WO2002015620A1

WO2002015620A1 - Upgrading a gsm system to a combined gsm/umts system

Info

Publication number: WO2002015620A1
Application number: PCT/SE2001/001754
Authority: WO
Inventors: Åke ARVIDSSON; Mikael Willgert; Håkan NISKA; Kyösti TOIVANEN
Original assignee: Telefonaktiebolaget L M Ericsson (Publ)
Priority date: 2000-08-16
Filing date: 2001-08-14
Publication date: 2002-02-21
Also published as: SE0002946D0; AU2001280397A1

Abstract

The present invention relates to methods and means for upgrading a cellular radio communication system. A GSM system with GSM MSC/VLR's is upgraded to a combined GSM/UMTS system that provides both GSM and UMTS services. A cluster of GSM/UMTS MSC/VLR's, a number of RNC's with corresponding Node B's are added (501, 502) to the GSM system. The cluster of GSM/UMTS MSC/VLR's are connected (503) to both the BSC's in the GSM access network and the RNC's in the UMTS acces network. An MSC/VLR address function is added (504) in which each MSC/VLR's in the system is identified by one or more core network node identifiers.

Description

Upgrading a GSM system to a combined GSM/UMTS system.

TECHNICAL FIELD OF THE INVENTION

The present invention relates in general to the field of cellular radio communication and, in particular, to a method and means for upgrading a cellular radio communication system.

DESCRIPTION OF RELATED ART

A cellular radio communication system is normally divided into an access network and a core network. The access network in GSM includes the Base Transceiver Stations (BTS) and control nodes such as the Base Station Controller (BSC) while the core network includes core network nodes such as the Mobile Switching Centre (MSC) , the Visiting Location Register (VLR) , the Home location Register (HLR) and also Serving GPRS Support Nodes (SGSN) if General Packet Radio Service (GPRS) is included in the system.

The GSM system belongs to the 2^nd generation radio communication systems. Today an evolution of the 2ⁿ generation GSM system (including General Packet Radio Service (GPRS) and Enhanced Data services for GSM Evolution (EDGE) ) and also 3^rd generation radio communication systems are on their way. The 3^rd generation systems (called IMT 2000 by ITU, the International Telecommunication Union) includes the Universal Mobile Telecommunication Systems (UMTS) .

The access network in UMTS includes the base transceiver stations (Node B) and control nodes such as the Radio Network Controller (RNC) while the core network includes the same types of nodes as the GSM system (as described above) . In known GSM and UMTS systems, each control node in the access network (BSC/RNC) is connected to one MSC. This means that each control node always sets up connections with one dedicated MSC. It is a common practice to combine/integrate each MSC with a VLR into an MSC/VLR which means that each control node sets up connections with one dedicated MSC/VLR. An MSC/VLR is also an example of a switching node that belongs to the core network nodes in the GSM and UMTS systems. The VLR part of the MSC/VLR is in charge of temporary storing subscription data as well as holding data on the location of subscribers at a more precise level than the HLR.

The GSM and UMTS systems are divided into different geographical areas to simplify the management of the subscribers. A first one is called a cell, which is the area that a BTS or a Node B is serving. A number of cells are then grouped into a second area, which is called a Location Area (LA) . Each Location Area is associated with one VLR or MSC/VLR. A third area is called an MSC Service Area, which is the area in which an MSC or an MSC/VLR is serving. An MSC Service Area may include one or several LA's.

A Mobile Station that is attached to the cellular system is registered in the MSC/VLR as present in one Location Area. An MSC handover or a Location Update is performed each time the Mobile Station is moved out of the current MSC service area into a new MSC service area (which is served by a new MSC or MSC/VLR) . A Location Update is performed each time an idle Mobile Station is moved to a new Location Area.

