Abstract:
The invention relates to a method for providing multicast and/or broadcast services to user terminals ( 8, 9 ) via a cellular network ( 2, 5 ). In order to increase the efficiency of multicast or broadcast transmissions, it is proposed that each of said user terminals ( 8, 9 ) willing to receive content data of at least one multicast and/or broadcast service enters at least once for a pre-determined time a state in which it is capable of receiving content data of said multicast and/or broadcast service. The invention equally relates to a corresponding user terminal, to a corresponding cellular network and to a corresponding mobile radio system.

Description:
FIELD OF THE INVENTION  
         [0001]    The invention relates to a method for providing multicast and/or broadcast services to user terminals via a cellular network. The invention equally relates to such a user terminal, to such a cellular network, to a network element of a cellular network, and to a mobile radio system comprising such a user terminal and such a cellular network.  
         BACKGROUND OF THE INVENTION  
         [0002]    Broadcast and Multicast are methods for transmitting data-grams from-a single source to several destinations, i.e. as point-to-multipoint transmission.  
           [0003]    As mentioned in the technical specification 3GPP TS 22.146 V5.0.0 (2001-10): “Multimedia Broadcast/Multicast Service; Stage 1 (Release 5)”, currently two such services are specified for cellular networks, the cell broadcast service (CBS) and the internet protocol (IP) multicast service.  
           [0004]    The cell broadcast service enables a transmission of low bit-rate data to all subscribers in a set of given radio cells of a cellular network over a-shared broadcast channel.  
           [0005]    The transmission of content data for IP multicast services, provided e.g. by an internet service-provider, to user terminals via a radio interface can currently be supported by GPRS (general packet radio system) based cellular networks. It is required, however, that a point-to-point PDP (packet data protocol) context is set up between every user terminal and a network element of the GPRS network, i.e. the GGSN (Gateway GPRS support node), and every user terminal receiving a IP multicast transmission occupies a separate radio channel. Several radio channels thus deliver the same content in a point-to-point way through a GPRS system.  
           [0006]    For UMTS, 3GPP (3rd generation partnership project) is developing new services for multicast and broadcast in UMTS. In the above mentioned technical specification 22.146, 3GPP provides for example a stage one description of broadcast and multicast services for UMTS. One desirable feature for new multicast services is for example to enable the transmission of an IP-multicast service content via a radio access network of a UMTS system to several user terminals with a shared radio channel. For both, new broadcast and new multicast services, a procedure for activating a broadcast or multicast session on the radio interface still has to be provided.  
           [0007]    From an IP multicast content source point of view, the handling of a multicast transmission by a radio access network should be similar to IP-multicast, in order to avoid a conflict with the IETF IP-Multicasting standards.  
           [0008]    In the RFC (Request for Comments) 2236: “Internet Group Management Protocol, Version 2X”, of November 1997, it is specified for example that multicast routers use IGMP (Internet Group Management Protocol) to learn which groups have members on each of their attached physical networks. It is requested that a multicast router keeps a list of multicast group memberships for each attached network, and a timer for each membership. The expression ‘Multicast group memberships’ means the presence of at least one member of a multicast group on a given attached network, not a list of all of the members.  
         SUMMARY OF THE INVENTION  
         [0009]    It is an object of the invention to enable an efficient delivery of multicast and/or broadcast services to mobile users by a cellular network. It is in particular an object of the invention to enable an activation of multicast or broadcast sessions only in such radio cells of the cellular network, in which the number of likely receivers exceeds a predetermined value.  
           [0010]    In order to reach the objects of the invention, a method for providing multicast and/or broadcast services to user terminals via a cellular network is proposed, in which method each of these user terminals which is willing to receive content data of at least one multicast and/or broadcast service enters at least once for a predetermined time a state in which it is capable of receiving content data of said multicast and/or broadcast service.  
