IoT energy management: eDRX vs PSM
eDRX (Extended Discontinuous Reception) and PSM (Power Saving Mode) are two features designed to minimise power usage and extend battery life for devices using Low Power Wide Area (LPWA) technologies such as NB-IoT and LTE-M.
Read on to learn how these two IoT energy management features work, the similarities and differences between them, and their suitability for a range of business IoT use cases.
What is eDRX?
Devices on a network generally need to listen for network notifications and send tracking information at regular intervals. As a rule, to keep battery drain to a minimum, it’s better to let a device remain in idle (sleep) mode for as long as possible, waking periodically to report to the network and to check if there is any pending data for it.
LTE networks usually provide a power-saving feature called Discontinuous Reception (DRX). This enables devices to frequently stop listening for data and sleep for very short durations at a time (usually 1-2 seconds).
DRX works fine for smartphones. However, many IoT devices don’t need to be as reachable or transmit data a lot less frequently than a phone. Extended DRX (eDRX) is essentially a variation on the DRX principle that allows an IoT device to stay in low-power sleep mode for longer.
With DRX, the network provider decides how long devices go into sleep mode. By contrast, with eDRX, the sleep time length (i.e. the eDRX cycle) can be configured by the device owner or application developer, albeit the networks will have final say on what timers are allowed. Nonetheless, this gives developers a lot of flexibility in striking a balance between ensuring device responsiveness and limiting power consumption to match the needs of specific use cases.
When in an eDRX cycle, a device cannot receive any data sent to it. That data is not delivered until the device wakes up. On waking, the device can listen for pending data without having to establish a full network connection, thereby further helping to preserve power.
What is PSM?
PSM (Power Saving Mode) is a feature on a LPWA network that enables a device to sleep for longer than with eDRX.
The length of the PSM period for a device is negotiated between the network provider and software in the device. Typically, networks will allow a minimum sleep time of 4 hours and a maximum of 413 days. When in PSM mode, a device remains registered with the network, but no longer transmits or receives data. Any data packets sent to the device when it is in PSM are stored by the network and delivered on wake-up. When waking up from PSM, a device is only active for a certain duration. This active time duration is also negotiated with the network.
During its PSM cycle, a device uses considerably less power than a device in an eDRX sleep. However, when it wakes, it has to be active for longer (around 100-200 milliseconds) compared to a device coming out of eDRX, in order to receive and transmit control messages with the network before receiving data. To achieve the maximum power savings, eDRX can be used along with PSM to reduce how much a device is active after it wakes up from PSM.
Design and testing is important when using PSM or eDRX. Users will have to make a trade-off between extended battery life and having devices offline.
What are the primary use cases of eDRX technology?
For devices using LTE-M networks, the minimum eDRX can be as short as 5.12 seconds, ranging up to 43 minutes.
For devices using NB-IoT networks, the minimum sleep time can be as short as 10.24 milliseconds, with a maximum of up to around three hours.
eDRX is therefore a good power-saving option for devices where you want to preserve battery power, but where you still need the device to be reachable throughout the day; whether it’s every few seconds, or up to several hours.
Example use cases include:
- Asset trackers for logistics
- Industrial performance monitoring systems
- Smart grid technology
A key advantage of eDRX is that cycle times can be altered depending on the application owner’s preferences. For example on a production line performance monitor, you could set the eDRX cycle to a couple of minutes for when the production line is running, and extend the cycle to several hours for non-working periods.
What are the primary use cases of PSM technology?
Sleep times for PSM are considerably longer than for eDRX. This makes PSM a good option for power saving where it’s acceptable for the device to be unreachable for long periods throughout the day.
Example use cases include:
- Water, soil or pressure monitoring sensors where you only need the device to transmit data perhaps once or twice a day.
Find out more
Wireless Logic enables organisations to make the best possible decisions regarding the suitability of 5G technology, in line with current needs and future goals.
To explore your options, speak to us today. For further education around all things IoT, our IoT glossary is full of definitions and explanations.