A known solution to upgrade a GSM system to a combined GSM/UMTS system is to upgrade all the GSM switching nodes, i.e. MSC's or MSC/VLR's, to combined GSM/UMTS MSC's (or GSM/UMTS MSC/VLR's) and then connect a number of Radio Network Controllers (RNC) to each GSM/UMTS MSC in the respective MSC Service Areas . The upgrade can be done by replacing the old GSM MSC's with new GSM/UMTS MSC's or by adding new UMTS units in the old GSM MSC's. The upgrade must be made in all MSC Service Areas with some UMTS coverage, which means that a GSM MSC must be upgraded to a GSM/UMTS MSC even if only a very small part of the MSC Service Area is to be used for UMTS. Hence, large investments have to be made even if the new UMTS services are only applied in limited areas.

The number of users that will use and pay for the new UMTS services provided by the GSM/UMTS systems might not be so big in the beginning. This means that the operators and/or network providers want to introduce the new UMTS services gradually, at low cost and possibly only in specific and limited areas where there are a high number of possible UMTS customers. This is not possible with the known solution mention above.

It is therefore desired to provide a method and means that allows a flexible and low cost introduction of UMTS services in a GSM system.

By a Mobile Station (MS) is meant all portable equipment intended for radio communication, like mobile phones, transceivers, pagers, electronic notebooks, laptops with integrated radios, communicators, tailored microchips connecting to radios or any other portable electronic equipment using a radio link as a mean of communication. SUMMARY

The present invention meets a problem related to a GSM system.

The problem is to upgrade a GSM system to a combined GSM/UMTS system.

In light of the foregoing, a primary object of the present invention is to provide a method and means for upgrading a GSM system to a combined GSM/UMTS system in a more flexible and less expensive way than what is known today.

Another object of the present invention is to provide methods and means for reducing the number of new switching nodes that are needed to provide UMTS services in a combined GSM/UMTS system.

Yet another object of the present invention is to provide methods and means for providing switching nodes with different services in one and the same cellular radio communication system.

Accordingly, the present invention provides a method for upgrading a GSM system to a combined GSM/UMTS system as claimed in claim 1.

The present invention further provides a combined GSM/UMTS system according to the present invention as claimed in claim 11.

Embodiments of the present invention are characterised as it appears from the subclaims .

An advantage with the present invention is that the upgrade of the system can be made gradually.

Another advantage is that the already installed GSM switching nodes do not need to be upgraded. Still another advantage is that the number of new switching nodes are kept to a minimum.

Yet another advantage is that it is possible to page a mobile station in only one of the access networks in the combined GSM/UMTS system.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is illustrating a view of a known GSM system.

Figure 2 is illustrating a view of a known GSM/UMTS system

Figure 3 is illustrating a flow chart of a first embodiment of a method according to the present invention.

Figure 4 illustrating a view of a first embodiment of a system according to the present invention.

Figure 5 is illustrating a flow chart of a second embodiment of a method according to the present invention.

Figure 6 is illustrating a view of a second embodiment of a system according to the present invention.

Figure 7 is illustrating a view of the second embodiment of a system with a packet network connection according to the present invention.

Figure 8 is illustrating a view of a TMSI information element . DETAILED DESCRIPTION OF EMBODIMENTS

The present invention relates to a method and means for upgrading a GSM system to a cellular radio communication system that can provide both GSM and UMTS services, i.e. a combined GSM/UMTS system.

Figure 1 illustrates a simplified view of a known GSM system 100 with a GSM radio coverage area 113. The GSM system 100 includes five Base Station Controllers (BSC) 101-105. BSC 101-103 are connected to a first Mobile Switching Centre/Visiting Location Register (MSC/VLR) 109 that is serving a first MSC Service Area 111 and BSC 104-105 are connected to a second Mobile Switching Centre/Visiting Location Register (MSC/VLR) 110 that is serving a second MSC Service Area 112. The BSC's 101-105 are access network nodes and part of the GSM access network and the MSC/VLR 109-110 are core network nodes and are part of the GSM core network. Other parts of the GSM system, such as the Base Transceiver Stations and Home Location Register, are excluded to simplify the figure.

