Friday 21 February 2020

EPS Fallback in 5G Standalone Deployments

It can be expected that later this year some mobile network operators will launch their initial 5G standalone (5G SA) deployments.

Nevertheless there will remain areas with temporary or permanently weak 5G NR coverage. One possible reason might be that even when 5G and LTE antennas are co-located, which means: mounted at the same remote radio head, the footprint of the 5G NR cell is significantly smaller when it uses a higher frequency band than LTE - see figure 1.

Figure 1: Smaller footprint of co-located 5G NR cell with higher frequency

Especially UEs making Voice over New Radio (VoNR) calls from the 5G cell edge have a high risk of experiencing bad call quality, in worst case a call drop. To prevent this the UE is forced  during the voice call setup towards 5G core network (5GC) to switch to a LTE/EPS connection where the radio conditions are better for the voice service.

The same procedure for which the term "EPS Fallback" was coined by 3GPP also applies when the UE is served by a 5G cell that is not configured/not optimized for VoNR calls or when the UE does not have all needed VoNR capabilities.

Figure 2: Two options for EPS fallback

When looking at the RAN there are two options for executing the EPS Fallback as shown in figure 2.

In option A the 5G radio connection is released after the initial call attempt is successfully finished and with the 5G RRC Release the UE is ordered to reselect to a 4G cell where a new radio connection is started for the VoLTE call. In this case the UE context is transferred from the AMF to the MME over the N26 interface. 3GPP seems to use also the term "RAT fallback" for this option.

Option B is to perform a 5G-4G inter-RAT handover. Here the session management and user plane tunnels in the core network are handed over from SMF/UPF to MME/S-GW in addition. This is realized with the GTPv2 Forward Relocation procedure on N26 interface.

All in all the EPS fallback is expected to cause an additional call setup delay of approximately 2 seconds.

For the inter-RAT handover case it is easy to detect from signaling information that an EPS fallback was triggered. In the source-eNodeB-to-target-eNodeB-transparent-container sent by the gNB to the eNB a boolean "IMS voice EPS fallback from 5G" indicator will be found that is set to "true". This container is named according to the receiving entity and will be carried by the NGAP Handover Preparation, GTPv2 Forward Relocation Request and the S1AP Handover Request messages.

If a redirection for Voice EPS Fallback is possible or not is indicated in the NGAP Initial Context Setup Request, Handover Request (during 5G intra-system handover) and Path Switch Request Acknowledge (after Xn handover) messages, all sent by the AMF to the gNB.

Further the NGAP protocol provides the cause value "IMS voice EPS fallback or RAT fallback triggered" in the PDU Session Resource Modify Response message indicating that a requested VoNR session cannot be established.  

An excellent, very detailed description of N26 interface functionality and testing ia available here.


Sandeep said...

Thanks for the article. You have explained it very nicely. I was wondering if you know which direction/architecture 3GPP is going for VoNR solution - any reference spec you could refer to.

Kind regards,

Ralf Kreher said...

An older 3GPP meeting protocol from 2018 lists the following VoNR feature plan that spans activities across multiple protocols:

Unknown said...

For option B it would be helpful if the call flows were numbered.



Kar Ann said...

Hi, I read that VoNR CSFB will be supported in R16. Could you confirm please? Thanks, Kar Ann

Anonymous said...

Can you explain how the 5G and E-UTRAN fallback works?

Ralf Kreher said...

If I find some time I will look into further details for fallback solutions also for VoNR. I know that Rel. 16 defines e.g. SRVCC 5G->3G, but I need to study if this can be implemented as a Fallback solution as we have seen it LTE/EPS.

Anonymous said...

Hey you have explained it very nicely. I was wondering if you know solution for returning to NR after EPS fallback

Anonymous said...

So from your statements,can we deduce that option A will be used when the voice call is already established in NR.

Rajeev said...

A Network Operator providing both 5G NR and 4G LTE service, but initial it may not be possible to have 5G coverage everywhere and at some point when 5G coverage or service not available to 5G system (5GS) may attempt to fallback the RAT to 4G Evolved Packet System (4G-EPS). EPS Fallback can be defined as the mobility procedure where network trigger the procedure for UE to change Radio Access from 5G to 4G.

Techplayon has written an excellent article about EPS Fallback

prasad said...

UE initiated the EPS Call , after getting fallback to LTE ,calls are getting Disconnected what could be the reason any one can suggest please

Unknown said...

Any body have wireshark pcap for EPS fallback in 5G?