           [0011]    The state in which a user terminal willing to receive content data of a specific multicast or broadcast service is in addition capable of receiving such content data will also be referred to as READY state. In the state in which a user terminal is willing to receive content data of a specific multicast or broadcast service, it is in contrast not capable without any further action of receiving such content data will be referred to as STANDBY state.  
           [0012]    The object is equally reached with a user terminal, with a cellular network, with a network element for a cellular network, and with a communication system comprising means for realizing the proposed method.  
           [0013]    The invention proceeds from the idea that if a user terminal enters for a time a READY state when it is willing to receive content data of a multicast and/or broadcast service, it can be provided with such content without any further action being required for the transmission during this time, thus making the delivery of a service more efficient.  
           [0014]    In a preferred embodiment of a first approach for further developing the invention, at least some of the user terminals willing to receive content data of at least one multicast and/or broadcast service transmit a corresponding indication to the cellular network, which indication can provide the network in particular with an information on which multicast and/or broadcast services the respective user terminal is willing to receive. The indication can consist for example of an indication that a respective user terminal joins a group of user terminals willing to receive at least one specific multicast and/or broadcast service. As consequence, the cellular network can determine whether there is a sufficient number of likely receivers in a specific cell for the delivery of multicast and/or broadcast services.  
           [0015]    Advantageously, the cellular network has moreover knowledge about the time during which a user terminal remains in a READY state. In this case, the cellular network does not only know how many likely receivers there are for a service, but also many of them are currently in a READY state and for how long. The cellular network does not have an information on the exact location of user terminals in the STANDBY state. With the knowledge about the time during which each user terminal remains in a READY state, the cellular network can therefore determine with an increased certitude the number of likely receivers in a specific cell for the delivery-of multicast and/or broadcast services.  
           [0016]    The time during which a respective user equipment remains in a READY state can be fixed for all user terminals and known at the network. In this case, the network knows already only with the indication of a user terminal that it is willing to receive content data for a multicast and/or broadcast service, for how long this user terminal will stay in the READY state and thus be able to receive content data.  
           [0017]    Alternatively, at least some of the user terminals willing to receive content data of at least one multicast and/or broadcast service transmit in addition an indication to the cellular network for how long they will remain in the READY state. The indication of a time for how long a user terminal will be in the READY state could also be transmitted by itself to the cellular network. This is of particular interest for a later point of time, at which a user terminal has already switched from the READY state to the STANDBY state, and then returns to the READY state, e.g. because of some other transmission.  
           [0018]    The first approach of the invention enables a network element of the cellular network in particular to immediately respond to a polling by a multicast and/or broadcast router wishing to provide content data for a service, since it knows for how long how many user terminals willing to receive such content data will be in the READY state.  
           [0019]    In a preferred embodiment of a second approach for further developing the invention, a network element of the cellular network transmits a paging message when a multicast/broadcast router starts to deliver content data for a specific multicast or broadcast service. To this paging message, each user terminal, which is willing to receive content data of this specific multicast or broadcast service but for which said predetermined time has passed, can respond, in case it actually receives the paging message.  
           [0020]    This second approach is based on the idea that if user terminals in a STANDBY state are paged by the cellular network when a multicast or broadcast service is to be provided, the number of responses by these terminals indicates to the cellular network whether there is a sufficient number of likely receivers for the delivery of multicast and/or broadcast services.  
           [0021]    It is an advantage of the second approach of the invention that an evaluation of likely receivers is also possible, after the terminals in this cell have turned to the STANDBY state. The evaluation can be performed separately for each radio cell of the cellular network.  
           [0022]    Also in the second approach, each user terminal willing to receive content data of at least one multicast and/or broadcast service can transmit a corresponding indication to the cellular network. This indication can provide the cellular network with additional information for paging.  
           [0023]    Both approaches thus provide the cellular network with a reliable information on a minimum number of user terminals which are currently available for receiving content data of a broadcast or multicast service.  