Figure 2 illustrates a simplified view of a GSM/UMTS system 200 with the GSM radio coverage area 113 and an UMTS radio coverage area 206. The GSM/UMTS system 200 is an upgraded version of the GSM system 100 in figure 1. The GSM system 100 has been upgraded to provide UMTS services according to a known method and architecture. The GSM/UMTS system 200 includes the five Base Station Controllers (BSC) 101-105 and three (new) Radio Network Controllers (RNC) 201-203. BSC 101-103 and RNC 201 are connected to a first GSM/UMTS MSC/VLR 204 that is serving the first MSC Service Area 111 and BSC 104-105 and RNC 202-203 are connected to a second GSM/UMTS MSC/VLR 205 that is serving the second MSC Service Area 112. The GSM/UMTS MSC/VLR's 204-205 have replaced the GSM MSC/VLR's 109-110 in the upgraded GSM/UMTS system 200. A part of the first MSC Service Area 111 has been provided with UMTS services (i.e. via RNC 201) and the second MSC Service Area 112 has been provided with UMTS services (i.e. via RNC's 202-203). This means that the GSM radio coverage area 113 is larger than the UMTS radio coverage area 206 as seen in figure 2. The BSC's 101-105 are access network nodes in the GSM access network and the RNC's 201-203 are access network nodes in an UMTS access network. The GSM/UMTS MSC/VLR's 204-205 are core network nodes and part of the same GSM/UMTS core network. Other parts of the GSM/UMTS system, such as the Base Transceiver Stations (connected to the BSC's), Node B's (connected to the RNC's), the Home Location Register etc, are excluded to simplify the figure.

Figure 3 illustrates a flow chart of a first embodiment of the method according to the present invention. The method is applied on the GSM system 100 as illustrated in figure 1. An inventive combined GSM/UMTS system 400, that is created by this method of updating, is illustrated in figure 4. References below will be made to figure 3 as well as figure 1 and 4.

According to a step 301 in figure 3, one GSM/UMTS MSC/VLR 401 is added to the GSM system 100.

According to a step 302, one RNC 402 in MSC service area 111 and two RNC's 403-404 in MSC Service Area 112 are added to the GSM system 100. A number of Node B's (NB) , i.e. UMTS base stations, 406-410 are also added and connected to the RNC's. The Node B's are arranged in such a way that their corresponding UMTS cells 411-415 cover the UMTS radio coverage area 206. The UMTS cells 411-415 are defined to belong to (or grouped into) UMTS location areas, which are separated from the GSM location areas that the GSM cells belongs to. This will enable the system 400 to smoothly allocate GSM/UMTS Mobile Stations to a GSM/UMTS MSC/VLR. According to a step 303, the GSM/UMTS MSC/VLR 401 is connected to all RNC's in the system, i.e. RNC 402-404, and to all BSC's in the system, i.e. BSC 102-105, so that the GSM/UMTS MSC/VLR 401 can serve Mobile Stations in MSC service area 111 and 112 with both GSM and UMTS services. This means that two groups of MSC/VLR's (i.e. two pools of core network nodes) have been created. The GSM/UMTS MSC/VLR 401 forms a first pool 416 with GSM MSC/VLR 109 that can serve the MSC Service Area 111 with GSM and UMTS services. The GSM/UMTS MSC/VLR 401 forms a second pool 417 with GSM MSC/VLR 110 that can serve the MSC Service Area 112 with GSM and UMTS services. This means that each pool is serving a MSC service area in this embodiment.

According to a step 304, an MSC/VLR address function for the first and the second pool 416, 417 respectively is added to the system. The MSC/VLR address function enables the BSC's and RNC's to address the right one of the two MSC/VLR's in the respective MSC/VLR pool. More details regarding this address function are disclosed further on in this description. Another function that can be added is a core network node addressing co-ordination procedure to facilitate smooth and fault free address allocations in the system.

