WO2002025981A1

WO2002025981A1 - Method of performing roaming between networks and corresponding device

Info

Publication number: WO2002025981A1
Application number: PCT/EP2000/009307
Authority: WO
Inventors: Jyrki Tukiainen
Original assignee: Nokia Corporation
Priority date: 2000-09-22
Filing date: 2000-09-22
Publication date: 2002-03-28
Also published as: EP1319317A1; AU2000272891A1

Abstract

The present invention proposes a method of performing roaming of a subscriber (1) using a first transmission type from a first network (2) to a second network (5) being based on a second transmission type, said method comprising a first registering step (S4) for registering said subscriber (1) in said second network (5) on the basis of one of said first and said second transmission types, and in the case that said first registering step fails, a second registering step (S7) for registering said subscriber (1) in said second network (5) on the basis of the other of said first and said second transmission types. The present invention proposes also a corresponding device.

Description

METHOD OF PERFORMING ROAMING BETWEEN NETWORKS AND CORRESPONDING DEVICE

FIELD OF THE INVENTION

The present invention relates to a method of performing roaming of a subscriber between networks and also relates to a corresponding device. In particular, the present invention is directed to a method and corresponding device being used when a circuit switched (CS) subscriber, like a subscriber of a 2G (second generation) telecommunication network roams from one telecommunication network, like the 2G telecommunication network, to a packed switched (PS) network, like a 3G (third generation) telecommunication network.

BACKGROUND OF THE INVENTION

Mobile telecommunication systems such as the Global System for Mobile Communications GSM, the Universal Mobile Telecommunications System UMTS, and the like, have a cellular structure comprising several cells. For each cell at least one transceiver station (base station) is provided. Via this base station, a user equipment of a subscriber and elements of the telecommunication system (network elements such as exchange stations and registers) communicate. The subscriber is free to move within the network from one cell to another without loosing contact (handover) .

There is also the case that several telecommunication networks coexist, either in the same geographical area or in adjacent geographical areas. Subscribers are able to roam from one network (e.g. the respective home network) to another (visited) network. This is one commonly known feature of mobile telecommunication network systems.

The coexisting networks may be of different types. For example, one network (e.g. the home network) is a 2G (second generation) network (e.g. GSM) and the other network (the visited network) is a 3G (third generation) network (e.g. UMTS). Since the respective networks may offer different service types and/or communication modes, attention has to be paid to the establishment of a correct connection and communication paths, when e.g. a 2G subscriber wishes to roam into the 3G network.

As mentioned above, transmission of data can be divided into two different types. The circuit switched (CS) transmission type uses one transmission resource (e.g. one channel) for a communication until ending the communication. As a further transmission type, packet switched transmission has been developed. Here, data are collected in a buffer and then sent as one packet via a requested transmission resource to an indicated address.

In traditional 2G networks, such as the GSM network, CS transmission is used. As a further development for existing 2G networks, packet switched system, like the Generalized Packet Radio Service GPRS for GSM networks, are added in order to enable the usage of both transmission types for the subscribers. That is, there can be networks and/or subscribers either supporting only one of these transmission types (namely the "traditional" CS type) or both types (CS and PS type) . In the latter case, the subscriber may select which mode is to be used, when both are available from a network.

3G networks, and correspondingly 3G subscribers (i.e. user terminals) are designed to perform both CS and PS transmission communications. In particular for the packet switched mode, 3G networks, such as the UMTS, offers specific communication modes. One of them is the so-called Voice over Internet Protocol (VoIP) communication mode which is a CS-like packet data communication method. Due to the similarity to a CS communication, it is also possible that 3G networks are designed to support the usage of VoIP services by CS subscribers.

At least during a transitional period, when both the 2G system and the 3G system coexist, there are situations in which a subscriber with only CS capability may roam into the 3G network. In such a case, for example when the roaming subscriber requests to use the VoIP communication mode, it has to be decided in the 3G network, on the basis of which kind the subscriber is (CS only, CS and packet switched, and the like) , which operations in the network has to be executed in order to establish a proper communication connection for the subscriber. This may include for example transport layer registration or transport layer access, application layer registration or application layer access, authentication mechanisms, subscriber profile creations and the like.

