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Showing posts with label SRVCC. Show all posts
Showing posts with label SRVCC. Show all posts

Wednesday, 7 January 2015

Enhancing voice services using VoLTE


VoLTE has been a very popular topic on this blog. My overview of the LTE Voice Summit missed out narrowly from the Top 10 posts of 2014 but there were other posts related to VoLTE that made it.

In this magazine article, NTT Docomo not only talks about its own architecture and transition from 3G to 4G for voice and video, it provides some detailed insights from its own experience.

There is also discussion into technical details of the feature and examples of signalling for VoLTE registration and originating/terminating calls (control, session and user plane establishment), SMS, SRVCC, Video over LTE (ViLTE) and voice to video call switching.

The paper is embedded below and available from slideshare to download.



Related links:

Monday, 20 January 2014

Different flavours of SRVCC (Single Radio Voice Call Continuity)



Single Radio Voice Call Continuity (SRVCC) has been quietly evolving with the different 3GPP releases. Here is a quick summary of these different flavors

In its simplest form, SRVCC comes into picture when an IMS based VoLTE call is handed over to the existing 2G/3G network as a normal CS call. SRVCC is particularly important when LTE is rolled out in small islands and the operator decided to provide VoLTE based call when in LTE. An alternative (used widely in practice) is to use CS Fallback (CSFB) as the voice option until LTE is rolled out in a wider area. The main problem with CSFB is that the data rates would drop to the 2G/3G rates when the UE falls back to the 2G/3G network during the voice call.



The book "LTE-Advanced: A Practical Systems Approach to Understanding 3GPP LTE Releases 10 and 11 Radio Access Technologies" by Sassan Ahmadi has some detailed information on SRVCC, the following is an edited version from the book:

SRVCC is built on the IMS centralized services (ICS) framework for delivering voice and messaging services to the users regardless of the type of network to which they are attached, and for maintaining service continuity for moving terminals.

To support GSM and UMTS, some modifications in the MSC server are required. When the E-UTRAN selects a target cell for SRVCC handover, it needs to indicate to the MME that this handover procedure requires SRVCC. Upon receiving the handover request, the MME triggers the SRVCC procedure with the MSC server. The MSC then initiates the session transfer procedure to IMS and coordinates it with the circuit-switched handover procedure to the target cell.

Handling of any non-voice packet-switched bearer is by the packet-switched bearer splitting function in the MME. The handover of non-voice packet-switched bearers, if performed, is according to a regular inter-RAT packet-switched handover procedure.

When SRVCC is enacted, the downlink flow of voice packets is switched toward the target circuit-switched network. The call is moved from the packet-switched to the circuit-switched domain, and the UE switches from VoIP to circuit-switched voice.

3GPP Rel-10 architecture has been recommended by GSMA for SRVCC because it reduces both voice interruption time during handover and the dropped call rate compared to earlier configurations. The network controls and moves the UE from E-UTRAN to UTRAN/GERAN as the user moves out of the LTE network coverage area. The SRVCC handover mechanism is entirely network-controlled and calls remain under the control of the IMS core network, which maintains access to subscribed services implemented in the IMS service engine throughout the handover process. 3GPP Rel-10 configuration includes all components needed to manage the time-critical signaling between the user’s device and the network, and between network elements within the serving network, including visited networks during roaming. As a result, signaling follows the shortest possible path and is as robust as possible, minimizing voice interruption time caused by switching from the packet-switched core network to the circuit-switched core network, whether the UE is in its home network or roaming. With the industry aligned around the 3GPP standard and GSMA recommendations, SRVCC-enabled user devices and networks will be interoperable, ensuring that solutions work in many scenarios of interest.

Along with the introduction of the LTE radio access network, 3GPP also standardized SRVCC in Rel-8 specifications to provide seamless service continuity when a UE performs a handover from the E-UTRAN to UTRAN/GERAN. With SRVCC, calls are anchored in the IMS network while the UE is capable of transmitting/ receiving on only one of those access networks at a given time, where a call anchored in the IMS core can continue in UMTS/GSM networks and outside of the LTE coverage area. Since its introduction in Rel-8, the SRVCC has evolved with each new release, a brief summary of SRVCC capability and enhancements are noted below

3GPP Rel-8: Introduces SRVCC for voice calls that are anchored in the IMS core network from E-UTRAN to CDMA2000 and from E-UTRAN/UTRAN (HSPA) to UTRAN/GERAN circuit-switched. To support this functionality, 3GPP introduced new protocol interface and procedures between MME and MSC for SRVCC from E-UTRAN to UTRAN/GERAN, between SGSN and MSC for SRVCC from UTRAN (HSPA) to UTRAN/GERAN, and between the MME and a 3GPP2-defined interworking function for SRVCC from E-UTRAN to CDMA 2000.