The combined GSM/UMTS system 400 is now able to serve Mobile Stations with combined GSM/UMTS capabilities as well as GSM only capabilities. Mobile Stations with combined GSM/UMTS capabilities will be served by MSC 401 as long as they remain in the MSC Service Areas 111 and 112, apart from a possible initial phase. If, as an example, a Mobile Station with GSM/UMTS capabilities is initially connected to a GSM cell in MSC Service Area 111, it may be served by MSC 109. If the Mobile Station at a later stage selects an UMTS cell (in an UMTS location area) , it will perform a location update through e.g. RNC 402 to GSM/UMTS MSC/VLR 401. The GSM/UMTS MSC/VLR 401 will recognise that the Mobile Station is currently served by MSC 109 (e.g. by the core network node identifier assigned by the GSM MSC/VLR 109) and then assign a new core network node identifier to the Mobile Station. The new core network node identifier is allocated to GSM/UMTS MSC/VLR 401 (but not yet assigned to any Mobile Stations) and selected from its own sub-range of identifiers. From this point in time the Mobile Station will be supported with GSM and UMTS services by the GSM/UMTS MSC/VLR 401 as long as it remains in the MSC Service Areas 111, 112 respectively.

Figure 4 illustrates a simplified view of a first embodiment of an inventive combined GSM/UMTS system 400 that has been upgraded from the GSM system 100 in figure 1 according to the first embodiment of the inventive method (figure 3) .

The combined GSM/UMTS system 400 includes the five Base Station Controllers (BSC) 101-105 from the GSM system 100 and the three new Radio Network Controllers (RNC) 402-404 with Node B's 406-410 and their corresponding UMTS cells 411-415. The BSC's 101-105 are connected to a number of BTS's (illustrated as dashed squares) that serve one GSM cell each (illustrated as dashed ellipses) . This means that the GSM access network includes the BSC's with their BTS's and that the UMTS access network includes the RNC's with their Node B's. The RNC's 402-404 are connected to the single GSM/UMTS MSC/VLR 401 that is serving both the first MSC Service Area 111 and the second MSC Service Area 112 with GSM and UMTS services. The BSC's 101-105 are still connected to their respective GSM MSC/VLR as in figure 1 but also connected to the GSM/UMTS MSC/VLR 401. There is no need in this inventive system to upgrade any existing switching nodes, e.g. GSM MSC/VLR 109 and 110, to provide GSM/UMTS services. The GSM MSC/VLR's 109 and 110 continue to give GSM services to Mobile Stations in their respective service areas. Mobile Stations requiring UMTS services are directed to the GSM/UMTS MSC/VLR 401. The inventive GSM/UMTS system 400 has the same GSM radio coverage area 113 and UMTS radio coverage area 206 as the known GSM/UMTS system 200 as illustrated in figure 2 but will only require one new GSM/UMTS MSC/VLR in stead of two, i.e. the inventive system "saved" one new GSM/UMTS switching node. The number of "saved" GSM/UMTS switching nodes can be higher in larger GSM/UMTS systems with more than two GSM MSC's. The number of saved GSM/UMTS switching nodes is of course dependent on the UMTS capacity that is desired in the upgraded system.

The connection 405 between the GSM/UMTS MSC/VLR 401 and the control nodes 102-105 and 402-404, i.e. the RNC's and BSC's, may be arranged by circuit connections as illustrated in figure 4 or by packet connections (e.g. an IP network) . All the control nodes 102-105 and 402-404 in the access network can access the GSM/UMTS MSC/VLR 401 in the GSM/UMTS system 400.

The number of GSM/UMTS MSC/VLR's in the inventive system can be increased gradually in a flexible way by using a second embodiment of the inventive method, if there is a need to increase the UMTS (and GSM) capacity compared to the system illustrated in figure 4.

Figure 5 illustrates a flow chart of the second embodiment of the method according to the present invention. The method is applied on the GSM system 100 as illustrated in figure 1. A second inventive combined GSM/UMTS system 600 that is created by this method of updating is illustrated in figure 6. References below will be made to figure 5 as well as figure 1 and 6.

According to a step 501 in figure 5, three GSM/UMTS MSC/VLR's 401a-c are added to the GSM system 100. These three GSM/UMTS MSC/VLR's 401a-c are arranged as a cluster 601 of GSM/UMTS MSC/VLR's. According to a step 502, the RNC 402 in MSC service area 111 and the two RNC's 403-404 in MSC Service Area 112 are added to the GSM system 100. A number of UMTS base stations (Node B's with their corresponding UMTS cells) are also added (not illustrated) and connected to each RNC in system 600 (as in system 400 according to figure 4) . The Node B's are arrange in such a way that their corresponding UMTS cells cover the UMTS radio coverage area 206. These UMTS cells do not need to have any correspondence with GSM cells in the GSM system 100. The UMTS cells are therefore defined to belong to (or grouped into) UMTS location areas, which are separated from the GSM location areas that the GSM cells belongs to .