A prior art solution for a method of roaming between two networks is described below with reference to Figs. 4A and 4B. Assuming that a CS subscriber like a 2G subscriber intends to roam from a 2G network to a PS network like a 3G network in order to perform a packet switched communication such as VoIP, then first a transport layer registration is performed to define relevant data structures, communication protocols, Quality of Service (QoS) and the like. The transport layer registration is made by the 2G subscriber e.g. by a PS attach (e.g. GPRS attach) to the 3G network and a GPRS location update to the 2G network. The transport layer registration is completed in the 3G network wherein the GPRS profile of the subscriber created and used in the GPRS location update depends on whether the subscriber is a CS only subscriber (option I: M204A, M205A) or a CS and PS subscriber (option II: M204B, M205B) . Next, a packet data protocol (PDP) context activation is executed between the 2G subscriber and the 3G network. Finally, an application level registration is executed to define the connection parameters (e.g. VoIP, All-IP) , wherein an normal location update at the 3G network and the 2G network is performed.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improved roaming method and an improved device capable of performing a roaming.

According to the present invention, this object is achieved by a method of performing roaming of a subscriber using a first transmission type from a first network to a second network being based on a second transmission type, said method comprising a first registering step for registering said subscriber in said second network on the basis of one of said first and said second transmission types, and in the case that said first registering step fails, a second registering step for registering said subscriber in said second network on the basis of the other of said first and said second transmission types.

Furthermore, the present invention proposes a device for performing roaming of a subscriber using a first transmission type from a first network to a second network being based on a second transmission type, said device is adapted to perform a first registration of said subscriber in said second network on the basis of one of said first and said second transmission types, and in the case that said first registration fails, a second registration of said subscriber in said second network on the basis of the other of said first and said second transmission types.

Advantageous further developments of the present invention are as set out in the respective dependent claims.

According to the present invention, the proposed method and/or device are easy to implement. Since the required functions can be executed by one network element, namely the roaming element such as a roaming signaling gateway functionality element R-SGW of the 3G network, and changing or adaptation of other network elements (e.g. of 2G or 3G networks) is not necessary. The present invention can be implemented by software, but also a hardware solution with a corresponding circuitry can be used.

One further advantage of the present invention is that the signaling load of the network in certain situations is reduced. Namely, when , for example, the 2G network indicates that no subscription for the requested transport layer is present, the roaming element immediately starts a normal location update towards the registration element of the home network of the subscriber, e.g. the 2G home location register, or ends the transaction immediately. When the 2G network does not accept the location update, e.g. since the subscriber is not allowed to roam to the 3G network, the transaction is cancelled before the transport layer access is completed and an application layer access in the 3G network is started.

Furthermore, it is possible to avoid granting access to services to a subscriber which actually has no allowance to use these 3G services. When the 2G network responds to the normal location update request with a message that the subscription of the subscriber prohibits a roaming to the 3G network, even the establishment of the transport layer is stopped. That is, the usage of services only requiring the transport layer registration by invalid CS subscribers is completely prevented.

Additionally, by using the roaming element as a buffer for storing the subscriber data (e.g. the subscriber profile) sent by the 2G registration element when the normal location update is accepted, a further contacting of the 2G network during the execution of the normal location update procedure in the 3G network at the application layer registration can be avoided. It is possible to buffer the subscriber data merely for a predetermined period of time so as to reduce the required memory capacity. However, when the data are needed after said predetermined period of time (i.e. after they are cleared), the data can be fetched again from the 2G network.

Preferred embodiments of the invention are described herein below in detail by way of example with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 shows a block circuit diagram illustrating a network environment with network elements used in the present invention.

Figs. 2A, 2B and 2C show flow charts illustrating an embodiment of the method according to the present invention.