           [0024]    It is a further advantage of both approaches of the invention, that from the content source point of view, the handling can be made similar to the IETF specified IP multicasting.  
           [0025]    The invention can be employed in particular, though not exclusively, for UMTS.  
           [0026]    Further, the invention can be employed in particular for supporting IP multicasting via the radio interface of a cellular network, but also for other types of multicast services or for broadcast services.  
           [0027]    Preferred embodiments of the invention are included in the subclaims. 
       
    
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0028]    In the following, the invention is explained in more detail with reference to a drawing, of which the only figure shows an environment in which an embodiment the invention can be employed. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0029]    The only figure shows selected elements of a communication system that are involved in embodiments of the invention according to the first or the second approach, which both support IP-multicasting via a cellular network to user terminals. The system comprises the internet, a PLMN as cellular network providing UMTS, and user terminals.  
         [0030]    In the figure, the internet  1  is connected to a GGSN  3  of a core network  2  of the PLMN. The GGSN  3  is further connected via an SGSN (switching GPRS support node)  4  of said core network  2  to at least one RNC  6  of an UTRAN S of the PLMN. Within the UTRAN  5 , the RNC  6  has a controlling access to at least one node B  7 . The node B  7 , finally, can provide via a radio interface a connection to user terminals  8 ,  9  which are located in a radio cell defining the coverage area of the node B  7 .  
         [0031]    A user terminal  8 ,  9  that is subscribing a specific multicast service provided by an internet service provider and that is currently willing to receive this service informs the cellular network that it joins a multicast group defined for receiving this specific service. The mobile joins this group using a specific multicast PDP context or a normal PDP context which is transmitted to the cellular network.  
         [0032]    After joining the group, the user terminal  8 ,  9  remains in a READY state for a predetermined time, i.e. until a timer T 1  provided in the user terminal  8 ,  9  runs up. Then, the user terminal  8 ,  9  enters a STANDBY state. The READY state will be entered again for the predetermined time when a PDP context is activated for a data transmission. Only in the READY state, a user terminal  8 ,  9  is able to receive multicast or broadcast transmissions without first having to activate a PDP context.  
         [0033]    A differentiation between a STANDBY state and a READY state has already been described for GPRS, e.g in the technical specification 3GPP TS 23.060 V3.6.0 (2001-01): “General Packet Radio Service (GPRS); Service description; Stage 2 (Release 1999)”. In the STANDBY state, paging messages for data or signaling information transfers may be received. It is necessary, however, to activate a PDP context before data can be transmitted or received. Such an activation can be initiated by the user terminal e.g. upon a paging message. For user terminals in the READY state, the network transmits no GPRS paging messages, since the user terminal is already able in this state to send and receive PDP PDUs (protocol data units) employed for transmitting content data. The READY state is supervised by a timer. A user terminal moves from the READY state to the STANDBY state when the timer expires.  
         [0034]    The timer T 1  of the user terminals  8 ,  9  depicted in the figure may be identical with the timer currently defined for GPRS. Alternatively, it may be a new timer introduced specifically for multicast services.  
         [0035]    Within the cellular network, the information that a user terminal  8 ,  9  has joined a multicast group is received by the node B  7  and forwarded via the RNC  6  and the SGSN  4  to the GGSN  3 . The GGSN  3  thus knows which user terminals  8 ,  9  have joined a multicast group for a specific service.  
         [0036]    In the embodiment according to the first approach, a user terminal  8  joining a multicast group transmits at the same time a timer value MC 1 , which indicates for how long this user terminal  8  will be able to receive an IP multicast transmission for this multicast group, i.e. for how long it will remain in the READY state.  
         [0037]    Equally, another user terminal  9  may join the same multicast group and transmit a corresponding timer value MC 2 .  
         [0038]    The timer values MC 1 , MC 2  are forwarded together with the indication that a user terminal  8 ,  9  wishes to join a specific multicast group to the GGSN  3 . The RNC  6  and the SGSN  4  also know the user terminals  8 ,  9  that are still able to receive multicast messages after having joined a corresponding multicast group.  
