13 Subsequent channel assignment using a circuit connection between MSC‑A and MSC‑B

23.0093GPPHandover proceduresRelease 17TS

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

13.1 GSM handover

If a circuit connection has to be set up (for example for a Mobile Originated or Mobile Terminated Call Establishment) after an Inter-MSC handover without circuit connection, MSC‑A shall request a Handover Number using a MAP‑PREPARE-HANDOVER request, containing the A-ASSIGNMENT-REQUEST, on the established MAP connection. For speech calls, MSC-A shall also include the Iu Supported Codecs List to be used by MSC-B for subsequent intra-MSC-B intersystem handover to UMTS and intra-MSC-B SRNS relocation. If MSC‑B indicates to MSC‑B and to MSC‑A that at least one of two procedures assignment or Handover Number allocation can not be completed, then MSC‑A shall terminate the circuit establishment attempt. The existing connection to the MS shall be maintained, if possible.

Upon receipt of the MAP-PREPARE-HANDOVER request MSC‑B shall perform the requested assignment operation towards the BSS. In addition it shall retrieve a Handover Number from VLR-B. If a failure occurs in the assignment or Handover Number allocation then it shall be reflected in the MAP-PREPARE-HANDOVER response that at least one of these two procedures has not been completed (i.e. either by a MAP-PREPARE-HANDOVER result with the assignment procedure outcome and the Handover Number allocation outcome or by a MAP-PREPARE-HANDOVER error).

If MSC-A supports A interface over IP, then for speech calls MSC-A may include the AoIP-Supported Codecs List (Anchor) in the MAP-PREPARE-HANDOVER request.

If the BSS-B supports A over IP then MSC-B shall include a Codec List (MSC preferred) in the A-ASSIGNMENT-REQUEST message to BSS-B. MSC-B may select the codecs for the Codec List (MSC preferred) from the channel type information and the AoIP‑Supported Codecs List (Anchor), if this list was provided by 3G_MSC-A in the MAP-PREPARE-HANDOVER request. For a detailed description of the handling of these codec lists by MSC-A and MSC-B see 3GPP TS 23.153 [25]. If the AoIP-Supported Codecs List (Anchor) was not provided or MSC-B does not support the selection of codecs from the AoIP-Supported Codecs List, then MSC-B shall create the Codec List (MSC preferred) using the channel type information received from 3G_MSC-A in the A-ASSIGNMENT-REQUEST message included in the MAP-PREPARE-HANDOVER request.

If MSC-A provided an AoIP-Supported Codecs List (Anchor) in the MAP-PREPARE-HANDOVER request and MSC‑B selected the codecs for the Codec List (MSC preferred) from the AoIP-Supported Codecs List (Anchor), MSC‑B may include the AoIP-Selected Codec (Target) and AoIP-Available Codecs List (MAP) in the MAP-PREPARE-HANDOVER response.

When MSC‑A receives a successful MAP-PREPARE-HANDOVER response it shall establish a circuit connection to MSC‑B by using the appropriate network supported procedures. In figure 36 this is indicated by the IAM (Initial Address Message) and ACM (Address Complete Message). MSC‑B shall also send the Answer message if appropriate to the signalling system. Upon receipt of the Answer MSC‑A shall consider the circuit connection establishment phase complete. If a failure occurs during the cirucit establishment phase then the existing connection to the MS shall be maintained, if possible.

NOTE: Can be sent at any time after the reception of IAM.

Figure 36: Successful circuit-switched call establishment after a Basic Handover without circuit connection

13.2 UMTS to GSM handover

If a circuit connection has to be set up (for example for a Mobile Originated or Mobile Terminated Call Establishment) after an Inter-3G_MSC UMTS to GSM handover without circuit connection, 3G_MSC‑A shall request a Handover Number using a MAP-PREPARE-HANDOVER request, containing the A-ASSIGNMENT-REQUEST, on the established MAP connection. For speech calls, 3G_MSC-A shall also include the Iu Supported Codecs List to be used by MSC-B for subsequent intra-MSC-B intersystem handover to UMTS and intra-MSC-B SRNS relocation. If MSC‑B indicates to MSC‑B and to 3G_MSC‑A that at least one of two procedures assignment or Handover Number allocation can not be completed, then 3G_MSC‑A shall terminate the circuit establishment attempt. The existing connection to the UE/MS shall be maintained, if possible.

If 3G_MSC-A supports A interface over IP, then for speech calls 3G_MSC-A may include the AoIP-Supported Codecs List (Anchor) in the MAP-PREPARE-HANDOVER request.

