Does UE need to indicate FGI bit 3,7 as supported for SMS over IMS?

Hello Experts.

For VoLTE support UE need to indicate FGI bit 3,7 as supported.

Do we need same indication for SMS over IMS as well?


In my understanding no.

Support of VoLTE usually implies support for SMS over IMS.

Does we have call flow for it (Call flow for SMS on IMS)?

Is it same like VoLTE call?

Sharetechnote (@Jaeku_Ryu) has it:

IMS/SIP - SMS over IMS - Sharetechnote

Basically, IMS call flows don’t change with underlying network.

So, be it a LTE network or 5G network, the call flow will be same.


Here over ShareTechNote no indication for any FGI bit for SMS over IMS.

As I mentioned, there is no separate bit for SMS over IMS.

Volte FGI implies the SMS over IMS support.

Support for these IMS services is indicated in IMS register message in the form of feature tags.


Similar feature tags exist for other services like chat, file transfer etc.

For which SIP message is it?

Register message.

If IPMS is not there then how will be call flow?

If IMS is not there, SMS is sent on SG interface:

From UE end no FGI bit indication.

IMS is there but IPMS is not there then what will be call flow? (That was question)

Second query: if IMS is not there then UE in LTE must do CSFB for MO MT msg?

What if IMS is there but IPMS is not there then UE will still do CSFB for MO MT sms?

What do you mean by IPMS?

First time I am hearing about this…

CSFB is only for voice calls. SMS can still be send as NAS payload in LTE on SGS interface.

I mean to IP-SM-GW.
Like in TDM SMSC

SMS over SGs is an interim solution to realize the transmission of native SMS from CS infrastructure over the LTE radio network.

Without SGs support, when a UE is camped on the LTE network, it would need to change its radio access technology back to 2G/3G, so as to process SMS service.

SG is a new interface between the MME and the MSC.

The SMS is transferred via SGs to MME and carried in NAS signaling to the UE, avoiding the need to execute fallback to a 2G/3G network.

In my understanding, CS interface is mandatory for the infras to deploy LTE CSFB. That means SGS support will be there. I never saw a use case where UE moves to 2G/3G just to send SMS.

In any case, VoLTE coverage is very rich now, so this discussion on SG interface is kinda outdated. :blush:

It was written here :point_up_2:

They are talking about a hypothetical use case on what would happen when there is no SG interface and IMS service.

In general, an LTE network with CS support has support for SG interface.

“+g.3gpp.smsip” parameter in Contact header indicates UE’s ability of IP SMS.

I just came across a case where UE indicates SMS over IMS support using “+g.3gpp.smsip” in the contact header, but the SIP/2.0 200 OK from the network does not contain the same.
The UE in this case still tries to send SMS using SIP, but gets a SIP/2.0 501 Not Implemented response.
VoLTE is supported by the network though.

I do not know if I can publically name the affected UE and network, so I leave that information out.

Thanks Waffle, this was a very useful remark. SMS over IMS can be extremely cumbersome - with no benefit for the users. After all, even with all the extra functionality and server infrastructure - SMS over IMS is just a big wrapper for the actual good-ole SMS TPDU.

So it is actually a rational approach to disable SMS over IMS in the IMS-REGISTER and IMS-SUBSCRIBE flows.

The network then responds with 501 Not Implemented to phones that try to send SMS over IMS despite it not being active, then those phones will fall back to sending the message via S1AP interface instead. This likely results in less bandwidth wasted and faster message delivery anyway.

In effect this means that the handsets will do Voice calls (VoLTE) and SMS on 4G - without wrapping the SMS TPDU in layers of SIP and sending it on the grand tour of SIP Servers.

Add another network to the SMS over IMS “SIP2/0 501 Not Implemented” list :slight_smile: