VCC is most often contrasted with UMA. Both promise seamless voice mobility, but UMA has some specific features that set it apart, such as the UNC (UMA Network Controller), which replicates the functionality of a BSC (Base Station Controller) in a GSM network. There are about half a dozen dual-mode handsets that support UMA, which has given a head start to carriers such as T-Mobile in the United States and BT in the United Kingdom.
VCC originated within the 3GPP community and is targeted for inclusion in Release 7. The main driver was developing a standards-based FMC plan built around IMS that maintained session continuity across diverse access technologies--wireless or wireline. Two architectures were proposed, but ICSA (IMS-Controlled with Static Anchoring) won out. As the diagram below shows, a dual-mode phone can communicate over either wireless network to a new core element, the CCCF (Call Continuity Control Function), which resides inside the IMS domain. This CCCF acts as the anchoring point between the circuit-switched cellular and packet-data networks and directs incoming calls over the appropriate link.
UMA started with the UMA Technology Group and was eventually folded into 3GPP, but VCC has its own champion with an industry body called MobileIgnite, led by BridgePort Networks. BridgePort Networks offers a product that fulfills the role of the CCCF. Last September MobileIgnite issued a functional specification for VCC that describes minimum expectations in terms of features and functionality. Although VCC is several months away from completion, vendors can build products and perform basic interoperability testing that puts them in a good position when the standard is approved.
VCC has several drawbacks in comparison to UMA as well on its own. From the carrier's perspective, it lags 18 months behind UMA and is still incomplete. While UMA is enjoying more than a dozen trials and deployments, VCC is just getting off the ground in terms of trials and handset availability. MobileIgnite claims 15 handsets that meet their functional specifications--which is different than VCC-compliancy. The existing dual-mode handsets on the market, whether UMA or VCC, are also handicapped with a short battery life.
