How VoLTE Works
VoLTE uses the IP Multimedia Subsystem (IMS) to manage voice sessions over 4G LTE. SIP handles call signalling via the P-CSCF and S-CSCF. Voice media travels as RTP/UDP packets over a dedicated GBR bearer with QCI 1, ensuring packet delivery within 100ms budget. The AMR-WB codec carries the audio.
The three components
A VoLTE call involves three core network elements working together:
- E-UTRAN – the 4G LTE radio access network (base stations / eNodeBs) that carries the radio connection to the device
- EPC (Evolved Packet Core) – the 4G core network including the MME, SGW, and PGW that manages LTE bearers and connectivity
- IMS (IP Multimedia Subsystem) – the multimedia control layer that handles SIP signalling, call routing, and supplementary services
IMS network elements
| Element | Role |
|---|---|
| P-CSCF | Proxy Call Session Control Function. First IMS contact point for the device. Handles SIP registration forwarding and QoS authorisation for the voice bearer. |
| I-CSCF | Interrogating CSCF. Queries the HSS to determine which S-CSCF should serve the registering subscriber. |
| S-CSCF | Serving CSCF. Central IMS registration and session routing function. Applies subscriber service profiles and routes calls to the TAS. |
| HSS | Home Subscriber Server. Contains subscriber profiles, IMS identities (IMPI/IMPU), and authentication vectors. |
| TAS | Telephony Application Server. Implements supplementary services: call forwarding, call waiting, voicemail access, conference. |
| MGCF | Media Gateway Control Function. Interworks VoLTE calls with the legacy PSTN using SIP-ISUP translation. |
| PCRF | Policy and Charging Rules Function. Authorises the dedicated QCI 1 bearer for each VoLTE call, ensuring priority bandwidth allocation. |
Call setup sequence
A VoLTE call setup proceeds as follows:
- The originating UE sends a SIP INVITE to its P-CSCF, including SDP offer with supported codecs (AMR-WB, AMR-NB).
- The IMS routes the INVITE via I-CSCF to the terminating S-CSCF. The PCRF authorises a dedicated GBR bearer (QCI 1) for the call.
- The terminating UE receives a SIP INVITE and rings. RTP packets can begin flowing on the pre-established bearer even before the call is answered (for ringing tone).
- The terminating UE answers. It sends a SIP 200 OK with SDP answer confirming the codec selection.
- The originating UE sends SIP ACK. The RTP voice stream is established end-to-end.
- Either party sends SIP BYE to end the call and release the bearer.
QoS bearer – QCI 1
VoLTE voice calls run on a GBR (Guaranteed Bit Rate) EPS bearer with QCI 1. QCI 1 has these parameters:
| Parameter | Value | Meaning |
|---|---|---|
| QCI | 1 | Conversational voice |
| Resource type | GBR | Guaranteed bit rate – bandwidth reserved |
| Priority level | 2 | Second highest priority in LTE |
| Packet delay budget | 100 ms | One-way network delay target |
| Packet error rate | 10^-2 | 1 in 100 packets may be lost |
SRVCC – maintaining calls at the cell edge
SRVCC (Single Radio Voice Call Continuity) is the mechanism that hands an active VoLTE call over to a 2G or 3G network if LTE coverage is lost mid-call. The eNodeB detects a coverage threshold is being crossed and triggers an SRVCC handover, transferring the IMS session to a circuit-switched call on the available legacy network without dropping the call.
The interruption is typically under 300ms with eSRVCC (enhanced SRVCC). With all UK 3G networks now closed, SRVCC to 2G is theoretically possible on networks that retain 2G. In practice, most installations should be checked for LTE coverage adequacy before relying on VoLTE, particularly for safety-critical applications like lifts.