Voice over LTE - VoLTE Tutorial

- operation of Voice over LTE VoLTE system for providing a unified format of voice traffic on LTE, and other systems including CSFB, and SV-LTE.

The Voice over LTE, VoLTE scheme was devised as a result of operators seeking a standardised system for transferring traffic for voice over LTE.

Originally LTE was seen as a completely IP cellular system just for carrying data, and operators would be able to carry voice either by reverting to 2G / 3G systems or by using VoIP in one form or another.

from around 2014 phones like this Apple iPhone 6 incorporated VoLTE as standard
From around 2014 Phones like this iPhone6 incorporated VoLTE as standard

However it was seen that this would lead to fragmentation and incompatibility not allowing all phones to communicate with each other and this would reduce voice traffic. Additionally SMS services are still widely used, often proving a means of set-up for other applications.

Even though revenue from voice calls and SMS is falling, a format for voice over LTE and messaging, it was as necessary to have a viable and standardized scheme to provide the voice and SMS services to protect this revenue.

Options for LTE Voice

When looking at the options for ways of carrying voice over the LTE system, a number of possible solutions were investigated. A number of alliances were set up to promote different ways of providing the service. A number of systems were prosed as outlined below:

  • CSFB, Circuit Switched Fall Back:   The circuit switched fall-back, CSFB option for providing voice over LTE has been standardised under 3GPP specification 23.272. Essentially LTE CSFB uses a variety of processes and network elements to enable the circuit to fall back to the 2G or 3G connection (GSM, UMTS, CDMA2000 1x) before a circuit switched call is initiated.

    The specification also allows for SMS to be carried as this is essential for very many set-up procedures for cellular telecommunications. To achieve this the handset uses an interface known as SGs which allows messages to be sent over an LTE channel.
  • SV-LTE - Simultaneous Voice LTE:   SV-LTE allows packet switched LTE services to run simultaneously with a circuit switched voice service. SV-LTE facility provides the facilities of CSFB at the same time as running a packet switched data service. It has the disadvantage that it requires two radios to run at the same time within the handset which has a serious impact on battery life which is already a major issue.
  • VoLGA, Voice over LTE via GAN:   The VoLGA standard was based on the existing 3GPP Generic Access Network (GAN) standard, and the aim was to enable LTE users to receive a consistent set of voice, SMS (and other circuit-switched) services as they transition between GSM, UMTS and LTE access networks. For mobile operators, the aim of VoLGA was to provide a low-cost and low-risk approach for bringing their primary revenue generating services (voice and SMS) onto the new LTE network deployments.
  • One Voice / later called Voice over LTE, VoLTE:   The Voice over LTE, VoLTE scheme for providing voice over an LTE system utilises IMS enabling it to become part of a rich media solution. It was the option chosen by the GSMA for use on LTE and is the standardised method for providing SMS and voice over LTE.

Voice over LTE, VoLTE formation

Originally the concept for an SMS and voice system over LTE using IMS had been opposed by many operators because of the complexity of IMS. They had seen it as far too expensive and burdensome to introduce and maintain.

However, the One Voice profile for Voice over LTE was developed by a collaboration between over forty operators including: AT&T, Verizon Wireless, Nokia and Alcatel-Lucent.

At the 2010 GSMA Mobile World Congress, GSMA announced that they were supporting the One Voice solution to provide Voice over LTE.

To achieve a workable system, a cut down variant of IMS was used. It was felt that his would be acceptable to operators while still providing the functionality required.

The VoLTE system is based on the IMS MMTel concepts that were previously in existence. It has been specified in the GSMA profile IR 92.

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 seemlessly rather than having several disparate applications operating concurrently.

Click for an IMS tutorial

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 - focussing 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. Read more about SRVCC - Single Radio-Voice Call Continuity

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.

By Ian Poole

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