Content for TS 23.221 Word version: 18.0.0
6.11 Signalling connection establishment
6.11a CS Domain Signalling Requirements (in particular relating to handover)
6.12 Relations between SRNS relocation and location registration
6.13 Requirements on identifiers for UMTS and GSM
6.14 Use of MM system information
6.15 Signalling procedures
6.16 RAN coordination
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6.11 Signalling connection establishment p. 30
A signalling connection between the UE and a CN node refers here to a logical connection consisting of an RRC connection between UE and the RAN and an S1 or Iu signalling connection between the RAN and the CN node. The signalling connection is used for transfer of higher layer (MM, CM) information between the UE and the CN node.
At a service request to one of the CN domain types and when no such connection exists towards the applicable CN domain type, the UE shall request establishment of a new signalling connection.
If no RRC connection exists, this is established in conjunction with (before) the transfer of the signalling establishment request. At the RRC connection establishment, an UE context is built up in the SRNC.
If an RRC connection is already established, the UE shall send the signalling establishment request using that RRC connection.
At reception of the signalling establishment request, the RAN will establish an S1 or Iu connection towards the CN node indicated by the CN service domain type received from UE.
6.11a CS Domain Signalling Requirements (in particular relating to handover) p. 31
Correct operation of the Call Control, Mobility Management and Call Independent Supplementary Service protocols requires that downlink messages from the MSC shall be delivered in the correct order and shall not be lost, duplicated or delivered in error.
The RAN and Iu/A interfaces shall provide this functionality in all cases except for when the Iu/A interface SCCP connection is being changed, e.g. at SRNS relocation or inter-BSC (external) handover.
When the SCCP connection is being changed, the MSC shall buffer downlink CC, MM and CISS messages. Specifically, the MSC shall buffer messages from these protocols after transmission of a (BSSMAP) Handover Command or RANAP-Relocation Command message and until receipt of a Handover Complete, Relocation Complete, Handover Failure or Relocation Cancel message.
In the uplink, the UE is responsible for delivering the CS domain messages across the radio interface. Once the message has been received by part of the network, it is the network's responsibility to deliver the message to the MSC. This can result in duplicate message delivery to the CN. The RAN shall ensure that the protocol used between UE and RAN permits any duplicate messages that are delivered to the CN, to be correctly discarded by N(SD) mechanism specified in TS 24.007 for the uplink CC, MM and CISS messages.
6.12 Relations between SRNS relocation and location registration p. 31
This clause clarifies the need for separate handling of MM registration area (LA and RA) information in RRC idle mode respective in RRC connected mode. The following example illustrates relations between SRNC relocation, registration area (LA/RA) change and LA/RA updates. As shown in the example, this is equally applicable for a combined MSC + SGSN as well as the 3G-MSC/VLR and 3G-SGSN.
Preconditions (see Figure 6.7):
- LA1 is handled by 3G_MSC/VLR1 and LA2 is handled by 3G_MSC/VLR2 ;
- RA1 is handled by 3G_SGSN1 and RA2 is handled by 3G_SGSN2 ;
- UE is registered in LA1 in 3G_MSC/VLR1 and in RA1 in 3G_SGSN1;
- the UE is in PS-CONNECTED state and a signalling connection exists between UE and 3G_SGSN1;
- the UE is in CS-IDLE state and no signalling connection exists between UE and 3G_MSC/VLR1;
- RNC1 is acting as SRNC and RNC2 is acting as DRNC;
- UE is in RRC cell connected state and with dedicated channels established to cells within both RNC1 and RNC2. UE does not listening to the PCH;
- the registration area information sent to the UE indicates LA1 and RA1.
Figure 6.7: Illustration of the preconditions in the described example
(⇒ copy of original 3GPP image)
(⇒ copy of original 3GPP image)
In Figure 6.7 MSC stands for 3G_MSC/VLR and SGSN for 3G_SGSN.
The UE moves now further towards the right, leaving the coverage area of cells controlled by RNC1, and resulting in that the UE has dedicated channel(s) established to cell(s) within only RNC2. This can result in the following sequence of events:
- the SRNC (RNC1) can decide to perform an SRNC relocation resulting in that the RNC2 becomes SRNC. In this example, the change of SRNC also implies a change of SGSN with an update of the UE location registration for the PS domain;
- after this SRNC relocation or combined with this procedure, the MM registration area information sent to the UE is changed and indicates now LA2 and RA2;
- the UE initiates a LA update, which results in a registration change from LA1 in 3G_MSC/VLR1 to LA2 in 3G_MSC/VLR2 and results in changed MM registration information.
