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An Introduction to Voice Call Continuity (VCC)An Introduction to Voice Call Continuity (VCC) Clinton M. Banner May 2nd, 2007 Abstract The IP Multimedia System, or IMS, defines a sophisticated 3G  service delivery infrastructure that is access independent – i.e. the service provider can deliver consistent blended services across multiple User Equipment (UE) types which access the network using different technologies. promise of a “seamless” user experience. Major telecommunications vendors are positioning VCC as an important part of their IMS value proposition1. This position is also being touted by key industry analysts2,3. VCC specifications define an interworking architecture and methods which allow the UE to move between different access technologies while maintaining a consistent user experience. VCC functionality includes support for automatic network selection by the UE, and how to perform an in-call handoff (HO) between access technologies. This paper provides an overview of VCC, and examines the impact of VCC compliance at both the network and the UE. Single & multiple access mode devices Single access mode devices provide specific UE capabilities through only one type of network. Each access technology has unique air interface characteristics and a dedicated network infrastructure to register and validate users and provide services including telephony and messaging. Within each access technologies there is already a mobility aspect; for example: • CDMA devices access and register in both home and visited CDMA networks, and can initiate and maintain active call sessions when the UE moves between cell sites using handoff capabilities • GSM devices similarly access and register in both home and visited GSM networks, and also maintain active call sessions using handoff capabilities• WiFi (News - Alert) based devices establish IP connectivity with Access Points, and then register and support packet based communications. Some WiFi Based devices support AP-AP roaming, while others do not. In these examples, each device works only in one type of network. A single mode CDMA device simply cannot communicate in a GSM network. The Radio Access Network (RAN) characteristics and protocols used for CDMA and GSM devices are not compatible. There devices which support multiple access technologies that are currently in widespread used. However, most of these devices do not provide a seamless service experience across different access technologies. For example, there are Personal Digital Assistants (PDAs) on the market now that support: • Circuit based telephony support (CDMA or GSM) • 3G data capabilities such as DOrA or HSDPA packet access.• WiFi (802.11 a/b/g) support The PDA supports multiple access modes, but many key PDA services work only in one of the networks. For example: • Voice calls are only supported in the circuit mode. If the user roamed out of the CDMA or GSM coverage area while on a call, the call would drop. The user can initiate a voice call only when the device is registered in a CDMA or GSM coverage area. • Short Message Service (SMS) messages can only be sent or received when the device is in a CDMA or GSM coverage area. • 3G data and WiFi data connections provide similar, but not seamless data services. If the user is streaming a video over the WiFi connection and then loses WiFi connectivity, the streaming video will most likely be interrupted or lost completely. IP Multimedia System (IMS) The IMS defines a packet based network infrastructure that supports the same applications and services that consumers have traditionally obtained from circuit based telephony providers. The IMS network also supports the rapid development and deployment of sophisticated data centric, location based, and converged 3G services. The IMS is defined for GSM by 3GPP4 and for CDMA by 3GPP2. The 3GPP and 3GPP2 standards have some functional differences as well as differences in terminology. For the sake of simplicity, this paper will use 3GPP (GSM) terms and concepts.5,6,7 Since these standards are still evolving, a number of industry groups are also defining best practices for VCC. 8 An IMS capable device, using packet based access technologies such as WiFi or WiMax, can provide the user with telephony and messaging support that is functionally equivalent with the services they receive through a packet switched network. However, the IMS packet-based capability still has the “single mode” problem we see above. If the device is an IMS UE using WiFi access, and the user is on a call and then leaves the WiFi coverage area, the call will drop. Seamless Convergence with VCC Voice Call Continuity adds a new level of convergence for multiple access mode devices. Let’s assume we have a multi-mode device that is VCC capable. To keep it simple, we’ll define it as a device with the following two modes: • CDMA Mode: The device has the radio and client functionality to access and register on a CDMA network. Once on the CDMA