LTE-M and NB-IoT both support the power saving techniques Power Saving Mode (PSM) and extended Discontinuous Reception (eDRX) to minimize power consumption.
The reason for employing power saving techniques, instead of just turning off the modem, is that the UE stays connected to the network and doesn’t have to go through the power consuming attach procedure to re-establish the connection. Power saving techniques allow the UE to switch off the radio while still maintaining connectivity with the network, mitigating the need for the connection re-establishing procedures.
RRC modes
The Radio Resource Control (RRC) protocol is used for communication between the UE and the eNB. The technical details of the RRC protocol and the RRC connection is beyond the scope of this course.
We will only focus on the fact that the UE can be in one of two RRC modes: RRC Connected and RRC Idle.
RRC Connected mode
RRC Connected mode refers to the time when the UE’s radio is actively sending or receiving messages. During RRC Connected mode, the UE must maintain its synchronization with the network, which consumes a lot of power.
In RRC Connected mode, the radio is mostly used for application-specific activity:
Sending uplink (UL) messages from the UE to the eNB, i.e. reporting a sensor value
Receiving downlink (DL) from the eNB, i.e. configuration updates
Neighbor cell measurements
RRC Idle mode
RRC Idle mode refers to the state when the UE is switched on, but the radio is not executing high battery-consuming tasks. In RRC Idle mode, the UE does the bare minimum to stay connected to the network and to stay reachable, so the battery consumption is low compared to RRC Connected mode, but still high compared to being in power saving mode.
In RRC Idle mode, the radio performs keep-alive activities:
Periodically listening to paging messages to be informed when the eNB requires a status update or value reporting
Periodically performing tracking area updates (TAUs) to update the eNB about the UE’s location for reachability
Paging
Paging is the mechanism in which the network tells the UE it has messages for it. The UE will then decode the content and initiates the appropriate procedure.
The UE must monitor paging, which is the process of monitoring whether the network is sending paging messages. This happens while the UE is in RRC Idle mode.
Since continuous page monitoring consumes power, the UE sleeps between monitoring pages. This method is called Discontinuous Reception (DRX).
Timers
This is a short explanation of the function of timers in cellular connections. The three timers that will become relevant later in this section are:
Periodic TAU timer (T3412): Time that the UE will stay in power saving mode before waking up to send a Tracking Area Update (TAU) to the network.
Active timer (T3324): Time that the UE is monitoring paging in RRC Idle mode, before going into PSM.
RRC Inactivity timer: Time that the UE stays in RRC Connected mode after transferring data. After this time, the UE receives the event RRC Release from the network
Understanding the different timers, and the different connection statuses and modes is important for understanding and calculating the power consumption in your application, and for debugging your cellular application.
Note
It is important to note that although some of these timer values can be requested from the network, the network ultimately decides which value to give and doesn’t have to follow a requested value.
Power saving mode (PSM)
Power Saving Mode (PSM) is a state where the UE reduces its power consumption to a bare minimum to stay connected, by going into sleep mode. In PSM, the UE is not monitoring paging and as a result, becomes unreachable. To avoid the re-connection procedure when waking up again, the UE is allowed to keep its connectivity status.
For this to happen the UE must wake up regularly and send a Tracking Area Update (TAU) to the network. This period is decided by the Periodic TAU timer. A UE in PSM will wake up for one of two reasons, either triggered by the Periodic TAU timer to send a TAU, or by the application to send UL messages.
Either way, the device wakes up, enters RRC Connected mode, sends and receives data packets, then stays in RRC Connected mode until the RRC Inactivity timer has run out. Then it will go to RRC Idle mode for the Active timer period to monitor paging, before going back to power saving mode.
The image below explains what this process looks like.
The difference between using PSM and simply turning off the modem is that in PSM the UE can still be configured to wake up to update the network about its status and stay reachable for a while. The advantage of PSM is that the UE goes into deep sleep and the radio only consumes a few microamps. The UE does not lose its connection information on the network, meaning that upon UE wakeup, the connection is established relatively quickly. This reduces the power consumption compared a powered-off modem waking up and having to re-establish a connection with the network from scratch.
eDRX
As we mentioned earlier DRX (Discontinuous reception) is a method where the UE and the network negotiate phases in which data transfer occurs. The UE wakes up regularly to check with the network if it has any incoming messages (pages), and will otherwise go into sleep mode. However, the sleep time is only around 2.5 seconds, which can work well for cell phones but when it comes to IoT devices, DRX is not enough.
Extended Discontinuous Reception (eDRX) takes this a step further, by extending the amount of time the UE can sleep before having to wake up and check for pages. In addition, the UE can request the duration it will sleep, referred to as the eDRX Cycle, and the paging time window.
