Low Power Modes: eDRX & PSM
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LTE Cat-M1 is often used for battery-powered devices. To extend the battery's lifetime, it is crucial to reduce the energy drain where possible. The way to do so is turn off the cellular's radio and let the device sleep, for as long as possible.
It has always been possible for cellular devices (also referred to as User Equipment or UE) to turn off their radio to preserve energy. However, before 3GPP Release 12 in 2015, devices had to turn on their radio and go through an energy-draining procedure to reattach itself to the network before sending a new data packet. This procedure includes frequency scanning, searching for mobile operators and selecting a cell tower. With novel cellular IoT technologies, the cumbersome reattachment procedure is no longer required as devices can turn off their radio while keeping the session with the cell tower alive. When a device decides to transmit a data packet, it simply activates its radio and transmits the message.
The two low power modes which are essential to running battery-powered devices are eDRX (Extended Discontinuous Reception) and PSM (Power Saving Mode). Both modes allow the UE to initiate a sleep state during which it consumes little energy, though it won't be available to receive any incoming traffic in the meantime.
Whether it is possible to use the low power modes, depend on your cellular modem, the mobile operator and the specific cell tower your modem is using.
Extended Discontinuous Reception, abbreviated to eDRX was introduced in 3GPP release 13, dating back to 2015.
Discontinuous Reception (DRX) refers to a power saving mode many smartphones are using to preserve battery. Smartphones usually don't have a continuous connection with a cell tower, instead it frequently checks with the cell tower for pending data packets from the network. The interval in which this happens is usually 2.56 seconds and is referred to as the paging cycle. Between the paging cycles, the device goes into micro-sleep before checking-in with the network again. These micro-sleeps during which the UE is not able to receive any traffic, is called Discontinued Reception, or DRX. In eDRX mode, UEs can be configured to extend their DRX cycle, allowing the device to sleep for a longer period of time. This reduces the power consumption, but increases the latency for incoming traffic from cell towers.
When enabling eDRX, two variable need to be defined: the Paging Time Window (PTW) and the eDRX Cycle. Together they form the Page Cycle Length (PCL).
The PTW is the period when the UE is reachable by the network and when it can receive incoming traffic. This period can be set between 1,28 and 20,48 seconds. The PTW is followed by the eDRX Cycle, the interval where the cellular modem is asleep and cannot be reached by the network. This cycle can be defined up to 43 minutes.
Some cellular modems allow you to set both the PTW and eDRX Cycle, some modems only allow you to set the eDRX cycle. Study the AT Commands manual from your cellular modem vendor to find out what applies to your modem.
Setting eDRX can be done using the AT Command AT+CEDRXS
Set the eDRX parameters
AT Command
AT+CEDRXS=<mode>,<AcTtype>,<Requested eDRX value>, <Requested paging time window>
Response
OK
Response unsolicited mode
AT+CEDRXP: <AcTtype>,<Requested eDRX value>,<NW provided eDRX value>,<Paging time window>
OK
Read the eDRX parameters
AT Command
AT+CEDRXS?
Example response
+CEDRXS: 4,"0001"
OK
Test the eDRX parameters
AT Command
AT+CEDRXS=?
