If you have been using the Voice over LTE (VoLTE) or Voice over WiFi (VoWiFi) services on your mobile phone, then you already have been served by the IMS platform that mobile operators use along with their mobile core network. IMS or IP Multimedia Subsystem is used in both 4G LTE and 5G NR networks.
IMS (IP Multimedia Subsystem) is a network element that works alongside 4G LTE core network (Evolved Packet Core – EPC) to enable Voice over LTE (VoLTE) in 4G networks; VoLTE is an IP-based technology enabled by EPC and IMS in packet-switched LTE networks to facilitate voice calls and texts (SMS).
Why IMS and EPC are required for VoLTE?
IMS (IP Multimedia Subsystem) and EPC (Evolved Packet Core – 4G core network) work together to combine IP telephony with data communication in VoLTE (Voice over LTE). This combination allows them to use the SIP protocol to enable voice calls and other rich communication services in 4G LTE networks.
Before the introduction of 4G LTE (Long Term Evolution) networks, voice calls and text messages (SMS) were delivered over the traditional circuit-switched part of the mobile network. 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 radio network resources for the whole 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 packet-switched and delivers all communication services over the IP network. The voice calling service in LTE is delivered by the VoLTE (Voice over LTE) technology, a managed VoIP (Voice over IP) technology that ensures Quality of Service (QoS) when for voice calls and SMS. IMS or IP Multimedia Subsystem is a network entity that enables VoLTE in 4G LTE networks. Voice over LTE (VoLTE) requires IMS (IP Multimedia Subsystem) to work with the 4G LTE core network, Evolved Packet Core (EPC), to combine IP telephony with data communication so that rich communication services can be delivered over the IP network.
How does IMS enable voice over LTE (VoLTE)?
IMS (IP Multimedia Subsystem) works with the 4G core network, Evolved Packet Core (EPC), to provide necessary integration with external voice networks such as PSTN and ISDN. IMS works with the network to ensure QoS requirements for HD voice calls, including bandwidth, latency, codecs etc., are met.
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 high-quality voice services (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 New Radio (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). IMS uses SIP (Session Initiation Protocol) to enable VoLTE for voice calls and other rich services over the IP part of the cellular 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. Unlike Over The Top (OTT) voice calling services like Skype that are delivered over the public internet, VoLTE is a managed VoIP service that requires a mobile operator to invest in IMS. The QoS parameters are defined by the benchmark set by the circuit-switched voice calls in 2G and 3G networks. 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.
What QoS and codec requirements are used by VoLTE?
VoLTE, according to ITU-T, requires the preferred maximum mouth-to-ear delay to be 150 milliseconds (ms); however, LTE networks can achieve roughly around 160 ms. The recommended codec for VoLTE is wideband Adaptive Multi-Rate (AMR-WB), but it can also use Enhanced Voice Services (EVS) for HD voice.
The User Equipment (UE or mobile phone) communicates with IMS through the radio and core networks. The mouth-to-ear delay must be below 200 milliseconds for voice calls to work in high quality. For VoLTE, according to [ITU-T G.114], a preferred maximum ear-to-mouth delay of 150 milliseconds has been specified. However, the average delay that network operators achieve for VoLTE networks may vary. LTE networks can achieve roughly around 160 milliseconds delay, which is slightly over 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.
VoLTE (Voice over LTE) is a technology that IMS (IP Multimedia Subsystem) enables in 4G LTE networks to support voice calls and other enriched communication services over the IP network. IMS can be used in both 4G LTE and 5G NR networks. It is a network entity that enables Voice over LTE (VoLTE) in 4G networks and Voice over NR (VoNR) in 5G networks. You can also get VoLTE in 5G networks, and you can find out more about that in this dedicated post on VoLTE, VoNR and VoWiFi.