6.4 Location management

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

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

6.4.1 General

The behaviour of a mobile station in idle mode shall be the same as when operating in class B, except that a GPRS simple class A mobile in idle mode can perform the RA update procedure in a DCCH. When the mobile station is in dedicated mode, the change of serving cell may trigger location procedures that require both domains of the mobile station to become active.

In dedicated mode the mobile station shall check the roaming restrictions (i.e. the LAI or the PLMN identity of the current cell is not contained in any of the lists of "forbidden LAs for roaming", "forbidden LAs for regional provision of service", "forbidden PLMNs for GPRS service" or "forbidden PLMNs" respectively, see 3GPP TS 23.122 and 3GPP TS 24.008).

Table 3 contains a summary of the procedures to be carried out by a GPRS mobile station operating in Class A when crossing a boundary.

Table 3: Location update procedures for a GPRS mobile station operating in class A

Mode			CS idle			CS dedicated
Boundary			PS stand-by	PS ready		PS stand-by	PS ready
Cell; same RA			Nothing	Cell Update		Nothing	Cell Update
RA; same LA	NMO	I	Combined RA/LA update		RA update. When the CS connection ends in a RA different than the original, a combined RA/LA update with IMSI attach is performed
		II, III	RA Update

LA	NMO	I	Combined RA/LA update			RA update. When the CS connection ends in a LA different than the original, a combined RA/LA update with IMSI attach is performed
		II, III	Parallel RA and LA updates			RA update. When the CS connection ends in a LA different than the original an LA update is performed.

The request from GMM to perform a location management procedure may trigger the request of packet resources, as described above. The contents of the request message (e.g. DTM Request) should help the BSS decide the resources to be allocated.

RA update and LA update procedures shall be supported in parallel in the main DCCH with SAPI 0. This helps reduce the congestion caused by GPRS signalling on GPRS TCHs that naturally exists in cells on the border of a RA or RA/LA without noticeably affecting the QoS of the CS connection.

In addition to crossing cell boundaries, a DTM capable mobile station in GMM Ready state shall perform a Cell Update procedure each time that it enters dedicated mode from packet idle mode in a cell that supports dual transfer mode.

The following clauses clarify how the mobile station performs the cell update and location/routeing area update procedures while in dedicated mode. As previously indicated, the request of the establishment of dual transfer mode may trigger a change of the RR resources in the cell or a change of the serving cell. To simplify the diagrams below, possible assignment or handover procedures are ignored.

The following diagrams consider the worst case (no packet resources allocated) as it requires the establishment of uplink and -for RA Update- downlink TBFs. If an uplink TBF already exists, the initial steps leading to the uplink TBF establishment are not necessary. If a downlink TBF already exists, the uplink TBF can also be established as currently by sending the Channel Request Description information element in the Packet Downlink Ack/Nack message on the PACCH; see 3GPP TS 44.060 [5].

In case of DTM Handover the cell update or (non-combined, as a CS connection exists) RA update procedure is defined as in 3GPP TS 43.129 [13] and 3GPP TS 23.009 [14].

6.4.2 Cell update

Figure 12 and figure 13 show the exchange of messages involved in a Cell Update procedure when the mobile station is in dedicated mode, packet idle mode and Ready state. The mobile station shall request uplink resources, indicating "Cell Update". Typically, the BSS will command the MS to perform the Cell Update procedure in single timeslot operation (figure 12), although it may allocate an uplink TBF on a different time slot (figure 13) if the LLC frame contains user data. In the latter case, a change of the radio resources as was described in the previous clauses may happen before the MS sends the LLC frame on the TBF.

Figure 12: Cell Update procedure in dedicated mode, packet idle mode
and Ready state; performed on the main DCCH

Figure 13: Cell Update procedure in dedicated mode, packet idle mode
and Ready state; performed ion a TBF

6.4.3 Routeing Area update

Figure 14 and figure 15 show the message flow during the Routeing Area Update procedure under the same conditions (the MS in CS dedicated mode, packet idle mode and Ready state). Figure 14 shows the procedures when the main DCCH is allowed, whereas two TBFs are used in figure 15. In this case, the uplink TBF is created to send the Routeing Area Update Request. The Routeing Area Update Accept from the SGSN needs the previous establishment of a downlink TBF.

It should be noted that the steps performed after the RA Update Complete message in Figure 15 are optional since it is not a requirement to move the TCH/F back to its original position.

Figure 14: Routeing Area Update procedure in dedicated mode,
packet idle mode and Ready state; performed on the main DCCH

Figure 15: Routeing Area Update procedure in dedicated mode,
packet idle mode and Ready state; performed on TBFs

6.4.4 Location update

6.4.4.1 Change of Location Area in dedicated mode

Figure 16 shows the exchange of messages when changing location area while in dedicated mode. It is identical to the Routeing Area Update procedure except for the final group of messages. As the CS domain is not updated in the MSC while the MS is in a CS connection, a Location Area Update procedure is initiated when the CS connection ends to align the MM contexts in the MSC and the SGSN. This procedure consists of a Combined RA/LA Update with IMSI attach when the network is in mode I or a Location Area Update for modes II and III.

If the MS and the network in mode I support enhanced DTM CS release procedure and the location area of the MS has changed while in dual transfer mode, the MS shall send an indication to the network that in this case the enhanced DTM CS release procedure shall not be used. This indication is sent in the PACKET SI STATUS or PACKET PSI STATUS message. After the receipt of the indication the network shall release the RR connection and PS resources. Upon receipt of a CHANNEL RELEASE message the MS shall initiate the Combined RA/LA Update procedure with update type combined RA/LA updating with IMSI attach.

If the MS and the network in mode II or III support enhanced DTM CS release procedure and the location area of the MS has changed while in dual transfer mode, the MS may perform the enhanced DTM CS release procedure and, after the release of the RR connection, request CS resources via the enhanced DTM CS Establishment procedure for performing the Location Area Update procedure.

a) for Network Mode of Operation I;

b) for Network Mode of Operation II and III.

Figure 16: LA Update and RA Update procedures in CS dedicated mode,
packet idle mode and Ready state

6.4.4.2 Simultaneous Location Area and Routeing Area update procedures

When the mobile station is in idle mode and crosses a LA boundary, and hence an RA boundary, the mobile station can perform both location procedures (LA and RA update) on the main DCCH. Figure 17 shows the case of the RAU procedure finishing before the LAU. If the LAU procedure finishes before the RAU procedure does, the SDCCH is released and the RAU is completed on standalone TBF(s), as shown in figure 18.

NOTE: Alternatively, the BSC may hold the DCCH for a few seconds until the RAU is finished. This is an improvement of the implementation and has not been standardised.

Figure 17: Parallel LA and RA Update procedures: the RAU finishes first

Figure 18: Parallel LA and RA Update procedures: the LAU finishes first