IMS Centralized Services (ICS)

Interesting to see which 3GPP Release 8 work items around the IP Multimedia Subsystem (IMS) are bearing fruit these days. While in Release 7, the hot application specifications for IMS services were Multi Media Telephony (MMTel) and device centric Voice Call Continuity (VCC), work in Release 8 seems to have to have shifted towards IMS Centralized Services (ICS) and a network centric Voice Call Continuity.

As the name implies, ICS offers centralized services, which means an IMS Application Server for ICS manages sessions for a user in several respects:

Multiple Devices – Several devices can be associated to the same user account and can be active simultaneously. Incoming media sessions can then be forwarded to one, several or all of these devices. Devices can be switched during ongoing sessions.

Including GSM mobiles – ICS has been designed not only to work with IP/IMS devices but also with "legacy" GSM  phones without any special software on board. When mobiles are switched on the MSC server communicates with the ICS Application Server (the Service Centralization and Continuity Application Server SCC AS) on behalf of the mobile. This is done either directly, in case the MSC server has been enhanced to act as an IMS client for the mobile, or via an Intelligent Network (IN) node that communicates with the MSC Server via CAP (CAMEL Application Part). The later one is probably preferred by many suppliers since IN nodes and CAP are used today for many applications such as prepaid billing. This has the advantage that the MSC software does not have to be extended for ICS.

Managing Supplementary Services – The ICS offers a standardized way of implementing services such as call forwarding, barring, hold, resume, 3-way calling.

Combination of Different Types of Media– ICS is not limited to voice telephony. Video calling, picture sharing and other media streams can be added or removed from a session at any time. 

Handover to 2G – And finally, the 3GPP ICS working group has also thought about how to hand over the voice portion of a session to a circuit switched bearer when the mobile reaches the limit of the broadband wireless networks. Currently, handovers from LTE are supported to GSM and 1xCDMA (hello Verizon!). Unlike the previous Voice Call Continuity (VCC) specification which requires the handover decision to be made by the mobile, the ICS handover to a circuit switched channel is network initiated and controlled. The advantage of network control is that the device does not have to be attached to two different radio networks simultaneously. This is important since like GSM/UMTS mobiles today, future LTE mobiles will also not be able to connect to two cellular network technologies simultaneously. From my point of view this network centric Single Radio Voice Call Continuity (SR VCC) approach is a major step towards solving the LTE voice gap. I'll describe the details in a future post.

For further information, here are links to the three main ICS specifications:

• 3GPP TS 23.292 – IMS Centralized Services
• 3GPP TS 23.237 – IMS Service Continuity
• 3GPP TS 23.216 – Single Radio Voice Call Continuity

And for some more general background reading, here are some more resources:

• Article in the Ericsson Review 2/2008 on Multi-access for IMS networks, which seems to be a pre-standard ICS.
• An article by Hao Zenwhu of ZTE on the history of IMS service specifications.