Note that the device is not reachable while it is sleeping, so the time it takes to reach the device depends on the duration of the eDRX Cycle.
The image below shows two different scenarios for eDRX connection states. On the left, the UE is waking up to monitor pages without having any. On the right, the UE wakes up to a page that requires a data transfer. Then is in RRC Connected mode for the duration of the RRC Inactivity timer, before monitoring paging and going back to sleep until the next eDRX Cycle.
PSM and eDRX
Although we presented PSM and eDRX as two separate power saving techniques, they can also coexist as they implement changes to different parts of the connection process.
The image below shows the connection states when using both PSM and eDRX.
When requesting values for the Active Timer and eDRX Cycle in this case, it is important to consider that the Active Timer must be larger than the eDRX Cycle by a few magnitudes. The eDRX Cycle indicates the time for one paging window and sleep interval, while the Active Timer (usually) indicates the time it takes for multiple intervals. Therefore if the eDRX Cycle requested is larger than the Active Timer, the value won’t be granted by the network.
Different use cases
To put it very simply, the difference between PSM and eDRX is how deep the device sleeps, and for how long it can sleep. PSM sleep times are much longer than eDRX, allowing the device to enter a deeper sleep which consumes less power. The downside being that the device will take longer to wake up and it will be active for a much longer period of time.
With “sleep”, we mean that in PSM, the radio turns off both RX and TX while it’s still registered to the network. In eDRX, TX is turned off and RX is intermittently turned on to listen for paging messages.
When the UE is sleeping, the application core can still be running, reading sensor data or other operations that don’t involve the modem.
Nordic Developer Academy Privacy Policy
1. Introduction
In this Privacy Policy you will find information on Nordic Semiconductor ASA (“Nordic Semiconductor”) processes your personal data when you use the Nordic Developer Academy.
References to “we” and “us” in this document refers to Nordic Semiconductor.
2. Our processing of personal data when you use the Nordic Developer Academy
2.1 Nordic Developer Academy
Nordic Semiconductor processes personal data in order to provide you with the features and functionality of the Nordic Developer Academy. Creating a user account is optional, but required if you want to track you progress and view your completed courses and obtained certificates. If you choose to create a user account, we will process the following categories of personal data:
Email
Name
Password (encrypted)
Course progression (e.g. which course you have completely or partly completed)
Certificate information, which consists of name of completed course and the validity of the certificate
Course results
During your use of the Nordic Developer Academy, you may also be asked if you want to provide feedback. If you choose to respond to any such surveys, we will also process the personal data in your responses in that survey.
The legal basis for this processing is GDPR article 6 (1) b. The processing is necessary for Nordic Semiconductor to provide the Nordic Developer Academy under the Terms of Service.
2.2 Analytics
If you consent to analytics, Nordic Semiconductor will use Google Analytics to obtain statistics about how the Nordic Developer Academy is used. This includes collecting information on for example what pages are viewed, the duration of the visit, the way in which the pages are maneuvered, what links are clicked, technical information about your equipment. The information is used to learn how Nordic Developer Academy is used and how the user experience can be further developed.
2.2 Newsletter
You can consent to receive newsletters from Nordic from within the Nordic Developer Academy. How your personal data is processed when you sign up for our newsletters is described in the Nordic Semiconductor Privacy Policy.
3. Retention period
We will store your personal data for as long you use the Nordic Developer Academy. If our systems register that you have not used your account for 36 months, your account will be deleted.
4. Additional information
Additional information on how we process personal data can be found in the Nordic Semiconductor Privacy Policy and Cookie Policy.
Nordic Developer Academy Terms of Service
1. Introduction
These terms and conditions (“Terms of Use”) apply to the use of the Nordic Developer Academy, provided by Nordic Semiconductor ASA, org. nr. 966 011 726, a public limited liability company registered in Norway (“Nordic Semiconductor”).
Nordic Developer Academy allows the user to take technical courses related to Nordic Semiconductor products, software and services, and obtain a certificate certifying completion of these courses. By completing the registration process for the Nordic Developer Academy, you are agreeing to be bound by these Terms of Use.
These Terms of Use are applicable as long as you have a user account giving you access to Nordic Developer Academy.
2. Access to and use of Nordic Developer Academy
Upon acceptance of these Terms of Use you are granted a non-exclusive right of access to, and use of Nordic Developer Academy, as it is provided to you at any time. Nordic Semiconductor provides Nordic Developer Academy to you free of charge, subject to the provisions of these Terms of Use and the Nordic Developer Academy Privacy Policy.
To access select features of Nordic Developer Academy, you need to create a user account. You are solely responsible for the security associated with your user account, including always keeping your login details safe.