Figs. 3A and 3B show diagrams illustrating the signaling between the network elements shown in Fig. 1 according to the present invention. Figs. 4A and 4B show diagrams illustrating the signaling between the network elements shown in Fig. 1 according to the prior art.

DESCRIPTION OF PREFERRED EMBODIMENTS

Fig. 1 is a block circuit diagram illustrating network structures of a 2G network 2 and a 3G network 5 as an illustrative example, in which the present invention is implemented. It is to be noted that for the sake of simplicity only network elements are shown which are directly involved. As commonly known, telecommunication network systems also comprise further elements such as a base station subsystem (BSS) comprising transceiver stations, registers, and the like, which are used for communications. The complete network structures as well as corresponding interconnections can be found e.g. in the GSM specifications and the UMTS specifications, respectively. Furthermore, it is to be noted that the described 2G/3G situation is only for illustration. A GPRS system can be part of the 2G network or existing in parallel. Moreover, the described method and/or device are applicable in general to situations in which a subscriber using a first transmission type roams to a network using a second transmission type, for example in the case that a CS subscriber roams to an all IP network, or the like.

According to Fig. 1, the 2G network 2 comprises a first switching element 3 such as a mobile switching center MSC (including a visitor location register VLR) and a registration element 4 such as a home location register HLR (including an authentication center AuC) . The MSC/VLR 3 serves a 2G subscriber 1 (represented by a user equipment/mobile station UE/MS) for controlling calls in the 2G network 2. The HLR 4 is used for storing subscriber data as well as for updating location information of the subscriber 1, and the like.

The 3G network 5 comprises switching elements such as an enhanced serving GPRS support node E-SGSN 6, an enhanced gateway GPRS support node E-GGSN 7, a control element such as a call state control function element CSCF 8 and a roaming element such as a roaming signaling gateway element R-SGW 9.

The E-SGSN 6 serves the subscriber in packet switched communication and establishes e.g. a mobility management context, containing information pertaining to mobility and security for the subscriber, and a PDP context. The E-GGSN 7 serves for packet switched communication and interconnection with other telecommunication networks. The E-SGSN and E-GGSN may be combined in one unity. Both comprise IP routeing functionality.

The CSCF 8 is used to perform call state control functions in the 3G network, such as an incoming call gateway function ICGW, a call control function CCF (e.g. processing of application layer registration) , an address handling AH, and the like.

The functionality of the R-SGW 9 is related to roaming of e.g. 2G/R99 (CS only and CS/GPRS) subscribers to/from e.g. a 3G/R00 UMTS teleservices domain and UMTS GPRS domain. In order to ensure a proper roaming, the R-SGW performs a signaling conversion at transport level (e.g. to IP based transport signaling) .

It is to be noted that the R-SGW 9 and the HLR/AuC 4 are usually not directly connected, as indicated by the dotted arrow in Fig. 1, but they are connected via several interconnecting and gateway elements known by a person skilled in the art, which are omitted in order to simplify the figure.

An embodiment of a roaming procedure will become apparent by the following description with reference to the flow charts shown in Figs. 2A to 2C and the message charts shown in Figs. 3A and 3B.

When a subscriber 1, e.g. a 2G/R99 subscriber, intends to use services offered by a 3G network, such as Voice over IP or the like, roaming from the 2G home network 2 to a 3G network 5 is to be performed. For this purpose, the E-SGSN 6 is contacted by said subscriber 1 in order to initialize a transport layer access (or transport layer registration) for using e.g. a packet switched communication such as

GPRS. To indicate its presence and intention to roam in the 3G network, a GPRS attach (PS attach) is sent from the user equipment of the subscriber 1 to the 3G network, which comprises for example the International Mobile Subscriber Identity IMSI (step S2; message M101) .

After receiving the GPRS attach, an authentication procedure using a GPRS mechanism is performed in the 3G network, in which the subscriber identity is checked (step S3; message M102) . In case the requesting subscriber 1 is verified, the E-SGSN 6 sends a GPRS location update message including the IMSI and the identification number of the E- SGSN 6 to the R-SGW 9 which in turn contacts the HLR/AuC 4 of the 2G network 2 (step S3; message M103) . This message M103 is sent e.g. by using a MAP (mobile application part) application context GPRS location update context.

