Wireless communications networks were initially conceived for enabling voice communications, similarly to the wired, Public Switched Telephone Networks (PSTNs), but between mobile users.

Wireless communications networks have experienced an enormous spread, especially after the introduction of second-generation mobile cellular networks, and particularly digital mobile cellular networks such as those complying to the Global System for Mobile communications (GSM) standard (and its United States and Japanese counterparts).

The services offered by these cellular networks in addition to plain voice communications have rapidly increased in number and quality; just to cite a few examples, Short Messaging System (SMS) and Multimedia Messaging System (MMS) services, and Internet connectivity services have been made available in the last few years.

However, these second-generation cellular networks, albeit satisfactory for voice communication, offer very poor data exchange capabilities.

Similarly to the PSTNs, second-generation cellular networks are in fact switched-circuit networks; this greatly limits the bandwidth that can be allocated for a given user.

On the contrary, data communications networks such as

computer networks and, among them, the Internet, adopt packet switching schemes, which allow extremely.. higher data transfer rates.

Some solutions have been proposed to overcome the limitations of conventional, switched-circuit cellular networks such as the GSM networks, so as to enable users of mobile terminals efficiently exploiting services offered through the Internet.

One of the solutions that have acquired a significant popularity is the General Packet Radio Service (shortly, GPRS). The GPRS is a digital mobile phone technology compatible with GSM networks (actually, build on the existing GSM network architecture) that enables data transfer at a speed higher than that allowed by pure GSM.

Essentially, the GPRS is a GSM add-up that supports and enables packet-based data communication.

Although third-generation wireless communications systems such as those complying with the Universal Mobile Telecommunication System (UMTS) are more promising in terms of data transfer rates, the GPRS is an almost ready-at-hand solution for enhancing the data exchange capabilities of already existing GSM networks, and is therefore gaining a certain popularity.

In GPRS communications networks the information content is usually transferred in a point-to-point modality (unicasting), upon activation of a session between a client (the GPRS mobile phone, or mobile station) and a service provider (the server) connected to a packet data network, e. g. a Web server connected to the Internet; the activation of such a session involves the setting up of a dedicated logic communications channel between the server and the GPRS mobile phone.

In such point-to-point communication mode, the radio

resources to be allocated for the exchange of data between the ground GPRS network and the GPRS. mobile phones depend on the number of different mobile phones simultaneously exploiting the GPRS services, irrespective of the fact that a same GPRS service is being exploited by two or more mobile phone users at the same time. Clearly, this limits the possibility of accessing GPRS services by several users, unless the radio resources are overdimensioned.

Thus, it would be desirable to have the possibility of having information contents related to a same GPRS service exploitable by two or more clients at a time, without for this reason having to increase the number of allocated resources in a way roughly proportional to the number of clients.

The problem of broadcasting relatively massive information contents such as multimedia (audio and/or video) contents to several mobile terminal users has already been faced in the art.

For example, the International application No. WO 02/11471 A2 describes an on-demand information service using wireless cellular communications system, in which the quantity of unused voice channels is identified, and such unused channels are used for broadcasting audio/video content.

The European patent application No. EP 944 275 A2 discloses a broadcast information delivery system in which one of the downlink traffic channels toward the mobile stations from the cellular wireless base station is employed as a broadcast channel, and the broadcast information is transmitted through this broadcast channel. The Applicant observes that, in existing cellular mobile networks, a small bandwidth is normally allocated to control channels and to channels such as the traffic channel, therefore, adopting

the solution proposed in the cited European patent application, the bandwidth allocated for broadcasting the multimedia information content is very. poor.

The International application No. WO 03/019840 A2 discloses a method of transmitting data packets to a cellular mobile unit, comprising: establishing a data transfer path for providing data from a data network gateway to a base station controller, for a first mobile station identity; and transmitting data received along the established path to at least one mobile station having at least one second identity different from the first identity.

The Applicant observes that the implementation of the proposed method is not free of costs, because it involves modifying in a significant way the network apparatuses, particularly those governing the radio communications between the base stations and the mobile stations.

In view of the state of the art outlined in the foregoing, it has been an object of the present invention to devise a solution to the problem of the waste of radio resources of a cellular mobile network supporting packet- based data exchange, particularly a GPRS network, arising when a plurality of users exploit a same GPRS service, particularly a service involving a relatively massive data exchange such as a service providing multimedia (audio/video) information contents to the GPRS users.

The Applicant has found that the waste of radio resources can be substantially eliminated if the data relating to a given GPRS service, such as multimedia information contents, are distributed"over the air" substantially in the conventional way, allocating a prescribed number of radio resources of a cell of the cellular mobile communications network, but tuning information, necessary to the mobile stations for tuning

onto that radio resources and capturing the relevant data of the GPRS service, are broadcasted within the cell, so that two or more mobile, stations located in the cell can exploit the GPRS service. In particular, the Applicant has found that an expedient way of broadcasting such tuning information is by broadcasting a message containing such tuning information exploiting one of the cell message broadcast channels normally provided in a cellular network for broadcasting messages to the mobile stations.

For the purposes of the present invention, by broadcasting there is intended a type of communication that is directed to a plurality of users, which users are capable of decoding an information carried by such a communication.

In this way, the GPRS service, being as usual distributed in point-to-point (unicast) modality, is nevertheless rendered exploitable by a two or more users at a time, in a point-to-multipoint (multicast) modality.

