What is the difference between IMS and VoLTE?

Before the fourth generation (4G), mobile networks used traditional circuit-switched technology for voice calls and text messages. The circuit-switched technology requires a dedicated connection between a mobile phone and the network for the entire duration of a call or session. The dedicated connection permanently engages network resources for the session duration, which isn’t efficient and limits the network’s ability to support more users simultaneously. The 4G network is based on the LTE (Long Term Evolution) technology which is IP-based and delivers all communication services over the IP network. The voice service in LTE is offered through VoLTE (Voice over LTE), which is a fully managed VoIP (Voice over IP) service that fulfils specific requirements defined by the mobile network to ensure the quality of service (QoS). IMS or IP Multimedia Subsystem is a network entity that enables VoLTE in 4G LTE networks. Without IMS, there is no VoLTE.

IMS – IP Multimedia Subsystem enables VoLTE

The voice capability in 2G and 3G networks has been an enormous success, and it will always be an essential service for mobile operators. The GSM and UMTS networks use the circuit-switched technology for voice calls and text messages (SMS) and the packet-switched technology for mobile data. Even though GSM and UMTS networks have the packet-switched part, the voice calls and text messages are sent over dedicated circuits. There is no way in 2G and 3G networks to deliver voice services in high quality (carrier-grade quality) using the data network. The aim of LTE networks is high-speed data with low latency based on IP, which is enabled by the packet-switched technology. As LTE networks do not have a circuit-switched part, the voice calling and other real-time communication services still need to be facilitated somehow. That is where IMS – IP Multimedia Subsystem comes in. IMS is the machinery behind voice calling in 4G and 5G networks, which combines IP telephony with data communication. IMS works alongside the mobile core network in 4G LTE and 5G NR to enable high-quality voice calls and other enriched communication services, including text and multimedia messages. IMS allows the necessary integration with external voice networks such as landline (PSTN – Public Switched Telephone Network) and ISDN (Integrated Services Digital Network). There are several Quality-of-Service (QoS) requirements in the LTE networks for high-quality (HD) voice calls, including latency, error correction, bandwidth, and codecs. Since voice calling in 2G and 3G networks is a mature and successful service, LTE networks use that as a baseline for offering voice services through VoLTE. IMS enables VoLTE and uses SIP (Session Initiation Protocol) for IP calling over the cellular network.

VoLTE relies on EPC and IMS

Voice over LTE (VoLTE) requires IMS (IP Multimedia Subsystem) to work with the 4G LTE core network, Evolved Packet Core or the EPC. The user equipment (phone) communicates with IMS through the radio and core networks. For voice calls to work in high-quality, the mouth-to-ear delay must be below 200 milliseconds. LTE networks can achieve roughly around 160 milliseconds delay, which is well within this range. Another critical requirement for voice services in LTE is the speech codec, which is based on AMR (Adaptive Multi-Rate) like the legacy GSM and UMTS networks. The recommended codec for VoLTE is the wideband variant of AMR, AMR-WB, but it also uses a more superior codec Enhanced Voice Services (EVS), for HD voice. For the speech codecs to work, both the network and the device must support them. The use of IP also creates a challenge around the size of IP headers, which Robust Header Compression (RoHC) addresses by essentially compressing the headers in the data packets.


IMS or IP Multimedia Subsystem is a network entity that enables Voice over LTE (VoLTE), Voice over NR (VoNR) and other enriched communication services, including text and multimedia messages. VoLTE or Voice over LTE is the technology IMS facilities in 4G LTE networks to support voice services in HD quality through the IP network instead of conventional circuit-switched lines.

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