Voice over LTE is an adaptation by 3GPP to use IMS over an LTE cellular network. The LTE EPS (Evolved Packet System) provides the functionality of the underlying IP-CAN.

As IMS is SIP/RTP over IP, and LTE a technology to provide mobile cellular IP access, isn't VoLTE just normal VoIP? Unfortunately it's not that simple.

First, IMS is much more than normal SIP/RTP. And in addition to that, there is a tight integration between the LTE system and the IMS on top of it.

VoLTE IMS Bearers

A VoLTE capable UE (Phone/Modem) establishes so-called EPS bearers through the LTE network. Those bearers are like logical pipes/tunnels for IP packets, terminating on the UE and PGW.

TODO: Diagram

There are two flavours of VoLTE related bearers: The IMS Default Bearer and IMS Dedicated Bearers.

VoLTE IMS Default Bearer

The IMS Default Bearer is usually established right away when the UE registers to the network. It is established at about the same time as the traditional "Internet" Default Bearer is established. The special part about the IMS Default bearer is that it is established against a well-known, fixed APN (Access Point Name) called ims.

A typical VoLTE phone hence has two default bearers established after network attach:
  • the internet default bearer for public internet access (what your various smartphone apps are using). The APN is configurable/flexible.
  • the IMS default bearer for IMS related signaling. The APN is fixed to ims.

The IMS default bearer is used by the IMS client on the phone to perform establishment of IMS signaling to the P-CSCF.

VoLTE IMS Dedicated Bearer

Once an actual IMS service (voice call, video call, ...) is being established, additional so-called dedicated bearers are established through the LTE network. Those dedicated bearers are for example used for the RTP streams containing the packetized voice or video streams.

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Updated by laforge about 2 years ago · 1 revisions

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