It is observed that this multicast distribution mode of the GPRS service data does not limit in any way the bandwidth that can be allocated thereto: in fact, substantially only the tuning information, very small, is broadcasted over the cell broadcast channel, the mass of the GPRS service data being instead put"over the air" substantially as usual, allocating to them a reasonable bandwidth.

According to an aspect of the present invention, there is therefore provided a method as set forth in appended claim 1, for distributing data packets to mobile stations through a wireless communications network.

Summarizing, the method comprises: establishing a data packet transfer path for said data packets from an information content provider to a first mobile station located in a network cell; and

allocating to the first mobile station physical communication resources of the network cell for transmitting said data packets to the first mobile station.

Tuning information are broadcasted through the network' cell for enabling at least one second mobile station capturing the data packets transmitted to the first mobile station through the physical communication resources allocated thereto.

In an embodiment of the invention, the first mobile station is a virtual mobile station, emulating a real mobile station and dedicated to establishing said data packet transfer path.

The tuning information may comprise an indicator labeling radio data blocks delivering the data packets to the first mobile station.

In particular, said indicator comprises a Temporary Flow Identity (TFI) labeling radio data blocks.

In an embodiment of the present invention, said broadcasting includes exploiting a broadcast channel of a GSM/GPRS network.

In a preferred embodiment of the invention, said broadcasting the tuning information includes encrypting the tuning information. The method may thus comprise enabling the second mobile station decrypting the tuning information.

In particular, said information contents include multimedia information contents.

According to another aspect of the present invention, a system as set forth in appended claim 9 is provided for distributing information contents provided in data packets by an information content provider to mobile stations through a wireless communications network.

Summarizing, the system comprises: a network base station subsystem enabling wireless

communications with a plurality of mobile stations located in a network cell; 'a network gateway structure interfacing the base station subsystem to the information content provider, the network gateway structure controlling the establishment of a data packet transfer path for said data packets from the information content provider to a first mobile station of said plurality, and the network base station subsystem allocating to the first mobile station physical communication resources of the wireless communications network for transmitting said data packets to the first mobile station; a cell broadcasting subsystem adapted to broadcasting through the network cell tuning information for enabling at least one second mobile station of said plurality capturing the data packets transmitted to the first mobile station through the physical communication resources allocated thereto.

In an embodiment of the present invention, the first mobile station is a virtual mobile station, interacting with the base station subsystem of the network, emulating a real mobile station and dedicated to causing the network gateway structure establishing said data packet transfer path.

In particular, the base station subsystem may distribute the information contents received in data packets in radio data blocks labeled by an indicator of the destination mobile station, and said tuning information broadcasted through the network cell may comprise said indicator.

In particular, said indicator comprises a Temporary Flow Identity (TFI) labeling radio data blocks.

In an embodiment of the present invention, the tuning information is broadcasted over a broadcast channel of a

GSM/GPRS network.

In a preferred embodiment of the. invention, said cell broadcasting subsystem encrypts the-tuning information. Said at least one second mobile station may be provided with a decryption key for decrypting the tuning information.

In an embodiment of the present invention, a provisioning center is provided, managing the subscription of users to the fruition of the information contents, the provisioning center causing the decryption key to be provided to the mobile stations of subscriber users.

In particular, said information contents may include multimedia information contents.

According to a third aspect of the present invention, a mobile station is provided as set forth in claim 18, for the use in a wireless communications network supporting the distribution to the mobile stations of information contents provided in data packets by an information content provider.

Summarizing, the mobile station is adapted to receiving through a cell broadcast channel of the network a broadcast message including a tuning information, and to use the received tuning information for capturing radio data blocks by means of which the data packets are transmitted to a different mobile station through physical communication resources of the network allocated thereto.

In particular, said tuning information comprises an indicator labeling radio data blocks delivering the data packets to the different mobile station; even more particularly, said indicator comprises a Temporary Flow Identity (TFI) labeling radio data blocks.

The mobile station may be adapted to receiving said broadcast message over a broadcast channel of the wireless communications network, particularly a broadcast channel of a GSM/GPRS network.

The broadcast message may be encrypted, and the mobile station be adapted to receive a decryption key for decrypting the encrypted message containing the tuning information. The. received decryption key may be stored locally to the mobile station.

These and other features and advantages of the present invention will be made apparent by the following detailed description of an embodiment thereof, provided merely by way of non-limitative example, description that will be conducted making reference to the attached drawings, wherein: Figure 1 is a schematic view of a GPRS network adapted to implement a method according to an embodiment of the present invention; Figure 2 schematically shows how data relating to a same GPRS service are multicast to several mobile stations, in one embodiment of the present invention; Figure 3 is a schematic flowchart illustrating the operation of the GPRS network of Figure 1, in an embodiment of the present invention; Figure 4 schematically shows, in terms of functional blocks relevant to the understanding of the exemplary invention embodiment herein considered, a mobile station adapted to exploit a multicast GPRS service; and Figure 5 is a schematic flowchart illustrating the operation of the mobile station of Figure 4, in an embodiment of the present invention.

With reference to the drawings and, particularly, to Figure 1, a cellular mobile communications network 100, particularly a GSM network, is schematically shown. The mobile communications network 100 comprises a plurality of Base Station Subsystems (BSSs), each one providing coverage for cellular communications in a respective geographic

region; for simplicity of the drawing, only one BSS is shown in Figure 1, identified by 105.

