April 29, 2021

VoLTE Steering

MNOs have begun to look in earnest towards the 2G/3G sunset, where they can finally switch off these legacy technologies. This is not easy, and MNOs face many obstacles to doing this. One major obstacle is the dependence on these technologies for voice calls using Circuit Switched Fallback. The solution to removing this dependency is Voice over LTE (VoLTE). 

A large number of networks have implemented VoLTE, and a significant number of these have also enabled VoLTE roaming in partner networks. As time goes by, more roaming subscribers will depend on VoLTE for voice service as VoLTE is enabled in more markets, and subscriber handsets are upgraded to support it. This proliferation of VoLTE is a positive for the industry, but it does come with risks that operators must mitigate, and Steering of Roaming is an important part of this.

“VoLTE Steering” has been appearing on RFPs for a number of years now, but it seems that many MNOs do not know what they are actually looking for, and what they are going to need going forward. In this article, we will take a closer look at these points.

VoLTE Registration

VoLTE service is provided by the IMS. When a device connects to a network (at home or roaming) it will attach to an MME. Once this registration is complete, the device will establish a data tunnel for data usage, which is what enables the subscriber to access the internet. When VoLTE is in use, a separate dedicated data tunnel will be established between the device and the home network (assuming S8HR is used rather than LBO). Once this data tunnel is established, the device will use SIP to register with the IMS. Once this registration is successful, the subscriber can now make and receive VoLTE calls.

So what does this have to do with steering? On the face of it – very little. Steering of roaming systems traditionally operate based on mobility signalling (SS7 and Diameter), using this signalling to try and force the subscriber to attach to the network we want them on. Once they are attached to the network, the job of the steering system is arguably done. From here it is assumed that the VoLTE registration will occur. But what if it doesn’t?

VoLTE Registration Failure

A VoLTE registration could fail for numerous reasons. It could be a misconfiguration in the HSS, or the IMS, or some misconfiguration in the visited network. This does not have anything to do with the Steering of Roaming system, but it is important that the steering system is aware of it. This is very important data that must be considered by the SoR system when it is deciding which network to steer towards.

As a specific example of where this matters, let’s imagine we have one of our subscribers roaming in a country with 3 networks – A, B and C. Network A is our preferred network, but it is a VoLTE only network. Networks B and C support Circuit Switched Fallback. The Steering system steers a subscriber towards Network A successfully. The subscriber should be capable of using VoLTE, but for some reason they are unable to register with the IMS. This subscriber is now without voice service. The steering system must be made aware of this fact so that it will know that it should steer this subscriber elsewhere to ensure their expected availability and quality of service is maintained. This type of use case becomes more and more significant as the circuit switched networks are turned off in each country/network over time.

Steer VoLTE subscribers

The steering system should also be aware of which subscribers are VoLTE capable, so that a specific steering policy can be applied for them. For example, if we have our sample network A above, we should only steer subscribers that can use VoLTE to that network. We should not steer non-VoLTE capable subscribers to that network since voice service would be unavailable.

To summarise, there are 3 things the Steering system should know for VoLTE:

  1. In which partner networks is VoLTE available (and in which networks is only VoLTE available without CSFB)
  2. Which subscribers are VoLTE capable
  3. Which subscribers are really using VoLTE when roaming

How to get this information:

  1. For knowing which networks have VoLTE available, this can be done through provisioning. It could also be learned through analysis of the Diameter signalling.
  2. For knowing which subscribers are VoLTE capable, this could be done through provisioning. It can also be learned through analysis of the SS7/Diameter signalling.
  3. For knowing which subscribers are really using VoLTE, the system must have visibility of subscribers attempting to register and register in the IMS for VoLTE. This means looking at SIP registrations, which is not a protocol typically processed by an SoR system.

What does VoLTE steering really mean?

In light of this, we come back to what “VoLTE Steering” means to an MNO in relation to Steering of Roaming. To date, this has been a tick the box item for a lot of MNOs with no clear idea of what they actually want or need. This has allowed vendors to claim support for “VoLTE Steering” even though they do not meet the real needs of the operator. So it is important that MNOs know what to expect from a Steering of Roaming system for VoLTE. In summary:

  • The system should be able to identify VoLTE capable networks
  • The system should be able to identify VoLTE capable subscribers
  • The system should have visibility on IMS registrations (successful and failed attempts) and deregistrations
  • The system should be able to take action on the above intelligence to make the right decisions to ensure availability and quality of service for their roamers
  • There is no change in steering mechanisms. Steering is still done by blocking attempts to non-preferred networks, or by OTA SIM based steering. No action is taken on the SIP protocol. But in deciding which networks to steer towards, which networks are preferred, which are not preferred etc. for a particular subscriber, this clear view of VoLTE is essential

VoLTE Steering is available in the Cellusys Steering of Roaming system. There are different mechanisms of integration for VoLTE that can be supported, each with their own benefits and drawbacks. Please reach out if you would like to discuss this in more detail.

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April 29, 2021