- Voice call continuity
3GPPhas defined the Voice Call Continuity (VCC) specifications in order to describe how a voice call can be persisted, as a mobile phone moves between circuit switched and packet switched radio domains (3GPP TS 23.206).
Many mobile phones are becoming available today that support both cellular and other broadband radio technologies. For example, the
NokiaN Series and E Series devices support both GSMand WiFi. Similar devices from Sony Ericsson, BlackBerry, Samsung, Motorolaand even the Apple iPhoneprovide comparable dual mode technology. WiMAXsupport is also being added and further handsets are emerging from Kyoceraand other vendors, which provide dual mode technology in CDMAphones. A wide range of Internet applications can then be accessed from mobile devices using wireless broadband technologies like WiFi and WiMAX. For example, VoIPtraffic can be carried over these alternative radio interfaces.
Whereas VoIP calls from mobile devices are controlled by IP infrastructure, according to the VCC specifications, calls to and from a cellular phone in the circuit switched domain are also anchored in an IP domain, for example the
IP Multimedia Subsystem(IMS). As the handset becomes attached and detached from wireless access points such as WiFi hotspots, a client application in the device provides notifications of the radio conditions to a VCC platform in the network. This allows circuit switched and IP call legs to be originated and terminated such that the speech path is transferred between domains, transparently to the end user.
This technology is of interest to users as an example of the benefits that are achievable through
Fixed Mobile Convergence(FMC). Since most WiFi and WiMAX access points will use fixed backhaul technologies, seamlessly moving between for example WiFi and GSM domains allows the best quality and most cost efficient radio to be used at any given point in time, irrespective of the transport technology used for the media. Similarly, service providers are interested in VCC in order to offer FMC products towards specific market segments, such as enterprise users. Cellular operators in particular can offer bundled services that consist of for example, a broadband connection with a WiFi router and a set of dual mode devices. This supports a Fixed Mobile Substitution(FMS) business case where calls from the office can be carried as VoIP over WiFi and a broadband connection, while VCC technology allows these calls to be seamlessly handed over to cellular networks as the device moves to areas of poor WiFi coverage.
One limitation of VCC however, relates to the focus on voice service. In order to preserve the cellular telephony experience while users are WiFi attached, other features need to be replicated in the packet switched domain. For example, the 3GPP has defined SMS over IP specifications (3GPP TS 23.204) in order to describe how messaging functionality can be provided to end users that are present within IP based access networks. However, over several years a range of other business logic, such as GSM Supplementary Services within the
Home Location Register(HLR) has been embedded within cellular networks. This functionality must also be realised within the IP domain in order to provide full service continuity between multiple access networks.
In the context of the Release 8 of
3GPPstandards, studies are ongoing to make VCC evolve into a wider concept that would extend VCC beyond voice services. This work started under the name MMSC (Multimedia Session Continuity) [ [http://www.3gpp.org/ftp/Specs/html-info/23893.htm TR 23.893 - Feasibility study on multimedia session continuity] ] , but the final name will be IMS Service Continuity (IMS_Cont) [ [http://www.3gpp.org/ftp/Specs/html-info/23237.htm TS 23.237 - IP Multimedia Subsystem (IMS) Service Continuity; Stage 2] ] .
* [http://www.compactpci-systems.com/columns/spec_corner/pdfs/2006,04.pdf Article by John Meredith]
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