The generic BSS, such as the BSS 105 shown, comprises a plurality of Base Transceiver Stations (BTSs), each one covering a respective geographic area (a so-called cell) within the region covered by the BSS; again, for simplicity of the drawing, only one BTS is shown in Figure 1, schematically represented by an antenna identified by 110, and the associated cell is schematically depicted and identified by 115. The generic BTS, such as the BTS 110, communicates with users'Mobile Stations (MS), typically cellular phones, which are located in the BTS's cell, such as the two MSs 120m and 120s shown in Figure 1 within the cell 115.

Typically, a plurality of BTSs are connected to a same Base Station Controller (BSC) that controls the BTSs ; for example, all the BTSs of a same BSS are connected to a same BSC, such as the BSC 125 shown in Figure 1. Roughly speaking, the BTSs handle the actual transmission/reception of signals to/from the MSs, whereas the BSC instructs the different BTSs of which data to transmit on specified radio communication channels, and performs at least part of the operations involved in the authentication of the MSs.

The BSCs of the different BSSs, such as the BSC 125 of the BSS 105, are typically connected to a same Mobile Switching Center (MSC), such as the MSC schematically represented in Figure 1 and identified by 130. The MSC 130 is mainly responsible of the authentication of the MSs, and performs the functions of gateway of the cellular network 100 to other communications networks such as one or more fixed telephony networks (PSTNs) and/or one or more other mobile telephony (cellular) networks, all these other communications networks being globally identified by 135 in

the drawing. As known in the art, the MSC operates on the basis of a Home Location Register (HLR), containing subscription data of users subscriber to the. mobile communications network 100, particularly mobile phone numbers, and a Visitor Location Register (VLR), containing information on where the users are currently located, i. e. under which cell they are currently registered.

In Figure 1 elements are also schematically shown that, according to the GPRS standard, enables the MSs connected to the cellular mobile communications network 100 accessing an external packet-based data communications network (shortly, packet data network) such as, for example, the Internet and/or an Intranet, more generally any data communications network in which data are exchanged in packets, i. e. according to a packet-switched scheme instead of a switched- circuit one, particularly but not limitatively any network adopting the Internet Protocol (IP); in the drawing, the external packet data network is shown only schematically, and it is identified globally by 140; hereinafter, it will be assumed that the external packet data network 140 is the Internet, but this is not to be intended as a limitation, being merely an example.

Without entering into excessive details, known per-se in the art and not relevant to the understanding of the invention embodiment herein described, at least one Gateway GPRS Support Node (GGSN) 145 is provided to act as an interface between the cellular network 100 and one or more external packet data network, such as the Internet 140. The GGSN 145 exchanges data packets, through a GPRS backbone network 150, with one or more Serving GPRS Support Nodes (SGSN), such as the SGSN 155 shown in the drawing. The generic SGSN, such as the SGSN 155, routes the data packets received, through the GGSN 145 and the GPRS backbone network

150, from the external packet-based communications network 140, to the proper destination MS,. such as the MS 120m. In particular, exploiting the information contained in the VLR, the SGSN 155 keeps track of the geographical location of the MSs (particularly of those MSs that are trying to open/have opened a GPRS session), so as to know the proper BTS 110 to which the data packets are to be directed to in order to be delivered to the destination MS 120m; in other words, the SGSN 155 knows in which cell 115 the destination MS 120m is currently located, and properly routes the data packets. It is observed that a single SGSN may communicate with a plurality of GGSN, for receiving data packets from different external packet data networks.

In order to provide data packet-based communications services, each BSC is associated with a respective Packet Control Unit (PCU), such as the PCU 160 shown in the drawing, which is associated with the BSC 125. The PCU 160 behaves as an interface of the BSC 125 to a packet-based data communications network 165 internal to the cellular network 100 and leading to a respective SGSN, such as the SGSN 155. The PCU 160 converts the data packets, received from the SGSN 155 through the internal packet-based data communications network 165 and directed to the destination MS 120m, into data streams adapted to being transmitted "over the air", by the BTS 110, exploiting the radio resources of the network, and converts data streams transmitted by the MS 120over the air"and received by the BTS 110 into data packets formatted according to the protocol supported by the internal packet-based data communications network 165, for transmission to the GGSN 145.

Conventionally, the procedure for enabling the user of a generic MS supporting GPRS communications (a GPRS MS), for

example the MS 120m (the client), to exploit services provided by a service provider or content provider 170 (the server) accessible through the Internet 140 or, shortly stated, the fruition by a GPRS client of a GPRS service essentially involves two steps: a first step (called Packet Data Protocol-PDP-context activation) in which a logic connection (the PDP context) is created between the client 120m and the server 170 providing the service contents ; and a second step (called Temporary Block Flow-TBF- activation/release) in which the cellular network 100 assigns to the client 120m prescribed physical communication resources, namely radio resources of the"over the air" radio link portion of the cellular network 100.

In greater detail, the MS 120m preliminary has to register at the SGSN servicing the cell 115 in which the MS 120m is located at that moment, in the example the SGSN 155.

To this purpose, the MS 120m sends to the SGSN 155 a registration request. The SGSN 155 receives the registration request and checks whether the user is entitled to exploit the GPRS data communication services (for example, the user needs a preliminary subscription to the GPRS services); after having ascertained the entitlement of the MS 120m, the SGSN 155 registers the MS 120m. The SGSN 155 assigns to the MS 120m a Packet Temporary Mobile Subscriber Identity (P- TMSI) with which the MS 120m is identified, for security purposes, within the cellular network 100.

