6.1 Establishment

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

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

6.1.1 General

The existent establishment procedures for class A mode of operation rely on the capability of the mobile station to be able to operate in different frequencies in the same timeslot, e.g. to listen to the (P)BCCH while in dedicated mode. New procedures need to be added to the specifications to allow mobile stations without such capabilities to be able to enter the dual transfer mode.

The new cases are marked with "" in table 1 and explained in detail in this clause.

Table 2: Summary of establishment cases

			Requested
			CS	PS
				MO	MT
					Ready state	Standby state
Active	Nothing		Normal establishment
	CS		Engaged			
	PS	MO		Same TBF	Normal: PACCH	Not applicable
		MT		Normal: PACCH	Same TBF

6.1.2 PS establishment while in dedicated mode

6.1.2.1 Principles

The mobile station may request packet resources while in dedicated mode by sending the DTM Request message to enter the dual transfer mode.

Two DTM assignment messages are defined:

– the DTM Assignment Command message: this message shall describe both the CS and packet resources when a reallocation of the CS resource is needed, e.g. when a multislot configuration cannot be accommodated or when an "TCH/H + PDCH/H" configuration is to be used.

– the Packet Assignment message: this message describes the allocated packet resources when no reallocation of the CS resource is necessary, e.g. on an adjacent timeslot.

A mobile station that supports multiple TBF procedures can determine whether or not the network supports multiple TBF procedures by reading the GPRS Cell Options IE included within the DTM Assignment Command and the Packet Assignment messages.

When there is reallocation of the CS timeslot:

– if the mobile station successfully establishes the new CS connection, it shall send an Assignment Complete message on the new main DCCH.

– if the mobile station fails to establish the new CS connection, it shall go back to the old timeslot, send a DTM Assignment Failure message on the (old) main DCCH and continue the CS operation. The mobile station shall assume that the old PS resources were released and attempt its re-establishment.

If the network wants to move the mobile station to another cell, it shall send a Handover Command message on the main DCCH. After the handover procedure is completed, the network supporting DTM shall send the DTM Information message on the main DCCH in order to speed up the resumption of the dual transfer mode of operation by the mobile station.

As described above, the main DCCH can be used in either direction with no prior assignment provided that the required conditions are met. Otherwise, the procedures here described apply.

6.1.2.2 MO session: packet request procedure

If the serving cell of the CS connection indicates that supports DTM, the mobile station may request the establishment of a PS session by sending a DTM Request message on the main DCCH.

The network may answer the request with one of the two defined DTM assignment messages, sent on the main DCCH. If the network cannot allocate the packet resources, it shall answer with a DTM Reject message on the main DCCH. The DTM Reject message shall indicate a waiting time after which the mobile station is allowed to reattempt the packet establishment in the same cell.

Figure 3 shows the successful case of the allocation of an uplink TBF when the reallocation of the CS timeslot is needed. The mobile station informs the network about the correct seizure of the new CS resource by sending an Assignment Complete message on the main DCCH of the new resource.

Figure 3: Establishment of a MO PS session while in dedicated mode
with reallocation of the CS resource; successful case

Figure 4 shows one failure case. If there is an error when establishing the main signalling link in the new timeslot, the mobile station shall send a DTM Assignment Failure message on the old main DCCH and then it may re-attempt the establishment of the packet session. The timers in the assignment procedure are reused.

Figure 4: Establishment of a MO PS session while in dedicated mode
with reallocation of the CS resource; failure case

In figure 5, the packet resource is mapped onto adjacent timeslot(s) and thus the Packet Assignment message is used. There is no release/re-establishment of the main signalling link, successful and failure messages are not needed. The successful and failure cases for the establishment of the TBF are determined as in normal GPRS (see 3G TS 04.60 [5]).

Figure 5: Establishment of a MO PS session in multislot configuration
while in dedicated mode; successful case

Figure 6 shows the case of the main DCCH being used as the uplink resource.

