Open Source IMS Client

h1. VoLTE

*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.

h2. 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_.

h3. 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]].

h3. 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.