Handover procedures
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 and vSRVCC, 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 sub-procedures 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 clause 6.3 for clarity. The MSC in this category can be any of MSC-A, MSC-B, 3G_MSC-A or 3G_MSC-B.
In both cases i) and iii) the same procedures as defined in the 3GPP TS 48.008 [5] and the 3GPP TS 24.008 [10] shall be used on the A‑interface and on the Radio Interface, respectively.
In case ii) the same procedures as defined in the 3GPP TS 25.413 [11] and the 3GPP TS 24.008 [10] shall be used on the Iu‑interface. If the 3G_MSC in case ii) also supports the A-interface, the 3GPP TS 48.008 [5] and the 3GPP TS 24.008 [10] shall be used on the A‑interface.
In case iii) the handover procedures shall transport the A-interface messages between MSC‑A and MSC‑B described in the Mobile Application Part (MAP), 3GPP TS 29.002 [12].
In case iv) the handover procedures shall transport the A-interface messages between 3G_MSC and MSC described in the Mobile Application Part (MAP), 3GPP TS 29.002 [12].
In case iv) the relocation procedure shall transport the Iu-interface messages between 3G_MSC‑A and 3G_MSC‑B described in the Mobile Application Part (MAP), 3GPP TS 29.002 [12].
The interworking between the 3GPP TS 29.002 [12] protocol and the 3GPP TS 48.008 [5] protocol is described in the 3GPP TS 29.010 [8].
Multicall supplementary service is not applicable in GERAN Iu mode and relocation of Multicalls is therefore only possible within UTRAN.
Enhanced SRNS relocation is possible only within UTRAN between two RNSs conntected to the same 3G_MSC, i.e. in case ii).
Handovers, which take place on the same MSC are termed Intra-MSC handovers; this includes both Inter-BSS and Intra‑BSS handovers.
Handovers, which take place on the same 3G_MSC are termed Intra-3G_MSC handovers; this includes Inter-RNS handovers and optionally RNS to BSS and BSS to RNS handovers.
In the context of this specification the term InterSystem handover can also refer to a handover which takes place between a Base Station serving a mobile station in Iu mode and a Base Station serving a mobile in A/Gb mode.
"Flexible Iu interface for handover/relocation" Option (see 3GPP TS 23.221 [19], clause 4.2.1): Up to release 99 an RNS can be connected only to one 3G_MSC. From release 4 onwards, as a network option, an RNS can have Iu interfaces to more than one MSC. Such an additional Iu interface may be selected by an MSC during an intra-PLMN relocation or intra-PLMN BSS to RNS handover procedure. This allows the MSC to use an Intra-3G_MSC handover procedure according to case ii) instead of an Inter-3G_MSC handover procedure according to case iv). The decision whether to use the Intra-3G_MSC handover procedure is implementation and configuration dependent. In a network implementing this option, a global title based on the Global RNC-Id may optionally be used for the addressing of the Iu interface messages.
"Intra Domain Connection of RAN Nodes to Multiple CN Nodes" Option (see 3GPP TS 23.236 [18]): when applied, a BSS or an RNS can be connected to more than one MSC.
The present document also covers the requirements for handover in ongoing GSM voice group calls, directed retry and handover without a circuit connection between (U)MSCs. The present document does not consider the case of handovers between radio channels on the same BSS (Intra-BSS handover) or the handover of packet radio services except for case v), the "BSS Internal Handover with MSC Support" for Intra-BSS handover in AoIP, involving the MSC as described in clause 6.3. The Inter-RNS handover case that results in a relocation is covered by the present document, but not other Inter-RNS or Intra-RNS handover cases.
For voice broadcast calls in GSM, the speaker uses normal point-to-point handover procedures, whilst the listeners use idle mode cell reselection procedures, as for the voice group call listeners.
Voice group calls is only applicable to GSM and handover of voice group calls is therefore only possible in GSM.
Inter-MSC hand-over imposes a few limitations on the system. After inter-MSC hand-over:
- call re-establishment is not supported.
The list of 3GPP TS 48.008 [5] features supported during and after Inter-MSC handover is given in 3GPP TS 49.008 [7].
In the Inter-MSC handover case, the interworking between a Phase 1 BSSMAP protocol possibly used by one MSC and the Phase 2 BSSMAP protocol used in the Phase 2 MAP protocol on the E-interface is performed by this MSC.
This specification assumes TDM based Core Network and therefore PCM, ITU-T G.711 [16] encoded, voice channel for speech calls between MSC-A and MSC-B and toward the other party. For bearer independent CS Core Network architecture implementations see 3GPP TS 23.205 [23] and 3GPP TS 23.231 [24]. For handover including Out-Of-Band transcoder control and transcoder free operation see 3GPP TS 23.153 [25]. For handover with Local Call Local Switch (LCLS) see 3GPP TS 23.284 [29].
NOTE 1: The message primitive names used in the SDL diagrams and message flows in the present document do not represent the actual messages specified in the GSM or 3GPP stage 3 technical specifications. The primitive names are only intended to be indicative of their use in the present document.
The MSC server enhanced for SRVCC and the MSC server enhanced for vSRVCC as specified in 3GPP TS 23.216 [26] follows the procedures defined for 3G_MSC-A in the present specification with the exceptions and additions as specified in clause 4.5.