Figure 6: Use of the main DCCH for GPRS information while in dedicated mode

6.1.2.3 MT session

6.1.2.3.1 Ready state: packet downlink assignment

If the mobile station is in the Ready state, the SGSN may send an LLC frame to the BSS parenting the mobile station’s serving cell. The downlink LLC PDU shall include the IMSI if it is known. As the IMSI of the mobile station was previously stored, the BSS is able to identify that the mobile station to which the data is sent is in dedicated mode. The BSS shall use the main signalling link to send the downlink assignment command instead of the (P)CCCH. Note that a mobile station in dedicated mode does not listen to the (P)CCCH unless it is "unrestricted class A" capable.

Editor’s note : the consequences on the procedures currently defined for the DTM feature shall be analysed if the IMSI can not be provided in the BSSGP DL-UNITDATA PDU.

The assignment is done with one of the DTM assignment messages, sent on the main DCCH.

Figure 7 shows the successful case, when a downlink TBF is assigned without reallocation of the CS resource.

Figure 7: Example of a successful establishment of a PS MT session
while in dedicated mode, packet idle mode and Ready state

6.1.2.3.2 Standby state: packet notification

If the mobile station is in the Stand-by state and the SGSN has something to send, it shall send a page to the BSS(s) parenting the RA where the mobile station is, in order to find out the actual serving cell/BVCI. As the mobile station has an established signalling connection with the BSS, the BSS shall not page the mobile station. Instead, the BSS shall inform the mobile station that it is being paged for packet services. This is done by sending the Packet Notification message on the main DCCH. The mobile station shall answer the notification with a Cell Update procedure: sending an LLC frame to act as a "Packet Paging Response".

For that purpose, the GMM layer shall request the establishment of uplink resources. If the LLC frame is dummy (i.e. does not convey user data information) and it is short enough, the mobile station shall send it on the main DCCH. Otherwise, an uplink TBF is needed and its establishment shall be done.

Once the LLC frame is sent, the mobile station moves to the GMM Ready state. The SGSN understands the LLC frame as a valid page response and starts sending the downlink information. In order to forward this information to the mobile station, the BSS shall send a (second) assignment message as soon as it receives the data from the SGSN.

The procedure is shown in figure 8.

Figure 8: Example of a successful establishment of a PS MT session
while in dedicated mode, packet idle mode and Standby state

6.1.3 CS establishment while in packet transfer mode

When in packet transfer mode, either the mobile station or the network may initiate a CS connection establishment. In both cases, the packet session may be aborted and the establishment of the CS connection is initiated.

When the establishment of the CS connection is initiated by the network, the CS paging message may come directly from the MSC or via the SGSN if the Gs interface is present. The BSS shall be able to verify in both cases if the paged mobile station is in packet transfer mode and shall send the CS page on the PACCH.

NOTE 1: This paging co-ordination can be reused for GPRS mobile stations in mode of operation B, so that the mobile station does not need to listen to the PCH.

NOTE 2: This feature breaks the link between the presence of the Gs interface and the network capability to perform paging co-ordination. Alignment of 3G TS 23.060 is needed.

Once on the DCCH, the mobile station may request the re-establishment of the packet resources by sending a DTM Request message. The procedure to re-establish an aborted uplink TBF shall be identical to the MO session request. The procedure to re-establish an aborted downlink TBF shall be identical to the MT session request.

Figure 9 shows this procedure graphically.

NOTE: The IMSI is sent when available at the MSC and if the BSS supports the DTM feature.

Figure 9: Successful establishment of a CS connection while in packet transfer mode

Upon receiving the ASSIGNMENT REQUEST message from the MSC, the BSS may send one of the following messages to the MS:

– Channel Mode Modify message to modify the existing CS channel’s mode, as shown in Figure 9.

– DTM ASSIGNMENT COMMAND message to reallocate the CS resource and maintain some PS resources.

– ASSIGNMENT COMMAND message to reallocate the CS resource and drop the PS resources.

Figure 10: void (deleted)

If the mobile station and the network support enhanced CS establishment a CS connection may be established while in packet transfer mode, without release of the packet resources.

A mobile station that supports enhanced CS establishment can determine whether or not the network supports enhanced CS establishment by reading the GPRS Cell Options IE included within system information messages (see 3GPP TS 44.018 and 44.060).

