4 Class A capabilities

3GPP43.055Dual Transfer Mode (DTM)Release 17Stage 2TS

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

4.1 Main DCCH with SAPI=0

4.1.1 General

The main DCCH (with layer 2 SAPI=0) is used for GSM signalling. GPRS signalling shall be able to use this resource. User data shall not be sent on the main DCCH.

The use of the main DCCH is only allowed when the mobile station is in dedicated mode. In dual transfer mode (i.e. the mobile station has resources allocated for an RR connection and for one or more TBFs), the main DCCH shall not be used and the current procedures described in 3GPP TS 44.060 [5] apply.

When upper layers request to send a message uplink, the mobile station shall send the message on the main DCCH if:

– the mobile station is in dedicated mode;

– the information contained in the message is signalling; and

– the number of LAPDm frames is smaller than a certain value specified by the network. When the parameter defining the maximum number of LAPDm frames has not been received by the MS in the serving cell (e.g. immediately after a handover), the MS assumes the default value defined in 3GPP TS 44.018

Otherwise, the mobile station shall request an uplink TBF as specified in 3GPP TS 44.018 [4]. Even though the mobile station shall not send messages on the main DCCH in dual transfer mode, the network shall not reject the received messages.

NOTE: This is needed to prevent erroneous cases caused by race conditions (e.g. if an uplink message sent on the main DCCH is not completely acknowledged on layer 2 level by the network before the mobile station leaves the dedicated mode and enters the dual transfer mode, the network would reject the message).

On the other hand, the network should not use the main DCCH to send messages that exceed the maximum length specified for the uplink. The mobile station, however, shall not reject messages that exceed the maximum length. Similarly, the network should not use the main DCCH when the mobile station is in dual transfer mode, although the mobile station shall not reject the received messages.

NOTE: This is needed to prevent erroneous cases caused by race conditions (e.g. if the mobile station leaves the dedicated mode and enters the dual transfer mode at the same time as the network sends a downlink message on the main DCCH, the mobile station would reject the message).

4.1.2 MS-SGSN tunnelling

The GPRS information from upper layers (i.e. GMM or SM) is always sent inside an LLC frame. This LLC frame can now be passed down:

– to RLC and transmitted on a TBF; or

– to RR, if the MS is in dedicated mode, and transmitted on the main DCCH.

The procedures for the transmission of an LLC frame via RLC are defined in 3GPP TS 44.060 [5]. The procedures for the transmission of an LLC frame on the main DCCH are defined in 3GPP TS 44.018 [4]. The new tunnelling mechanism for the transmission of the LLC frame is shown graphically in figure 2.

Figure 2: Transmission of an LLC PDU on the main DCCH

In the uplink, the LLC PDU is inserted in a new Layer 3 message^[2]. This Layer 3 message is sent to the BSC on the main DCCH, with the existing Layer 2 mechanisms. The BTS re-assemblies the Layer 3 message and sends it to the BSC. The BSC extracts the TLLI and the LLC PDU, which are then put into a BSSGP UL-UNITDATA.

In the downlink, when the BSS receives a downlink BSSGP PDU, it can identify:

– if the PDU contains signalling information ("T bit" in the QoS profile IE);

– if the length of the LLC PDU meets the requirements; and

– if it has an RR connection to the addressed MS (with the IMSI);

in which case, it sends the LLC using the same procedure as described above. If any of the conditions above is not met, the BSC sends the information on a downlink TBF.

4.2 Single slot operation

4.2.1 General

A mobile station in dual transfer mode has one timeslot allocated for the CS services. It is possible to reuse the same timeslot for the transmission of GPRS signalling and user data.

It is desirable to be able to use the same timeslot as the CS connection for GPRS data, due to the impossibility for the network to allocate a TBF in some circumstances (e.g. congested cell, multislot capabilities not supported in the serving cell).

The proposed solution for single timeslot operation is the "TCH/H + PDCH/H" configuration (see 3GPP TS 45.002 [6]).

4.2.2 TCH/H + PDCH/H

A "TCH/H + PDCH/H" configuration implies the multiplexing of CS information and RLC/MAC blocks in the same timeslot of the TDMA frame. Which domain uses each half shall be flexible and indicated in the assignment command.

On the ‘TCH/H’ part, the support of AMR as the speech codec is mandatory. Support of other circuit switched half rate traffic channels are indicated in the bearer capability IE (see 3GPP TS 24.008).

The PDCH/H is a resource dedicated to the mobile station in both directions. For instance, if an uplink TBF is established, the network may send a control message in any of the downlink blocks. No downlink data, however, shall be sent without a previous downlink assignment.

