Voice over LTE - call without fallback with a very quick call. How we did it

    We recently completed VoLTE tests in Moscow. This is a continuation of the evolution of the LTE network. As you know, LTE is characterized by a higher data transfer rate compared to the networks of GSM and 3G standards that existed before it. Initially, the architecture of LTE networks deployed in a 2G / 3G environment in Russia was not “tuned” to the voice. To receive a voice call, the LTE terminal suspends the packet session and temporarily switches to 2G / 3G networks (a procedure called Circuit Switched FallBack). It takes up to several seconds to complete this transition and return back to the LTE network after the end of the conversation.

    The evolution of the voice mobile network.

    The chassis of Huawei OSTA 2.0, on which the tested IMS core is built.

    Among the main advantages - the time from pressing the “call” button in the LTE network to the first call from another subscriber in most cases will be no more than a second. Speech intelligibility will also be improved compared to traditional networks through the use of broadband codecs.

    The evolution of mobile networks leads to the fact that in the fourth generation networks, which are LTE networks, telephony is implemented on the basis of data transfer, and traditional networks with channel switching (Circuit Switched) cease to develop. Everything goes to data.

    Roughly speaking, from the point of view of an LTE network, voice is just one of the data transfer functions. But unlike, for example, Internet pages, voice transmission imposes stringent requirements on network quality. The essence of the requirements is the presence of certain thresholds for packet loss and delay. At the same time, the quality should be end-to-end, along the entire route of the packages - and this: the radio subsystem, PS Core, transport. Transferring voice to a packet network makes a 2G / 3G network handover unnecessary - if the subscriber is registered in an LTE network and the operator infrastructure supports VoLTE, the call will be processed in the LTE network.

    Transferring communications to a single IP transport environment allows you to expand the range of services, for example, supplement a voice video call and, at the same time, send mail or participate in a chat. By the way, we also tested video calls. Unlike 3G networks, video resolution is no longer limited to a channel of 64K and the picture can be of very decent quality (limited by the allowed codecs and the capabilities of the terminals themselves).
    Naturally, we can talk about these advantages if both subscribers use VoLTE in a conversation with each other.


    The tests were carried out in the test zone of VimpelCom OJSC on Huawei equipment. During the tests, calls between subscribers in the LTE network, calls between subscribers located in different LTE and 2G / 3G networks, as well as forwarding to subscribers located in the coverage area of ​​networks of any of these generations, were checked. We checked how the network and devices behave when switching from an LTE network to a 3G network coverage area during a conversation (when the LTE coverage worsens / loses, a seamless handover by technology is made - Single Radio Voice Call Continuity).

    Here is the schematic diagram of the LTE-2G / 3G

    connection : Establishing a connection between a VoLTE subscriber and a subscriber located in a traditional network.

    And this is the LTE-LTE call:

    Voice packets go from your high priority phone and come to the called party’s phone. That is how a fully packet network should work.


    To fully work with VoLTE technology, the terminal requires support for the standard at the chipset level. It is this approach that will allow to save the experience accumulated by subscribers in traditional networks and optimize such an important parameter as the battery’s battery life.
    At the moment, most of the phones sold in Russia do not have such support. For other phones, it is still closed at the software level. For tests, we used Android phones, of which particular manufacturer - for now, let's not say that they would not be considered advertising.

    How does it work now (before VoLTE)?

    Currently, voice services and SMS are possible in the LTE networks of Russia thanks to the use of CSFB (Circuit Switched FallBack) technology - temporary switching to 3G / 2G networks to establish a voice connection. After the call ends, the devices return to LTE, which entails significant delays in establishing a connection and inefficient use of network resources. This technology will exist for quite some time, since it will be a long time before all operators implement VoLTE support in their LTE networks and enter into agreements between themselves, until this point CSFB will be required for roaming to work.

    What does it mean?

    We are the first to implement full-featured (with QoS and SR VCC support) VoLTE technology in Russia (as part of a test operation). If you are already connected to one of our LTE networks, after a while you can feel for yourself how nice it is to get rid of the overhead of many old technologies.

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