6.13 Requirements on identifiers for UMTS and GSM p. 32
1a)
The format of the UMTS Location Area Identifier and UMTS TMSI shall not prevent a dual mode GSM-UMTS mobile which was last location updated over the GSM radio interface (i.e. has a GSM LAI and GSM TMSI), from performing a location update (or other signalling) over the UMTS radio interface to a UMTS MSC.
1b)
The format of the UMTS Location Area Identifier and UMTS TMSI shall not prevent a dual mode GSM-UMTS mobile which was last location updated over the UMTS radio interface (i.e. has a UMTS LAI and UMTS TMSI), from performing a location update (or other signalling) over the GSM radio interface to a GSM MSC.
1c)
The format of the UMTS Routing Area Identifier and UMTS P-TMSI shall not prevent a dual mode GSM UMTS mobile which was last routing area updated over the GSM radio interface (i.e. has a GSM RAI and GSM P-TMSI), from performing a routing area update (or other signalling) over the UMTS radio interface to a UMTS SGSN.
1d)
The format of the UMTS Routing Area Identifier and UMTS P-TMSI shall not prevent a dual mode GSM UMTS mobile which was last routing area updated over the UMTS radio interface (i.e. has a UMTS RAI and UMTS P-TMSI), from performing a routing area update (or other signalling) over the GSM radio interface to a GSM SGSN.
2)
The standard shall support means by which an operator can configure GSM and UMTS cells to be members of the same registration area (i.e. the mobile can receive paging from whichever cell it is camped on and does not need to location update (or routing update) just because the mobile has changed from a UMTS to a GSM cell).
3a)
The standard shall support means by which an operator can allocate GSM and UMTS LAIs which enable GSM MSCs to be able to contact UMTS MSCs and vice versa.
3b)
The standard shall support means by which an operator can allocate GSM and UMTS RAIs which enable GSM SGSNs to be able to contact UMTS SGSNs and vice versa.
4)
The standard shall support means by which an operator can ensure that the IMSI does not need to be sent over the radio interface when the mobile station moves from a GSM cell to a UMTS cell (and vice-versa).
5)
The standard shall support means by which an operator can ensure that the IMSI does not need to be sent over the radio interface when a USIM is moved from a UMTS mobile station to a GSM mobile station (and vice versa).
6)
The standard need not support means by which an operator can ensure that the IMSI is not sent over the radio interface when a GSM SIM is moved from a GSM mobile station to a UMTS mobile station (and vice-versa).
6.14 Use of MM system information p. 33
In each cell the network broadcasts MM system information on the broadcast channel. . The "current MM system information" is used by the MM functionality in the UE respecting the rules for the UE service state of the respective MM state machine, see clause 6.6. The "current MM system information" is identified as follows:
- In RRC idle mode, when the UE camps on one cell, it receives all MM system information valid for this cell on the broadcast channel of the cell. The received MM system information is then the "current MM system information";
- In UTRAN RRC connected mode, the SRNS shall control the current MM system information valid for the UE. e.g. at SRNS relocation, the new SRNS may send applicable MM system information to the UE. The established RRC connection shall be used for transferring any new MM system information to the UE. The UE shall use any new MM system information received from the SRNC on the established RRC connection as the "current MM system information";
- In E-UTRAN RRC connected mode, the UE receives all MM system information valid for this cell on the broadcast channel of the cell. The received MM system information is then the "current MM system information";
- At the RRC connection establishment, the UE uses the broadcasted MM system information of the cell where the establishment is made as the "current MM system information";
- When the UE leaves the RRC connected mode and enters RRC idle mode, the UE uses the broadcasted MM system information of the chosen cell, which is determined by the UE idle mode cell selection/re-selection process that is then performed, as the "current MM system information".
6.15 Signalling procedures p. 33
6.15.1 Idle mode procedures p. 33
The signalling procedures shown in the following clauses do not represent the complete set of possibilities, nor do they mandate this kind of operation. This document specifies a set of elementary procedures for each interface, which can be combined in different ways in an implementation. Therefore these sequences are merely examples of a typical implementation. By default the combined procedures as defined in TS 23.060 or TS 23.272 are also applicable when using Gs or SGs.
The list of parameters should be regarded as examples of possible information carried by the messages and not as a complete list.
6.15.1.1 Location Area update p. 34
Figure 6.8 shows location registration when changing LA including change of 3G-MSC/VLR and when the UE is in MM idle state towards the 3G_MSC/VLR.
The illustrated transfer of MM signalling to/from the UE uses an established RRC connection. This RRC connection can have been established beforehand due to ongoing interwork between UE and 3G-SGSN or be established only for this location registration procedure towards the 3G_MSC/VLR.