Then, the MS 120m sends to the SGSN 155 a request for activation of a PDP context; the PDP context defines the packet data network to be used (in the example, the Internet 140), the GGSN to be used for accessing the Internet 140 (the GGSN 145 in the present example), and other parameters possibly including an address (e. g., an IP address) for identifying the MS 120m in the Internet 140.

The PDP context request triggers a PDP context activation procedure in which the MS 120m, the SGSN 155 and the GGSN 145 exchange information useful to negotiate PDP context parameters.

The PDP context activation procedure results in the definition of a data packets transfer path between the GGSN 145 and the PCU 160 servicing the BTS 110 covering the celA in which the MS 120m is located. A PDP context, i. e. a logic connection (depicted schematically in Figure 1 as a dash- and-double dot line identified by 175) between the client 120m and the server 170 is thus created.

The activation of the PDP context does not involve per- se the allocation of physical communication resources of the cellular network 100, but merely establishes a logic connection between the client and the server; thus, the PDP context, once activated, may be kept active for hours, in principle forever, even when there are no data to be exchanged between the server 170 and the client 120m, until the client 120m (or, possibly, the server 170) deactivates it.

After the PDP context has been activated, the BSC 125 checks whether there are data received from the server 170 through the PCU 160 to be transmitted to the client MS 120m; in the affirmative case, i. e. when there are data to be exchanged between the server 170 and the client MS 120m, a TBF is activated by the BSC 125.

The activation of the TBF determines the allocation of physical radio communications resources of the cellular network 100, i. e. of a radio channel (an airlink channel), to the MS 120m for enabling the exchange of the data packets from the data transfer path (converted in a suitable data stream by the PCU 160) through the proper BSC and BTS 125 and 110 over the air to the MS 120m.

When the data have been exchanged, the TBF is deactivated and the radio resources are released; provided the PDP, context is not closed, the BSC 125 waits for new data'to be exchanged. Thus, differently from the PDP context, the TBF, i. e. the physical, radio communications resources of the GSM network 100 are kept allocated for the MS 120m only as long as there are data to be transmitted/received to/from it, and are released as soon as the data have been exchanged, keeping the radio resources free for other uses/users. In other words, a TBF is temporary and is maintained only for the duration of the data transfer.

According to the GPRS standard, to each TBF there is univocally assigned an indicator, called Temporary Flow Identity (TFI). Roughly speaking, the TFI is exploited for managing the scheduling of the data to be transmitted in downlink (i. e. , from the BSC and BTS 115 and 110 to the destination MS 120m); different GPRS service users within a same cell are assigned different TFIs. In terms of the ISO's Open System Interconnect (OSI) stack of layers model, the TFI allows discriminating between different entities at the Radio Link Control (RLC) layer, that is, a given TFI univocally addresses a respective RLC entity and is inserted in a header portion of all the RLC data blocks transmitted.

When the TBF is activated, a TFI is assigned thereto and such a TFI is communicated to the destination MS 120m.

At the physical level, the GPRS is based on the physical layer of the GSM standard.

As known in the art, the GSM standard provides, for the communications between the BTSs and the MSs, a plurality of radio channels of 200 KHz bandwidth, associated with a plurality of radio carriers ; particularly, 124 radio carriers are provided, and a hybrid Frequency Division

Multiple Access (FDMA) /Time Division Multiple Access (TDMA) access scheme associates eight, time slots (eight physical channels) with each carrier.

The transmission to/from a given MS-takes place discontinuously, only within the time slot cyclically assigned to that MS; a cycle is the succession in time of all the eight time slots, and is referred to as a radio frame. A"buffer and burst"transmission results: once the MS has been granted access to a given carrier (which is done during the initial registration of the MS in the GSM network, and which in the GSM standard is subject to changes according to a frequency hopping scheme), the transmission and reception are accomplished in distinct time slots.

The physical level used by the GPRS system is based on the GSM one, superimposing thereto a different logic structure. Control and data traffic"logic"channels (defined at the MAC level, which is commonly implemented in the BTS 125) are multiplexed in time and frequency division on a single GPRS physical channel, called Packet Date CHannel (PDCH).

A PDCH corresponds to a physical GSM channel, and is defined, in the frequency domain, by a radio carrier number (one of the 124 carriers), and, in the time domain, by a timeslot number (one of the eight timeslots within that carrier). The timing and duration of the timeslots are identical to those defined for the GSM system.

The control and data traffic GPRS logic channels are multiplexed in time so as to share the same PDCH at physical level.

As pictorially shown in Figure 2, given a prescribed radio carrier ("CARRIER within the drawing), which is one of 124 radio carriers of the GSM standard, the radio frames (each one including eight time slots TS1 to TS8) are grouped

in groups of fifty-two radio frames, so as to form a so- called multiframe, such as the multi'frame MFRj depicted in the drawing. Each multiframe is subdivided'into twelve frame blocks, such as the frame block FRB1 shown in the drawing, including each one four radio frames. Between adjacent triplets of frame blocks, an idle frame IFR is interposed, left.. deliberately free of data.

