Cellular IoT – Part 6 – High Latency Communication

One of the main requirements of many IoT scenarios is extremely low power consumption of a device in exchange for only sending and receiving very little data and very long intervals in which no data is exchanged at all. If a device does not need to react instantly to incoming requests it doesn’t make sense to keep the radio module powered up all the time. If, for example it is enough to check once every half hour for incoming IP packets, the radio module can be completely switched off for most of this time which saves a lot energy. The downside is, of course, that in the worst case it takes 30 minutes for a device to respond to an incoming IP packet. To cater for such scenarios the 3GPP specifications were extended by a number of features for “High Latency Communication”. Let’s have a quick look at them.

Extended Idle Mode Discontinuous Reception (DRX)

When a mobile device is in idle state it has to listen on the LTE paging channel for incoming paging messages. These are sent when no active radio link is established and IP packets arrive from the Internet for the device. The device then answers to the paging, a radio channel is established and the IP packets are delivered. A typical paging interval in LTE networks today is 1.28 seconds, i.e. the radio chip of an IoT device has to wake up once every 1.28 seconds and check the paging channel. While for smartphones the amount of power required to check for incoming paging messages once a second is negligible compared to the overall power consumption of the device it can be a significant part of the power requirement of an IoT device.

If it is acceptable for an IoT device to extend the paging interval it can signal this to the network during the LTE attach and tracking area update procedures. During these procedures it can request the network to extend the paging interval to values between 5.12 seconds and 2621.44 seconds (43.69 minutes). The network can accept, deny or modify the value. Once the attach or tracking area update procedure is finished and the network has released the radio bearer, the device can power-off the radio for the extended DRX time without releasing its bearer context, i.e. the device keeps its IP addresses.

Extended Buffering of MT (mobile terminated) Data

If the mobile device is in idle state when IP packets from the Internet arrive, the S-GW requests the MME to page the device and to establish a radio channel. When the MME recognizes that the device is in extended Idle Mode DRX it will ask the S-GW to buffer the IP packets until the device can be paged again. The MME then waits for the remainder of the DRX cycle and then pages the device, which can take up to 43 minutes. At this point in time or if the mobile device wants to send mobile originating IP packets before the DRX interval expires, a radio channel is established and the waiting packets are delivered.

Power Save Mode

Another option for turning off the radio for prolonged amounts of time is the Power Save Mode (PSM) feature. To activate this mode the mobile device requests an active time during which it will still listen to the paging channel once it has entered the idle state on the radio network. Once this time expires the devices is no longer reachable by the network as it powers down the radio until it has to perform a periodic tracking area update or it has outgoing data to send. In addition the device can request to extend the periodic tracking area update timer which is per default set to a value between one and several hours. If granted by the network the device can receive a periodic tracking area update timer (T3412) in the order of several days. This makes especially sense for devices that only push data to a receiver in the network but do not expect to be contacted from the outside.