Voice call continuity requires maintaining a voice call when a mobile terminal moves from one cell to another for second generation Global System for Mobile Communications (GSM) digital cellular communications systems. Operational for many years, this technique enables a conversation to continue when the Circuit-Switched (CS) call reroutes to use a new basestation as the mobile moves from one coverage area to another. The parties will perceive no break whatsoever.
Today, the scenario is rather more complicated, with calls being handed over not only from 2G to 2G cells and from 3G to 3G cells, but also between 2G GSM and 3G Universal Mobile Telecommunications System (UMTS) cells. This is relatively easy from an administrative point of view, given that generally the same cellular network is involved throughout.
Earlier work carried out within the 3rd Generation Partnership Project (3GPP) envisaged telephony using packet-switched connections – Voice over Internet Protocol (VoIP) – using either the 3GPP-defined IP Multimedia Subsystem (IMS) on the 3G Universal Terrestrial Access Network (UTRAN), or Wireless Local Area Network (WLAN) radio access technology based on IEEE 802.11, and other standards. This was covered by the WLAN interworking work items.
However, until now, handover between CS and IMS (packet-switched) calls was not addressed. 3GPP is now investigating the problem of handing over a voice (or potentially video or other multimedia conversational service) call between the cellular network and a WLAN, possibly operated by a completely different service provider. Again, for conversational service, the hand-over has to be seamless, with no break in service perceived by either party to the call. Until recently, such handover had only been considered for services that are not real-time, such as file-transfer, where short breaks during the handover process are acceptable and probably go unnoticed by the user.
The approach taken by 3GPP is to have the WLAN operator use the information registered by the home operator for the mobile terminal subscriber in this sequence:
1. Validate the eligibility of the handover to happen at all 2. Manage charging for the call that is effectively transferred from one network operator to another
It is generally, though not necessarily, the case that WLAN hotspots are also well covered by cellular service. Thus, such handover may take place when cellular coverage is reduced to an unacceptable level, yet an adequate WLAN hotspot service is available. The handover is more likely to occur when spare bandwidth exists on the WLAN but where excess demand for cellular channels exists.
The goal is to maintain the conversational service call, thus optimizing the service to the users, which in turn will maximize the revenue accruing to the operator(s). 3GPP embarked on the technical activity required to enable this service by approving a work item on Voice Call Continuity (VCC) in the June 2005 meeting of its Technical Specification Group System Aspects and Architecture (TSG SA). In order to be accepted onto the 3GPP work plan, any work item needs to have the support of at least four supporting member companies, and no sustained opposition. The VCC work item has no fewer than 16 supporters, and its progress can be tracked on the 3GPP website, www.3gpp.org. It is intended that this work be achieved in the Release 7 time frame.
3GPP TR 23.806: Voice Call Continuity between CS and IMS Study (Release 7) 3GPP TS 23.206: Voice Call Continuity (VCC) between Circuit Switched (CS) and IP Multimedia Subsystem (IMS); Stage 2 (Release 7) 3GPP TS 24.206: Voice Call Continuity between the Circuit-Switched (CS) domain and the IP Multimedia Core Network (CN) (IMS) subsystem; Stage 3 (Release 7) 3GPP TS 24.216: Communication Continuity Management Object (MO) (Release 7)
IMS 2.0 world forum is a must attend event .. I learnt a lot from it. Here is a brief synopsis of where I see IMS is heading to .. Seek your thoughts and feedback especially you can identify other Operators with an interesting strategy and / or if you attended this event
As I could gather, there are six broad strategies: a) Voice call continuity(VCC) / fixed to mobile convergence b) Blended voice : voice tied contextually to messaging or rich media c) SIP without IMS (Naked SIP) d) Strategies from device manufacturers(especially Nokia and Motorola) e) Real time IMS applications (multiplayer games and other such applications that need near real time blended media interaction within a session) f) Abstraction of the core network
Most of the focus is around (a) Voice call continuity(VCC) / fixed to mobile convergence
This is a pity – but also understandable Operators are most familiar with voice In its broadest sense, voice call continuity pertains to roaming within cellular and non cellular networks(such as roaming between cellular and wifi networks). A specific instance of this is Fixed to mobile convergence for instance BT fusion My personal view is:
a) I don’t quite know if I would be interested in FMC as a customer .. b) I think its being sold on cost – which is not a good idea c) I think it fulfils an industry goal(fixed and mobile networks trying to get new subscribers from each other’s networks in mature markets) d) In general, voice is becoming cheap .. so I am not sure that a pure voice play is a good idea
Blended voice(b) and real time applications(e) are interesting but need device support. Devices supporting IMS fully are conspicuous by their absence!
In contrast, devices supporting SIP(c) - but not IMS are very much here and so are applications – for instance movial
Abstracting the network layer through software APIs(f) – is the most interesting – but I felt very few Operators had the vision to embrace this strategy at the moment. The two big exceptions being TIM and Telia sonera - who are doing some very interesting work.
To recap, by abstracting the network layer, I mean : In an IP world, as the Mobile Internet mirrors the Internet, the Operator should focus on the core of the network and leave the edge of the network to third parties. Specifically, this means – identify the elements that can be performed ONLY in the core and then abstract them through APIs. This approach gets us away from the dichotomy of the ‘pipe’ vs. ‘no pipe’. It also means that the Operator retains control.
Finally, Operators in emerging markets like Globe telecom from Philippines were also impressive i.e. they understood the space, the issues specific to their market and how they could leverage IMS in their markets. Harvey G Libarnes, Head of innovation and incubations program , Globe Telecom, gave a very thorogh presentation
To conclude: a) At Operator level, IMS is still largely about voice and a defensive approach(such as FMC) b) Lack of devices is the key question mark c) Device manufacturers on the other hand have significant leverage(more on that soon) d) Some operators are going to be very innovative – TIM and Teliasonera from amongst the attendees