The multiframe periodically repeats every fifty-two radio frames. The different GPRS control and data traffic logic channels are multiplexed together based on a subdivision into blocks (radio data blocks) of the data to be transmitted. The radio data blocks are the basic transport structure of a GPRS logic channel: a given radio data block is thus univocally dedicated to a respective GPRS logic channel, being it a data traffic channel or a control channel. The assignment scheme of the radio data blocks to the different logic channels is transmitted, together with other control information, over a GPRS control logic channel PBCCH, having a fixed position within the multiframe.

As for voice communications, radio transmission takes place as a sequence of four"normal bursts", according to the GSM standard specification. Every radio data block, such as the radio data block RDBk shown in the drawing, is transmitted during four consecutive radio frames of the same PDCH, such as the frames FR1 to FR4 in the drawing, exploiting one (possibly more, depending on the radio resources allocated to that logic channel) time slot in each frame, such as the timeslot TS3 in the drawing.

Still according to the GPRS standard, every radio data block includes a header portion containing the TFI that univocally identifies the TBF, in addition to other parameters. In this way, two or more TBFs can be multiplexed on a same PDCH in downlink to the MSs.

Each MS in a cell listens to every radio data block transmitted by the BTS'of that cell; however, an MS destination of GPRS data, that-is an MS such as the MS 120m that has activated a PDP context and has been assigned a TBF, e. g. TBFl, only captures those radio data blocks that, similarly to the radio data block RDBk in the drawing, are labeled by that TFI TFI1, i. e. by the TFI that has previously been communicated thereto, when the TBF was activated; all the other radio data blocks, not labeled by the correct TFI TFIl, are discarded by the MS 120m. The TFI is thus used substantially as a tuning information, which the MS uses for tuning onto the physical communication channel on which the GPRS data directed to the MS are transmitted.

In a conventional GPRS network, if a different MS in the same cell as the MS 120m, for example the MS 120s shown in the Figure 1, wishes to exploit, through the GPRS network, the same services made available by the server 170 as those being already exploited by the MS 120m, a procedure identical to that described in the foregoing has to be performed, leading to the activation of another PDP context and, even worse, to the activation of different TBFs when data have to be exchanged with the MS 120s, i. e. to the allocation of additional radio resources different from those already allocated by the GSM network 100 for the MS 120m. This is clearly a waste of resources, especially when the services exploited are relatively heavy in terms of data to be downloaded to the MSs, such as in the case of delivery of multimedia contents, audio or video (e. g. television) streaming, and the availability of GPRS services offered to the users may be severely limited, unless the GSM network infrastructure is greatly overdimensioned.

In order to avoid such a waste of resources, according

to an embodiment of the present invention, the other MSs in the same cell'as the MS 120m, for example the MS 120s shown in the drawing, that wish to exploit through the GPRS network the same. services made available by the server 170 as those exploited by the MS 120m, are communicated by the GPRS/GSM network the TFI TFI1 assigned to the TBF activated within the PDP context in respect of the MS 120m and labeling the radio data blocks directed thereto, exploiting any one of the broadcast communications channels normally available in the GSM and/or GPRS networks, for purposes such as broadcasting information messages to the cellular phones located in a cell.

In particular, and by way of example only, the GSM Cell Broadcast CHannel (cell CBCH, depicted schematically in Figure 2 and identified therein by BCCH) can be exploited to broadcast through the cell a message including the TFI s identifying the TBFs relating to the GPRS service which is to be offered in multicast. As known in the art, the GSM cell broadcast channel is a common channel exploited for example to implement a GSM network cell broadcast service according to which information is transmitted on the common channel, so that all or a plurality of users in the gell can receive the information. More generally, the TFI TFI1 may be broadcasted exploiting any broadcast channel, either GSM or GPRS.

Since the information broadcasted over the CBCH CBCH are readable by every MS in the cell, the TFI TFI1 that identifies the radio data blocks such as the radio data block RDBk containing the data streams corresponding to the data packets received from the server 170 is thus made available to potentially all the MSs in the cell 115. Based on the TFI TI ? II received over the CBCH CBCH, an MS 120s (slave MS) other than the MS 120m (master MS) that activated

the PDP context 175, can thus receive the proper radio data blocks and exploit the same service already being exploited by the MS 120m, without for this reason causing a duplication of the radio resources that the GSM network 100 needs to allocate.

In other words, according to an embodiment of the present invention, a PDP context is activated as usually, upon request by a MS in a cell (the first MS in that cell, i. e. the master MS) that wishes to get access to the services provided by a given service provider 170 through the Internet 140; once this"pilot"context is activated, two or more other MSs (the slave MSs) in that same cell are provided information adapted to allowing them to"tune"onto the TBF activated for delivering the service information content to the master MS; in particular, such information, essentially consisting of the TFIs that identify the radio data blocks transmitted in the TBF, is included in a message that is broadcasted in the cell exploiting one of the cell broadcast channels, expediently one of the cell broadcast channels already existing in the GSM/GPRS network. Thus, even an MS that did not activate a PDP context can receive data relating to a GPRS service corresponding to the activated PDP context.

In Figure 1 the elements adapted to implement the exemplary and non-limitative embodiment of the invention herein considered are schematically indicated.

