Imagine a not-too distant future, one without flying cars – just one where 2G and 3G networks have been switched off.

And the imagine a teenage phone user, who has almost run out of their prepaid mobile data allocation, and so has switched mobile data off, or a roaming scenario where the user doesn’t want to get stung by an unexpectedly large bill.

In 2G/3G networks the Circuit Switched (Voice & SMS) traffic was separate to the Packet Switched (Mobile Data).

This allowed users to turn of mobile data (GPRS/HSDPA), etc, but still be able to receive phone calls and send SMS, etc.

With LTE, everything is packet switched, so turning off Mobile Data would cut off VoLTE connectivity, meaning users wouldn’t be able to make/recieve calls or SMS.

In 3GPP Release 14 (2017) 3GPP introduced the PS Data Off feature.

This feature is primarily implemented on the UE side, and simply blocks uplink user traffic from the UE, while leaving other background IP services, such as IMS/VoLTE and MMS, to continue working, even if mobile data is switched off.

The UE can signal to the core it is turning off PS Data, but it’s not required to, so as such from a core perspective you may not know if your subscriber has PS Data off or not – The default APN is still active and in the implementations I’ve tried, it still responds to ICMP Pings.

IMS Registration stays in place, SMS and MMS still work, just the UE just drops the requests from the applications on the device (In this case I’m testing with an Android device).

What’s interesting about this is that a user may still find themselves consuming data, even if data services are turned off. A good example of this would be push notifications, which are sent to the phone (Downlink data). The push notification will make it to the UE (or at least the TCP SYN), after all downlink services are not blocked, however the response (for example the SYN-ACK for TCP) will not be sent. Most TCP stacks when ignored, try again, so you’ll find that even if you have PS Data off, you may still use some of your downlink data allowance, although not much.

The SIM EF 3GPPPSDATAOFF defines the services allowed to continue flowing when PS Data is off, and the 3GPPPSDATAOFFservicelist EF lists which IMS services are allowed when PS Data is off.

Usually at this point, I’d include a packet capture and break down the flow of how this all looks in signaling, but when I run this in my lab, I can’t differentiate between a PS Data Off on the UE and just a regular bearer idle timeout… So have an irritating blinking screenshot instead…