Voice or CS (circuit Switched) calls are an important part of telecommunication systems. Though data traffic already overshadowed voice traffic in telecom systems still for a successful mobile system to exists voice is very essential service.
When 3GPP started designing LTE system most of the importance were given to create a system which can achieve highest data throughput with lower latency. Which all the importance was given to data, traditional service like voice was ignored.
Since LTE is an all IP network means traditional circuit switched calls are handled bit differently in LTE than in older mobile systems. The solution to deliver voice over internet protocol in LTE is IMS (IP Multimedia Subsystem).
IMS exists in 3GPP from the year 2000 in thousands of documents but the problem was there was no clear instructions how to use IMS in LTE to deliver voice. This means it was left upon to the vendors or companies how they are going to implement different features required for VOIP (Voice over IP) calls. For example no clear instructions were there how to do authentication, session setup, supplementary service execution and bearer setup for voice over IP calls.
Because of absence of a clear voice solution in LTE during the initial deployment it was decided that 3GPP will opt of CSFB (CS Fall Back) as the intermediate solution. With CSFB when a voice call is started in LTE network the UE move to legacy systems like UMTS or GSM to start voice call and remain there during the entire duration of the call. After the call is released UE moves back to LTE. Though this solution works it create a big problem when it comes to user experience. For example when UE moves to 2G system because of CS fallback the user can not do any data call because of the limitation of GSM network.
Another alternative to IMS based VoLTE was VoLTE over Generic Access proposed by VOLGA forum.
VoLTE Timeline – From No Voice to Voice over LTE
In the year 2009 12 telecom companies which includes some big names from network vendors, operators and UE manufacturer proposed a solution for IMS based VoLTE. This initiative was call One Voice and the proposed solution was to provide unambiguous and a common solution for voice over IP network.
In February 2010 GSMA (Global System for Mobile Communication) took the solution and started their work to make this a official solution to provide next generation voice over LTE network. GSMA VoLTE initiative was supported by more than 40 organizations around the world.
By March GSMA published IR.92 on IMS profile for voice and short message service. This was an improved version of One Voice profile.
In September 2010, GSMA freeze IR.92 reference document.