If the BSS-B supports A over IP, then MSC-B shall include a Codec List (MSC preferred) in the A-ASSIGNMENT-REQUEST message to BSS-B. MSC-B may select the codecs for the Codec List (MSC preferred) from the channel type information and the AoIP‑Supported Codecs List (Anchor), if this list was provided by 3G_MSC-A in the MAP-PREPARE-HANDOVER request. For a detailed description of the handling of these codec lists by 3G_MSC-A and MSC-B see 3GPP TS 23.153 [25]. If the AoIP-Supported Codecs List (Anchor) was not provided or MSC-B does not support the selection of codecs from the AoIP-Supported Codecs List (Anchor), then MSC-B shall create the Codec List (MSC preferred) using the channel type information received from 3G_MSC-A in the A-ASSIGNMENT-REQUEST message included in the MAP-PREPARE-HANDOVER request.

When 3G_MSC‑A receives a successful MAP-PREPARE-HANDOVER response, it shall establish a circuit connection to MSC‑B by using the appropriate network supported procedures. In figure 37 this is indicated by the IAM (Initial Address Message) and ACM (Address Complete Message). MSC‑B shall also send the Answer message if appropriate to the signalling system. Upon receipt of the Answer 3G_MSC‑A shall consider the circuit connection establishment phase complete. If a failure occurs during the circuit establishment phase then the existing connection to the UE/MS shall be maintained, if possible.

NOTE 1: Can be sent at any time after the reception of IAM.

Figure 37: Successful circuit-switched call establishment after a Basic UMTS to GSM Handover without circuit connection

13.3 GSM to UMTS handover

If a circuit connection has to be set up (for example for a Mobile Originated or Mobile Terminated Call Establishment) after an Inter-3G_MSC GSM to UMTS handover without circuit connection, MSC‑A shall request a Handover Number using a MAP-PREPARE-HANDOVER request, containing the A-ASSIGNMENT-REQUEST, on the established MAP connection. If 3G_MSC‑B indicates to 3G_MSC‑B and to MSC‑A that at least one of two procedures assignment or Handover Number allocation can not be completed, then MSC‑A shall terminate the circuit establishment attempt. The existing connection to the UE/MS shall be maintained, if possible.

If MSC-A supports A interface over IP, then for speech calls MSC-A may include the AoIP-Supported Codecs List (Anchor) in the MAP-PREPARE-HANDOVER request to be used by 3G_MSC-B for subsequent intra-3G_MSC-B intersystem handover to an A over IP capable BSS. For a detailed description of the handling of this codec list by MSC‑A and 3G_MSC-B see 3GPP TS 23.153 [25].

Upon receipt of the MAP-PREPARE-HANDOVER request 3G_MSC‑B shall perform the requested assignment operation towards the RNS. In addition it shall retrieve a Handover Number from VLR-B. If a failure occurs in the assignment or Handover Number allocation then it shall be reflected in the MAP-PREPARE-HANDOVER response that at least one of these two procedures has not been completed (i.e. either by a MAP-PREPARE-HANDOVER result with the assignment procedure outcome and the Handover Number allocation outcome or by a MAP-PREPARE-HANDOVER error).

For speech calls, if 3G_MSC-B supports the selection of codec based on the Iu-Supported Codecs List, 3G_MSC‑B shall select a codec from the Iu Supported Codecs List, generate associated RAB parameters and connect a transcoder. If the Iu Supported Codecs List was not received or 3G_MSC-B does not support the selection of codec based on the Iu-Supported Codecs List, 3G_MSC‑B shall select the appropriate default speech codec.

For an assignment in UTRAN Iu mode, 3G_MSC-B shall also generate a NAS Synch Indicator for the Iu-RAB-ASSIGNMENT-REQUEST message. If the Iu Supported Codecs List was received by 3G_MSC-B and 3G_MSC-B supports the selection of codec based on the Iu-Supported Codecs List, then the Iu Selected codec shall be indicated in the MAP-PREPARE-HANDOVER response, sent from 3G_MSC‑B to MSC‑A.

When MSC‑A receives a successful MAP-PREPARE-HANDOVER response, it shall establish a circuit connection to 3G_MSC‑B by using the appropriate network supported procedures. In figure 38 this is indicated by the IAM (Initial Address Message) and ACM (Address Complete Message). 3G_MSC‑B shall also send the Answer message if appropriate to the signalling system. Upon receipt of the Answer MSC‑A shall consider the circuit connection establishment phase complete. If a failure occurs during the circuit establishment phase then the existing connection to the UE/MS shall be maintained, if possible.

NOTE 1: Can be sent at any time after the reception of IAM.

Figure 38: Successful circuit-switched call establishment
after a Basic GSM to UMTS Handover without circuit connection

13.4 SRNS Relocation

13.4.1 Without circuit connection

If a circuit connection has to be set up (for example for a Mobile Originated or Mobile Terminated Call Establishment) after an Inter-3G_MSC relocation without circuit connection, 3G_MSC‑A shall request a Handover Number using a MAP-PREPARE-HANDOVER request, containing the IU-RAB-ASSIGNMENT-REQUEST, on the established MAP connection.

