6 Interoperability between GSM and UMTS

23.1213GPPArchitectural requirements for Release 1999Release 1999TS

The requirements for GSM – UMTS interoperability is defined in 22.129.

UMTS is a system supporting handovers between GSM and UMTS in both directions. To support these handovers effectively, the following is required from a dual mode MS/UE supporting simultaneous ISDN/PSTN and packet service in GSM/UMTS.

Depending upon the solution adopted for GSM-UMTS handover, the MS/UE supporting simultaneous ISDN/PSTN and packet service may be required to perform appropriate update into CN depending on the activity of the UE once the handover between GSM and UMTS is completed. This update is needed to avoid any severe interruptions on the accessibility of packet services after the handover.

The nature of the update to be made after the handover in both direction, i.e., from GSM to UMTS and from UMTS to GSM, from MS/UE depends on the activity of the UE in the following way:

– ISDN/PSTN connection: RA update only (if RA is changed);

– Packet connection: LA and RA update (if RA and LA are changed).

Both ISDN/PSTN and packet connection: RA update only (if RA is changed).

6.1 Circuit Switched Handover and Roaming Principles

Introduction of a UMTS Core Network necessitates the inter-connection with legacy systems to allow inter-PLMN roaming and handover.

For ease of convergence with the existing networks and the introduction of dual mode handsets, roaming and handover to/from UMTS should be performed in the simplest manner that requires as little change as possible to the legacy networks and standards, i.e. inter-MSC handover functionality.

These principles provide – from a user perspective – transparency of handover and roaming. In addition, operators providing UMTS services should also allow access to legacy networks using existing subscriber profiles and network interfaces.

Illustrated in figure 6.1 shows the introduction of a UMTS Core Network for UMTS phase 1 network configuration. Notice that it leaves the current GSM specifications mainly untouched whereupon the UMTS core network acts towards the GSM MSC like a GSM MSC by providing for example MAP/E for handover purposes. Further, it should be observed that GSM subscriptions belong to the HLR whilst UMTS subscriptions exist in the HLR release 99.

Figure 6.1: Inter-Operability between GSM and UMTS

NOTE 1: No physical implementation should be taken from the figure. As a further note, no interworking functions are shown to ease clarity, but however should not be precluded.

From figure 6.1 it can be seen that the information exchanged over the Iu must provide the necessary parameters to enable the core networks to communicate via for example the MAP interface for handover purposes.

NOTE 2: From the above diagram, existing interfaces are used towards the HLR to allow for subscription management based on today’s principles using the already defined user profile, providing seamless roaming between the 2nd generation system and UMTS.

The existing GSM handover procedures should be re-used to minimise the effects on existing GSM equipment.

– The anchor concept in GSM for inter-MSC handover should be used for inter-system handover between UMTS and GSM.

– The signalling over the A-interface and over the MAP/E-interface should be the same as in GSM phase 2+ with possibly addition of some new or updated information elements in some messages.

– For the set up of the handover leg (user plane) standard ISUP/POTS should be used in line with the principles used in GSM.

– The control signalling over the Iu-interface at handover between UMTS and GSM should be based on the A‑interface signalling at inter-MSC handover in GSM.

– The signalling over the Iu-interface at call set up to/from a dual mode UMTS/GSM mobile station, shall include GSM information elements needed for handover from UMTS to GSM. In the corresponding way the signalling over the A-interface at call set up to/from a dual mode UMTS/GSM mobile shall include UMTS elements needed for handover from GSM to UMTS. The data are needed to initiate the handover towards the new BSS/RNC.

– A target cell based on CGI is sent to the MSC from UTRAN at handover from UMTS to GSM. The CGI points out the target MSC and target BSC.

– A target cell based on CGI is sent to the MSC from the BSS at handover from GSM to UMTS. The CGI points out the target UMTS MSC and target RNC (UMTS MSC does the translation from CGI to RNC identity).

6.1.1 UMTS to GSM handover for circuit switched services

UMTS to GSM handover for circuit switched services is detailed in 23.009.

6.1.2 GSM to UMTS handover for circuit switched services

UMTS to GSM handover for circuit switched services is detailed in 23.009.

6.2 Packet Switched Handover and Roaming Principles

The introduction of a UMTS core Network illustrates the requirement for inter-connection with the legacy GSM system to allow inter-PLMN roaming and handover.

Even though there is no current GPRS deployment, the operator may decide to deploy a GPRS network prior to the deployment of a UMTS network. Therefore, the introduction of a UMTS Core Network may require to be inter-connected to the legacy packet network.

As in the circuit switched case, roaming and handover to/from UMTS should be performed in the simplest manner that requires as little change as possible to the GPRS network and standards, i.e. inter-GSN handover functionality. In addition, access is provided to the GPRS network using the existing subscriber profiles and current network interfaces.

A similar figure to figure 6.1 is illustrated in figure 6.2. Notice that it also leaves the current GPRS specifications mainly untouched whereupon the UMTS core network acts towards the GSN like a GSN by providing for example Gn. Further, it should be observed that GPRS subscriptions belong to the HLR whilst UMTS subscriptions exist in the HLR release 99.

Figure 6.2: Inter-Operability between GSNs and UMTS

NOTE 1: No physical implementation should be taken from figure 6.2. As a further note, no interworking functions are shown to ease clarity, but however should not be precluded.

From figure 6.2 it can be seen that to provide inter-working between legacy packet switched and UMTS packet switched services, the information exchanged over the Iu must provide the necessary parameters to enable the core networks to communicate via for example the Gn interface for handover purposes.

NOTE 2: From the above diagram, the same principles are used as in the circuit switched services to provide seamless roaming.

6.2.1 Implications

– The active PDP context resides in the same GGSN even after a handover between GSM and UMTS (both directions). This corresponds in principle to the anchor concept on the circuit switched side, but note that whereas packet sessions are long lived, the anchor MSC remains only for the duration of a CS call (typically much shorter than a packet session).

– Assuming an internal structure in UMTS CN that contains logical GGSN and SGSN nodes, the signalling over the inter-system GGSN-SGSN interface should be a joint evolution of Gn for the GSM system and UMTS. I.e. when Gn evolves in the sequence of GSM releases, Gn should include any new or updated information necessary for interoperation.

– The corresponding SGSN-SGSN inter-system interface (also Gn) should also be evolved together. However, in this case the changes relative to the current GPRS release may possibly be more profound.

6.2.2 Signalling procedures

The signalling procedures shows how handover UMTS <-> GSM GPRS can be done. The parameters carried by each message is not complete and shall be seen as examples of important information carried be the messages.

The signalling sequences shows the case when the UMTS 3G_SGSN and the GPRS 2G_SGSN are located in separate "physical" nodes.

If the 3G_SGSN and 2G_SGSN are located within the same "physical" node, no signalling are needed between 3G_SGSN and 2G_SGSN.

For handover in the UMTS to GSM GPRS direction the intention is to re-use the handover principles of GSM GPRS today in order to limit the changes in GSM GPRS and to take the changes if any on the UMTS side. The below specified messages is standard GSM 2+ messages (when applicable).

6.2.2.1 Handover from UMTS to GSM GPRS

Handover from UMTS to GSM GPRS is detailed in 23.060.

6.2.2.2 Handover from GSM GPRS to UMTS

Handover from GSM GPRS to UMTS is detailed in 23.060.

Annex A (informative):
Reduction of UMTS signalling