The further processing depends on processing results of the HLR 4 concerning the GPRS location update from the 3G network. Namely, the HLR 4 checks whether it supports the application context GPRS location update context (message M103) , and whether the subscriber 1 has a subscription for GPRS. If YES (step S4), i.e. the subscriber has a PS subscription in the HLR 4 meaning that he/she is e.g. an 2G/R99 CS and GPRS subscriber, a procedure may follow which corresponds to the steps shown in Figs. 4A and 4B, option II (i.e. messages M204B to M217). Namely, in step S10, the HLR 4 accepts the GPRS location update and sends a corresponding message comprising the IMSI and the 2G/R99 profile of the subscriber 1 to the R-SGW 9. Then, the 3G network (e.g. in the R-SGW 9) modifies the subscriber profile in such way that the requested transport layer for the requested service (for example GPRS for VoIP) can be used. The new subscriber profile is sent to the E-SGSN 6, which in turn contacts the subscriber 1 in order to accept the GPRS attach. The GPRS location update is completed and the transport layer registration is confirmed.

Next, a PDP context activation is performed in the 3G network 5. Then, in step Sll, an application layer access (or application layer registration) is performed. In this connection, a normal location update between the 3G network 5 and the 2G network 2 is executed. The subscriber 1 sends a resource request RRQ comprising subscriber information for the application layer (ALSI) to the CSCF 8 in order to initialize an application layer registration. The RRQ message can be, for example, a H.323 protocol message, a session initiation protocol (SIP) message, or the like. After a further authentication check in the 3G network by using 2G parameters and mechanisms, a location update message comprising the ALSI and the CSCF address is sent from the CSCF 8 to the R-SGW 9. The R-SGW 9 forwards the location update message including an R-SGW identification number to the HLR 4. The HLR 4 cancel the location of the subscriber 1 in the 2G network (i.e. at the MSC/VLR 3) and sends a location update reply message comprising the 2G profile of the subscriber to the R-SGW 9. After receiving the 2G profile, in step S12, a new connection profile (e.g. an All-IP profile) is created in the 3G network by using the 2G profile in order to enable an establishment of the requested connection mode. The new connection profile is transmitted to the CSCF 8 by means of a location update reply message. The CSCF then completes the application layer registration and the normal location update (step S13) by transmitting a resource confirmation RCF message to the user equipment of the subscriber 1 in order to acknowledge the registration. The RCF message is, similar to the RRQ message mentioned above, for example, a H.323 protocol message, a session initiation protocol (SIP) message, or the like.

The procedure executed in the case that the HLR 4 replies to the GPRS location update (steps S3, S4) from the 3G network 5 with a message (M104) indicating that the GPRS location update is not acceptable (e.g. since it is a 2G/R99 CS only subscriber without GPRS subscription, or the like) differs to that described above. The R-SGW 9 checks •'the message content from the HLR 4 to decide whether the transaction has to be ended in this stage (step S5) .

There can be different reasons why the GPRS location update is not acceptable by the HLR 4. One reason is that the 2G HLR 4 does not support the application context GPRS location update context used in message M103. In this case the message returned to the R-SGW 9 represents an application context not supported message (in correspondence with message M104). This circumstance however also represents the fact that the subscriber 1 has no GPRS subscription. Another reason is that the subscriber 1 is known by the 2G HLR 4, which supports the application context GPRS location update context used in message M103, but the GPRS subscription is unknown. In both cases, an initialization of a normal location update is performed by the R-SGW 9 (step S6, to be described below) .

