5 General aspects
3GPP43.130GSM/EDGE Iur-g interfaceRelease 17Stage 2TS
5.1 Network architecture
5.1.1 General
The Iur-g interface is the logical interface between two BSSs in Iu mode or between an RNS and a BSS in Iu mode or between a BSS in A/Gb mode and a UTRA-NTDD RNS. The different Iur-g interface combinations are depicted in Figure 1 and Figure 1a.
Figure 1: Reference architecture for the Iur-g interface (BSS in Iu mode).
Figure 1a: Reference architecture for the Iur-g interface (BSS in A/Gb mode)
5.1.2 MS Identifiers in Iu mode
The Radio Network Temporary Identities (RNTI) are used as MS/UE identifiers within the GERAN/UTRAN and in signalling messages between MS/UE and GERAN. Four types of RNTI exist:
1) Serving BSS/RNC RNTI (S-RNTI);
2) Drift BSS/RNC RNTI (D-RNTI);
3) Cell RNTI (C-RNTI);
4) UTRAN RNTI (U-RNTI) or GERAN RNTI (G-RNTI);
The S-RNTI is used:
– by the MS/UE to identify itself to the Serving BSS/RNC;
– by the SBSS/SRNC to address the MS/UE; and
– by the DBSS/DRNC to identify the MS/UE to serving BSS/RNC.
The S-RNTI is allocated for all MSs/UEs having an RRC connection. It is allocated by the serving BSS/RNC and it is unique within the serving BSS/RNC. The S-RNTI is reallocated always when the serving BSS/RNC for the RRC connection is changed.
The D-RNTI is used by the serving BSS/RNC to identify the MS/UE to the drift BSS/RNC.
NOTE: The D-RNTI is never used on the Um/Uu interface.
The D-RNTI is allocated by the drift BSS/RNC upon drift MS/UE contexts establishment and it shall be unique within the drift BSS/RNC. Serving BSS/RNC shall know the mapping between the S-RNTI and the D-RNTIs allocated in the drift BSS for the same MS/UE. The drift BSS/RNC shall know the S-RNTI and SBSS-ID/RNC-ID related to the existing D-RNTI within the drift BSS/RNC.
The C-RNTI is used this identifier is used only in UTRAN:
– in case of the Iur-g interface between UTRAN and GERAN (specifically the case of a SBSS and DRNC);
– by the UE to identify itself to the controlling RNC; and
– by the controlling RNC to address the UE.
The C-RNTI is allocated by the controlling RNC upon UE accessing a new cell. The C-RNTI shall be unique within the accessed cell. The controlling RNC shall know the D-RNTI associated to the C-RNTI within the same logical RNC (if any).
The G-RNTI/U-RNTI is allocated to an MS/UE having an RRC connection and it identifies the MS/UE within GERAN/UTRAN. The G-RNTI/U-RNTI is composed of:
– SBSS/SRNC identity; and
– S-RNTI.
Each BSS/RNC has a unique identifier within the GERAN/UTRAN part of the PLMN, denoted by BSS/RNC identifier (BSS-ID/RNC-ID). This identifier is used to route GERAN/UTRAN interface messages to the correct BSS/RNC. The BSS-ID/RNC-ID of the serving BSS/RNC together with the S-RNTI is a unique identifier of the MS/UE in the GERAN/UTRAN part of the PLMN.
From this architecture, there are three possible scenarios for a particular MS/UE:
a) Both the serving and the controlling RAN nodes are BSCs.
b) The serving RAN node is a BSC and the controlling RAN node is an RNC.
c) The serving RAN node is an RNC and the controlling RAN node is a BSC.
These scenarios are shown in Figure 2.
Figure 2: Scenarios using the Iur-g interface (BSS in Iu mode)
5.1.3 MS Identifiers in A/Gb mode
The Radio Network Temporary Identity (RNTI) is used as a MS/UE identifier within the GERAN/UTRAN. One type of RNTI exists:
– Drift RNTI (D-RNTI)
The D-RNTI is allocated by the TBSS and is used to identify the MS/UE during a handover via the Iur-g interface.
There is one possible scenario in which the D- RNTI is used:
a) The serving RAN node is a UTRA-NTDD RNC and the target RAN node is a BSS
Figure 2a: Scenario using the Iur-g interface (BSS in A/Gb mode)
5.2 Iur-g interface capabilities
The Iur-g interface provides capability to support radio interface mobility between BSSs or between a BSS and an RNS of UEs having a connection with the GERAN or the UTRAN. This capability includes the support of paging, cell update, registration area update and handover between BSSs or between a BSS and an RNS. Further, Iur-g interface provides capability to support information exchange between two BSSs or between a BSS and RNS.