According to a step 503, the GSM/UMTS MSC/VLR's 401a-c are connected to all RNC's in the system, i.e. RNC 402-404, and to all BSC's in the system, i.e. BSC 102-105, so that all the GSM/UMTS MSC/VLR's 401a-c in cluster 601 can serve Mobile Stations in both MSC service area 111 and 112. This means that a first and a second pool of switching nodes 602, 603 respectively have been created. The GSM/UMTS MSC/VLR's 401a-c, i.e. the cluster 601, forms the first pool 602 with GSM MSC/VLR 109. The cluster 601 is also part of the second pool 603 which it forms with GSM MSC/VLR 110. This means that the cluster of switching nodes 601 is a common cluster of GSM/UMTS switching nodes that is arranged to serve the whole system 600, i.e. both in the first pool 602 and in the second pool 603, while the "old" GSM MSC/VLR's 109-110 are still arranged to serve their respective MSC service area, i.e. dedicated parts of system 600.

According to a step 504, an MSC/VLR address function is added to the system. The MSC/VLR address function enables the BSC's and RNC's to address the right MSC/VLR in the first and second pool of switching nodes 602 and 603 respectively. More details regarding this address function are disclosed further on in this description. A core network node addressing co-ordination procedure can also be added to facilitate a smooth and fault free address allocations in the system 600.

The combined GSM/UMTS system 600 is now able to serve Mobile Stations with combined GSM/UMTS capabilities as well as GSM only capabilities in the same way as the GSM/UMTS system 400 illustrated in figure 4. One of the GSM/UMTS MSC/VLR's in the cluster 601 will be selected at a location update through one of the RNC's, e.g. RNC 402. The RNC may select one of the MSC/VLR's through the address function (e.g. dependent on the service that the mobile station requires) or by a load sharing mechanism (to select the least loaded core network node) .

Figure 6 illustrates a simplified view of the inventive combined GSM/UMTS system 600 that has been upgraded according to the second embodiment of the inventive method. The GSM/UMTS system 600 includes the -^■ five Base Station Controllers (BSC) 101-105 from the GSM system 100 and the three Radio Network Controllers (RNC) 402-404 (the BTS's, with corresponding GSM cells, connected to the BSC's and the Node B's, with corresponding UMTS cells, connected to the RNC's are not illustrated). The GSM access network includes the BSC's with their BTS's and the UMTS access network includes the RNC's with their Node B's in system 600 as in system 400. The RNC's 402-404 are connected to the cluster of switching nodes 601, i.e. GSM/UMTS MSC/VLR's 401a-401c. The cluster 601 is serving both the first MSC Service Area 111 and the second MSC Service Area 112 with GSM and UMTS services, i.e. the whole system 600. The BSC's 101-105 are still connected to their respective GSM MSC/VLR as in figure 1 but also connected to the cluster of GSM/UMTS MSC/VLR's 601 (and forming the first and second pool 602, 603 respectively as described in connection with step 503 above) . Hence, there is no need here to upgrade any existing switching nodes, e.g. GSM MSC/VLR 109 and 110, because the

GSM and GSM/UMTS switching nodes can co-exist in the inventive combined GSM/UMTS system 600 (as they do in the combined GSM/UMTS system 400) .

The UMTS cells can be arranged according to a cell plan that is different from the cell plan in the existing GSM system. The UMTS Location Areas are different from the GSM Location Areas of the existing GSM system. This makes it possible to page for dual GSM/UMTS Mobile Stations in only one access network, either the GSM access network or the UMTS access network as the Location Areas, i.e. the GSM LA's and UMTS LA's, do not have "mixed" GSM/UMTS cells.