On the other hand, when the reason why the GPRS location update is not acceptable is that the subscriber is unknown (i.e. not stored in the HLR 4) or deactivated (e.g. by the operator) , the 2G HLR 4 sends a message M104 unknown subscriber/IMSI unknown to the R-SGW 9. In this case, on the basis of this return message, the R-SGW 9 decides that the transaction has to be ended and sends a negative acknowledgement for the location updating (steps S5 and S8) . As a further option for this case, step S5 can be omitted (e.g. since a processing of the message IMSI unknown is not performed in this stage) . That is, a normal location update is initialized in any case. In this case, the transaction is ended after a further processing (normal location update, step Sβ) as in the case that the subscriber has no permission to roam to a 3G network (described below) . This latter procedure option would standardize the processing in the R-SGW 9.

When the reason for not accepting the GPRS location update results not in an immediate end of the transaction in step S5, the R-SGW 9 initializes immediately a normal location update towards the 2G network. This comprises the re- contacting of the HLR 4 and the sending of the IMSI and the R-SGW identification number (step S6; message M105) .

Now, the HLR 4 checks whether it can accept a normal location update of the subscriber (step S7) . That is, for example, it is checked whether the subscriber 1 is known to HLR (i.e. is the subscriber identity deleted from the database) , or a prohibition to roam in a 3G network is present, or the like. In the case that the HLR 4 indicates to the R-SGW 9 that the normal location update of the subscriber 1 is not accepted, and the R-SGW 9 sends an error message to the 3G network in order to indicate that the subscriber 1 can not roam to the 3G network. It is also possible to forward a corresponding error message to the subscriber 1. Then, the roaming transaction is terminated.

On the other hand, in the case that the HLR 4 decides that a location update of the subscriber 1 is acceptable, the location in the 2G network 2 (i.e. in the MSC 3) is cancelled (S20; M106) . A location update reply is sent to the R-SGW 9 including the 2G profile of the subscriber 1 (M107). There are means (not shown) such as a memory provided in the R-SGW 9 which are adapted to buffer this 2G profile sent from the HLR 4 for further processing.

In the 3G network, since the subscriber 1 has no GPRS subscription as determined in step S4, a "default" GPRS profile is created (step S21; M108) with a real-time

Quality of Service (QoS) support. For this purpose, for example, the 2G profile (2G GPRS profile, 2G CS service information, or the like) may be mapped to 3G services. The profile is stored in the R-SGW 9, wherein the subscriber database is indexed by the IMSI. It is possible to provide a link for each subscriber to one "default" profile, or one individual profile is stored for each subscriber.

Then, the new "default" GPRS profile is sent to the E-SGSN 6 (M109) , which forwards a GPRS attach reply to the user equipment of the subscriber 1 (MHO) . The GPRS location update is completed and the transport layer registration is confirmed (step S21) .

Next, a PDP context activation is performed between the user equipment of the subscriber 1 and the 3G network (Mill) . Then, in step S22, an application layer registration is executed, wherein a normal location update between the 3G network 5 and the 2G network 2 is executed (M112) . The subscriber 1 sends the resource request RRQ comprising subscriber information for the application layer (ALSI) to the CSCF 8 in order to initialize the application layer registration. After a further authentication check (M113) in the 3G network by using 2G parameters and mechanisms, a location update message comprising the ALSI and the CSCF address is sent to the R-SGW 9 (M114) .

The R-SGW 9 reads the buffered 2G profile of the subscriber 1 from said buffer means, and a new connection profile (e.g. an All-IP profile) is created in the 3G network by using the 2G profile in order to enable an establishment of the requested connection mode (step S23; M115) . A re- contacting of the 2G network (i.e. of the HLR 4) is not required. The new connection profile is transmitted to the CSCF 8 by means of a location update reply message (M116) . The CSCF then completes the application layer registration and the normal location update (step S24) by transmitting the resource confirmation RCF message M117 to the user equipment of the subscriber 1 in order to acknowledge the registration.