In the mobile-originated case, the MS requests a CS connection by sending the PACKET CS REQUEST message on PACCH to the network.

If the contention resolution is not solved, the mobile station shall delay the transmission of the PACKET CS REQUEST message until contention resolution is solved.

If the countdown procedure has been started on all the ongoing uplink TBFs, none of those TBFs is operating in extended uplink TBF mode and there is no downlink TBF in progress, the mobile station may either send the PACKET CS REQUEST message, or may immediately release the ongoing TBF(s) and start an RR connection establishment as specified in 3GPP TS 44.018.

Upon receipt of the PACKET CS REQUEST message, the network replies to the MS with a PACKET CS COMMAND message on PACCH that encapsulates one of (RR) DTM ASSIGNMENT COMMAND, IMMEDIATE ASSIGNMENT, IMMEDIATE ASSIGNMENT REJECT messages as defined below:

– The network may allocate both PS and CS resources to the MS by sending a (RR) DTM ASSIGNMENT COMMAND message. When the MS receives this message it starts CS connection establishment and enters dual transfer mode. The network may also reallocate PS resources in the DTM ASSIGNMENT COMMAND message. In this case the resulting channel combination must be TCH + PDTCH, SDCCH + PDTCH is not allowed. By omitting the PS resource description in the DTM ASSIGNMENT COMMAND, the network indicates that the current PS Resources are maintained.

– The network allocates only CS resources to the MS and orders the release of PS resources by sending an (RR) IMMEDIATE ASSIGNMENT message. When the MS receives this message it releases the PS connection and establishes the CS connection. When in dedicated mode the MS may request PS resources by using the procedures specified in 3GPP TS 44.018.

– The network rejects the CS request by sending an (RR) IMMEDIATE ASSIGNMENT REJECT message. When the MS receives this message it continues in packet transfer mode normally. The mobile station may later reinitiate the CS connection request.

– If the PS resources have been dropped before the network has a chance to respond to the PACKET CS REQUEST, the network shall abort the current DTM procedure. If the mobile station does not receive a PACKET CS COMMAND message after it has sent a corresponding PACKET CS REQUEST message, the mobile station will drop any PS resources and start CS access procedures on the RACH.

If the network and mobile both support the extended RLC/MAC control message segmentation, the network may send the PACKET CS COMMAND message in more than two radio blocks, see 3GPP TS 44.060. If not, the network is responsible for ensuring that the PACKET CS COMMAND does not exceed two radio blocks in length.

Figure 10a illustrates succesful MS originated RR connection request procedure.

Figure 10a: MS originated RR connection request procedure

In the mobile-terminated case the BSS sends to the mobile station a PACKET CS COMMAND message on PACCH when receiving a CS paging message from the core network. The PACKET CS COMMAND message encapsulates one of (RR) DTM ASSIGNMENT COMMAND, IMMEDIATE ASSIGNMENT messages as defined below:

– The network may allocate both PS and CS resources to the MS by sending a (RR) DTM ASSIGNMENT COMMAND message. The network may also reallocate PS resources in the DTM ASSIGNMENT COMMAND message. In this case the resulting channel combination must be TCH + PDTCH, SDCCH + PDTCH is not allowed. By omitting the PS resource description in the DTM ASSIGNMENT COMMAND, the network indicates that the current PS Resources are maintained.

Figure 10b illustrates succesful MS terminated RR connection establishment.

Figure 10b: MS terminated RR connection establishment

6.1.4 PS establishment while in dual transfer mode

Once the mobile station is in dual transfer mode, the establishment of any further packet sessions shall be done with the existent mechanisms (see 3GPP TS44.060 [5]).

The network may send the DTM Assignment Command message to the mobile station at any time to reallocate one or more ongoing TBFs and the resources required for the RR connection. If the MS receives a DTM Assignment Command message after sending a request for one or more uplink TBFs but before receiving any uplink allocation in response to its request, it shall act on the DTM Assignment Command message and after successful reallocation of resources wait for the response from the network to the uplink TBF request on the newly allocated resources. All ongoing TBFs not addressed by the DTM Assignment Command message are released.