The connection 405 between the cluster of GSM/UMTS MSC/VLR's 601 and the control nodes 102-105, 402-404 respectively, i.e. the RNC's and BSC's, may be arranged by circuit connections as illustrated in figure 6 or by packet connections in a packet network 701 (e.g. an IP network) as illustrated in figure 7. The connections between the GSM MSC/VLR's 109, 110 respectively and their BSC's can also be included in the packet network (not illustrated) . All the control nodes 102-105 and 402-404 in the access network can access all the GSM/UMTS MSC/VLR's in the cluster 601 through the packet network 701.

The UMTS capacity in the system 600 can be gradually increased (or decreased) by adding (or removing) GSM/UMTS MSC/VLR's in the cluster 601. A new version of a GSM/UMTS MSC/VLR with a different performance characteristics than the already installed GSM/UMTS MSC/VLR's can easily be added to the system 600 without the need to upgrade the already installed GSM/UMTS MSC/VLR's so that they have the same performance characteristics as the new one. A Mobile station that requires a service which is only provided by the new version of GSM/UMTS MSC/VLR can then be assigned one of the core network node identifiers that have been allocated by the new GSM/UMTS MSC/VLR (see more below where the address function has been described) . Hence, this inventive combined GSM/UMTS system provides the possibilities for even further service differentiation in the core network than what is described.

The inventive system 600 also provides a possibility to temporarily increase the traffic capacity in one of the MSC service areas by dedicating one or more of the GSM/UMTS MSC/VLR's in the cluster 601 to only serve an MSC service area that needs the additional capacity. This is performed by letting the control nodes in the MSC service area that needs the additional capacity be the only control nodes in the system that knows the core network node identifiers for the dedicated GSM/UMTS MSC/VLR's in cluster 601.

As previously stated, the combined GSM/UMTS systems as illustrated in figure 4, 6 and 7 are provided with a core network node addressing function in which specific switching nodes can be identified so that the control nodes (i.e. RNC and BSC) can identify and address the right switching node in the pool/cluster. One solution is to extend the use of the Temporary. Mobile Subscriber Identity (TMSI) by letting some of the bits in the TMSI also represent a core network node identity. The TMSI identifiers could then be used as core network node identifiers that identify each switching node in the pool/cluster. The core network node identifiers can then be used to find the actual core network node address in e.g. a table that includes the identifiers and their corresponding core network node addresses.

The TMSI is an alias for the subscriber identity used in order to avoid sending the IMSI (International Mobile Subscriber Identity) "in clear" on the radio path. The TMSI is allocated by the system (MSC/VLR) on a location area basis, at a given moment, and it refers non-ambiguously to a specific subscriber when used in conjunction with the LAI (Location Area Identity) .

Figure 8 illustrates a view of a TMSI information element 800. The TMSI is a 32-bit number (divided into 4 octets). The TMSI is also used in GPRS where it is called P-TMSI, i.e. Packet-TMSI. The P-TMSI is distinguished from the "normal" TMSI by the combination ¹11' in the most significant bits 801 of octet 4. The TMSI, except P-TMSI, is managed by MSC/VLR's and further structuring is decided by the MSC/VLR implementation.

One way to use this TMSI as a core network node identifier is to let one or several parts of the TMSI number in the TMSI information element to represent a core network node identity number. One core network node can be associated with (identified by) one or more of these core network node identity numbers. In a first example, octet 4 of the TMSI information element 800 is defined to represent the core network node identifier number (as well as a part of the normal alias which includes all four octets) . As the combination '11' in the most significant bits 801 of octet 4 is reserved for a P-TMSI, the following TMSI identifier range can be used to identify individual core network nodes in a pool of core network nodes, namely 00000000-10111111, which means that up to 192 different core network nodes can be identified by the 4th octet. The rest of the TMSI information element 800 (the bits in octets 1-3) is still used as an alias for the subscriber identity (together with octet 4) . Further examples of how to include the core network node identifier number in the TMSI are to let one of octets 1-3, let more than one of the octets 1-4 or let a number of specific parts of each octet represent the core network node identity number.

