The Opportunities and Challenges Presented to Mobile Network Operators by Voice Over LTE (VoLTE)

Monitoring and Managing QoS for VoLTE

The LTE architecture redefines the radio access network (RAN) and specifies an all new flat IP infrastructure moving away from the old hierarchical network element model, decoupling signalling transport from data and supporting the utilization of IP-based protocols and IP networking principals. Operators now need to focus on monitoring and managing native-IP interfaces and IP-based protocols rather than traditional signalling and element specific metrics.

The successful delivery of a VoLTE service on a LTE network is dependent on the smooth transition of packets between distinct elements across the Network, Transport, GTP, IMS and Application layers of the LTE bearer path.

At a basic level, monitoring and managing QoS for a VoLTE session will include gathering key metrics from call establishment, call drops and call termination procedures. But, unlike legacy cellular technologies, subscriber services in LTE are non-integrated.  For VoLTE services, the VoIP “control plane” and “user plane” both are multiplexed on to the LTE user plane. Thus, VoLTE call control is performed outside the purview of the LTE network.

With SIP, on which IMS is based, call control and bearer (transport) control are essentially independent. This is by design. But in the context of the GSMA IR.92 specification for VoLTE services, there are two situations in which it is desirable to have some communication between the two domains.

The first is during call session establishment. It is possible that a terminating device could accept a call invitation (and alert the terminating subscriber), but subsequently turn out that there are insufficient bearer resources to support the call. This results in a so-called “ghost call”. This is not normally a problem in legacy circuit-switched systems because call and bearer control are relatively more coupled. In order to minimize the possibility of this occurrence, IR.92 requires the use of the SIP preconditions framework. This ensures that the terminating subscriber is not notified of the incoming call unless it is certain that there are sufficient resources to support the call (at the required QoS).

The second situation in which call and bearer control require some communication is in the context of call drops and call establishment failures. If, for example, communication between the LTE/SAE network and the internet is lost, or if the radio link fails, IR.92 requires the IMS call session to be re-established or cleared gracefully on both the mobile device side and on the network side.

 Measuring QoE for VoLTE

In the previous section, I have described how the QoS the VoLTE application is dependent on monitoring and managing the smooth transition of packets between distinct elements across the LTE bearer path. To understand the impact that variances in LTE network QoS settings can have, operators need to look to the actual subscriber or QoE performance measurement of all application flows.

The QoE of each and every VoLTE session for all subscribers can be grouped and measured in the following categories:

• Accessibility - whether the service is readily available to the subscriber when needed and what is the average delay in providing the access to the voice service.
• Retainability - whether the service is retained until the Voice call is terminated gracefully and deliberately by any of the end users.
• Integrity - the quality of an established and ongoing voice call.

To monitor and manage the QoS of any VoLTE session, operators must  record and co-relate these QoE measurements per subscriber, location and device type in all layers including the Network, Transport, GTP, IMS and Application layers.