Dialogic Blog

VoLTE Roaming – Are We There Yet?

by Thomas Schroer

Sep 22, 2015 11:06:00 PM


 VoLTE Roaming Are We There Yet

I recently did a webinar with Hot Telecom analysts Isabelle Paradis, and Steve Heap where the topic of discussion was on IPX Services and the Pathways to Innovation. The webinar covered concepts like how IPX operators should focus on providing more than just traditional connectivity, and pointed out ways an IPX operator can add value and expand revenue 

by focusing on and leveraging:

  • Its position in the network as a trusted intermediary
  • The security of its core network, and
  • Its longstanding experience in providing interoperability and interworking.

What was really evident as Isabelle, Steve, and I were collaborating on this presentation was the extent to which the industry has moved from TDM and SS7 to IP. And this evolution is disruptive; not only with changes to voice technology, but also to traditional voice charging models, and how content and applications can be delivered to networks and ultimately subscribers. There were also several interesting questions raised during the course of the webinar that I wanted to share with you.

One attendee asked when we thought that CSPs will offer VoLTE roaming support especially since we now have several VoLTE networks in service today; and secondly, what architectures would be used for VoLTE roaming.

I want to start off by answering the second part of the question. There are two fundamental architectures currently supported by the GSMA and 3GPP: S8HR and Visited Routed with Local Breakout or LBO.

S8HR is a VoLTE roaming method where the “S8” refers to the LTE interface between the visited Evolved Packet Core (EPC) network and the home EPC, and “HR” stands for Home Routing. This architecture has quickly gained acceptance and has been used for most of the VoLTE roaming trials that have occurred to date. In the S8HR method, both the SIP signaling traffic and RTP-based media are tunneled between the visited network and the home network using GTP protocol. The IMS infrastructure of the roamer’s home network is used to process the call. Home routing is very comparable to mobile data roaming scenarios, and it’s advantageous from the standpoint that roamers can use VoLTE even if the visited network has not implemented an IMS.

LBO with Visited Routing is also referred to as RAVEL which, believe or not, is somehow derived from the term Roaming Architecture for Voice over IMS with Local Breakout. This method was specified a few years ago by the 3GPP in TS 23.850 and calls for the local break out of VoLTE calls for inbound roamers by the visited network. In this approach, similar to circuit switched roaming, the media and SIP signaling go over the same path to the terminating network. In RAVEL, the way the call progresses for an inbound roamer is that the signaling first goes back to the home network to determine whether the call will be broken out locally. A new signaling platform in the visited network is then introduced called the TRF or Transit and Routing Function, and signaling sent to the home network for processing is then looped back to the visited network’s TRF which then directs both the SIP signaling and media to the appropriate destination.

The answer to the first part of the question is that service providers are already starting to test and roll out VoLTE roaming support for international calling, and a few operators have already come out with announcements on their VoLTE roaming plans, services or trials. LG UPlus and KDDI were the first to announce a commercial VoLTE roaming arrangement in April of 2015 for their subscribers in South Korea and Japan using the S8HR method over the Syniverse IPX networkDocomo, KT, and Verizon were involved earlier in the year in a trial of VoLTE roaming using S8HR as well leveraging Docomo’s IPX network, and they have plans to go commercial with the offering later this year. One of the earliest VoLTE roaming trials was way back in October of 2014 involving KPN, China Mobile and iBasis using a LBO approach.

The Opportunity for IPX

An IPX has an opportunity here to provide the any-to-any connectivity to interwork multiple CSP networks regardless of what roaming approach they want to take or even if one of the participants has not implemented a VoLTE network at all. There are examples of IPX operators on the forefront like iBasis providing a comprehensive interworking and regional break out capability that allows CSPs that have deployed different VoLTE roaming methods to actually interwork. In addition, they can help accelerate a CSP’s ability to start supporting inbound VoLTE roamers even if they have not implemented an IMS core so they can start to accelerate revenue opportunities more quickly and better monetize their investment.

By adding regional breakout capabilities, where the IPX can direct the IP voice (and even TDM) traffic to the appropriate destination network, the IPX can offload traffic to improve the latency on some of the more complex call scenarios as well as reduce the size of the backhaul needed by the mobile operators. By implementing a complement of equipment such as a Diameter Signaling Controller (DSC) for Diameter signaling hubbing, an IP Proxy that includes SBC-based session control to support the P-CSCF and TRF functionality, an MGCF and IMS MGW for TDM termination, and an Interconnection Border Control Function and Transition Gateway (IBCF/TrGW) for IP multimedia breakout, an IPX operator can not only help reduce the complexity of interconnecting different VoLTE roaming architectures, but also hand off calls regionally to improve routing efficiency, lower backhaul costs and help CSPs turn up services faster.

You can watch the entire archived webinar by clicking here. There’s also a free white paper you can download as well on Pathways to IPX Innovation. We’ll be sharing some other questions that came up in future blogs so keep checking back, and let us know how you think VoLTE roaming will evolve by tweeting us @Dialogic #VoLTERoaming.  

Topics: VoLTE/VoWifi & IMS, IPX, Guides: How-to's, Infographics, and more