What is Voice over LTE, VoLTE

Voice over LTE, VoLTE is now the standard means of carrying voice over an LTE radio link, providing many gains beyond that of just carrying voice.

4G LTE includes:
What is LTE     LTE OFDMA / SCFDMA     MIMO     LTE Duplex     LTE frame & subframe     LTE data channels     LTE frequency bands     LTE EARFCN     UE categories / classes     LTE-M (Machine to Machine)     LTE-LAA / LTE-U     VoLTE     SRVCC    

LTE Advanced topics:     LTE Advanced introduction     Carrier aggregation     Coordinated multipoint     LTE relay     Device to device, D2D    

Voice over LTE, VoLTE is the method of sending voice over an LTE bearer. Although there are many over the top applications for carrying voice over an LTE signal, VoLTE is the method that is provided by operators.

When LTE was launched, it was seen as a packet data based system for data only. Operators would be able to carry voice either by reverting to 2G / 3G systems or by using VoIP in one form or another. However this did not provide the operator revenue and the standardisation required to provide a really reliable service.

A further requirement was that many system relied on SMS messaging. It was therefore particularly important to have a viable and standardised format for voice over LTE and messaging to protect the revenue provided to operators by these services.

To overcome this Voice over LTE, was introduced as the global scheme to provide seamless integration of voice and short messaging into LTE networks.

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

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 had been specified in the GSMA profile IR 92.

Voice over LTE, VoLTE basics

VoLTE, Voice over LTE is an IMS-based technique. Adopting this approach, it enables the system to be integrated with the suite of other applications for LTE.

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. This considerably reduced the costs for network operators as this had been a major issue preventing he introduction of IMS.

IMS network for 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 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.

VoLTE is now included on most 4G smartphones as standard. The networks steadily deployed IMS either in its full form, or as the cut down version making voice calls over the LTE network common. This was particularly useful as 2G and 3G frequencies were re-farmed for 4G purposes.

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