You will able to receive an electronic certificate from Nordic Developer Academy upon completion of courses. By issuing you such a certificate, Nordic Semiconductor certifies that you have completed the applicable course, but does not provide any further warrants or endorsements for any particular skills or professional qualifications.
Nordic Semiconductor will continuously develop Nordic Developer Academy with new features and functionality, but reserves the right to remove or alter any existing functions without notice.
3. Acceptable use
You undertake that you will use Nordic Developer Academy in accordance with applicable law and regulations, and in accordance with these Terms of Use. You must not modify, adapt, or hack Nordic Developer Academy or modify another website so as to falsely imply that it is associated with Nordic Developer Academy, Nordic Semiconductor, or any other Nordic Semiconductor product, software or service.
You agree not to reproduce, duplicate, copy, sell, resell or in any other way exploit any portion of Nordic Developer Academy, use of Nordic Developer Academy, or access to Nordic Developer Academy without the express written permission by Nordic Semiconductor. You must not upload, post, host, or transmit unsolicited email, SMS, or \”spam\” messages.
You are responsible for ensuring that the information you post and the content you share does not;
contain false, misleading or otherwise erroneous information
infringe someone else’s copyrights or other intellectual property rights
contain sensitive personal data or
contain information that might be received as offensive or insulting.
Such information may be removed without prior notice.
Nordic Semiconductor reserves the right to at any time determine whether a use of Nordic Developer Academy is in violation of its requirements for acceptable use.
Violation of the at any time applicable requirements for acceptable use may result in termination of your account. We will take reasonable steps to notify you and state the reason for termination in such cases.
4. Routines for planned maintenance
Certain types of maintenance may imply a stop or reduction in availability of Nordic Developer Academy. Nordic Semiconductor does not warrant any level of service availability but will provide its best effort to limit the impact of any planned maintenance on the availability of Nordic Developer Academy.
5. Intellectual property rights
Nordic Semiconductor retains all rights to all elements of Nordic Developer Academy. This includes, but is not limited to, the concept, design, trademarks, know-how, trade secrets, copyrights and all other intellectual property rights.
Nordic Semiconductor receives all rights to all content uploaded or created in Nordic Developer Academy. You do not receive any license or usage rights to Nordic Developer Academy beyond what is explicitly stated in this Agreement.
6. Liability and damages
Nothing within these Terms of Use is intended to limit your statutory data privacy rights as a data subject, as described in the Nordic Developer Academy Privacy Policy. You acknowledge that errors might occur from time to time and waive any right to claim for compensation as a result of errors in Nordic Developer Academy. When an error occurs, you shall notify Nordic Semiconductor of the error and provide a description of the error situation.
You agree to indemnify Nordic Semiconductor for any loss, including indirect loss, arising out of or in connection with your use of Nordic Developer Academy or violations of these Terms of Use. Nordic Semiconductor shall not be held liable for, and does not warrant that (i) Nordic Developer Academy will meet your specific requirements, (ii) Nordic Developer Academy will be uninterrupted, timely, secure, or error-free, (iii) the results that may be obtained from the use of Nordic Developer Academy will be accurate or reliable, (iv) the quality of any products, services, information, or other material purchased or obtained by you through Nordic Developer Academy will meet your expectations, or that (v) any errors in Nordic Developer Academy will be corrected.
You accept that this is a service provided to you without any payment and hence you accept that Nordic Semiconductor will not be held responsible, or liable, for any breaches of these Terms of Use or any loss connected to your use of Nordic Developer Academy. Unless otherwise follows from mandatory law, Nordic Semiconductor will not accept any such responsibility or liability.
7. Change of terms
Nordic Semiconductor may update and change the Terms of Use from time to time. Nordic Semiconductor will seek to notify you about significant changes before such changes come into force and give you a possibility to evaluate the effects of proposed changes. Continued use of Nordic Developer Academy after any such changes shall constitute your acceptance of such changes. You can review the current version of the Terms of Use at any time at https://academy.nordicsemi.com/terms-of-service/
8. Transfer of rights
Nordic Semiconductor is entitled to transfer its rights and obligation pursuant to these Terms of Use to a third party as part of a merger or acquisition process, or as a result of other organizational changes.
9. Third Party Services
To the extent Nordic Developer Academy facilitates access to services provided by a third party, you agree to comply with the terms governing such third party services. Nordic Semiconductor shall not be held liable for any errors, omissions, inaccuracies, etc. related to such third party services.
10. Dispute resolution
The Terms of Use and any other legally binding agreement between yourself and Nordic Semiconductor shall be subject to Norwegian law and Norwegian courts’ exclusive jurisdiction.