The existent RLC/MAC block format is used. In the downlink, the mobile station shall only pass to upper layers those blocks with the TFI indicated in the assignment message. In the uplink, the mobile station may transmit in any of the blocks of the PDCH/H, irrespective of the USF in the previous blocks in the dynamic allocation case, if the USF was present in the (uplink) assignment message. The mobile station, however, stores the USF for possible multislot configurations where dynamic allocation is supported.

The PDCH/H can be used for both GPRS signalling and user data. A PDCH/H shall not be assigned to a DTM capable mobile station in packet transfer mode.

Apart from the different mapping onto physical resources, the PDCH/H has the same characteristics as a PDCH/F. A PDCH/H is always used in exclusive allocation.

If a mobile station and the network support multiple TBF procedures (see 3GPP TS 44.060) and the mobile station is DTM capable, the network may establish multiple downlink TBFs using the PDCH/H. Only one uplink TBF may be established on a PDCH/H since exclusive allocation is used.

4.3 Multislot operation

4.3.1 General

In multislot operation, the packet data is sent on one or more PDCHs. The number of timeslots (i.e. PDCHs) used to carry packet data is decided by the network after taking into account the DTM multislot capabilities supported by the mobile station.

4.3.2 Shared PDCH

The PDCH/F may be shared with other GPRS mobile stations. The existent procedures in 3GPP TS 44.060 [5] apply. In the case of GPRS, EGPRS and EGPRS2 MSs multiplexed on the same PDCH, the same restrictions as described in 3GPP TS 44.060 [5] shall apply.

If a mobile station and the network support multiple TBF procedures (see 3GPP TS 44.060) and the mobile station is DTM capable, the network may establish multiple downlink TBFs for that mobile station using one or more shared PDCHs. In this case the mobile station may request the establishment of multiple uplink TBFs and the network may allocate uplink TBF resources on one or more shared PDCHs.

4.3.3 Exclusive use of PDCH/H

The PDCH/H shall not be shared with other GPRS mobile stations. An uplink PDCH/H shall be assigned in exclusive mode, where the correspondent mobile station is always granted the right to transmit. The existent RLC/MAC block structure shall be kept. The procedures specified in 3GPP TS 44.060 [5] shall apply.

Despite the dedicated characteristics of the uplink PDCH/H, the network shall allocate and use a valid USF, in order to satisfy the signalling requirements defined in 3GPP TS 44.060 [5].

4.3.4 TCH/H + PDCH/F

For this configuration, on the ‘TCH/H’ part the support of AMR as the speech codec is mandatory. Support of other circuit switched half rate traffic channels is indicated in the Bearer Capability IE (see 3GPP TS 24.008).

4.3.5 Dual Carrier in the Downlink

DTM configurations which use two radio frequency channels in the downlink shall obey the following restrictions:

– Uplink PDCHs shall be assigned on the same radio frequency channel as the uplink CS timeslot.

– Only EGPRS or EGPRS2 TBFs shall be assigned to the packet resources.

4.4 Bearer capability

The decision of which of the class A capabilities shall be used shall be always made by the network after considering:

– the supported capabilities (by both the network and the mobile station);

– the type of data to be sent;

– the length of the data; and

– the requested QoS parameters;

shows the GPRS data supported by the different class A capabilities.

Table 1: Support of GPRS data by the different class A capabilities

Bearer GPRS data		Main DCCH with SAPI 0	Single slot operation	Multislot operation
GPRS signalling	Short frames	Yes	Yes	Yes
	Long frames	No
User data		No

NOTE: The use of the main DCCH with SAPI 0 has the following properties:

– it delays RR commands;

– it harms speech quality;

– it places load onto the A-bis LAPD signalling links;

– it has a maximum length of 251 bytes.

4.5 Indication of the DTM capabilities supported by the MS

4.5.1 Definition of MS DTM classes

4.5.1.1 MS DTM classes

Different mobile stations may support different DTM capabilities and thus they need to be communicated to the network so that they can be taken into account for the allocation of radio resources. The DTM multislot capabilities are independent from the currently defined 3GPP TS 45.002 multislot capabilities. When EGPRS is supported, DTM multislot capability for EGPRS operation (applicable also to EGPRS2 operation if supported) is indicated independently from DTM multislot capability for GPRS operation.

DTM multislot classes 5, 6, 9, 10, 11, 31 to 33, 36 to 38 and 41 to 44 are defined in this release (see 3GPP TS 45.002 [6]). Classes 31 to 33, 36 to 38 and 41 to 44 are supported only by mobile stations supporting DTM High Multislot Class capability.