For each indicated MM message sent in this case to/from UE, the CN discriminator indicates 3G_MSC/VLR.
Figure 6.8: Interface information transfer for location update when changing VLR area
(⇒ copy of original 3GPP image)
(⇒ copy of original 3GPP image)
Step 1.
The RRC connection is established, if not already done. The UE sends the initial message LA Update Request (old TMSI, old LAI, etc.) to the new 3G_MSC/VLR. The old TMSI and LAI are those that were assigned to the UE. The SRNS transfers the message to the 3G_MSC/VLR
The RAN shall add the RAC and the LAC of the cell where the message was received before passing the message to the MSC.
Step 2.
The new 3G_MSC/VLR sends an Send Identification Request (old TMSI) to the old 3G_MSC/VLR to get the IMSI for the UE. (The old LAI received from UE is used to derive the old 3G_MSC/VLR identity/address.) The old 3G_MSC/VLR responds with Send Identification Ack. (IMSI and Authentication triplets).
Step 3.
Security functions may be executed.
Step 4.
The new 3G_MSC/VLR inform the HLR of the change of 3G_MSC/VLR by sending Update Location (IMSI, MSC address, VLR number) to the HLR.
Step 5.
The HLR cancels the context in the old 3G_MSC/VLR by sending Cancel Location (IMSI). The old 3G_MSC/VLR removes the context and acknowledges with Cancel Location Ack.
Step 6.
The HLR sends Insert Subscriber Data (IMSI, subscription data) to the new 3G_MSC/VLR. The new 3G_MSC/VLR acknowledges with Insert Subscriber Data Ack.
Step 7.
The HLR acknowledges the Update Location by sending Update Location Ack. to the new 3G_MSC/VLR.
Step 8.
The new 3G_MSC/VLR validates the UE presence in the new LA. If due to regional, national or international restrictions the UE is not allowed to attach in the LA or subscription checking fails, then the new 3G_MSC/VLR rejects the LA update with an appropriate cause. If all checks are successful, then the new 3G_MSC/VLR responds to the UE with LA Update Accept (new TMSI, new LAI).
Step 9.
The UE acknowledges the new TMSI with a TMSI reallocation Complete. (TMSI can optionally be reallocated with the TMSI reallocation procedure).
Step 10.
When the location registration procedure is finished, the 3G_MSC/VLR can release the signalling connection towards the SRNS for the concerned UE. The SRNS shall then release the RRC connection if there is no signalling connection between 3G_SGSN and SRNS for the UE.
6.15.1.2 Routing Area Update p. 35
The routing area update procedure is detailed in TS 23.060.
6.15.1.2A Tracking Area Update |R8| p. 35
The tracking area update procedure is detailed in TS 23.401.
6.15.1.3 Periodic registration towards both CN nodes without use of Gs or SGs p. 35
Periodic registration for the CS domain is specified in TS 23.012. Periodic registration for the PS domain is covered in TS 23.060 and in TS 23.401.
6.15.1.4 Periodic registration with use of Gs or SGs p. 35
Periodic registration for the PS domain with use of Gs or SGs is covered in TS 23.060 or in TS 23.272. Only TA/RA Updates are required from the UE to the SGSN or MME.
6.15.1.5 UE initiated combined detach procedure when using Gs or SGs p. 35
UE initiated Combined Detach Procedure when using Gs or SGs is specified in TS 23.060 or in TS 23.272. The UE indicates which form of detach is required, i.e., PS Detach only, CS Detach only or combined Detach.
6.15.1.6 Forbidden LA/RA |R5| p. 35
The CN (SGSN and MSC/VLR) shall not send the COMMON Id message with SNA information to the UTRAN when the attach request, LA update, or RA update are rejected.
6.15.2 SRNS Relocation p. 35
6.16 RAN coordination p. 35
This applies to UTRAN only. The RAN coordinates the resource allocation of an UE attached to both PS and CS services. The UTRAN shall reject or downgrade a connection which cannot be granted, see TS 23.060. The cause might be congestion on the radio interface, or the existence of other connections between this UE and the other CN.
The RAN uses the IMSI to identify a UE. The IMSI is transferred from the CN to the RAN with the common ID procedure. When an Iu connection is established, the CN shall perform the RANAP common ID procedure toward RAN as soon as the UE is identified (IMSI). The IMSI is only stored in the RAN for the duration of the RRC Connection.
There are two functions that are co-ordinated.
- Paging coordination is described in clause 5.9.1 of TS 25.410; and
- Relocation coordination that is described in clause 8.7.5 in TS 25.413.
