4th Generation SRVCC Technology

As already described in this article, the ability to transmit voice data in the networks of the fourth generation (4G) were quite limited. With the development of technology, Voice over IP has come to 4G mobile networks, called Voice over LTE (VoLTE) and is tied to the IMS (IP Multimedia Subsystem) subsystem. In fact, this is not a new technology, because the very concept of IMS appeared back in 2000, when the 3GPP consortium started working on the Release 2000 standard. As a result, IMS is already available starting with the version of the 3GPP Release 5 standard (March 2002), although the architecture with all the functionality It was finally brought to the divine form only by the beginning of 2004. Later editions (Release 7) added support for various standards (for example, DOCSIS, xDSL, etc.). Starting with Release 8 (2008), IMS technology can be considered as a working model for transmitting voice data in LTE networks,

SIP was also seen in mobile networks before, but voice transmission from one subscriber to another through this protocol via a packet data network appeared relatively recently due to the tight integration of IMS with 4G and 3G networks.

Here is what a 4G network integrated with IMS looks like in general (Fig. 1):

Fig . 1 - 4G network and IMS

subsystem The IMS subsystem has quite a few elements with various functions, however, the focus in this article will be on the mechanism of subscriber transition from a 4G network to a 3G network with the current voice connection from the point of view of the 4G network, which at the moment is the best solution and indirectly replaces another technology - CSFB (Circuit Switch Fall Back), discussed in thisarticle. This type of transition from a 4G network to a 3G network with an active voice connection is called SRVCC (Single Radio Voice Call Continuity). To implement this service, the network must support the network architecture described since 3GPP Release 8. There are several types of SRVCC handover - CS_Only and CS_and_PS. This parameter is configured in the radio network, and is transmitted in the initial request for a handover, which determines the further actions of the MME. To simplify, I will consider only the first type, which is easier to understand and gives a general picture of the process without complications.

So, if a subscriber has an active voice connection in a 4G network, then from the point of view of a mobile network, this subscriber has a minimum of 3 sessions (bearers) - the default session (default bearer, EPS Bearer ID = 5) for data transmission with the quality parameter QCI = 8, which determines a maximum packet delay of 100ms and a maximum loss of 10 -6 without a guaranteed data transfer band (non-GBR); default session (EBI = 6) for monitoring voice transmission with a quality parameter QCI = 5, which determines the maximum delay of 100ms, the loss is not higher than 10 -6 without a guaranteed data transmission band; and a dedicated session (dedicated bearer, EBI = 7) for the actual voice traffic with the quality parameter QCI = 1, which determines the maximum delay of 100ms and the maximum loss of not more than 10 -2with a guaranteed data transfer band (GBR), which in our case is 30kbps, but it is not a constant and may vary depending on the network setting.

If for some reason, most likely associated with a significant deterioration in the 4G signal, the mobile phone decides to switch to the 3G network, it sends a handover request to the MME. Upon receiving such a request, the MME initiates the transfer of the session to the 3G network by sending a SRVCC PS to CS Request message to the ehanc Mobile Enhanced Mobile Switching Center containing the subscriber’s identification, the description of the current session, the RNC selected for this handover, and several other service parameters. If the eMSC successfully responds to this request with an SRVCC PS to CS Response messagewithout errors, the MME waits for final confirmation from the eMSC with the SRVCC PS to CS Complete Notification message , and in turn responds with the SRVCC PS to CS Complete Acknowledgment confirmation message . Further, when the mobile phone has switched to the 3G network, the MMS will receive a SGSN Context Transfer request to transfer all available sessions to the 3G network from SGSN. Here, the MME should divide the sessions into those that need to be transmitted and those that need to be closed. In this case, the MME must first delete the session that transmitted voice traffic (EBI = 7), because it now goes in a completely different way and the session is no longer needed, and only then transfer the first two sessions (EBI = 5, 6) to SGSN according to the standard procedure. As a result, the MME first sends an S / P-GW request to delete the sessionDelete Bearer Command and deletes the session when it receives a confirmation message Delete Bearer Request in response and responds with a confirmation message Delete Bearer Response . When the session is deleted, the MME responds with an SGSN Context Response message containing a description of the two sessions on the SGSN. And in the end, it receives a confirmation message from SGSN - SGSN Context Acknowledgment . On this, the transfer of sessions is over and the subscriber session on the 4G network is no more.

Figure 2 - Network Architecture with SRVCC Support

The 3GPP standard dictates that the entire procedure should not take more than 0.3 seconds. This interrupt is measured from the last packet sent in a 4G network and the first packet in the CS domain of a 3G network. Practice shows that this time varies depending on the type of procedure (CS Only, CS and PS), as well as on the state of the network elements. In general, the voice is transmitted without visible delay and frame drops, i.e. imperceptibly for the subscriber.

In conclusion, I would like to note that the technology has not yet been fully tested, although it is available for commercial use. For example, Alcatel-Lucent and Nokia Solutions and Networks in mid-September of this year applied for an improvement of 3GPP 23.216 - Single Radio Voice Call Continuity (SRVCC), noting that under certain conditions a 3G network can intercept a voice session before MME can delete it in the 4G domain, which will lead to a collision in the network and cause certain problems with the release of unused resources.

You can read about the following:
1. 3GPP 23.401 - General Packet Radio Service (GPRS) enhancements for Evolved Universal Terrestrial Radio Access Network (E-UTRAN) access
2. 3GPP 23.216 - Single Radio Voice Call Continuity (SRVCC)
3. Voice over LTE (VoLTE)

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