Voice Call Continuity - Straightforward but Challenging
It is the IMS (IP Multimedia Subsystem), SIP and the inexorable rise in voice over IP which is finally fuelling true fixed-mobile convergence. Following the introduction of the IMS, mobile networks (GSM and CDMA) will soon be able to support voice (and video) connection over WiFi and WiMax access points. Standards are in the final stages of definition and the first dual mode devices are appearing. Once the technology has stabilised, it can be expected that, like 3G handsets, there will be an exponential increase in their availability.
Voice call continuity is a fundamental feature of mobile networks. As a mobile moves from cell to cell, a call in progress should proceed with no noticeable interruption of the connection. As networks converge, this essential attribute of the mobile must be maintained.
The Business Requirement
Voice Call Continuity (VCC) now means the process of continuing a voice (or video) call as a user moves between a WiFi (SIP based) and a mobile network can be realised. Clearly, if handover is implemented, it is critical that it occurs quickly, seamlessly and reliably. There are a number of business and technical reasons why such handover would be required:
| a) | Mobile VoIP service may offer improved reception and fewer dropped calls via WiFi. Particularly in cities, cellular bandwidth and channel capacity will always be at a premium and in many cased WiFi may offer both a cheaper and a more reliable access method. |
| b) | From the user’s perspective, there may be a financial incentive to use WiFi, for example at home or in the office. As part of the drive towards the single phone concept with convergence, it is generally accepted that the use of one’s mobile handset needs to be to be charged differentially when using WiFi. Such differential charging may extend to a subscription basis when at home or office, but be charged per call when on the move. |
Standards are being defined for VCC. First, if transfer between GSM and WiFi is likely, the call is moved into the SIP domain if it is not already there. Then, the handset, identifying that transfer is possible and desirable, sets up a parallel call, which the SIP application can then manipulate to achieve the transfer.
So far, so good - in the general scheme of signalling associated with location updating, call set-up, roaming, etc, the VCC elements of procedure are relatively straightforward. There are, of course, issues associated with factors such as jitter (moving in and out of WiFi coverage) and speed of reaction associated with rapid loss of coverage, but we can confidently predict that these will be resolved as theory is put into practice.
Practical Voice Call Continuity
Call handover is the current focus of standards activity, however VCC does not exist in isolation and there are significant issues in its deployment. A practical implementation of VCC needs to consider the other factors such as scalability, adverse interaction with other services, and intelligent rating & charging.
Scalability - Intelligent Routing Needed
A call that trombones in and out of the IMS overlay uses unnecessary resources. It is important, therefore, that only calls that are likely to transfer are handled in the IMS. This requires intelligence and additional information, such as location and customer preferences, not just clever signalling.
Service Interaction
A capability that is only available to some subscribers will never realise its full potential. VCC, therefore, must not preclude the use of other services. A whole host of value added services are deployed by operators using IN technology and new ones will emerge as IMS/SIP takes hold. It is a classic problem of IN that many services are incompatible due to trigger conflicts in the switches; a similar challenge will be faced with SIP signalling. A VCC application will use both IN and SIP signalling at the same trigger points as other services. Service interaction management will be needed to resolve these conflicts and enable the services to inter-work.
Intelligent Charging and Rating
A prime reason for a user to subscriber to VCC is commercial – cost savings. A practical deployment of VCC must, therefore, properly handle the associated differential rating and charging. Take the case of a call starting on GSM, moving to WiFi and moving back to GSM.
Call supervision is needed to create the necessary charging records and, crucially, inter-work with real-time charging engines. Moreover, both the offline and real-time charging systems must be able to handle the events to charge appropriately.
Conclusions
Convergence is fast becoming a reality and the IMS is fundamental to it. First steps, such as British Telecom's Fusion have already been deployed. Voice Call Continuity, the seamless handover between wireless and wireline, is seen as an important feature in its commercial success. The current focus of VCC is on the signalling technology and associated standards. Whilst it is important to get this right, the real challenge lies in the commercial deployment of the capability. If it is to be seamless it must not affect other services, so all the interactions with other services, whether intelligent network based or SIP based, need to be handled. Furthermore if the VCC capability is to be of any use to anyone, charging must be properly handled both for pre- and post- paid subscribers. The approach to VCC must address all these challenges, not just the signalling.
For more information, click on the links below to download our Voice call continuity - IP Voice Call Control brochure. You must be registered to download.
Click to download Voice call continuity - IP Voice Call Control brochure