In particular, a Cell Broadcasting Center (CBC) 180, depicted as internal to the MSC 130 although this is not to be construed as a limitation, since the CBC might as well be thought as external to the MSC, is provided in the GSM network 100 for managing the broadcasting of information to the different MSs in the different cells, over the dedicated cell broadcast channels. The CBC 180 is selective towards

the cells, i. e. it is capable of selectively broadcasting different information for different cells. In conventional GSM/GPRS networks the CBC 180 manages for example the broadcasting to the MSs of information such as the name of the geographic area wherein they are presently located, traffic information and other information of public utility, and the like. Conventionally, the CBC 180 holds a set of cell broadcast messages to be broadcasted within the different cells of the network, and transmits the messages to the network BSCs in accordance with location areas that may be predefined for each message.

The CBC 180 needs to know, for each cell, which pilot PDP contexts are currently active; to this purpose, as schematically depicted in Figure 1 by means of a dash-and- double dot line 185, the CBC 180 dialogues with the SGSNs and/or the GGSNs of the GPRS network, such as the SGSN 155 and the GGSN 145 shown in the drawing. From the SGSNs and/or the GGSNs, the CBC 180 gets information adapted to determine, on a cell by cell basis, which pilot PDP contexts are currently active. The CBC 180 may discriminate between pilot PDP contexts and non-pilot PDP contexts, i. e. between PDP contexts relating to GPRS services that are intended to be offered in multicast modality, and PDP contexts that are instead not intended to be offered in such modality, but only in the traditional unicast mode, looking e. g. at the service name and/or the IP address of the server 170 providing it.

Having established that a pilot PDP context is active in, e. g., the cell 115, the CBC 180 gets from the respective BSC 125 the TFI identifying the TBF assigned to that pilot PDP context, and includes the TFI in a broadcast message that is broadcasted in the cell 115 over a selected one of the cell broadcast channels, such as the CBCH CBCH for that

cell. The BSC 125 thus causes the BTS 110 to broadcast the message including. the TFI in the-cell 115.

Thus, referring to the simplified flowchart 300 of Figure 3, similarly to the conventional procedure described above, the master MS 120m sends to the SGSN 155 servicing the cell 115 in which the MS 120m is located at that moment a registration request, to register at the SGSN (block 305); the SGSN 155, after checking its entitlement, registers the master MS 120m (block 310).

Then, the master MS 120m sends to the SGSN 155 a request for activation of a PDP context (block 315).

The PDP context request triggers the PDP context activation procedure, resulting in the definition of a data packets transfer path between the GGSN 145 and the PCU 160 servicing the BTS 110 covering the cell in which the MS 120m is located. A pilot PDP context is thus created, and the activation of the pilot PDP context is communicated to the CBC 180 (block 320). Possibly, before activating the PDP context that corresponds to a multicast GPRS service, the SGSN 155 checks whether a pilot PDP context is already active for that GPRS service (for example because the PDP context activation request has been received from a MS different from the MS herein considered) and, in the affirmative case, the new PDP is not activated.

After the pilot PDP context has been activated, the BSC 125 checks whether there are data received from the server 170 through the PCU 160 to be transmitted to the master MS 120m (decision block 325); in the affirmative case, (exit branch Y of decision block 325), a TBF is activated for the master MS 120m (block 330), and physical radio communications resources of the cellular network 100 are allocated for enabling the exchange of the data packets from the data transfer path (converted in a suitable data stream

by the PCU 160) through the proper BSC and BTS 125 and 110 over the air to the MS 120m. Also, a TFI identifying the TBF is generated by the BSC 125.

Under instructions of the CBC 180, the BCS 125 causes the BTS 110 to broadcast a message including the TFI, identifying the TBF assigned to the pilot PDP context, over the cell broadcast channel CBCH (block 335). Preferably, the message is broadcasted periodically.

The master MS 120m receives the TFI in the usual way, while the slave MSs, such as the MS 120s, receive the TFI over the cell broadcast channel CBCH (block 340).

Data are transmitted by the BTS 110 in radio data blocks over the selected physical channel, the radio data blocks being labeled by the TFI assigned to the TBF (block 345); the master MS 120m and the slave MS 120s can both tune onto the proper physical channel and capture the radio data blocks pertaining to the desired multicast GPRS service thanks to the fact that they both posses the correct TFI (block 350).

When the data have been exchanged, the TBF is deactivated (block 355) and the radio resources are released; provided the PDP context is not closed (exit branch N of decision block 360), the BSC 125 waits for new data to be exchanged, otherwise the procedure ends.

In an alternative embodiment of the invention, the TFI assigned to the TBF activated for distributing GPRS data to the master MS 120m is not retrieved by the CBC 180 from the BSC 125, being instead communicated to the CBC 180 (or to a suitable network apparatus to which the CBC 180 is connected) directly by the master MS 120m, when it receives from the BSC 125 and the BTS 110 the TFI labeling the radio data blocks directed thereto, together with an indication enabling the CBC 180 to identify the network cell wherein

the master MS 120m is located; this requires that a suitable application logic runs on the master MS 120m, adapted to intercepting the TFI communicated thereto, and to send information including the TFI and the cell identifier to the CBC to the network apparatus, exploiting for example a signaling GSM channel.

In an embodiment of the present invention, accession of the MSs in a cell to a multicast GPRS service is not granted indiscriminately to every MS in the cell, but on a selective basis, particularly only to MSs of those users that have previously subscribed to such a GPRS service multicasting.

