4 Motivation, principles and assumptions
3GPP43.130GSM/EDGE Iur-g interfaceRelease 17Stage 2TS
4.1 Motivation
As part of the work to connect GERAN via the Iu interface to the CN, the GRA and RNTI concepts have also been adopted for the GERAN. With these concepts, an inactive mobile in the RRC-GRA_PCH state will perform signalling to the network whenever it changes the GRA. Since GRAs need not be constrained by BSC coverage areas, it is possible that the MS reselects a cell on a different BSC but on the same GRA. In order maintain the ability to page the mobile in the GRA with the correct RNTI, as well as to enable a path for a GRA update procedure with the serving BSC, it is necessary to have an Iur-g interface between the BSSs.
The same principle applies for dual-RAN capable mobiles. The architecture specifications permit network operators to design networks that alleviate the potential problem caused by dual mode mobiles frequently toggling between UTRAN and GERAN coverage areas (e.g. in indoor coverage situations): for instance, common LAIs and RAIs for GERAN and UTRAN cells in the same geographical area. Defining GERAN and UTRAN cells in common LAs and RAs permits an inactive mobile to change from GERAN to UTRAN coverage or, vice versa, without any signalling to the network. Prior to Release 5, this is possible because GSM/GPRS is does not use the connection oriented Iu interface. From Release 5 on, an inactive MS would signal to the network when a cell change involving change of RAN takes place. To prevent this, operators may configure GRA IDs and URA IDs to indicate the same registration area. To enable paging across these registration areas, as well as to enable the possibility to perform GRA/URA update procedures with the serving BSC/RNC, it is necessary to have an Iur-g interface between the BSS and the RNC.
The definition of GRAs/URAs has the following benefits:
– It increases the MT call success rate: by reducing the frequency of updates, it reduces the time during which the MS is believed by the RAN to be in the old registration area, when paging requests would fail.
– It reduces the amount of signalling on the radio interface, since the mobile does not need to indicate cell changes when moving within a GRA/URA.
– For the same reason, it reduces the amount of signalling in the network, thus decreasing the number of updates and of SRNC/SBSC relocation procedures.
The Iur-g may bring other benefits:
– Support for the SRNC/SBSC relocation procedure: the current SRNS relocation procedure defined for UTRAN using the Iur interface could be reused, improving the performance of the procedure. The Cell Update and Relocation Commit messages could be transferred over this interface.
4.2 Principles
The Iur-g interface shall be designed based on the following principles:
– The Iur-g interface shall be open.
– From a logical standpoint, this interface is a point to point interface between one BSS and one BSS or RNC within a PLMN. From a physical point of view, the interface could share Iu or other transmission resources.
– The Iur-g interface is optional. The presence of the Iur-g shall be transparent to the MS: the 3GPP specifications shall ensure that all mobiles function correctly irrespective of the presence or absence of the Iur-g interface.
– This interface shall support the exchange of signalling information between a BSS and a BSC/RNC. The Iur-g interface shall not carry user information.
– A GRA contains one or more GERAN cells and zero or more UTRAN cells. A URA contains one or more UTRAN cells and zero or more GERAN cells.
NOTE: The term RAN Registration Area (RRA) is used in this document to refer to a registration area irrespective of whether it contains GERAN cells, UTRAN cells or both types of cells. The terms GRA and URA will be used in GERAN and UTRAN specifications, respectively, where GRAs may contain UTRAN cells and URAs may contain GERAN cells.
– In Iu mode, the BSC has been allocated an identifier (BSC-Id) from the same pool of numbers as the RNC-Id.