For speech calls, 3G_MSC-A shall include the Iu Supported Codecs List in the MAP-PREPARE-HANDOVER request. 3G_MSC‑A shall configure the RANAP RAB parameters according to the appropriate default speech codec.

If 3G_MSC-A supports A interface over IP, then for speech calls 3G_MSC-A may include the AoIP-Supported Codecs List (Anchor) in the MAP-PREPARE-HANDOVER request to be used by 3G_MSC-B for subsequent intra-3G_MSC-B intersystem handover to an A over IP capable BSS. For a detailed description of the handling of this codec list by 3G_MSC-A and 3G_MSC-B see 3GPP TS 23.153 [25].

Alternatively, if 3G_MSC-B is known to support the use of the Iu Supported Codecs List, 3G_MSC-A may configure the RANAP RAB parameters according to the preferred codec and indicate this to 3G_MSC-B by including the RAB configuration indicator in the MAP-PREPARE-HANDOVER request. The decision to use this option is based on internal configuration information in 3G_MSC-A.

For an assignment in UTRAN Iu mode, 3G_MSC‑A shall also include the NAS Synch Indicator in the Iu-RAB-ASSIGNMENT-REQUEST.

If 3G_MSC‑B indicates to 3G_MSC‑B and to 3G_MSC‑A that at least one of two procedures (RAB) assignment or Handover Number allocation can not be completed, then 3G_MSC‑A shall terminate the circuit establishment attempt. The existing connection to the UE shall be maintained, if possible.

Upon receipt of the MAP-PREPARE-HANDOVER request, 3G_MSC‑B shall perform the requested RAB assignment operation towards the RNS. In addition it shall retrieve a Handover Number from VLR-B.

For speech calls, if 3G_MSC-B supports the selection of codec based on the Iu-Supported Codecs List, 3G_MSC‑B shall select an Iu Selected codec from the Iu Supported Codecs List and connect a transcoder. If the Iu Supported Codecs List was not received or 3G_MSC-B does not support the selection of codec based on the Iu-Supported Codecs List, 3G_MSC‑B shall select the appropriate default speech codec.

3G_MSC-B shall reconfigure the RANAP RAB parameters according to the Iu Selected codec:

– if the RAB configuration indicator is included in the MAP-PREPARE-HANDOVER request and the codec selected by 3G_MSC-B is different from the preferred codec; or

– if the RAB configuration indicator is not included in the MAP-PREPARE-HANDOVER request and the codec selected by 3G_MSC-B is different from the appropriate default speech codec.

Additionally, for an assignment in UTRAN Iu mode, 3G_MSC-B shall include the NAS Synch Indicator for the Iu Selected codec in the Iu-RAB-ASSIGNMENT-REQUEST. If the Iu Supported Codecs List was received by 3G_MSC-B and 3G_MSC-B supports the selection of codec based on the Iu-Supported Codecs List, then the Iu Selected codec shall be indicated in the MAP-PREPARE-HANDOVER response, sent from 3G_MSC‑B to 3G_MSC‑A.

If a failure occurs in the RAB assignment or Handover Number allocation then it shall be reflected in the MAP-PREPARE-HANDOVER response that at least one of these two procedures has not been completed (i.e. either by a MAP-PREPARE-HANDOVER result with the RAB assignment procedure outcome and the Handover Number allocation outcome or by a MAP-PREPARE-HANDOVER error).

When 3G_MSC‑A receives a successful MAP-PREPARE-HANDOVER response, it shall establish a circuit connection to 3G_MSC‑B by using the appropriate network supported procedures. In figure 39 this is indicated by the IAM (Initial Address Message) and ACM (Address Complete Message). 3G_MSC‑B shall also send the Answer message if appropriate to the signalling system. Upon receipt of the Answer 3G_MSC‑A shall consider the circuit connection establishment phase complete. If a failure occurs during the circuit establishment phase then the existing connection to the UE shall be maintained, if possible.

13.4.2 With circuit connection (Optional functionality)

If 3G_MSC-A and 3G_MSC-B support the optional supplementary service Multicall (See 3GPP TS 23.135 [17]), 3G_MSC-A and 3G_MSC-B shall have the following functionality additionally to the description in clause 13.4.1.

A new circuit connection shall be able to set up (for example for a new Mobile Originated or a new Mobile Terminated Call Establishment) after an Inter-3G_MSC relocation with one or several circuit connections. The procedures for the establishment of the additional circuit connection in 3G_MSC-A and 3G_MSC-B are the same as that described in clause 13.4.1.

NOTE 1: Can be sent at any time after the reception of IAM.

Figure 39: Successful circuit-switched call establishment
after a Basic Relocation without circuit connection