In a further embodiment, the buffering means for storing the subscriber data (2G profile) from the 2G registration element in the roaming element is arranged to store the data only for a predetermined period of time. When the application layer registration (the normal location update) in the 3G network is not detected by said roaming element 9 within this period of time, the subscriber data are cleared. When the roaming element 9 detects the normal location update in the 3G network after the subscriber data buffered are cleared, it is possible to request the respective subscriber data again from the 2G registration element 4. This can be executed for example by re-sending the location update message (step Sβ; M105) or by sending a specific message (not shown) for merely requesting this information. Since it is generally assumed that the application layer registration is performed almost immediately after the transport layer registration, the period of time can be set to be short. Hence, only a small storage capacity for the buffering means is required, even if several subscriber roaming procedures are to be handled by the roaming element 9 concurrently.

In yet a further embodiment, the normal location update is started by the roaming element 9 in any case immediately after the transport layer location update (steps S3, S4) request is responded by the registration element 4 of the

2G network 2. That is, also in the case that the subscriber 1 has a subscription for the transport layer registration, e.g. a GPRS subscription (YES in step S4), the roaming element 9 initiates a normal location update in the 2G network and buffers the subscriber data sent by the registration element 4. The following steps may correspond to the steps S20 to S24. Since the signaling procedure of this embodiment is equivalent in both cases (the case the GPRS subscription is present and the case that no GPRS subscription is present in the HLR) , the roaming functionality as a whole is simplified.

Furthermore, while it was described above to first try a transport layer access (PS) as a first registration and a location update as a second registration, the present invention is also applicable in the case that a location update is tried as a first registration and a transport layer access is tried as a second registration. Moreover, it is also possible to try a location update procedure or a transport layer access (for example with different parameters/subscriber data or the like) in both registrations. There can be executed even more than two registration steps, if necessary. Which on of the above possibilities is to be employed may depend for example on the respective network. The order of the respective substeps (S2 to S7) is then to be adapted accordingly (changing of the order and/or repeating of substeps) .

One advantage of the roaming procedure according to the present invention are first that the signaling load is significantly reduced in specific error cases. Namely, according to the conventional procedure, the transport layer registration is first completed before a normal location update in the application layer registration (and hence e.g. a check of the allowance for roaming of the subscriber 1) is executed. Furthermore, the usage of services only requiring a transport layer registration by invalid subscribers can be prevented.

It is to be noted that the present invention can be used also for other communication types, not only for the above described packed switched and/or circuit switched modes.

As described above, the present invention proposes a method of performing roaming of a subscriber 1 using a first transmission type from a first network 2 to a second network 5 being based on a second transmission type, said method comprising a first registering step S4 for registering said subscriber 1 in said second network 5 on the basis of one of said first and said second transmission types, and in the case that said first registering step fails, a second registering step S7 for registering said subscriber 1 in said second network 5 on the basis of the other of said first and said second transmission types. The present invention proposes also a corresponding device. It should be understood that the above description and accompanying figures are merely intended to illustrate the present invention by way of example only. The preferred embodiments of the present invention may thus vary within the scope of the attached claims. Also combinations of different embodiments are possible.

Claims

1. A method of performing roaming of a subscriber (1) using a first transmission type from a first network (2) to a second network (5) being based on a second transmission type, said method comprising a first registering step (S4) for registering said subscriber (1) in said second network (5) on the basis of one of said first and said second transmission types, and in the case that said first registering step fails, a second registering step (S7) for registering said subscriber (1) in said second network (5) on the basis of the other of said first and said second transmission types.

2. A method according to claim 1, wherein in said first registering step (S4) said second transmission type is used, and in said second registering step (S7) said first transmission type is used.

3. A method according to claim 1, wherein said first transmission type is a circuit switched transmission type and said second transmission type is a packet switched transmission type.

4. A method according to claim i, wherein in said first registering step and/or said second registering step an inquiry for a transport layer access is made.

5. A method according to claim 1, wherein in said first registering step and/or said second registering step an inquiry for a location update is made.

6. A method according to claim 1, further comprising a denying step (S8) for denying a roaming of said subscriber (1) in the case that both the first registering step and the second registering step fail.

7. A method according to claim 1, wherein said first and said second registering steps are initiated by sending a corresponding inquiry (S2, Sβ) from said subscriber (1) to a roaming element (9) executing said first and said second registering steps.

