LTE: Voice over LTE (VoLTE)

Voice over Long-Term Evolution (VoLTE)

Voice over LTE (VoLTE)
Voice over LTE (VoLTE)

Voice over Long-Term Evolution (VoLTE) is a standard for high-speed wireless communication for mobile phones and data terminals. It is based on the IP Multimedia Subsystem (IMS) network, with specific profiles for control and media planes of voice service on LTE defined by GSMA in PRD IR.92. This approach results in the voice service (control and media planes) being delivered as data flows within the LTE data bearer. This means that there is no dependency on (or ultimately, requirement for) the legacy circuit-switched voice network to be maintained. VoLTE has up to three times more voice and data capacity than 3G UMTS and up to six times more than 2G GSM. Furthermore, it frees up bandwidth because VoLTE’s packet headers are smaller than those of unoptimized VoIP/LTE.

Why is VoLTE necessary?

This technology is necessary mainly because LTE is a data-only networking technology.

Previous cellular networks, such as 2G and 3G, were designed mainly to carry voice calls–services added cellular data support later through methods that basically “tunneled” data inside of voice-call connections.

LTE turns the network around and uses Internet Protocol packets for all communications. As such, it doesn’t support traditional voice-call technology, so a new protocol and applications for Voice over LTE are needed.

Voice over LTE, VoLTE basics

VoLTE, Voice over LTE is an IMS-based specification. Adopting this approach, it enables the system to be integrated with the suite of applications that will become available on LTE.

Note on IMS:

The IP Multimedia Subsystem or IP Multimedia Core Network Subsystem, IMS is an architectural framework for delivering Internet Protocol, IP multimedia services. It enables a variety of services to be run seamlessly rather than having several disparate applications operating concurrently.

In order that IMS was implemented in fashion that would be acceptable to operators, a cut down version was defined. This not only reduced the number of entities required in the IMS network, but it also simplified the interconnectivity – focusing on the elements required for VoLTE.

Diagram showing the basic IMS network used for carrying Voice over LTE, VoLTE.
Reduced IMS network for VoLTE

As can be seen there are several entities within the reduced IMS network used for VoLTE:

  • IP-CAN IP, Connectivity Access Network:   This consists of the EUTRAN and the MME.
  • P-CSCF, Proxy Call State Control Function:   The P-CSCF is the user to network proxy. In this respect all SIP signalling to and from the user runs via the P-CSCF whether in the home or a visited network.
  • I-CSCF, Interrogating Call State Control Function:   The I-CSCF is used for forwarding an initial SIP request to the S-CSCF. When the initiator does not know which S-CSCF should receive the request.
  • S-CSCF, Serving Call State Control Function:   The S-CSCF undertakes a variety of actions within the overall system, and it has a number of interfaces to enable it to communicate with other entities within the overall system.
  • AS, Application Server:   It is the application server that handles the voice as an application.
  • HSS, Home Subscriber Server:   The IMS HSS or home subscriber server is the main subscriber database used within IMS. The IMS HSS provides details of the subscribers to the other entities within the IMS network, enabling users to be granted access or not dependent upon their status.

The IMS calls for VoLTE are processed by the subscriber’s S-CSCF in the home network. The connection to the S-CSCF is via the P-CSCF. Dependent upon the network in use and overall location within a network, the P-CSCF will vary, and a key element in the enablement of voice calling capability is the discovery of the P-CSCF.

An additional requirement for VoLTE enabled networks is to have a means to handing back to circuit switched legacy networks in a seamless manner, while only having one transmitting radio in the handset to preserve battery life. A system known as SRVCC – Single Radio Voice Call Continuity is required for this. 

VoLTE codecs

As with any digital voice system, a codec must be used. The VoLTE codec is that specified by 3GPP and is the adaptive multi-rate, AMR codec that is used in many other cellular systems from GSM through UMTS and now to LTE. The AMR-wideband codec may also be used.

The used of the AMR codec for VoLTE also provides advantages in terms of interoperability with legacy systems. No transcoders are needed as most legacy systems now are moving towards the AMR codec.

In addition to this, support for dual tone multi-frequency, DTMF signalling is also mandatory as this is widely used for many forms of signalling over analogue telephone lines.

VoLTE IP versions

With the update from IPv4 to IPv6, the version of IP used in any system is of importance.

VoLTE devices are required to operate in dual stack mode catering for both IPv4 and IPv6.

If the IMS application profile assigns and IPv6 address, then the device is required to prefer that address and also to specifically use it during the P-CSCF discovery phase.

One of the issues with voice over IP type calls is the overhead resulting from the IP header. To overcome this issue VoLTE requires that IP header compression is used along with RoHC, Robust Header Compression, protocol for voice data packet headers.

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