Even with the development and deployment of 5G, the roll-out of LTE continues apace (712 commercially launched networks – December 2018). Consequently, VoLTE (Voice over LTE) is the leading solution which allows the subscriber to conduct a phone call whilst maintaining LTE coverage; there is no requirement for the device to fall back to the 2G or 3G voice network at any point.
Figure 1 VoLTE End to End Call Flow.
The end to end architecture required for VoLTE is outlined in Figure 1. VoLTE is standardized by the GSMA (GSM Association) and is based on an IMS (IP Multimedia Subsystem), which must be deployed for call control. In particular, the service provider’s IMS will feature a TAS (Telephony Application Server), which is responsible for providing supplementary services to the subscriber base eg. Call Forwarding, Calling Line ID Hiding, Call Barring.
In VoLTE, when a subscriber wishes to make a call, the device must generate SIP (Session Initiation Protocol) based signalling and send it to the IMS using the LTE network. The SIP signalling will include the Called and Calling Party address information, as well information related to the voice traffic that will eventually be flowing eg High Definition or Standard audio. SIP signalling will be used to not only initiate the call, but also provide information during the call and eventually tear down the call when necessary.
As can be seen in Figure 1, SIP signalling is exchanged on an end to end basis via the IMS. It is the role of the IMS to ensure that requests for calls are sent to the correct recipient. Moreover, the IMS must interact with the LTE network (via Policy and Charging Control) in order to ensure that sufficient network resources are reserved for the call. This ultimately means that the subscriber will be allocated a bearer with a guaranteed bitrate, across which the voice packets will eventually flow. Like signalling, voice packets are also delivered on an end to basis (there is no requirement for these voice packets to flow via the IMS, although they often do due to media anchoring).
Service providers who deploy VoLTE could also use the same IMS infrastructure to offer IM (Instant Messaging) services to the customer, akin to traditional SMS services. In this case, the instant message will be carried on an end to basis between subscriber devices using SIP (assuming both devices are using VoLTE). In addition, an IMS based system called SR-VCC (Single Radio – Voice Call Continuity) is routinely deployed with VoLTE to support seamless call continuity when moving from LTE to 2G/3G access (typically when LTE coverage has been lost).