All the available TMSI identifiers form a TMSI range. This TMSI range may be divided into sub-ranges and distributed among the core network nodes so that each core network node, e.g. the MSC/VLR's 109, 110 and 401 in figure 4, can be allocated one sub-range each for each served MSC service area, according to a traffic allocation plan at the initial set up of the system. As an example, the TMSI identifiers beginning with 00 and 01 are allocated to GSM MSC/VLR 109 and the TMSI identifiers beginning with 10 are allocated to GSM/UMTS MSC/VLR 401 for the MSC service area 111. The TMSI identifiers beginning with 01 and 10 are allocated to GSM MSC/VLR 110 and the TMSI identifiers beginning with 00 are allocated to GSM/UMTS MSC/VLR 401 for the MSC service area 112. These sub-ranges do not need to comprise TMSI identifiers in consecutive order. They can be selected in any possible way. The important thing is that each TMSI identifier is only present in one of these sub-ranges, for the same location area, to avoid any double allocation. To simplify the example above, the TMSI is made unique for the switching nodes in each MSC service area.

The problem of providing a core network node identification in an addressing function between access network nodes (in one or more access networks) and core network nodes (in at least one pool of core network nodes) is solved by letting at least one part of the TMSI identifier represent a core network node identity. The TMSI is an already existing mechanism that can be used for this purpose together with its original purpose as subscriber alias. This means that the introduction of this addressing function can be done without changing any standards or existing protocols.

A combined GSM/UMTS system includes at least one pool of core network nodes that is connected to at least one access network node in the GSM/UMTS system. The core network nodes are identified by core network node identifiers, which are allocated to the core network nodes according to a first traffic allocation plan. The core network node identifiers are used by control nodes in the access network (s) to find the address to the core network nodes in the pool(s). TMSI identifiers can as an example be used as core network node identifiers by letting at least a first part of the TMSI identifier to represent a core network node identity number at the same time as the whole TMSI is used as the alias for the subscriber identity.

When a Mobile Station is attached to or moved into the service area of a pool of MSC/VLR's, it is assigned a first TMSI identifier by a first MSC/VLR. The first MSC/VLR is the MSC/VLR that is going to serve the Mobile station and the first TMSI identifier is one of the TMSI identifiers that have been allocated to the first MSC/VLR in the pool. The control nodes in the access network can by reading the TMSI identifier assigned to the Mobile Station in combination with knowledge of which location area (LAI) the mobile station is associated with identify the serving MSC/VLR. The control nodes can find the right address to the serving MSC/VLR in the pool, e.g. by mapping the TMSI to the MSC's in the pool, and then set up a connection between the Mobile Station and the serving MSC/VLR in the pool (i.e. the MSC/VLR that the Mobile Station has been assigned to) .

The embodiments of the inventive method according to figure 3 and 5 can also be used to upgrade a GSM system that has a pool of GSM MSC/VLR's, to a GSM/UMTS system with both GSM and GSM/UMTS MSC/VLR's. The inventive combined GSM/UMTS system according to figure 4, 6 and 7 can easily be modified to include a GSM system with a pool of GSM MSC/VLR's instead of the GSM system 100 with "un-pooled" GSM MSC/VLR's.

The invention can be completely or partially implemented as software in at least one microprocessor.

Claims

1. A method of upgrading a GSM system (100) to a combined

GSM/UMTS system (400,600), where said GSM system (100) includes GSM switching means (109,110) connected to GSM control means (101-105) in a GSM access network, and where said method comprises the step of; adding (302,502) an UMTS access network to said GSM system, where said UMTS access network includes UMTS control means (402-404); c h a r a c t e r i s e d in that said method further comprises the following steps: adding (301,501) GSM/UMTS switching means (401,401a-c) to said GSM system (100); and connecting (303,503) said GSM/UMTS switching means to said UMTS control means (402-404) in said UMTS access network and to at least one of said GSM control means (101-

105) in said GSM access network to create said combined

GSM/UMTS system (400,600).

2. The method as claimed in claim 1 , wherein said method also comprises the step of: adding an addressing function to said combined GSM/UMTS system to enable said GSM and UMTS control means to address specific switching means in said combined GSM/UMTS system (400,600).

