PSM and eDRX in IoT Connectivity

 

One of the important consideration in IoT is the capabilities of IoT device to run with a low battery consumption. This will increase the operational period of the device, lowering the maintenance demand, and at the end lowering the cost. Some of the Low Power Wide Area Network (LPWAN) technologies like NBIOT and LTE-M has widely known has this capability. They deliver this advantage is with protocol innovations to the standard that enable edge devices to communicate with cellular networks in a variety of different way. One of the example is eDRX or Extended Discontinuous Reception and PSM or Power Saving Mode.

eDRX

Introduced by 3GPP in Release 13, eDRX is an extension of the DRX feature that is used by IoT devices to reduce power consumption. It’s a mechanism in which a device goes into sleep mode for a certain period and then wakes up after a fixed interval to receive signals. The basic principle for eDRX is to extend DRX cycles to allow a device to remain in a power-saving state for a longer period of time.

How about other cellular technologies like 4G and 5G? As mentioned realier that it was introduced by 3GPP Release 13 so eDRX was also known in 4G and 5G.

In SIM card / eSIM the eDRX setting can be found in EF Elementary File Administrative Data (EFAD). This EF contains information concerning the mode of operation according to the type of USIM. It also provides an indication about how some IoT device features shall work during normal operation. In this EF it’s defined whether the IoT device is authorized or not to modify the polling interval and/or disable the UICC interface during extended DRX cycle. Even in 5G SIM, the eDRX value described further in EF5GSEDRX. It contain Extended idle mode DRX cycle length.

 

In case the SIM  supports the UICC suspension mechanism (SUSPEND UICC command), the IoT device may suspend the SIM during the extended idle mode DRX cycle. In this case, the IoT device shall resume the UICC successfully before the end of the extended idle mode DRX cycle or before any other transmission to the network.

 

How to enable eDRX

First, the IoT device requests the eDRX parameters during the ATTACH and TAU procedures. Then, the mobile network may or may not accept the IoT device’s request based on the network configuration. The network can also propose different eDRX parameters to the requested ones. But the device must always apply the parameters transmitted by the network.

Once the IoT device and network have successfully negotiated the eDRX parameters, the network will know when to reach the device. During eDRX, any data or messages sent to the device will be queued and delivered to the IoT device when its receiver turns back on.

 

PSM

 

How about power saving mode (PSM)? PSM is a feature of cellular networking  designed for IoT devices. As its name implies, it allows idling devices to be placed in a low-power mode, conserving a maximum amount of energy when the IoT device is not in use. PSM is key to keeping energy-related costs low, reducing traffic on a particular network, and lengthening the lifecyle of IoT devices.

Regarding the PSM capability on SIM, if the UICC supports the UICC suspension mechanism (SUSPEND UICC command), the IoT device may suspend the UICC after entering the PSM. In this case, the IoT device shall successfully resume the UICC before it can leave the PSM.

 

How to enable PSM

Both the network and mobile device need to support this feature to enable PSM. During the Attach or Tracking Area Update procedures, the mobile device activates PSM by negotiating two timers with the network the T3324 Active Timer, it’s determine the duration IoT device reacheabled by the network  and T3412 Extended periodic Tracking Area Update timer, it’s to periodically notify the availability of the IoT device to the network. The network may accept these timer values or set new ones. The network can also reject the request entirely if the network does not support PSM. The difference between these two timers is the time a device can remain in Power Saving Mode (T3412 – T3324).

The Use of PSM or eDRX are depends on applications. As per analysis and evaluation, for given triggering interval eDRX performs better than PSM mode when shorter reachability is needed. PSM performs better than eDRX when reachability requirement is in same range as trigger interval.

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