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IMS Service Mobility -- Beyond Voice Call Continuity
[August 17, 2006]

IMS Service Mobility -- Beyond Voice Call Continuity

TMCnet Spotlight on FMC Columnist
The ability to seamlessly move an active voice session between the IP Multimedia Subsystem (News - Alert) (IMS) domain and the circuit-switched (CS) cellular network is currently being defined by 3GPP as the Voice Call Continuity (VCC) effort. There is a great deal of interest behind VCC from both the operator and vendor communities as the ability to provide session continuity across heterogeneous access domains is seen as a critical step towards fixed-mobile convergence (FMC).


During 2005, various approaches for enabling VCC were proposed to 3GPP, with two of the leading architectures being Original Domain Controlled (ODC) and IMS-Controlled with Static Anchoring (ICSA). It was determined that both options had pros and cons, and finding a way to merge the two options was not possible due to fundamental architectural differences. As a result, 3GPP decided to focus on a single architecture, and the ICSA approach was selected as the basis for future VCC standardization efforts.
The principle behind the ICSA approach is that all voice calls are controlled by the Call Continuity Control Function (CCCF) element residing within the IMS domain. This means that calls originating from the CS cellular domain that are traditionally controlled by the Mobile Switching Center (MSC) will now be controlled by the IMS domain. The MSC is required to route all calls originated in the CS cellular domain to the CCCF element in IMS.
To support domain transfer between IMS and CS cellular networks for an active voice call, the mobile device is required to initiate a second call towards the CCCF to trigger and execute the mobility event.
The ability for VCC to support domain transfer between IMS and CS cellular networks for active voice sessions is an important step towards fixed-mobile convergence (FMC). However, as the market evolves and real-world performance and implementation issues become clear, additional requirements surrounding IMS service mobility must be met and addressed.
Not Just Voice, But Also Multimedia Session Mobility
The first letter in VCC stands for “voice,” and as such its sole objective is to specify a method to enable domain transfer of voice sessions between IMS and CS cellular domains. IMS service mobility for data and video/multimedia sessions is therefore beyond the scope of VCC.
However, with the advent of IPTV (News - Alert), MobileTV, triple-play services, etc., combined with the ever-increasing popularity of online activities such as interactive gaming, there is a clear need to mobilize real-time multimedia sessions. The full potential of IMS will not be unleashed until the ability to provide seamless continuity of multi-media sessions across heterogeneous access domains is achieved.
Therefore, operators evaluating IMS service mobility solutions must anticipate requirements associated with data and multimedia mobility, and ensure that their chosen solution is extensible and future-proofed to support more than just voice mobility. Standardized methods for enabling multimedia session continuity must be provided and implemented by the vendor community. Issues such as how to move an encrypted IPSec session carrying multimedia traffic flows across different network domains must be addressed.
Ensure Transparent Support for Supplementary Services
End-users are accustomed to using supplementary services such as call forwarding, call hold, call waiting, etc., and they expect to use those same services no matter which network domain they are attached to. Simply offering the ability to make and receive voice calls and be able to perform domain transfer of those voice calls is not enough to meet end-user requirements.
Therefore, a key consideration in launching a successful FMC service offering is the ability to ensure transparent support for supplementary services across different network access domains.
However, there is no clear approach or consensus for supporting supplementary services in VCC as it is defined today. Many outstanding issues still remain such as whether “selective anchoring” is a good thing, or how mid-call supplementary services should be handled during domain transfer.
Although 3GPP has identified these issues as items requiring further work, a timely resolution to these issues is not expected for inclusion in the 3GPP Release 7 timeframe. VCC may have support for supplementary services in 3GPP Release 8, which is still some time away. Therefore, operators looking to deploy FMC services that include domain transfer and supplementary support will likely need to partner with solution vendors that are tackling these issues today.
Carrier Control over Mobility Events
The mobility approach as currently defined in VCC requires the mobile device to be the decision maker of mobility events. The mobile device continually monitors received signal strengths from both IMS and CS cellular network domains. When the received signal’s strengths reach or exceed pre-configured thresholds on the mobile device that favors a particular access network, the mobile device makes the decision to initiate a mobility event towards that access network.
This approach is sensible and may prove to be sufficient at least for smaller scale deployments. However, as the number of user sessions, mobility events, and bandwidth consumption increases, the impact on operator network resource consumption must be considered. Therefore, the network—and, ultimately, the operator—needs to have control over domain transfer events instead of the mobile device in order to take into consideration any impact on network resource utilization.
As such, in addition to received signal strengths, operators need to take into account network resource related issues such as load-balancing and available bandwidth or QoS threshold for triggering a mobility event. The mobile device has no visibility into the overall network characteristics other than received signal strengths, and therefore cannot appropriately make handoff decisions based on network resource related attributes.
The ideal IMS service mobility solution should therefore give operators the ability to establish network resource related handoff policies in addition to signal strength based triggers. Furthermore, the solution should be able to provide the capability to dynamically enforce the operator-established handoff policies based on prevailing network resource conditions. The enforcement of handover policies should be implemented at the network as opposed to the mobile device.
IP Bearer-Awareness in Supporting Mobility Events
Today’s VCC definition focuses on the signaling aspects of domain transfer. While defining the signaling call flows to enable seamless domain transfer is absolutely necessary, a perhaps overlooked aspect of domain transfer has to do with bearer path anchoring and handling during mobility events.
More importantly, as discussed previously, network resource related attributes need to be taken into consideration when making mobility event decisions. Simply looking at received signal strengths from different access domains is not sufficient.
Consider the scenario where an active session is accessing the IMS domain via Wi-Fi across a DSL line. Suppose the DSL line becomes corrupt but the signal strength received from the Wi-Fi access point remains at 100 percent, full strength. Using today’s VCC method for making domain transfer decisions, the mobile device will decide to remain on the Wi-Fi access domain based on the 100 percent signal strength indicator, even though the DSL link has become inactive!
As a result, due to the lack of IP bearer awareness, the mobile device will make the wrong domain transfer decision and remain connected to a faulty network domain.
To address such scenarios, the IMS service mobility solution needs to have visibility into the IP bearer and be aware of those attributes that serve as domain transfer triggers. The only way to have such IP bearer awareness is through close coupling of bearer and mobility signaling capabilities. Operators looking to maximize end-user satisfaction for seamless domain transfer needs to seek out a complete solution that provides such close synchronization between bearer handling and signaling functions.
IMS service mobility has been taking shape by way of the VCC approach. It represents a good first step towards introducing a standardized approach for supporting domain transfer between IMS and CS cellular network domains.
However, there is more to IMS service mobility than what VCC can provide. Operators must consider how they will deliver services beyond voice, support supplementary services, and manage mobility events in a scalable fashion to fully realize the promise and potential of IMS.
More importantly, given that complete VCC standardization that includes supplementary service support will not be available until the 3GPP Release 8 timeframe, operators need to consider the time-to-market impact of waiting to deploy FMC services. 
First movers in FMC will have a significant advantage, and operators that harness infrastructure solutions complying with standards—while offering innovative approaches to the requirements beyond the VCC effort—are most likely to garner the majority of FMC customers and revenue. 
Allan Baw is Director of Product Management in the Mobility Solutions department at Stoke, a company dedicated to accelerating the adoption of IP-based communications by solving the service delivery challenges associated with network convergence and mobility.

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