3.4.4d CS to PS SRVCC procedure

3GPP44.018GSM/EDGE Radio Resource Control (RRC) protocolMobile radio interface Layer 3 specificationRelease 17TS

Tools: ARFCN - Frequency Conversion for 5G NR/LTE/UMTS/GSM

This procedure is only valid for a CS to PS SRVCC capable MS. In dedicated mode or dual transfer mode, a change to E-UTRAN / UTRAN PS bearer can be requested by the network RR sublayer. This change is performed through the CS to PS SRVCC procedure.

The CS to PS SRVCC procedure includes:

– The suspension of normal operation except for RR management (layer 3);

– The disconnection of the main signalling link, and of the other links via local end release (layer 2), and the disconnection of the TCH(s) if any;

– The disconnection and the deactivation of previously assigned channels and their release (layer 1);

– The abortion of the packet resources (see 3GPP TS 44.060), if applicable;

– The establishment of E-UTRAN / UTRAN PS bearers, see 3GPP TS 36.331, 3GPP TS 25.331.

The SRVCC to E-UTRAN/UTRAN procedure is always initiated by the network. The decision to perform CS to PS SRVCC procedure and the choice of the new cell is out of the scope of this technical specification.

In case of dual transfer mode, when this procedure is initiated there is no handover functionality related to the PS domain (i.e. no PS Handover is initiated and the CS to PS SRVCC procedure is therefore the same for both dedicated mode and dual transfer mode).

3.4.4d.1 CS to PS SRVCC procedure initiation

The network initiates the CS to PS SRVCC procedure by sending an INTER SYSTEM TO E-UTRAN HANDOVER COMMAND or INTER SYSTEM TO UTRAN HANDOVER COMMAND message to the mobile station on the main DCCH. The network then starts timer T3121.

If the INTER SYSTEM TO UTRAN HANDOVER COMMAND refers to an unknown cell (see 3GPP TS 25.133 and 3GPP TS 25.123), this shall not be considered as an error.

When sending one of these messages on the network side, and when receiving it on the mobile station side, all transmission of signalling layer messages, except for those RR messages needed for this procedure and for abnormal cases, is suspended until resumption is indicated. These RR messages can be deduced from sub-clause 3.4.3 and 8.5.1 "Radio Resource management".

Upon receipt of the INTER SYSTEM TO E-UTRAN HANDOVER COMMAND or INTER SYSTEM TO UTRAN HANDOVER COMMAND message, the mobile station initiates, as described in sub-clause 3.1.4, the release of link layer connections and disconnects the physical channels (including the packet resources, if applicable).

Switching to the assigned cell(s) and physical channel establishment in E-UTRAN or UTRAN is described in 3GPP TS 36.331 and 3GPP TS 25.331.

3.4.4d.2 CS to PS SRVCC procedure completion

NOTE: After lower layer connections are successfully established, the mobile station sends a RRCConnectionReconfigurationComplete message on E-UTRAN channels(s), see 3GPP TS 36.331 or a Handover to UTRAN Complete message on UTRAN channel(s), see 3GPP TS 25.331.

When receiving the RRCConnectionReconfigurationComplete message (3GPP TS 36.331), the network stops timer T3121 and releases the old channels.

When receiving the Handover to UTRAN Complete message (3GPP TS 25.331), the network stops timer T3121 and releases the old channels.

If there are any upper layer messages for which LAPDm has not yet received acknowledgement from the network, the MS shall, after the handover has been successfully completed, discard these messages.

3.4.4d.3 Abnormal cases

For CS to PS SRVCC to UTRAN (HSPA) the same abnormal cases as defined in 3.4.4a.3 apply.

If the INTER SYSTEM TO E-UTRAN HANDOVER COMMAND message instructs the mobile station to use a frequency that it is not capable of, then the mobile station shall stay on the current channel(s) and return a HANDOVER FAILURE message with cause "frequency not implemented".

If connection is not possible on the E-UTRAN channel(s) (see 3GPP TS 36.331), the MS reactivates the old channel(s) and reconnects TCHs and triggers the establishment of the main signalling link. It then sends a HANDOVER FAILURE message on the main signalling link and resumes normal operation.

When the HANDOVER FAILURE message has been received, the network releases the E-UTRAN dedicated bearer and stops timer T3121.

If a lower layer failure happens while attempting to connect back to the old channels, the standard rules are applied (cf. sub-clause 3.4.13.2 for dedicated mode).

On the network side, if timer T3121 elapses before either the RRCConnectionReconfigurationComplete (3GPP TS 36.331) message is received on the E-UTRAN channel(s), or a HANDOVER FAILURE message is received on the old channels, or the mobile station has re-established the call, the old channels are released if they were dedicated channels and all contexts related to the connections with that mobile station are cleared.

On the network side, lower layer failures occurring on the old channels after the sending of the INTER SYSTEM TO E-UTRAN HANDOVER COMMAND message are ignored.