To this purpose, in an embodiment of the present invention shown schematically in Figure 1, a Provisioning Center (PC) 190 is provided, managing the subscription of MS users to the exploitation of one or more multicast GPRS services. In order to subscribe to a selected multicast GPRS service, a MS user has to register at the PC 190, through any prescribed provisioning channel, such as for example by sending to the PC 190 a Short Message System (SMS) message, or accessing a specified provisioning Web site, possibly accessed by the GPRS MS, or through a cellular phone supporting the WAP protocol, or by phone, or the like. In order to become subscriber of a given multicast GPRS service, the user has to provide to the PC 190 information such as the telephone number of the MS.

It is observed that although the provisioning center has been depicted as external to the MSC, this is not to be construed as a limitation, because the provisioning center might as well be part of the MSC. Also, the PC 190 may be connected to the Internet 140, or at least include a Web site, accessible through the Internet, and/or a call center.

The PC 190 keeps a register of the users that have subscribed to the multicast GPRS services. Registration to a

given multicast GPRS service may be free of charge or conditioned to the payment of a price.

Once the user has registered at-the PC 190, the PC 190 communicates to the CBC 180 the registration data, particularly the MS telephone number and the multicast GPRS service to which the user has subscribed.

An encryption/decryption key pair is generated. The encryption key is made available to the CBC 180. The decryption key may be sent over the air to the MS of the subscriber user, for example the MS 120s in the drawing, for example by means of an SMS, or in any other way, possibly including the shipment to the user of a new, substitutive Subscriber Identity Module (SIM) to be used in the mobile phone for exploiting the multicast GPRS service. The encryption/decryption key pair may be generated by any suitable network equipment, e. g., the PC 190, or the CBC 180.

As mentioned in the foregoing, the CBC 180 detects when a TBF is assigned to the PDP context relating to the multicast GPRS service, and gets the respective TFI that has been generated for labeling the corresponding radio data blocks (alternatively, the CBC receives the TFI directly from the master MS). The CBC 180 encrypts the broadcast message including the TFI using a suitable encryption algorithm and the encryption key, and causes the BSC/BTS 125/110 to broadcast the encrypted broadcast message including the TFI over the cell CBCH. The slave MSs which, similarly to the slave MS 120s, are subscribers to the service and have received the decryption key are able to decrypt the encrypted broadcast message including the TFI, extract the TFI from the decrypted message and use the extracted TFI for tuning onto the correct channel and receive the desired radio data blocks.

Figure 4 schematically shows, in an extremely simplified way and in terms of'functi-onal blocks relevant to the understanding of the-exemplary invention embodiment herein considered, an MS such as the MS 120s shown in the drawing, adapted to exploit a multicast GPRS service. As known, an MS comprises, in addition to specific elements allowing radio communications, a programmable data processing unit, particularly a microprocessor, with dynamic and non-volatile memory resources, and strictly interacting with the SIM, which is a removable smart-card module having its own data processing and storage capabilities. It is intended that at least some of the functions that will be described are implemented in terms of software run by the data processing units of the MS and/or of the SIM.

The MS 120s comprises a physical layer unit 405 handling the low-level (physical level) details of the radio communications, compliant to the GSM standard; this unit comprises in particular the transmitter/receiver circuits of the MS.

The physical level unit 405 communicates with an RLC/MAC (Media Access Control) level unit 410, managing the communications at the immediately higher TLC/MAC level of the ISO OSI model, particularly controlling the access of the MS to the physical communication medium. In extremely simplified terms, sufficient for the purposes of the present description, the RLC/MAC level unit 410 receives the radio data blocks from physical level-unit 405, and reconstructs the various GPRS logic channels mentioned in the foregoing.

In particular, the RLC/MAC level unit compares (as schematized by the AND logic gate 415) the TFI labeling the received radio data blocks to the locally-stored tuning TFI TFI1, which the MS uses for establishing whether the radio data blocks are directed thereto, and are therefore to be

captured and retained, or discarded. In case the MS is the master MS, i. e. the MS that has activated the PDP context, the tuning TFI TFIi is communicated to the MS by the BCS when a TBF is activated within the PDP context associated with the MS. If the TFI labeling the received radio data blocks does not coincide with the tuning TFI TFIi, the radio data blocks are discarded (as schematized by the switch 420 open), otherwise they are captured, the data traffic channel is reconstructed, and the data are passed over to the higher levels of the OSI model 425, up to the OSI application layer.

Differently from radio data blocks relating to data traffic logic channels, the data received over the broadcast channel such as the GSM CBCH are not filtered by the TFI.

The received data relating to the GPRS service are then passed to an application software 430, such as a content viewer or an MP3 player or the like and, through the proper I/O peripheral (display, loudspeaker), are made available to the user. Alternatively or in combination, the data may be stored in a local storage of the MS, for a background fruition.

According to an embodiment of the present invention, a decrypter application software 435 runs in the MS 120s adapted to decrypt the encrypted broadcast message including the TFI, received over the CBCH, exploiting the decryption key 440 of the encryption/decryption key pair. The TFI extracted from the decrypted broadcast message is fed to the RLC/MAC level unit 410, which stores it as a tuning TFI for that specific type of GPRS service. In particular, a TFI database may be provided in the MS, in which several TFIs may be stored, associated with indications suitable to identify the related GPRS services ; the information broadcasted over the CBCH include both the TFI, identifying

the data blocks of a given GPRS service exploitable in multicast, and an identifier of the GPRS service itself, for example-the Internet address of the service provider 170.

