5.2.1 Call establishment procedures

23.0343GPPHigh Speed Circuit Switched Data (HSCSD)Release 17Stage 2TS

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

5.2.1.1 Mobile originated call establishment (A/Gb mode)

Figure 3 depicts the procedures for a successful HSCSD call establishment in mobile originated case in A/Gb mode.

The Multislot class is sent from MS to network using the early classmark sending.

At the call setup the mobile station sends a set of parameters describing the HSCSD characteristics to the network. These parameters and their presence in the Setup message in transparent (T) and non‑transparent (NT) calls are as follows:

– Other Modem Type, OMT (T/NT);

– Fixed Network User Rate, FNUR (T/NT);

– Acceptable Channel Codings, ACC (including ACC ext.) (T/NT);

– maximum number of traffic channels, Max TCH/F (T/NT);

– User Initiated Modification Indication, UIMI (NT);

– wanted Air Interface User Rate, AIUR (NT), and

– channel coding ASYMmetry indication, ASYM (NT).

In reply the network responds in Call Proceeding with the Other Modem Type, OMT, Fixed Network User Rate, FNUR, and User Initiated Modification Indication, UIMI (NT only), parameters it is prepared to give to the mobile station.

Figure 3: Mobile originated call establishment (A/Gb mode)

The MSC requests the BSC to allocate the channel configuration using parameters derived from the HSCSD related parameters agreed in the setup phase. Based on these parameters and operator preferences the BSC then allocates a suitable number of channels and a suitable channel coding for the connection.

The following rule for the channel allocation apply:

– The BSS shall try to reach but not exceed, with one exception, the wanted AIUR. The exception is the case when the chosen configuration can reach the wanted AIUR with lower number of TCH/F, e.g. in case AIUR=14,4 kbit/s, max number of TCH/F=3, ACC=TCH/F4.8 and TCH/F9.6, the network shall choose 2 x 9,6 over 3 x 4,8 if the TCH/F9.6 is available in the cell.

– A separate channel activation is applied for each of the HSCSD channels before the selected channel configuration with information of the channel coding is forwarded to the mobile station. When the preference for downlink or uplink biased channel coding asymmetry is indicated by the user, and an asymmetric channel coding connection is set up based on this indication, the BSC shall always assign a TCH/F14.4 channel on the unbiased link of the connection.

– At assignment completion, the BSS informs the MSC of the chosen HSCSD configuration and the MSC may seize the IW resources accordingly.

5.2.1.1a Mobile originated call establishment (GERAN Iu mode)

Figure 3a depicts the procedures for a successful HSCSD call establishment in mobile originated case in GERAN Iu mode.

The MS GERAN Iu mode radio access capabilities, including the multislot capabilities of the MS, are sent from the MS to the network during the establishment of the RRC connection.

The BSC includes the GERAN classmark in the RANAP Initial UE message to inform the MSC about the BSS capabilities of the serving cell:

– Acceptable Channel Codings, ACC (including ACC ext.); and

– maximum number of traffic channels, Max TCH/F.

The parameters in the Setup message and their presence requirements for transparent (T) and non-transparent (NT) calls are the same as for A/Gb mode, however, for non-transparent calls in GERAN Iu mode the channel coding asymmetry indication will be ignored. When negotiating the parameters for a transparent call, the MSC shall take the GERAN classmark into account.

Figure 3a: Mobile originated call establishment (GERAN Iu mode)

The MSC requests the BSC to allocate a radio access bearer by sending a RANAP RAB Assignment Request message. The RAB parameters in this message are set according to the parameters negotiated between MS and MSC and the GERAN classmark of the serving cell. For non-transparent calls the RAB parameters contain the description of all RAB subflow combinations allowable for the maximum bit rate at the Iu interface, and thus for the maximum possible AIUR that can be allocated by the BSC for this call.

Additionally, the MSC includes a GERAN BSC container, indicating the Max TCH/F allowed and the Allowed radio interface data rates. In this version, the MSC shall indicate only one radio interface data rate as allowed.

Based on these parameters and operator preferences the BSC then allocates a suitable number of channels for the allowed channel coding.

After the successful setup of the radio access bearer, for non-transparent calls the BSC initiates the Iu user plane protocol. In the Iu UP initialisation message the first RAB subflow combination proposed in the list of RAB subflow combinations indicates the RAB subflow combination and thus the AIUR to be used by the IW function when starting the communication phase (see 3GPP TS 25.415 [20]). The BSC shall use the same RAB subflow combination in uplink direction.

5.2.1.2 Mobile terminated call establishment (A/Gb mode)

Figure 4 depicts the procedures for a successful HSCSD call establishment in mobile terminated case in A/Gb mode.

At the call setup the network sends the Other Modem Type,OMT, Fixed Network User Rate,FNUR, and User Initiated Modification Indication,UIMI (NT only), parameters to the mobile station.

In reply the mobile station responds to the network with the set of parameters describing the HSCSD characteristics. These parameters and their presence in the Call Confirmed message in transparent (T) and non-transparent (NT) calls are as follows:

– wanted Other Modem Type, OMT (T/NT);

– wanted Fixed Network User Rate, FNUR (T/NT);

– Acceptable Channel Codings, ACC (including ACC ext.) (T/NT);

– maximum number of traffic channels, Max TCH/F (T/NT);

– User Initiated Modification Indication, UIMI (NT);

– wanted Air Interface User Rate, AIUR (NT), and

– channel coding ASYMmetry indication, ASYM (NT).

Figure 4: Mobile terminated call establishment (A/Gb mode)

The same channel allocation rules as in mobile originated case apply.

The same channel activation rules as in mobile originated case apply.

At assignment completion, the BSS informs the MSC of the chosen HSCSD configuration and the MSC may seize the IW resources accordingly.

5.2.1.2a Mobile terminated call establishment (GERAN Iu mode)

Figure 4a depicts the procedures for a successful HSCSD call establishment in mobile terminated case in GERAN Iu mode.

Figure 4a: Mobile terminated call establishment (GERAN Iu mode)

For the Setup message and the Call Confirmed message the same rules apply as in A/Gb mode.

The signalling for the setup of the radio bearer and the radio access bearer, and the initialisation of the Iu user plane (NT only) is as in the mobile originated case.