scope:

The present document contains a detailed description of the handover procedures to be used in PLMNs. The purpose of the handover procedures, as described in the present document, are to ensure that the connection to the Mobile Station (MS) or User Equipment (UE) is maintained as it moves from one cell or radio network to another. The document defines the circuit switched handover functionality based on the service requirements in 3GPP TS 22.129 [9]. For the circuit switched handover functionality related to SRVCC, it is based on the service requirements in 3GPP TS 23.216 [26].

The present document considers the following five handover cases:

i) handover between Base Station Subsystems (BSS) connected to the same MSC, this is termed an Intra-MSC handover;

ii) handover between Radio Network Subsystems (RNS) connected to the same 3G_MSC, this is termed an Intra- 3G_MSC handover/relocation. This case also includes inter-system handover between RNS and BSS if the 3G_MSC supports the A-interface. In the context of this specification the term RNS refers also to a BSS when serving a mobile station in Iu mode. Furthermore, this case includes Intra-3G_MSC enhanced SRNS relocation between two RNSs;

iii) handover between Base Station Subsystems connected to different MSCs, this is termed an Inter-MSC handover. This category can be sub-divided into three further procedures:

a) the Basic Inter-MSC Handover procedure, where the MS is handed over from a controlling MSC (MSC-A) to another MSC (MSC-B);

b) the Subsequent Inter-MSC Handover procedure, where the MS is handed over from MSC-B to a third MSC (MSC-B');

c) the Subsequent Inter-MSC handback, where the MS is handed back from MSC-B to MSC-A.

iv) handover between Radio Network Subsystems connected to different 3G_MSCs, this is termed an Inter- 3G_MSC handover/relocation. In the context of this specification the term RNS also refers to a BSS when serving a mobile station in Iu mode. This category can be divided into three further sub-procedures:

a) the Inter-3G_MSC Handover procedure from UMTS to GSM, where the UE/MS is handed over from a controlling 3G_MSC (3G_MSC-A) to an MSC (MSC-B);

b) the Inter-3G_MSC Handover procedure from GSM to UMTS, where the UE/MS is handed over from a controlling MSC (MSC-A) to a 3G_MSC (3G_MSC-B);

c) the Inter-3G_MSC Relocation procedure, where the UE is relocated from 3G_MSC-A to 3G_MSC-B. This procedure can also be combined with a hard change of radio resources (Hard Handover with switch in the core network).

The MSC in items a) and b) this category can optionally be a 3G_MSC supporting the A-interface. The three subprocedures also cover subsequent handover/relocation to a third MSC-B' or 3G_MSC-B' and subsequent handover/relocation back to MSC-A or 3G_MSC-A. v) handover within one BSS connected via AoIP, supported by the same MSC, this is termed "BSS Internal Handover with MSC Support". It is in fact a kind of external handover from MSC perspective and therefore a subset of i) but described in detail in separate subclause 6.3 for clarity. The MSC in this category can be any of MSC-A, MSC-B, 3G_MSC-A or 3G_MSC-B.