03.553GPPDual Transfer Mode (DTM)Release 1999Stage 2TS
4.1 Main DCCH with SAPI=0
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 a TBF), the main DCCH shall not be used and the current procedures described in 3G TS 04.60  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.
Otherwise, the mobile station shall request an uplink TBF as specified in 3G TS 04.18 .
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 3G TS 04.60 . The procedures for the transmission of an LLC frame on the main DCCH are defined in 3G TS 04.18 . 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. 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
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 3G TS 05.02 ).
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. Basic half rate is optionally supported.
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 or the allocation bitmap in the fixed allocation case, if that information was present in the (uplink) assignment message. The mobile station, however, stores this information for possible multislot configurations where fixed or 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.
4.3 Multislot operation
In multislot operation, the GPRS data is sent on a PDCH. The number of timeslots comprising the PDCH is decided by the network after taking into account the class A 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 04.60  apply. In the case of GPRS and EGPRS MSs multiplexed on the same PDCH, the same restrictions as described in 3GPP TS 04.60  shall apply.
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 has always granted the right to transmit. The existent RLC/MAC block structure shall be kept. The procedures specified in 3GPP TS 04.60  shall apply.
Despite the dedicated characteristics of the uplink PDCH/H, the network shall allocate and use a valid USF or allocation bitmap, in order to satisfy the signalling requirements defined in 3GPP TS 04.60 .
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
with SAPI 0
Single slot operation
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
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 3G TS 05.02 multislot capabilities. When EGPRS is supported, DTM multislot capability for EGPRS operation is indicated independently from DTM multislot capability for GPRS operation.
The following DTM multislot classes are proposed:
- DTM multislot class 5: the mobile station supports DTM multislot operation; the restrictions of multislot class 5 apply (cf. 3G TS 05.02 ).
- DTM multislot class 9: the mobile station supports DTM multislot operation; the restrictions of multislot class 9 apply (cf. 3G TS 05.02 ).
Other classes can be defined in the future if strictly required.
The single slot DTM operation is optional and supported if indicated by Single Slot DTM capability bit in the Classmark 3 and MS Radio Access Capabilities.
The support of the following two 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.
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. The 8-PSK capability of the MS are the same in DTM and non-DTM mode;
- a mobile station supporting EGPRS as well as the DTM mode of operation shall support EGPRS DTM.
4.6 Indication of the capabilities
The mobile station DTM class is indicated in the Classmark 3 and MS Radio Access Capabilities. 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).
4.7 Compatibility issues
The mobile station shall indicate in its classmark whether it is DTM capable or not. The network shall not allocate resources for DTM operation unless the mobile is DTM capable. 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. 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.