8. A method according to claim 7, further comprising a buffering step for buffering subscriber data in said roaming element (9) in the case that at least one of said first and said second registering steps is successful.

9. A method according to claim 1, wherein said first and said second registering steps are initiated by sending a corresponding inquiry (S2, Sβ) from said subscriber (1) to a home network (2) of said subscriber.

10. A method according to claim 1, wherein said first network (2) comprising a registration element (4) storing subscriber data of said subscriber (1), and said second network (5) comprising a roaming element (9) enabling roaming of subscribers from networks different to said second network (5) by performing signaling conversion, said method further comprising the steps of: initializing (S2) a transport layer access and sending subscriber data to said second network (5) via said roaming element ( 9) , contacting (S3) said registration element (4) of said first network by said roaming element (9) to determine by means of said subscriber data whether said subscriber (1) has a subscription for the transport layer access, in the case that the registration element (4) of said first network (2) indicates to said roaming element (9) of said second network (5) that there is no subscription of said subscriber (1) for the initialized transport layer access (S4) , deciding (S5) on the basis of the indication of said registration element (4) whether to end the roaming, and if not initializing (Sβ) a location update procedure by said roaming element (9) at said registration element (4) of said first network (2) , and receiving (S7) and processing (S8; S20 to S24) an indication being based on a check (S7) by said registration element (9) whether a location update of said subscriber (1) from said first network (2) to said second network (5) is possible.

11. A method according to claim 10, further comprising the steps of: in the case that said registration element (4) indicates to said roaming element (9) after said checking step that a location update of said subscriber (1) from said first network (2) to said second network (5) is not possible (Sβ) , sending (S8) an error indication to said second network (5) by said roaming element (9), and ending the transport layer access without establishing a connection.

12. A method according to claim 10, further comprising the steps of: in the case that the decision step (S5) indicates an ending of the roaming, sending (S8) an error indication to said second network (5) by said roaming element (9), and ending the transport layer access without establishing a connection.

13. A method according to claim 11, wherein the reason for the indication of no subscription by said registration element (9) is that the subscriber (1) is unknown to said registration element (9) , or is deactivated in said registration element (9), or is not permitted to access said second network (5) .

14. A method according to claim 10, further comprising the steps of: in the case that said registration element (4) indicates to said roaming element (9) after said checking step (S7) that a location update of said subscriber (1) from said first network (2) to said second network (5) is possible, confirming (S20) the location update by sending subscriber data from said registration element (4) to said roaming element (9), buffering said subscriber data in said roaming element

(9) for further processing of the location update procedure, canceling location of said subscriber (1) in said first switching element (3) of said first network (2) , creating (S21) a subscriber profile for said transport layer registration of said subscriber (1) at the roaming element (9) and completing said transport layer access in said second network (5) .

15. A method according to claim 14, further comprising the steps of: after completion of said transport layer access, initializing (S22) an application layer access in said second network (5) , executing said location update procedure in said second network (5) by using the subscriber data buffered in said roaming element (9), creating (S23) a new subscriber profile to use services offered by said second network (5) , completing (S24) said application layer access by using said new subscriber profile and completing said location update procedure in said second network (5) .

16. A method according to claim 15, wherein said subscriber data are buffered in said roaming element (9) for a predetermined time.

17. A method according to claim lβ, further comprising a steps of: in the case that the application layer access is initialized after said predetermined time, re-contacting said registration element (4) of said first network (2) by said roaming element (9) of said second network (5), and fetching said subscriber data from said registration element (4) and buffering them in said roaming element (9).

18. A method according to claim 10, wherein said first network (2) comprises a home location register (4) and a mobile switching center (3) , and said second network (5) comprises an enhanced serving GPRS support node (β), an enhanced gateway GPRS support node (7), a call state control function element (8) and a roaming signaling gateway function element (9).

19. A device for performing roaming of a subscriber (1) using a first transmission type from a first network (2) to a second network (5) being based on a second transmission type, said device is adapted to perform a first registration of said subscriber (1) in said second network (5) on the basis of one of said first and said second transmission types, and in the case that said first registration fails, a second registration of said subscriber (1) in said second network (5) on the basis of the other of said first and said second transmission types.

