Understanding Voice over LTE (VoLTE)
Voice over LTE (VoLTE) represents a standards-based method for achieving true all-IP voice communications. Prior to VoLTE, a portion of voice calls may have traversed over IP, but the radio link (e.g. the RF channel) was not IP-based). With VoLTE, LTE is leveraged in conjunction with IMS using specific profiles for control and media planes of voice service on LTE defined by GSMA.
In the new LTE Radio and Evolved Packet Core (EPC) architecture, there is no circuit-switched domain to handle voice calls in the traditional 2G/3G way. There is a need for a Voice over VoLTE solution that enables use of the 4G packet based bandwidth.
There are many issues and challenges with 4G from a technology and operational perspective. One of those issues is spotty coverage. To manage this issue, VoLTE is dependent upon the Service Centralized and Continuity Application Server (SCC).
The SCC is an IMS Application Server (AS) that provides functionality required to enable IMS Centralized Services and Service Continuity for multi-media sessions. The SCC AS is in the signaling path of Session Initiation Protocol (SIP) messaging and is the connection point for packet bearer media. When the target wireless device is crossing over into an area that does not have sufficient 4G/LTE coverage, the SCC AS manages hand-over to 3G for voice.
Voice over LTE Alternative: CSFB uses 3G
VoLTE is not to be confused with so called Circuit Switched Fallback (CSFB) solutions. CSFB is known by its 3GPP designation as specification 23.272. It uses various network elements and procedures to move the handset radio down to 2G or normal 3G connection, before initiating a circuit-switched voice call.
Therefore, the terminology of fallback is somewhat of a misnomer as there is no handover with a CSFB solution. Instead, all voice calls start on 3G.
CSFB requires modifications to existing elements within the network (MSCs) as well as specific support on new devices.
Voice over LTE Alternative: VoLGA uses Circuits with LTE
Another alternative solution for voice over LTE that was discussed is the so called Voice over LTE Generic Access (LGA) or VoLGA solution. VoLGA relies upon establishing a virtual circuit within the LTE packet bandwidth. Therefore, like CSFB, VoLGA is not 4G packet based but rather uses a circuit connection.