returns after an implicit detach usually receives a reject with EMM cause #10 "Implicitly detached", and it has to attach again.

• The mobile reachable timer follows T3412 : if the operator changes T3412, the reachability window on the MME changes with it.
• Paging stops before the UE is detached : the MME first stops paging on the mobile reachable timer, and detaches only on the implicit detach timer.
• An implicit detach is silent : the UE learns about it only when its next request is rejected, typically with cause #10.

What has changed in later releases ?

The table above matches an early release of 24.301. 24.301 v20.0.0, Release 20, keeps every row of it and adds three more timers on the network side. It also gives most retransmission timers a second value for coverage enhancement.

The new timers are listed below. Two of them belong to the power saving features eDRX and PSM, and one belongs to service gap control.

The retransmission timers also have a longer value in WB-S1/CE mode. This applies when the UE supports CE mode B, and in some satellite access types. T3422, T3460 and T3470 grow from 6 seconds to 24 seconds, and T3450 grows from 6 seconds to 18 seconds. In NB-S1 mode, the timer value is calculated as described in clause 4.7 of 24.301.

Two existing rows have also changed. T3413 now also stops when the paging procedure is aborted. And the mobile reachable timer has more rules in its note. If the UE is attached for emergency bearer services, the timer is set equal to T3412, and on expiry the MME detaches the UE. If the MME sends T3346 larger than T3412 in a reject, it sets the two reachability timers so that their sum is greater than T3346.

• PSM and eDRX change what reachable means : the active timer and T3415 let the MME stop paging a UE that is in a power saving period by design.
• CE mode lengthens the retransmission timers : a UE with many repetitions needs more time to answer each message.
• T3447 runs on both sides : the UE and the MME each run T3447 for service gap control, so both sides agree when uplink user data is allowed again.

Reference

• 3GPP TS 24.301 v20.0.0 - clause 10.2, Table 10.2.1, EPS mobility management timers - UE side, and Table 10.2.2, EPS mobility management timers - network side