3. The method as claimed in claim 1 or 2, wherein a first one of said GSM switching means (109,110) is serving a first MSC service area (111,112) and a second one of said GSM switching means (109,110) is serving a second MSC service area (111,112), and said GSM/UMTS switching means is serving at least parts of said first and second MSC service areas (111,112).

4. The method as claimed in claim 3, wherein said GSM/UMTS switching means forms a first pool of switching means (416,602) with said first one of said GSM switching means and a second pool of switching means (417,603) with said second one of said GSM switching means, and where said first pool is serving said first one of said MSC service areas and said second pool is serving said second one of said MSC service areas .

5. The method as claimed in one of claims 2-4, wherein said GSM and GSM/UMTS switching means in said GSM/UMTS system are identified by core network node identifiers.

6. The method as claimed in claim 5, wherein a first part of a TMSI identifier is arranged to represent a core network node identity number so that said TMSI identifier can be used as said core network node identifier.

7. The method as claimed in one of claims 1-6, wherein said GSM/UMTS switching means is a GSM/UMTS switching node (401) and said first and second GSM switching means are GSM switching nodes (109,110).

8. The method as claimed in one of claims 1-6, wherein said GSM/UMTS switching means is at least two GSM/UMTS switching nodes (401a-c) arranged in a cluster (601) and said first and second GSM switching means are GSM switching nodes (109,110) .

9. The method as claimed in claim 7 or 8, wherein said GSM and GSM/UMTS switching nodes (109, 110, 401, 401a-c) are MSC ' s .

10. The method as claimed in claim 7 or 8, wherein said GSM and GSM/UMTS switching nodes (109, 110, 401, 401a-c) are

MSC/VLR's.

11. A combined GSM/UMTS system (400,600) including a GSM access network with GSM control means (101-105) and an UMTS access network with UMTS control means (402-404) , c h a r a c t e r i s e d in that said combined GSM/UMTS system (400,600) also includes GSM switching means

(109,110) connected to said GSM control means and GSM/UMTS switching means (401, 401a-c) connected to said UMTS control means and to at least one of said GSM control means

(101-105) .

12. The system as claimed in claim 11, wherein said system provides means for addressing that enable said GSM and UMTS control means to address specific switching means in said combined GSM/UMTS system (400,600).

13. The system as claimed in claim 11 or 12, wherein a first one of said GSM switching means (109,110) is arranged to serve a first MSC service area (111,112) and a second one of said GSM switching means (109,110) is arranged to serve a second MSC service area (111,112), and said GSM/UMTS switching means is arranged to serve at least parts of said first and second MSC service areas (111,112) .

14. The system as claimed in claim 13, wherein said GSM/UMTS switching means is arranged to form a first pool of switching means (416,602) with said first one of said GSM switching means and a second pool of switching means (417,603) with said second one of said GSM switching means, and where said first pool is arranged to serve said first one of said MSC service areas and said second pool is arranged to serve said second one of said MSC service areas .

15. The system as claimed in one of claims 12-14, wherein said GSM and GSM/UMTS switching means in said GSM/UMTS system are identified by core network node identifiers.

16. The system as claimed in claim 15, wherein a first part of a TMSI identifier is arranged to represent a core network node identity number so that said TMSI identifier can be used as said core network node identifier.

17. The system as claimed in one of claims 11-16, wherein said GSM/UMTS switching means is a GSM/UMTS switching node (401) and said first and second GSM switching means are GSM switching nodes (109,110).

18. The system as claimed in one of claims 11-16, wherein said GSM/UMTS switching means is at least two GSM/UMTS switching nodes (401a-c) arranged in a cluster (601) and said first and second GSM switching means are GSM switching nodes (109,110).

19. The system as claimed in claim 17 or 18, wherein said GSM switching nodes (109,110) are GSM MSC's and said GSM/UMTS switching nodes (401,401a-c) are GSM/UMTS MSC's.

20. The system as claimed in claim 17 or 18, wherein said GSM switching nodes (109,110) are GSM MSC/VLR's and said GSM/UMTS switching nodes (401, 401a-c) are GSM/UMTS MSC/VLR's.