We are supposedly on the IoT event horizon, ready to realize the promise of billions of connected things. Many of these things will find current mobile networks perfectly suited to their needs: high resolution sensors and HD video cameras will need the low latency and high bandwidths available with today’s LTE network.
A large number of IoT devices (some suggest the majority) will have different needs, demanding long battery life, low cost and better coverage. The mobile ecosystem has been busily preparing for these use cases through the MTC initiatives.
Today I will focus on Power Saving Mode (PSM), a 3GPP Release 12 feature which forms the backbone of 3GPP’s MTC power-saving strategy, and is an important part of LTE CAT-0 (which I first discussed in February). With this approach, the UE decides how often it needs to be active in order to transmit and receive data, entering PSM in between.
The mechanics of PSM are relatively simple. The UE activates PSM simply by including two proposed timer values in the Attach or Tracking Area Update (TAU). The first proposed timer is the T3324, which will be the time the UE stays in idle mode following the Attach or TAU procedure. The second proposed timer is an extended T3412. Network acceptance of PSM comes when the network provides the actual values of the T3324 and T3412 to be used in the Attach Accept.
Following the Attach or TAU procedure, the new PSM process begins. Once the network releases the RRC connection after the Attach/TAU, T3324 and T3412 are started, and the device behaves as any registered, idle LTE device would. This is called the Active Time and is there to provide the opportunity for a remote server to contact the device. The more certain a specific application is that no downlink data is pending, the shorter T3324 can be. Once T3324 expires, the device will then enter PSM for the duration of T3412. Here the device should enter its lowest power mode. It should not look for paging or any other network signaling. The network should not page the device, and moreover, should hold any data that arrives for the UE while in PSM. It is important to note that while in PSM mode, the device and network maintain all UE states, including Radio Bearers, registration state, and temporary identities. The maximum duration of PSM (T3412) is 12.1 days. PSM mode can be cancelled anytime by the device by sending a TAU to the network that does not include the PSM timers.
Figure 1: PSM Flow
PSM puts the UE largely in the driver’s seat in terms of the power management needed for its application, which is very important. IoT applications will be wide ranging, and this type of flexibility is needed. With the network in sync with the device, it can better manage the data intended for the device. A properly-implemented network solution should be able to avoid congestion by managing the timers of all the devices using PSM, and adjusting the wake-up periods to be offset as much possible. This way all of the devices will not wake at the same time and attempt to access the network.
As with all new features, there are still some potential issues and concerns. Most networks today expect contact with a mobile every 2-4 hours, otherwise the mobile is considered “not reachable” and quietly detaches it from the network. This helps the network constrain the number of mobiles it has to track. Infrastructure vendors will now need to support tracking for millions of devices that will only contact the network once every week or two.
On the device side, there will be challenges as well. Sleep times are increasing from seconds to weeks. There will be engineering challenges in keeping accurate timers, at the same time pushing down power consumption to new levels. IoT will revive the need for highly-skilled embedded engineering.
The question lurking in PSM is this: why not just shut off or cut power to the LTE modem for the long inactive periods? This is always a possibility. A full power-down would likely involve a detach procedure (extra signaling), losing track of the cell and having to re-acquire SIB information, then having to re-establish all the bearers (again more signaling and active time). So if power consumption during PSM can be driven to near “off” levels, then if could be even more efficient than when off. Again, this is where good engineering will improve either solution and make the correct choice for the particular application.
PSM is a key feature and a big step forward for Cellular IoT. It was defined for 2G, 3G and LTE networks and, at a high level, is a simple solution. But, as with most things, the devil will be in the details of the implementation. For some of my earlier thoughts on Cat-0, please refer to this post.