Back in July this year I had a post on a GSMA and 3GPP initiative to think about a simpler VoLTE roaming approach. Instead of the quite complicated VoLTE roaming with local breakout, some members of the two fora openly pushed for for a much simpler solution called S8 Home Routing (S8HR). For the details have a look at my previous post. There are surprisingly few public documents that can be found on the web even today which leads me to believe that it's not really a very popular topic amongst network operators. Japanese network operator NTT Docomo certainly doesn't belong to those delaying a standard based on S8HR and perhaps they are even tired of waiting for an emerging industry standard as they have announced that they will launch their S8HR VoLTE roaming with KT in South Korea TODAY.
That's quite NTT Docomo like. Already in the days of 3G, they had a UMTS network up and running well before anyone else. They suffered a bit later on as their network wasn't quite compatible with the final 3GPP standards so they had to evolve their equipment over time. When LTE was first launched they were again one of the very early adopters. The same goes for VoLTE, they launched their service already back in mid-2014. And now they are pushing the state of the art again with VoLTE roaming.
Fortunately, VoLTE S8HR in it's very basic form requires no support from the roaming network as all signaling and even the voice data packets are tunneled back to the home network. On mobile devices, however, some things need to be modified. Today, mobile devices deactivate VoLTE when not in their home network which is the first thing that has to change.
In addition, the following things need to be considered on the mobile side:
- IMS registration should work like in the home network, no things to change.
- When establishing a call, forget about preconditions and dedicated bearer establishment.
- For emergency calls just fall back to 2G and 3G even if it is done differently in the home network.
- There is no SR-VCC support to hand-over an ongoing call to 2G or 3G when running out of LTE coverage as the roaming is not aware of an ongoing VoLTE call (it's all home routed...). There are no modifications for this on the mobile devices side, as this is controlled by the network. In case of roaming, the SR-VCC command will simply not come and the call will drop. As I mentioned in my previous post, that's a pity but the competition can't do this either. On the other hand, in countries with good LTE coverage such as South Korea there should be little need for SR-VCC anyway.
If you can think of any other things that have to change on mobile devices to make S8HR VoLTE roaming work, please leave a comment, I'd be quite interested.
In the home IMS network, quite a number of changes were probably required. Roaming calls are likely to be charged differently so the components involved in call charging were probably dapted. That shouldn't have been much of a challenge though. Somewhat more effort was likely spent on the IMS call establishment logic. In the home network, dedicated LTE bearers are established to ensure constant bandwidth and priority for voice packets on the radio interface and in the network which requires interaction between the IMS and the network infrastructure. As the IMS system in the home network can't interact with the network components of the visited network, the IMS call flow for roaming calls needs to be modified. Compared to the VoLTE local breakout solution that requires such an international interaction, such changes are pretty much a no-brainer.
Kudos to Docomo for racing ahead, it's another way to make others follow.