In Chapter 1, the structure of the GSM network was described in combination with voice telephony. It is difficult to separate the GSM voice service from the GSM network as the (voice) service and the network are completely integrated. Even in UMTS this is still the case to a significant degree. For LTE, however, 3GPP decided, with only a few exceptions, to completely separate the network from any kind of higher‐layer service, including voice telephony. This is the reason why the description of LTE in the previous chapter could be done without mentioning an integrated voice telephony solution.
Voice telephony, however, is still an important service and the CS‐Fallback solution described in the previous chapter was only meant to be a temporary solution on the path to an all‐IP network in which all services including voice telephony are based on the Internet protocol. After many years this has finally been accomplished with the VoLTE (Voice over LTE) IP Multimedia Subsystem (IMS) profile standardized in GSMA IR.92 , which is based on the Session Initiation Protocol (SIP).
In practice, it can be observed today that LTE networks have not yet reached the same level of geographic coverage as GSM networks and it is likely to remain that way for some time to come. Therefore, a fallback for an ongoing voice call to a classic circuit‐switched channel is still required. This functionality is referred to as Single Radio ...