Example response
+CEDRXS: (0-3),<AcT-type(2-5)>,<Requested_eDRX_value(0-15) in bits>,<Requested_PTW_value(0-15) in bits>
OK
Syntax Values
mode
0: Disable the use of eDRX
1: Enable the use of eDRX
2: Enable the use of eDRX and enable the unsolicited result code
3: Disable the use of eDRX and discard all parameters for eDRX
AcTtype
1 for EC-GSM-IoT
2 for GSM, GPRS/eGPRS
3 UTRAN
4 for E-UTRAN (LTE Cat-M1)
5 for E-UTRAN (NB-IoT)
Requested_eDRX_value
4 bit value (see below table for details)
NW-provided_eDRX_value
4 bit value (see below table for details)
Paging_time_window
4 bit value
eDRX cycle length duration values
0 0 0 0
5,12 seconds
0 0 0 1
10,24 seconds
0 0 1 0
20,48 seconds
0 0 1 1
40,96 seconds
0 1 0 0
61,44 seconds
0 1 0 1
81,92 seconds
0 1 1 0
102,4 seconds
0 1 1 1
122,88 seconds
1 0 0 0
143,36 seconds
1 0 0 1
163,84 seconds
1 0 1 0
327,68 seconds
1 0 1 1
655,36 seconds
1 1 0 0
1310,72 seconds
1 1 0 1
2621,44 seconds
Page Time Window (PTW) values PTW is the duration during which the modem can receive data
0 0 0 0
1,28 seconds
0 0 0 1
2,56 seconds
0 0 1 0
3,84 seconds
0 0 1 1
5,12 seconds
0 1 0 0
6,4 seconds
0 1 0 1
7,68 seconds
0 1 1 0
8,96 seconds
0 1 1 1
10,24 seconds
1 0 0 0
11,52 seconds
1 0 0 1
12,8 seconds
1 0 1 0
14,08 seconds
1 0 1 1
15,36 seconds
1 1 0 0
16,64 seconds
1 1 0 1
17,92 seconds
1 1 1 0
19,20 seconds
1 1 1 1
20,48 seconds
Power Saving Mode enables the modem to sleep and detach itself from the mobile network while keeping the session with the cell tower alive. If any data packets are sent to the device during its PSM Cycle, the cell tower will store the data and send it to the device once it reactivates itself. Once it wakes up, the UE sends its TAU (Tracking Area Update), informing the network of its current registration and status. The periodic TAU (pTAU) is the period between two TAUs, and contains the Active Time (known as T3324) in which the UE can receive incoming data, and the PSM Cycle which is the duration of the UE being in sleep mode.
The Active Time has to be at least 16 seconds. Once the Active Time expires, the device starts its PSM cycle, also known as the Hibernate state which can be (depending on the operator) up to 413 days (as defined in TS 24.008). Note that some operators only allow PSM cycles of 4 hours minimum.
Setting PSM can be done using the AT Command AT+CPSMS
Set the PSM parameters
AT Command
AT+CPSMS=<mode>,<Requested_Periodic_RAU>,<Requested_GPRS_READY_timer>,<Requested_Periodic_TAU>,<Requested_Active_Time>
Response
OK
Read the PSM parameters
AT Command
AT+CPSMS?
Syntax
+CPSMS: <mode>,<Requested_ Periodic_RAU>,<Requested_ GPRS_READY_timer>,<Requested_Periodic_TAU>,<Requested_Active_Time>
Example response
+CPSMS: 1,,,"01000011","01000011"
OK
Syntax Values
mode
0: Disable the use of PSM
1: Enable the use of PSM
Requested Periodic RAU
Not used for LTE-M
Requested GPRS READY timer
Not used for LTE-M
Requested Periodic TAU
8 bit value (see below table for details)
Requested Active Time
8 bit value (see below table for details)
To request the Periodic TAU and Active Time, an 8 bit value needs to be created and sent to the cellular modem.
The 8 bit value is constructed out of 2 parts: the first 3 bits are the multiplier, the last 5 bits are binary value. The multiplier bits are shown in the table below
0 0 0
10 minutes
2 seconds
0 0 1
1 hour
1 minute
0 1 0
10 hours
6 minutes
0 1 1
2 seconds
1 0 0
30 seconds
1 0 1
1 minute
Mind that the first 3 bits represent different multipliers, depending on setting the TAU (T3412) or Active Time (T3324).
The binary value is simply a binary number, ranging from 00000 (0) to 11111 (31).
Be aware that when the UE is in Hibernate state, it becomes unresponsive to AT Commands.
If your mobile network supports LTE Cat-M1 doesn't necessarily mean it also supports eDRX and/or PSM. Whether it is possible to use the low power modes depend on your cellular modem, mobile operator as well as the specific cell tower your modem is connected to. Make sure to validate if all 3 elements and thoroughly test the modes before deploying your devices in the field.
4 minutes TAU
1 hour multiplier is: 001 (see table below)
4 is: 00100
20 seconds Active Time
2 seconds multiplier is: 000
10 is: 01010
