Assuring a VoLTE call essentially means being able to monitor two related sessions simultaneously. Before a voice over LTE session can occur, the subscriber device must connect to both the LTE and IMS networks and then establish a VoLTE session which effectively runs over-the-top (OTT) of the LTE network.
To connect to the LTE network, the subscriber must first have their device attach to the radio network then, get authenticated, authorized and have accounting procedures performed, domains translated to route signaling messaging to switches (DNS query) and a session established before the bearer is modified. Once the subscriber is connected to the LTE network, the subscriber must be registered to the IMS network and then the SIP session signaling will commence. The SIP signaling to establish a VoIP session with RTP for the media will run over the LTE user plane interfaces and then onto the IMS network via the Mp or Gm interface.
Accordingly, a service assurance solution must provide both an end-to-end and hop-by-hop view to effectively monitor and perform service triage on VoLTE sessions. With that level of visibility it’s possible to take a proactive network monitoring approach where you deconstruct the LTE/VoLTE/IMS sessions into their essential procedures: radio attachment, authentication, session establishment in LTE, registration in IMS and so on. With this approach network operations and engineering can effectively monitor the sessions for all subscribers with a “top down” approach. If there is degradation in any step of the session model proactive measures can be taken to mediate that issue before service degradation or outage occurs.
The “top down” approach allows network operations and engineering to take a very complex service and break it down into the essential pieces or procedures to simplify service assurance and triage. In contrast, the “bottoms up” approach works off individual subscriber sessions where the session messages are traced for each hop to identify the network or service issue. Other sessions then must be found to see if that single subscriber session is indicative of a wider ranging issue or a single instance. The “top down” approach addresses VoLTE service impacting issues for the entire subscriber base holistically and reduces the MTTR.