Voice over LTE - VoLTE

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

Operators, however saw the fact that a voice format was not defined as a major omission for the system. It was seen that the lack of standardisation may provide problems with scenarios including roaming. In addition to this, SMS is a key requirement. It is not often realised, that SMS is used to set-up many mobile broadband connections, and a lack of SMS is seen as a show-stopper by many.

As mobile operators receive over 80% of their revenues from voice and SMS traffic, it is necessary to have a viable and standardized scheme to provide these services and protect this revenue.

Options for Voice over LTE

When looking at the options for ways of carrying voice over LTE, 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:

• VoLGA, Voice over LTE via GAN
• CSFB, Circuit Switched Fall Back
• One Voice / later called Voice over LTE, VoLTE

Issues for Voice services over LTE

Unlike previous cellular telecommunications standards including GSM, LTE does not have dedicated channels for circuit switched telephony. Instead LTE is an all-IP system providing an end-to-end IP connection from the mobile equipment to the core network and out again.

In order to provide some form of voice connection over a standard LTE bearer, some form of Voice over IP, VoIP must be used.

The aim for any voice service is to utilise the low latency and QoS features available within LTE to ensure that any voice service offers an improvement over the standards available on the 2G and 3G networks.

VoLGA

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.

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 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. The

In addition to this CSFB requires modification to elements within the network, in particular the MSCs as well as support, obviously on new devices. MSC modifications are also required for the SMS over SGs facilities. For CSFB, this is required from the initial launch of CSFB in view of the criticality of SMS for many procedures.

Voice over LTE, VoLTE basics

The One Voice profile for Voice over LTE, VoLTE 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.

VoLTE, Voice over LTE is an IMS-based specification. Adopting this approach will enable it to integrate into the suite of applications that will become available on LTE.

To provide the VoLTE service, three interfaces are being defined:

• User Network interface, UNI: This interface is located between the user's equipment and the operators network.
• Roaming Network Network Interface, R-NNI: The R-NNI is an interface located between the Home and Visited Network. This is used for a user that is not attached to their Home network, i.e. roaming.
• Interconnect Network Network Interface, I-NNI: The I-NNI is the interface located between the networks of the two parties making a call.

Work on the definition of VoLTE, Voice over LTE is ongoing. It will include a variety of elements including some of the following:

• It will be necessary to ensure the continuity of Voice calls when a user moves from an LTE coverage area to another where a fallback to another technology is required. This form of handover will be achieved using Single Radio Voice Call Continuity, or SR-VCC).
• It will be important to provide the optimal routing of bearers for voice calls when customers are roaming.
• Another area of importance will be to establish commercial frameworks for roaming and interconnect for services implemented using VoLTE definitions. This will enable roaming agreements to be set up.
• Provision of capabilities associated with the model of roaming hubbing.
• For any services, including LTE, it is necessary to undertake a thorough security and fraud threat audit to prevent hacking and un-authorised entry into any area within the network..

In many ways the implementation of VoLTE at a high level is straightforward. The handset or phone needs to have software loaded to provide the VoLTE functionality. This can be in the form of an App.

The network then requires to be IMS compatible.

While this may appear straightforward, there are many issues for this to be made operational, especially via the vagaries of the radio access network where time delays and propagation anomalies add considerably to the complexity.