LTE-M Timers: Understanding T3324 and T3412

This page provides an overview of LTE-M Timers, specifically T3324 and T3412. It details their functions within the LTE-M technology landscape.

Introduction to LTE-M

LTE-M, short for Long Term Evolution for Machine-Type Communications, adheres to 3GPP specifications, similar to standard LTE technology. It caters specifically to IoT and machine-type communication applications.

LTE-M devices are categorized into Cat-0, Cat-M1, and Cat-M2, based on different 3GPP releases. These categories align with 3GPP TS 36 series of rel.12, rel.13, and rel.14, respectively.

LTE-M employs two power-saving modes: PSM (Power Saving Mode) and eDRX (extended Discontinuous Reception). PSM utilizes two crucial timers: T3324 and T3412.

T3324 Timer: Active/Idle Mode Duration

The T3324 timer governs the duration a User Equipment (UE), also referred to as an “IoT Module,” remains in ACTIVE or IDLE mode after waking up due to a Periodic Tracking Area Update or when initiating a Mobile Origination (MO) event. Mobile Origination refers to the UE transmitting data or SMS to the network, destined for a backend Application Server.

This timer involves a trade-off:

• Lower values: Conserve battery life by enabling the UE to enter HIBERNATE state (the remaining duration of the T3412 Extended timer) more quickly.
• Higher values: Allow more time for the Application Server (AS) to respond to UE/MO data, such as acknowledgements or network-initiated data.

It is recommended to carefully test this parameter to determine the optimal value for your specific use case.

Recommendation: Aim for a ratio of T3324 Active Timer to T3412 Extended Timer where:

(T3412 Extended Timer - T3324 Active Timer) / T3412 > 90%

This configuration helps maximize battery savings when using the PSM feature. While there isn’t a network-recommended value, the value cannot be below the following:

• Minimum: 16 Seconds (Calculated as: 2 * DRX cycles (MNO LTE DRX cycle value, e.g., 2.56sec) + 10 seconds (buffer time) = 2 * 2.56sec + 10 seconds = 16 seconds)

T3412 Timer: Extended Hibernate Period

The T3412 Extended Timer specifies the interval at which the device informs the network about its continued registration via Periodic Tracking Area Updates (pTAU). The period between the T3324 Timer expiry and the next pTAU instance represents the HIBERNATE period.

Important Note: The UE/Device is unreachable for Mobile Terminated messages/SMS while in the PSM/Hibernate state.

A recommended value for this setting is:

• Minimum: 4 Hours

Note: The maximum T3412 Extended value is 413 days, as defined by 3GPP (TS 24.008).

Regarding mobile-terminated SMS/Data delivery, it’s advisable for enterprise applications to implement a “pull” model for MT Data. This means the device initiates MO data transmission to the application server, and the application server responds with the downlink payload.

Note on Mobile Originations: An Application Processor running on a UE, which controls the UE Module/Radio, can initiate a Mobile Origination at any time, even if the device is in a PSM state. Therefore, Mobile Originations are not governed by the use of PSM. Furthermore, T3412 resets after MO events.