         [0039]    In case an IP-multicast router wants to transmit content data for a specific IP multicast service received from an internet service provider, it first performs a polling with the GGSN  3 . Only if the GGSN  3  responds to the polling, the content data will actually be transmitted. Since the GGSN  3  knows the timer values MC 1 , MC 2  of all user terminals  8 ,  9  that have joined a specific multicast group, it is at any time aware of whether there are user terminals  8 ,  9  in its coverage area, which are still in the READY state. As long as there are such user terminals in the READY state, it can respond to a polling by a multicast router.  
         [0040]    In the embodiment according to the second approach, a determination of user terminals  8 ,  9  which have joined a specific multicast group and are still available for receiving content for this service is carried out by the RNC  6 .  
         [0041]    When an IP multicast router starts to deliver content data for a specific multicast service, the GGSN  3  forwards the content data via the SGSN  4  to the RNC  6 . The RNC  6  shall activate a multicast session in those cells for which it controls the radio resources, in which there are user terminals  8 ,  9  that have joined the corresponding multicast group.  
         [0042]    User terminals  8 ,  9  that are in the READY state simply receive a start message from the RNC  6  informing them about the start of a multicast session.  
         [0043]    The RNC  6  is not aware, however, whether a user terminals  8 ,  9  in the STANDBY state, which have joined the respective multicast group, is still located in a radio cell supplied by the RNC  6 . Therefore, the user terminals  8 ,  9  in the STANDBY state are first paged by the RNC  6 , in order to enable the RNC  6  to determine in which radio cell they currently are.  
         [0044]    In the presented embodiment, the employed paging is a special paging for multicast services. It contains specific multicast indicators, more specifically a multicast group indication, a multicast channel indication, and possibly some other information. The number of bits that can be used on the GPRS paging channel is limited. For this reason, all indicators are defined in a shortened form. Because the paging normally serves more than one user terminal, however, the paging message can be e.g. 2 to 3 times larger than the normal paging to a single user terminal.  
         [0045]    All user terminals  8 ,  9  that are in the coverage area of the RNC  6  and that have joined the multicast group indicated in the paging message can respond to the paging after a random delay. This random delay ensures that collisions between a large number of simultaneous responses are avoided. The multicast channel indication in the paging message indicates to the user terminal  8 ,  9  which radio channel is to be employed for the transmission of content data for the service via the radio interface.  
         [0046]    In the RNC  6 , a number is defined which corresponds to a sufficient number of responses to a paging message. This number may for instance be  3 , but it may also be lower or higher. As soon as the RNC  6  has received a number of responses which is equal to the predetermined number, the RNC  6  transmits a “paging finished” message on the paging channel. As a result, the user terminals  8 ,  9  which are still waiting to send their paging response due to the respective random delay, will refrain from sending a response. This function is of particular importance in large scale applications, in which there may be hundreds of user terminals  8 ,  9  which have joined a specific multicast group in a single radio cell.  
         [0047]    When the RNC  6  receives only one or two multicast paging responses from a radio cell, or in general terms less responses than the predetermined number, it forwards these back to the core network  2 . In the core network  2 , a multicast service center, the GGSN  3  or the SGSN  4  can then decide, whether point-to-point connections using the normal PDP context mechanism should be set up to the corresponding user terminals  8 ,  9 . The decision may be based on operator capacity preferences or on user preferences given e.g. by different tariff levels.  
         [0048]    This second approach can also be combined with the first approach. A paging could for instance be carried out only in cases in which there are not sufficient number of user terminals in the READY state. Alternatively, the paging could be employed as a control function also when there are still enough timers active which indicate that user terminals are in a READY state.  
         [0049]    It is to be noted that even though both approaches were described for IP multicasting, they are equally applicable for other types of multicast and broadcast services.