In Figure 5 a schematic flowchart is provided illustrating, at a very high level, the operation of the generic MS, according to an embodiment of the present invention.

The MS continuously listens to the broadcast channels, such as the cell CBCH CBCH, over which the preferably encrypted broadcast message including the TFI of the multicast GPRS service is communicated to all the MSs in the cell. The MS gets the encrypted broadcast message including the TFI, decrypts it (through the decrypter software 435 and using the decryption key 440 previously stored) extracts the TFI from the decrypted message and stores the TFI into the TFI database, together with the identifier of the GPRS service (block 505).

When the MS user requests to access a GPRS service (decision block 510, exit branch Y), the MS firstly checks whether the GPRS service identifier is already present in the TFI database (decision block 515). In the negative case (exit branch N), meaning that the desired GPRS service is not a multicast service, or that no pilot PDP context has yet been created for that GPRS service, or that the user is not a subscriber to the multicast reception of the GPRS service, a conventional PDP context activation request is performed; the PDP context is created and, when there are data to be transmitted to the MS, a TBF is activated; the corresponding TFI is conventionally communicated to the MS (block 520), and this TFI is used by the MS for tuning onto the proper physical channel and identifying the radio data blocks pertaining to the desired GPRS service. If instead the MS finds the GPRS service identifier in the TFI database

(exit branch Y of decision block 515), meaning that the GPRS service is currently being offered in multicast, modality and can be exploited without activating another. PDP context, being a pilot PDP context already active, and the user is a subscriber to the multicast GPRS service, the MS does not send any PDP context activation request, but gets the proper TFI from the TFI database (block 520), and exploits such a TFI for tuning onto the proper physical channel and receiving the radio data blocks relating to the multicast GPRS service (525).

The network apparatuses, such as the SGSN 155 and/or the GGSN 145, may implement a suitable mechanism adapted to enabling the slave MSs, such as the MS 120s, to continue receiving the GPRS service data even in case the original PDP context, activated by the master MS 120m, is closed, for example responsive to a PDP context closure command from the master MS 120m, or because the master MS 120m leaves the network cell. For example, when the SGSN 155 detects that the PDP context relating to a multicast GPRS service is closed, it informs the CBC 180, which broadcasts a suitable message to the slave MSs in the cell, informing that the PDP context has been closed ; it is sufficient that one of the slave MSs in the cell, upon receiving such a notification from the CBC, requests to the SGSN a new PDP context activation for that service, thereby that MS takes the role of master MS in substitution of the former master MS. It is observed that since audio/video, particularly TV streaming, is often offered slightly delayed compared to the live event, the service may undergo no substantial interruptions.

It is pointed out that while in the invention embodiment described so far the pilot PDP context was activated by one MS in a given cell wishing to exploit the services of the service provider 170, in the example the

master MS 120m, this is not to be construed as a limitation of the present invention.. In an alternative solution, a dedicated MS may be provided for in the cell, depicted schematically in phantom in Figure 1 and identified by 120v, dedicated to the activation a pilot PDP context with a predetermined content provider such as the server 170, and such an MS 120v may even be a virtual MS, possibly an MS emulating apparatus of the GPRS network, i. e. an apparatus of the ground GSM/GPRS network.

Thanks to the invention embodiment described in the foregoing, GPRS service data can be distributed to, so as to be exploited by, a plurality of users at a same time, particularly users within the same cell of the cellular network, in a point-to-multipoint modality (i. e. , in multicast), and the physical radio resources to be allocated do not depend on the number of users simultaneously exploiting the GPRS services: in particular, only one PDP context and one TBF are to be activated. This is of great benefit especially in case of GPRS services relatively heavy from the viewpoint of the quantity of data to be transferred, such as in GPRS services involving the distribution of multimedia (audio and/or video) contents.

The GPRS service contents can be transferred either in live or in background mode, i. e. the generic MS, either master or slave, may store the multimedia contents received from the provider 170 for a subsequent fruition. For example, for a specific service, the GPRS network may communicate to the MS of users subscriber to the service that there are contents available to be downloaded ; in response, the MSs start a PDP context activation procedure; the first MS doing this will act as the master MS, while the other will receive the TFI assigned to the TBF in the way described above. The GPRS data may be distributed either

encrypted or not.

It is also pointed out that more than one GPRS services can be distributed in. multicast at a same time: in this case, two or more TFIs will be broadcasted over the cell broadcast channel, each one labeling radio data blocks of a respective TBF, corresponding to a respective pilot PDP context.

Thanks to the preferred embodiment described in the foregoing, the access to a multicast GPRS service by a given user is conditioned to the subscription of the user to the service, and only user which are subscribers to the multicast GPRS service can decrypt the tuning data necessary for tuning onto the multicasting GPRS service.

The solution according to the embodiment of the invention herein described has the significant advantage of not necessitating modifications of the standard GSM/GPRS apparatuses, already deployed on field.

Even though in the preceding description reference has been made to a GSM network, this is not to be intended as a limitation, because the invention can be applied in general to any wireless, mobile communications network, and particularly to networks realized according to the United States and Japanese counterparts of the GSM standard, as well as to third-generation networks such as those exploiting UMTS.

Although the present invention has been disclosed and described by way of an embodiment, it is apparent to those skilled in the art that several modifications to the described embodiment, as well as other embodiments of the present invention are possible without departing from the scope thereof as defined in the appended claims.