3GPP Rel-9: Introduces the SRVCC support for emergency calls that are anchored in the IMS core network. IMS emergency calls, placed via LTE access, need to continue when SRVCC handover occurs from the LTE network to GSM/UMTS/CDMA2000 networks. This evolution resolves a key regulatory exception. This enhancement supports IMS emergency call continuity from E-UTRAN to CDMA2000 and from E-UTRAN/UTRAN (HSPA) to UTRAN/ GERAN circuit-switched network. Functional and interface evolution of EPS entities were needed to support IMS emergency calls with SRVCC.

3GPP Rel-10: Introduces procedures of enhanced SRVCC including support of mid-call feature during SRVCC handover (eSRVCC); support of SRVCC packet-switched to circuit-switched transfer of a call in alerting phase (aSRVCC); MSC server-assisted mid-call feature enables packet-switched/ circuit-switched access transfer for the UEs not using IMS centralized service capabilities, while preserving the provision of mid-call services (inactive sessions or sessions using the conference service). The SRVCC in alerting phase feature adds the ability to perform access transfer of media of an instant message session in packet-switched to circuit-switched direction in alerting phase for access transfers.

3GPP Rel-11: Introduces two new capabilities: single radio video call continuity for 3G-circuit-switched network (vSRVCC); and SRVCC from UTRAN/GERAN to E-UTRAN/HSPA (rSRVCC). The vSRVCC feature provides support of video call handover from E-UTRAN to UTRAN-circuitswitched network for service continuity when the video call is anchored in IMS and the UE is capable of transmitting/receiving on only one of those access networks at a given time. Service continuity from UTRAN/GERAN circuitswitched access to E-UTRAN/HSPA was not specified in 3GPP Rel-8/9/10. To overcome this drawback, 3GPP Rel-11 provided support of voice call continuity from UTRAN/GERAN to E-UTRAN/HSPA. To enable video call transfer from E-UTRAN to UTRAN-circuit-switched network, IMS/EPC is evolved to pass relevant information to the EPC side and S5/S11/Sv/Gx/Gxx interfaces are enhanced for video bearer-related information transfer. To support SRVCC from GERAN to E-UTRAN/HSPA, GERAN specifications are evolved to enable a mobile station and base station sub-system to support seamless service continuity when a mobile station hands over from GERAN circuit-switched access to EUTRAN/ HSPA for a voice call. To support SRVCC from UTRAN to EUTRAN/ HSPA, UTRAN specifications are evolved to enable the RNC to perform rSRVCC handover and to provide relative UE capability information to the RNC.

NTT Docomo has a presentation on SRVCC and eSRVCC which is embedded below:



Wednesday, 16 May 2012

Friday, 27 May 2011

Dual Radio Solution for Voice in LTE

I did mention in the Twitter conversations post from LTE World Summit 2011 that there are now certain analysts and players in the market who think that it should be possible to have two radios. Here is a slide from ZTE that shows that they are thinking in this direction as well.


Click on the pic to enlarge.

Tri-SIM phones have been available for quite a while but now there are Quad-Sim Shanzhai phones that are available in China. I am sure there is a market for these kind of phones.

With the battery life and the mobile technology improving, these are no longer the concerns when talking about dual radio possibility in the devices. Another common argument is that there may be additional interference due to multiple radios simultaneously receiving/transmitting. I am sure these can be managed without much problem.

Another problem mentioned is we may need multiple SIM cards but the SIM cards used is actually a UICC. There can be multiple SIM applications and IMSI's on it. The network may need some very minor modifications but they should be able to manage this with no problems. In the good old days, we used to have mobiles with built in Fax. The mobile number used to be different from the Fax number. It was a similar kind of problem but managed without problem.

So there may still be time to keep LTE simple by standardising the dual-radio solution rather than having CSFB, VoLTE, SRVCC, VoLGA, etc.

Any thoughts?

Monday, 8 November 2010

Single Radio Voice Call Continuity (SR‐VCC)

From a 3GPP presentation by Hannu Hietalahti

1. SR-VCC use case
1a. IMS call initiated in LTE can continue in CS domain after moving outside of LTE coverage area
1b. SR-VCC is invoked if no other VoIP capable PS system (e.g., HSPA/eHRPD) is available for VoIP PS-PS HO (Handovers)
1c. Only HO of a single voice bearer from PS to CS is specified
1d. Requires overlapping with 1xRTT/GSM/WCDMA coverage

2. SR-VCC allows a voice calls are anchored in IMS
2a. One-way HO from PS to CS systems (LTE to GSM/UMTS or LTE to 1xRTT)
2b. No simultaneous operation of different radio transceivers needed

3. Rel-9 SR-VCC improvements
3a. IMS support of mid call services (e.g., HOLD, MPTY)
3b. SR-VCC support for emergency calls

4. Video calls, reverse direction from CS call to IMS and optimisations are being studied in Rel-10