network, this device can support basic and supplementary telephony features as well as SMS. • WiFi IMS Mode: The device has the radio and client functionality to access and register on a WiFi network. Once on the WiFi network, this device can support basic and supplementary telephony features as well as SMS by registering with and using the IMS network. VCC specifications require: • the UE detect, select, and register on the appropriate access network. This network selection decision is based on operator policy and UE design; for example the operator may desire that the UE prefer WiFi access over CDMA (Packet) access when both are available. • a network element is responsible for determining which access network a given subscriber is currently using in order to properly route all incoming calls to them. This includes a definition of how and where this decision is made, as well as the criteria for the decision. This network element must be involved in the routing decisions for all incoming calls so that the call can be sent to the correct network. • Support for calls to the UE from: o Other WiFi subscribers o Other circuit switched subscribers ? CDMA ? GSM/UMTS (News - Alert) ? Public Switched Telephone Networks (PSTN) • Voice Call Continuity support for all calls when the UE moves between the CDMA (Packet) network and the WiFi network. o Call initiated with UE in CDMA. User roams out of CDMA coverage area and WiFi coverage is available. The call is seamlessly handed over from the CDMA to WiFi domain. The call participants do not take any action and should not notice that the HO occurred. o Call initiated with UE in WiFi. User roams out of WiFi coverage area and CDMA coverage is available. The call is seamlessly handed over from the WiFi to CDMA domain. Again, the call participants do not take any action and should not even notice that the HO occurred. o Handback scenarios: ? Call previously handed off to CDMA is handed back to WiFi. ? Call previously handed off to WiFi is handed back to CDMA. With VCC properly implemented, the user can now: • Be reached by any caller using one Directory Number (DN) independent of which access network the UE is currently using. The calling party dials a single number; the VCC compliant network determines how to complete the call using the correct access network • Experience expanded service coverage as long as one of the access networks is present. The user does not have to do anything to make this happen; the UE will automatically detect and access the correct network. • Use their device (UE) for telephony and messaging without having to pay attention to the current access network. The device provides a consistent interface that supports voice and data services in the same way across both the CDMA and WiFi networks. VCC: Network Perspective VCC requires the following functionality from the IMS Network: • A new network element - the Voice Call Continuity Application Server (VCC-AS) - has been defined. • The VCC-AS must be aware of all in-progress calls to a managed device. This information is needed so the VCC-AS can control and execute the functions required to handoff calls between access networks. Therefore, all mobile originated and mobile terminated calls must be routed through the VCC-AS. o This is a simple problem when the device is in the WiFi (IMS) network. The IMS already defines “filters” that are applied at the S-CSCF which can perform this function. o For MO/MT calls when the device is in the CDMA domain, the serving MSC must have a mechanism to route the calls to the VCC-AS. This can be provided by provisioning Intelligent Networking (IN) triggers at the MSC. • The Home Subscriber Service (HSS,) which is the subscriber profile repository for the IMS network must now persist information defining what network a VCC subscriber can be reached through. o Whenever the UE registers on a network, the information showing which network they are on must be propagated to the HSS. o The HSS can then tell an S-CSCF (IMS Proxy) which network to use to reach a given subscriber • All incoming calls should be anchored through the IMS network o The S-CSCF can determine which network to use to complete a call to any subscriber o If the subscriber is in the CDMA domain call processing will be passed to the CDMA network. As mentioned above, the CDMA network must still route the call through the VCC-AS • The VCC-AS is notified by the UE when a HO is required. The VCC-AS is then responsible for setting up a new call leg on the access technology that the call will be switched over to. The model used is make-before-break. The VCC-AS will ensure that the new path is in place before the call audio is switched over to the new path. This minimizes audio path interruption and makes the HO as seamless as possible. VCC: UE (device) Perspective The UE plays a critical role in providing VCC. The device must be enhanced to support the following functionality: • Enforce network preference algorithms: The UE must understand and support the set of rules or “modes” that define which specific network types will be used by the UE. The operator may provide requirements specifying which modes must be supported. o CDMA only mode: The device is restricted to the CDMA network o WiFi only mode: The device is restricted to the WiFi network o CDMA preferred mode: When both CDMA and WiFi networks are available, the UE will use CDMA. WiFi access is only used when CDMA is not available. o WiFi preferred mode: When both CDMA and WiFi networks are available, the UE will use WiFi. CDMA access is only used when CDMA is not available. o Manual Mode: The user manually indicates a specific network type. • Handoff triggering: The UE must recognize the conditions to initiate a call handoff between access technologies. The UE must detect and respond to handoff triggers such as: o Loss of connectivity on the current domain: For example, the Access Point (News - Alert) (AP) the UE is connected to suddenly goes down. The UE should detect this and initiate HO to the circuit path (CDMA) immediately. o Loss of RF Signal: the UE detects that the signal on the current network is deteriorating. Specific thresholds and conditions are defined that will trigger the HO. o Decrease in voice quality: The UE may monitor link and audio quality (independent of RF signal) and use this information to trigger an HO. o Manual request: The user may explicitly request the call be handed over to the other network type. o Need for specialized services: If E911 service is not available in WiFi, and the user dials 911 in the WiFi mode, the UE may change modes in order to complete the call.9 • There are also UE requirements to support intra-network HO. In an enterprise environment, the UE may be required to maintain call connectivity during an HO between different APs. This type of HO does not involve the VCC-AS, but does require sophisticated interactions between the access points and the UE. Adding VCC support to a UE will impact the size, price, and performance of a device. This will have a profound impact on the desirability and target consumer audience for such devices. • A VCC compliant device requires radio transmitter/receiver hardware for each access technology. This impacts device size (UE components and antennas) and price when compared to a single mode UE. • Battery Life: The need to have multiple radio systems operating during a handoff, coupled with the need for the UE to check for the presence of the other network type in both in-call and out-of-call situations, places a great strain on the UE battery. Add the higher energy consumption seen initially for the first generation compact WiFi devices, and you have significantly decreased the nominal battery life of the UE. This can also have a negative impact on the desirability of the device. Conclusion The examples above focused on a dual mode device using CDMA and WiFi. This same technology could be used for other multi-mode devices10: • GSM and WiFi • CDMA and WiMax • CDMA, GSM, and WiMax VCC can support all of these and more. Adding VCC support to the network and to the UE is a key step in the evolution of 3G and beyond networks. VCC provides a tangible and understandable value to the user, and may help drive the larger scale deployment of IMS infrastructures. VCC may very well become a service that is ubiquitous in future networks. References [1] Nortel (News - Alert) Inc. Position Paper, “Why Nortel for Fixed Mobile Convergence?” http://www.nortel.com/solutions/cablemso/collateral/nn115820.pdf [2] MobileIN.com Research, “Voice Call Continuity and Multimedia Independent Handover Report 2006-2011: Enabling true FMC services.” http://www.mobilein.com/reports/VG/VCC.php [3] TMCNet, “IMS Service Mobility -- Beyond Voice Call Continuity” http://www.tmcnet.com/news/it/2006/06/02/1667856.htm [4] 3GPP, Specification Series, 23-Series, http://www.3gpp.org/ftp/Specs/html-info/23-series.htm [5] 3GPP, Work Item Description: Feature: “Voice call continuity between CS and IMS (incl. I-WLAN)” http://www.3gpp.org/specs/WorkItem-info/WI--32091.htm [6] 3GPP, Specification detail, 3GPP TS 23.206 , Voice Call Continuity (VCC) between Circuit Switched (CS) and IP Multimedia Subsystem (IMS); Stage 2” http://www.3gpp.org/ftp/Specs/html-info/23206.htm [7] S. Hayes, 3GPP Conference proceedings, TSG-SA, “3GPP and Fixed Mobile Convergence” http://www.3gpp.org/ftp/PCG/PCG_16/PCG%20Beijing%20Workshop%20Presentations/3GPP %20Beijing%20workshop%20Hayes%20TSG%20SA%20Chairman.ppt [8] MobileIGNITE MoU Industry Group, “Voice Call Handover Service: Functional Specification”, Version 1.0, September 21, 2006, http://www.mobileignite.org/documents/MobileIGNITE_Handover_FS_v1.0.doc [9] 3GPP, Specification detail, 3GPP TR 23.826, “Feasibility study on Voice Call Continuity (VCC) support for emergency calls” http://www.3gpp.org/ftp/Specs/html-info/23826.htm [10] J. Meredith, CompactPCI and AdvancedTCA (News - Alert) Systems, April 2006, “Voice call continuity in 3GPP” http://www.compactpci-systems.com/columns/spec_corner/pdfs/2006,04.pdf |