eDRX(Extended DRX)

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Hi Guys today we are learning about eDRX in NB-IOT. If you are interested in learning more about DRX please click here.

What is eDRX?

eDRX(Extended DRX) is a feature which was introduced by 3GPP in Rel. 13.
As the name suggests, it is an enhancement/extension of DRX, a Release 8 feature. It was introduced to reduce Power Consumption of IoT devices.

What is the difference between DRX and eDRX?

eDRX is an extension of DRX in the sense that the duration for which the UE is in sleep mode has been extended considerably.

DRX is used in smartphones and other LTE devices to reduce power consumption and extend battery life.

In NB-IoT, eDRX allows the UE to sleep for a long duration of time, and wake up periodically to check Paging. Since the sleep duration in eDRX is considerably longer than in DRX, hence there can be lot of power saving.

For eDRX to work there should be perfect timing synchronization between UE and NW.
Just like DRX, there are two types of eDRX:
  • Connected Mode
  •  Idle Mode

But while the Connected Mode DRX can lasts up to 2.56 s, Connected Mode eDRX lasts up to 9.22 s.

Why eDRX is useful for IoT applications?

If the device is sleeping for a long duration of time, it won’t be reachable to the network during this period. NB-IoT devices are designed for applications where it is acceptable for them to remain unreachable for long periods, and check paging every few hours. So eDRX works well for NB-IoT devices.

E.g.: think of an IoT device which works as a temperature sensor and uploads the temperature of the surroundings every few hours. There won’t be a scenario where some information has to reach this UE urgently, and it can afford to be in sleep mode for hours at end, only waking up to check Paging, and upload data every few hours.

What is PSM and can we use eDRX and PSM simultaneously?

PSM is a Release 12 feature which is also used in NB-IoT devices to reduce power consumption. While both features are aimed at reducing power consumption, PSM is different from eDRX.
eDRX and PSM can be used simultaneously to obtain additional power savings. 

How eDRX is an extension of DRX?

In LTE, the clocks which keep the UE and NW synchronized tick in units of Subframes
1 Subframe = 1 ms.
The next unit of time is System Frame Number (SFN).
1 SFN = 10 Subframes = 10 ms.

In NB-Iot,  since UEs can be in sleep mode for hours at end, it does not make sense to count the time in milliseconds or even tens of milliseconds. So, a new unit of time was introduced called Hyper Frame Number (HFN).

1 HFN = 1024 SFN = 2.91 hours.      

(1HFN = 1024 SFN's = 1024 *10 ms = 10240ms = 10.24seconds
1024 HFN's = 1024 * either 10.24s or 10240 ms = 10485.76 /3600 = 2.913 hrs.)

What are the important messages and IEs which convey information about eDRX to UE?

  • MIB – It carries the SFN.
  • SIB1

      hyperSFN ::= BIT STRING SIZE(10) [0000000000] OPTIONAL:Exist
      eDRXAllowed ::= ENUMERATED [true] OPTIONAL:Exist

eDRX Allowed : eDRX Allowed Indicates that if idle mode extended DRX is enabled in the cell. The DUT (devices) should not operate in idle mode extended DRX only if eDRX Allowed is set to TRUE.

  • Paging

      systemInfoModification-eDRX-r13 ::= ENUMERATED [true] OPTIONAL:Exist
systemInfoModification-eDRX : If present: indication of a BCCH modification other than SIB10, SIB11, SIB12 and SIB14 for UEs in extended DRX. This indication applies only to those devices having eDRX cycle longer than the BCCH modification period.

  • RRC Connection Reconfiguration

      longDRX-CycleStartOffset ::= CHOICE [sf2560]
      sf2560 ::= INTEGER (0..2559) [0]

      eDRX-Config-CycleStartOffset-r13 ::= CHOICE [setup] OPTIONAL:Exist
      setup ::= CHOICE [sf10240]
sf10240 ::= INTEGER (0..3) [3]

eDRX-Config-CycleStartOffset : Indicates longDRX-Cycle and drxStartOffset in TS 36.321. The value of longDRX-Cycle is in number of subframes.

The value of drxStartOffset (expressed in number of sub-frames) signifying by the value. of eDRX-Config-Cycle Start Offset multiplied by 2560.plus the offset value configured.in longDRX-Cycle-Start-Offset. E-UTRAN only configures value setup when the value in longDRX-CycleStartOffset is sf2560.

Reference: TS 36.304


  1. Correction to : "1 HFN = 1024 SFN = 2.91 hours. "

    1HFN = 1024 SFN's = 1024 *10 ms = 10240ms = 10.24seconds
    1024 HFN's = 1024 * either 10.24s or 10240 ms = 10485.76 /3600 = 2.913 hrs.

    1. Do you have a link where i can read more upon this? Currently i am studying about 5G-NR, so any new information will be helpful.


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