20. A device according to claim 19, wherein in said first registration said second transmission type is used, and in said second registration said first transmission type is used.

21. A device according to claim 19, wherein said first transmission type is a circuit switched transmission type and said second transmission type is a packet switched transmission type.

22. A device according to claim 19, wherein in said first registration and/or said second registration an inquiry for a transport layer access is made.

23. A device according to claim 19, wherein in said first registration and/or said second registration an inquiry for a location update is made.

24. A device according to claim 19, said device is adapted to deny a roaming of said subscriber (1) in the case that both the first registration and the second registration fail.

25. A device according to claim 19, wherein said first and said second registration are initiated by sending a corresponding inquiry (S2, Sβ) from said subscriber (1) to said device.

26. A device according to claim 25, wherein said inquiry in said first and said second registration is send to a home network (2) of said subscriber (1).

27. A device according to claim 25, said device is adapted to buffer subscriber data in the case that at least one of said first and said second registrations is successful.

28. A device according to any of the claims 19 to 27, wherein said device is comprised in a roaming element (9) .

29. A device according to claim 19 for performing roaming of a subscriber (1) from a first network (2) to a second network (5) , said first network (2) comprising a registration element (4) storing subscriber data of said subscriber (1) , and said second network (5) comprising a roaming element (9) enabling roaming of subscribers from networks different to said second network (5) by performing signaling conversion, wherein said subscriber (1) initializes transport layer access and sends subscriber data to said second network (5) via said roaming element (9), said roaming element (9) contacts said registration element (4) of said first network which determines by means of said subscriber data whether said subscriber (1) has a subscription for the transport layer access, in the case that the registration element (4) of said first network (2) indicates to said roaming element (9) of said second network (5) that there is no subscription of said subscriber (1) for the initialized transport layer access, said device is adapted to decide on the basis of the indication of said registration element (4) whether to end the roaming, and if not, to initialize a location update procedure at said registration element (4) of said first network (2), and to receive and process an indication of said registration element (9) after checking whether a location update of said subscriber (1) from said first network (2) to said second network (5) is possible.

30. A device according to claim 29, wherein said device is adapted, in the case that said registration element (4) indicates to said roaming element (9) after said checking step that a location update of said subscriber (1) from said first network (2) to said second network (5) is not possible, to send an error indication to said second network (5), and to end the transport layer access without establishing a connection.

31. A device according to claim 29, wherein said device is adapted, in the case that the decision indicates an ending of the roaming, to send an error indication to said second network (5) by said roaming element (9), and to end the transport layer access without establishing a connection.

32. A device according to claim 29, wherein the reason for the indication of no subscription by said registration element (9) is that the subscriber (1) is unknown to said registration element (9) , or is deactivated in said registration element (9), or is not permitted to access said second network (5) .

33. A device according to claim 29, wherein, in the case that said registration element (4) indicates after checking that a location update of said subscriber (1) from said first network (2) to said second network (5) is possible by sending subscriber data, said device comprises means for buffering said subscriber data for further processing of the location update procedure.

34. A device according to claim 33, wherein during an application layer access said location update procedure in said second network (5) is executed by using the subscriber data buffered in said means for buffering said subscriber data.

35. A device according to claim 33, wherein said means for buffering said subscriber data buffers said subscriber data for a predetermined time.

36. A device according to claim 35, wherein in the case that the application layer access is initialized after said predetermined time, said device is adapted to re-contact said registration element (4) of said first network (2) , and to fetch and to buffer said subscriber data from said registration element (4).

37. A device according to claim 29, wherein said first network (2) comprises a home location register (4) and a mobile switching center (3), and said second network (5) comprises an enhanced serving GPRS support node (6), an enhanced gateway GPRS support node (7), a call state control function element (8) and a roaming signaling gateway function element (9).

38. A device according to any of the claims 29 to 37, wherein said device is incorporated in said roaming element (9) .