4.5.1.2 Use of full and half rate

The mix of full and half rate packet data channels is not allowed in the uplink. This mix is only defined for the downlink direction and only supported by mobile stations indicating support for Extended GPRS DTM Multi Slot Class or Extended EGPRS DTM Multi Slot Class respectively (See 3GPP TS 24.008). The half rate packet data channel is only allowed on the same time slot as the circuit switched channel. Due to the different rate of the full and half rate channels used for GPRS during DTM, the network shall take care that the RLC/MAC blocks are sent in such an order that the reception is in sequence when using RLC unacknowledged mode.

4.5.1.3 Incremental support

In order to reduce the number of possibilities and the length of the coding, incremental support shall be used; that is, a mobile station that supports a certain level of capabilities shall support the capabilities of the less restrictive DTM classes. Annex B defines the incremental support for mobile stations supporting Extended DTM GPRS Multi Slot Class or Extended DTM EGPRS Multi Slot Class. For other mobile stations supporting DTM, the implicit support of the less restrictive DTM classes shall be as indicated in Figure 2a; for these mobile stations the single slot DTM operation is optional and supported if indicated by Single Slot DTM capability bit in the MS Classmark 3 and MS Radio Access Capability IEs.

Figure 2a – Incremental support of DTM multislot classes

4.5.2 Options

The support of the following four capabilities has to be indicated independently from the DTM class:

– Single Slot DTM: single slot DTM operation supported or not;

– E-GPRS: supported or not;

– Enhanced CS establishment and release: supported or not;

– DTM Handover: supported or not.

The mobile station also indicates support of the following capabilities which, if supported, require the indication of an additional DTM class:

– Extended DTM: supported or not. If supported, the Extended DTM multislot class shall also be indicated; a separate indication is provided for GPRS and EGPRS;

– DTM High Multislot Class: supported or not. If supported, the DTM high multislot class shall also be indicated; a separate indication is provided for GPRS and EGPRS.

In addition the following rules apply:

– exclusive allocation in the PDCH/H shall always be used; a mobile station supporting E-GPRS shall support GPRS.

4.6 Indication of the capabilities

The mobile station DTM class is indicated in the MS Classmark 3 IE and MS Radio Access Capability IE. The absence of this information shall indicate that the mobile station does not support simple class A (i.e. either it supports unrestricted class A or it cannot operate in mode of operation A at all). The support of enhanced CS establishment and release is indicated in the MS Classmark 3 and MS Radio Access Capability IEs. For mobile stations supporting DTM High Multislot Class capability, the mobile station DTM high multislot class is indicated in the MS Classmark 3 and MS Radio Access Capability IEs. The support of DTM handover is indicated in the MS Radio Access Capability and MS Classmark 3 IEs.

4.7 Compatibility issues

The mobile station shall indicate in its capabilities whether it is DTM capable or not and if so whether it supports enhanced CS establishment and release or not. The mobile station shall indicate in the MS Radio Access Capability and MS Classmark 3 IEs whether or not it supports DTM Handover. The network shall not allocate resources for DTM operation unless the mobile is DTM capable. The network shall not use enhanced CS establishment (respectively release) unless the mobile supports it. The network shall not use DTM Handover procedures unless the mobile station also supports it. The resources allocated by the network shall meet the requirements imposed by the classmark.

The network indicates on the BCCH or PBCCH whether or not the cell supports DTM and if so whether or not it supports enhanced CS establishment and release. It shall also indicate it on the SACCH for DTM capable mobile stations in dedicated mode or dual transfer mode. It may also indicate it on the PACCH for DTM capable mobile stations in packet transfer mode. A cell level indication is needed because adjacent BTSs may be in the same RA and LA but may be parented by different BSCs (from different vendors or different releases). The indication in the SACCH is needed to enable/suppress the transmission of packet resource requests when the mobile is in dedicated mode and cannot read the BCCH data. A mobile station shall not attempt to enter the DTM unless DTM is supported in the cell.

The network shall allocate resources taking into account the capabilities commonly supported with the mobile station. In order to avoid situations where both the mobile station and the network are DTM capable but no class A capabilities are shared, a core set of capabilities has been defined and shall be supported by the mobile station and the network, consisting of:

– the main DCCH with SAPI 0 for GPRS signalling, with a length restriction controlled by the network;

– the TCH/F + PDCH/F configuration (DTM multislot class 5).

In addition, the mobile station supporting DTM shall support TCH/H + PDCH/F configuration with AMR-HR.