Page 333 - From GMS to LTE

Long Term Evolution (LTE) and LTE-Advanced Pro 319

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 attach and tracking area update procedures. During these pro-
cedures 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 net-
work 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 Mobile‐Terminated Data
If the mobile device is in idle state when IP packets arrive from the Internet, 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 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 before 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 negotiates an active
time with the network 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 device is no
longer reachable by the network as it powers down the radio until it has to perform a
periodic tracking area update or 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
particular sense for devices that only push data to a server in the network and expect a
response only during and shortly after such events.

4.19.12 NB‐IoT – Optimizing IP‐Based and Non‐IP‐Based Data Transmission
While methods to reduce power consumption while no data is transmitted were dis-
cussed in the previous section, 3GPP has also specified how to transfer small amounts
of data more efficiently over the air interface to reduce the device’s power consumption
and the amount of overhead caused by the user data transfer. Especially when a single
eNode‐B serves hundreds or even thousands of NB‐IoT devices, it is absolutely essential
to keep the overhead to a minimum. 3GPP TS 23.401 chapter 4.1 describes three ways
to optimize the data transfer of very small amounts of data.

Resuming RRC Connections
A straightforward enhancement that has been specified as part of User Plane CIoT
(Cellular IoT) EPS optimizations is that an RRC connection can be suspended and
resumed. In LTE an RRC connection is usually released after 10 to 20 seconds of