At the 3GPP TSG RAN #19 meeting, the Work Item Description on "Network Assisted Cell Change from UTRAN to GERAN – Network Aspects" was approved.
In today's GPRS networks (without NACC), cell re-selection can causes a service interruption in the region of 4 – 8 seconds, which obviously has an impact on the user experience. Similar interruption times can be expected in mixed UMTS and GPRS networks, during UE cell re-selection from UTRAN to GERAN.
Consequences of this: e.g. TCP applications may time-out at cell change and suffer from the slow-start mechanism, streaming applications may stop at cell change due to client buffer depletion. All such problems will lead to an unacceptable user experience.
This "Network Assisted Cell Change" feature has already been introduced in the GERAN specifications and the appropriate changes have been to the RLC/MAC protocol [TS 44.060
] within Release 4. Additional enhancements were approved in Release 5 in order to exchange (Packet) System Information between BSSs, so that NACC can work across BSS boundaries.
Currently, there are procedures defined on the Gb and Gn interfaces to enable signalling of GERAN SI/PSI between BSSs. This RAN Information Management (RIM) mechanism was defined initially for the use of NACC, although in a manner that could be extended for applications other than NACC. It consists of the following messages:
RAN INFORMATION REQUEST - from Source BSS to Target BSS – requests GERAN SI/PSI.
RAN INFORMATION – from target BSS to source BSS – analogous to the Information Exchange over Iur and includes GERAN SI/PSI for one or more GERAN cells.
RAN INFORMATION ACKNOWLEDGE – from Source BSS to Target BSS.
RAN INFORMATION ERROR - to inform about e.g. message syntax errors.
In Release 5, TSG RAN approved the provision of the GERAN (P)SI messages in the CELL CHANGE ORDER FROM UTRAN message. In order for this feature to work successfully, a standardised method is required to signal relevant GERAN information across the relevant UTRAN interfaces.