1. Managing Retransmission Timers in Sleep Mode
Group Name: WG2 / ARC 27
Source: Catalina Mladin, Convida Wireless,
Mike Starsinic, Convida Wireless,
Meeting Date:

2. Agenda Background on PSM Background on S-GW Buffering
Use Case / Considerations
Conclusions

3. PSM (1/3
Power Savings Mode allows a UE to move in and out of a deep sleep state.
When the UE is in the “deep sleep” state, it does not listen to the paging channel.
The UE periodically leaves the “deep sleep” state, enters a connected mode, and then enters an “active time” where it listens to the paging channel.
How long the UE stays in connected mode depends on if it has MT data to send.
From TS : “A UE in PSM is not immediately reachable for mobile terminating services. A UE using PSM is available for mobile terminating services during the time it is in connected mode and for the period of an Active Time that is after the connected mode. The connected mode is caused by a mobile originated event like data transfer or signalling, e.g. after a periodic TAU/RAU procedure.”
Note: If the UE has data to send, it is free to leave the power savings mode at any time. It’s active timer will re-start when it is done sending data.
Active Time
Power Saving Mode
Active Time
Power Saving Mode
Connected
Mode
UE is listening to the paging channel.
The UE is not listening to the paging channel.

4. PSM (2/3)
The UE’s Tracking Area Update timer and Active Timer control how long the UE stays in each state.
After the UE does a TAU it goes to idle mode (assuming that it has not Mobile Originated Data to send), then the active timer is started when it does an attach or TAU.
Attach
TAU
TAU
T3412
Periodic TAU Timer
T3412
Periodic TAU Timer
Active Time
Power Saving Mode
Active Time
Power Saving Mode
T3324
Active Timer
T3324
Active Timer
UE is listening to the paging channel.
The UE is not listening to the paging channel.

5. PSM (3/3)
The SCEF allows the SCS to provides some input into how these timers are set.
From TS : “Maximum Latency indicating maximum delay acceptable for downlink data transfers. Maximum Latency is used for setting the periodic TAU/RAU timer for the UE as it sets the maximum period after which a UE has to connect to the network again and thereby becomes reachable. Determined by the operator, low values for Maximum Latency may deactivate PSM.”
From TS : “Optionally, Maximum Response Time indicating the time for which the UE stays reachable to allow the SCS/AS to reliably deliver the required downlink data. Maximum Response Time is used for setting the Active Time for the UE. When the UE uses extended idle mode DRX, the Maximum Response Time is used to determine how early this monitoring event should be reported to the SCS/AS before the next Paging Occasion occurs.”
So for example….
Active Timer can be set equal to Maximum Response Time.
Periodic TAU Timer can be set equal to Maximum Response Time + Maximum Latency.

6. S-GW Buffering
What happens if data is sent to the UE while it is in PSM?
It could be dropped by the network.
It could be buffered for an extended amount of time in the network. The buffering is done in the S-GW.
From TS : “The MME/S4-SGSN asks the Serving GW to buffer downlink data until the UE is expected to wake up from its power saving state.”
The SCEF provides the SCS with some control over the buffer.
From TS : “The SCS/AS may include the following parameters in the Monitoring Event configuration request to the SCEF: ……………Optionally, Suggested number of downlink packets indicating the number of packets that the Serving Gateway shall buffer in case the UE is not reachable.”

7. Use Case A reasonable configuration could be:
Active Time = 10 Minutes
Power Saving Mode = 50 Minutes
Note: Power Saving Mode could be much longer.
What happens if an IN-CSE, using an underlying application layer protocol such as HTTP or CoAP sends data to the UE while in power savings mode?
If S-GW buffering is not enabled, the underlying application or transport layer protocol is likely to send many re-transmissions. The retransmissions will all be dropped by the network before the request times out.
If S-GW buffering is enabled, the underlying application or transport layer protocol is still likely to send many re-transmissions. The retransmissions will all flood the buffer and will all be delivered to the UE when it goes back to Connected Mode.
Active Time
Power Saving Mode
Active Time
Power Saving Mode

8. Conclusion
The configuration of an IN-CSE’s underlying application or transport layer (e.g. HTTP, CoAP) retransmission timers should take the following into account:
How the IN-CSE configured “Maximum Latency” and “Maximum Response Time”
i.e. the UE’s PSM Configuration (PSM and Active Time values)
How the IN-CSE configured “Suggested Number of Downlink Packets”
i.e. whether S-GW Buffering is enabled and how many packets may be buffered in the S-GW
What “Active Timer” and “Periodic Tracking Area Update Timer” was last assigned to the UE
This information is not currently exposed to the SCS