3.4.4a Handover to UTRAN procedure

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

Only valid for a UTRAN capable MS. In dedicated mode or dual transfer mode, a change to UTRAN channel(s) can be requested by the network RR sublayer. This change is performed through the handover to UTRAN procedure.

NOTE: The decision to do a handover to UTRAN and the choice of the new cell is out of the scope of this technical specification.

The handover to UTRAN 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 in class A mode of operation;

– The establishment of UTRAN channel(s), see 3GPP TS 25.331.

The handover to UTRAN procedure is always initiated by the network.

3.4.4a.1 Handover to UTRAN initiation

The network initiates the handover to UTRAN procedure by sending an INTER SYSTEM TO UTRAN HANDOVER COMMAND message to the mobile station on the main DCCH. It then starts timer T3121.

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

When sending this message 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 resuming 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 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 in class A mode of operation). Switching to the assigned cell(s) and physical channel establishment is described in 3GPP TS 25.331.

3.4.4a.2 Handover to UTRAN completion

NOTE: After lower layer connections are successfully established, the mobile station returns a Handover to UTRAN Complete message on UTRAN channels(s), see 3GPP TS 25.331.

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, send the messages to the network using the newly established radio connection to the UTRAN.

3.4.4a.3 Abnormal cases

If the INTER SYSTEM TO 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 the INTER SYSTEM TO UTRAN HANDOVER COMMAND message instructs the mobile station to use a UTRAN predefined configuration that the mobile station has not read or instructs to use a default configuration not implemented by the mobile station, then the mobile station shall stay on the current channel(s) and return a HANDOVER FAILURE message with cause "UTRAN configuration unknown".

If connection is not possible on the UTRAN channel(s) (see 3GPP TS 25.331), 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 sending a HANDOVER FAILURE message in response to an INTERSYSTEM TO UTRAN HANDOVER COMMAND, the mobile station shall erase all the UTRAN predefined configurations.

When the HANDOVER FAILURE message has been received, the network releases the UTRAN channel(s) if they were dedicated channels 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 Handover to UTRAN Complete (3GPP TS 25.331) message is received on the 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 UTRAN HANDOVER COMMAND message are ignored.