Abstract:
A subscription to a service provided in a communication network is managed by a first terminal. The communication network includes a second terminal, and the service includes transmission of digital information. An access is capable of obtaining a maximum rising throughput value allocated to the first terminal and a descending throughput value allocated to the second terminal. On the access of the network, an access request is received for subscribing the second terminal to the service provided by the first terminal. Then, a decision about access to or refusal of subscription of the second terminal is made on the basis of the maximum rising throughput value allocated to the first terminal and on the basis of the descending throughput value allocated to the second terminal. And finally, a response indicating the decision is sent.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is the U.S. national phase of the International Patent Application No. PCT/FR2010/052456 filed Nov. 19, 2010, which claims the benefit of French Application No. 09 58316 filed Nov. 24, 2009, the entire content of which is incorporated herein by reference. 
     FIELD 
     The present invention relates to admission control in a communication network, and more particularly to admission control relating to a subscription to a service, which may in particular be a multibroadcast service (or ‘multicast’). 
     BACKGROUND 
     Control of admission to a communication network makes it possible to decide to accept or to deny a request to use resources of the network, as a function of certain criteria. It may in particular allow effective use of the resources of the network, in particular by denying a request to use resources which would be liable to bring about congestion of the network. 
     It is beneficial to put such admission control in place in communication networks of IP (for ‘Internet Protocol’) type. Networks of this type make it possible to offer various services such as telephonic services (VoIP for ‘Voice over IP’), video telephony (V2oIP for ‘Voice and Video Internet Protocol’) or else video broadcasting (IPTV for ‘Internet Protocol TeleVision’). 
     In such IP networks, it is possible to perform transmissions of packets according to a point-to-point link, termed a unicast IP link, or else according to a point-to-multipoint link, termed a multicast IP link or multibroadcast link. According to the latter type of link, the multicast IP packets are conveyed over the IP network and are replicated from a location relatively close to the recipient multicast terminals. This mode of transport makes it possible to establish unidirectional point-to-multipoint connectivity between an arbitrary unicast IP source address and a plurality of IP terminals, and thus to broadcast packets to these terminals in particular within the framework of a multibroadcast service for digital contents such as video contents for example. 
     In order to receive a content which is broadcast in the network according to a multicast transmission, a terminal subscribes to a corresponding multicast IP address. Next, subsequent to this subscription, it receives the content through multi-broadcasting. 
     In certain cases, the interface for access of a terminal thus subscribed to one or more multibroadcast services may be congested. Such congestion may degrade the quality of the multibroadcast service at the level of this terminal. 
     In order to avoid this type of congestion, document US2008112324 proposes the putting in place of an admission control which decides to accept or to deny a new subscription of a terminal to a multibroadcast service by taking into account the number of multicast IP addresses (or addresses of multibroadcast services) to which it is already subscribed or the total bitrate that this terminal is already receiving. By proceeding in this way, the proposed system makes it possible to limit congestion in the case where it is possible to ascertain an estimate of the bitrate at the level of the access interface for each multicast IP address to which the terminal is already subscribed. 
     Such may be the case when the source IP address of the multicast transmission corresponds to a server implemented by a content provider or else by a network operator. Indeed, in this case it is possible to control the bitrate generated by the server. 
     However, this type of admission control does not make it possible to effectively avoid congestion on the access interface in the case where an arbitrary terminal of the network may itself become a source of multibroadcast service to which other terminals of the network may subscribe. 
     Indeed, under these conditions, it is not possible to estimate the bitrate generated by a source terminal of a multibroadcast service and that a subscriber might receive during the period of his subscription to this multibroadcast service. 
     SUMMARY 
     The present invention improves the situation. 
     A first aspect of the present invention proposes a method of managing subscription to a service offered in a communication network by a first terminal; said communication network comprising a second terminal and said service comprising a transmission of digital information; 
     in which an admission entity of the network previously obtains a maximum value of upload bitrate and a value of download bitrate respectively allocated to the first and second terminals; 
     said method comprising the following steps at the level of the admission entity of the network: 
     /1/ receiving an admission request in respect of a subscription of the second terminal to the service offered by the first terminal; 
     /2/ taking a decision regarding admission or denial of subscription of the second terminal on the basis of the maximum value of upload bitrate allocated to the first terminal and of the value of download bitrate allocated to the second terminal; and 
     /3/ emitting a response indicating said decision. 
     The term ‘communication network’ is intended to mean any type of network such as for example wireless, mobile, or else fixed communication networks. This network may in particular be an IP network according to the Internet protocol. 
     The term ‘terminal’ is intended to mean any type of terminal suitable for communicating in the communication network considered, such as a fixed telephone, a mobile telephone, a computer, etc. 
     The term ‘maximum value of upload bitrate allocated to a terminal’ is intended to mean the bitrate value that a terminal is not permitted to exceed for the outgoing streams that it emits to the communication network. 
     The term ‘digital information’ is intended to mean any information suitable for being transported in the communication network according to a suitable transmission protocol. This digital information may in particular correspond to documents, digital contents of video or audio types, images, etc. 
     In the network considered, it is possible to allocate maximum values of upload bitrate to various terminals. This allocation of maximum value makes it possible to limit the total stream bitrate emitted by a terminal on the communication network considered. Furthermore, in one embodiment of the present invention, values of download bitrate can moreover be allocated to some of the terminals of the network. The term ‘value of download bitrate allocated to a terminal’ is intended to mean the bitrate value that it is possible to guarantee in reception for a terminal from the communication network. Thus, it is possible that under certain conditions the terminal may receive a higher download bitrate but this bitrate is not guaranteed if its value is greater than the allocated value of download bitrate. 
     Under these conditions, provision is made to decide whether or not to admit an admission request in respect of a subscription for a determined terminal by taking into account at one and the same time:
         the maximum value of upload bitrate allocated to the first terminal, that is to say to the terminal which offers the service considered; and   the value of download bitrate allocated to the second terminal.       

     By proceeding in this way, it is advantageously possible to forestall a state of congestion on the network access interface. Indeed, in this context, the first terminal is the terminal which offers the service considered. A maximum value of upload bitrate is allocated to this first terminal. Thus, in this network the first terminal is limited to this maximum value in its outgoing bitrate to the network. 
     Consequently, whatever the service that this first terminal is able to provide, in no case will it be able to represent a data stream bitrate greater than this maximum value. It is appropriate to consider that in reception, a terminal subscribed to this service will not therefore receive, in the context of this service, a bitrate greater than the maximum upload bitrate that is permitted to the first terminal. 
     By considering these two bitrate values, it is easy to forestall, in a relevant way, congestion in respect of any new subscription to a service offered in the network by one of the terminals of the network. 
     These provisions can easily be applied to any type of service offered to one or more terminals by another terminal of the network. They are particularly advantageous in the context of a multibroadcast service offered in the network by a terminal. Indeed, it is all the more advantageous to take care to avoid congestion in the context of a multibroadcast service which is by nature more liable to bring about congestion on the network access interface. However, no limitation is attached to the type of service offered in the network. The present invention covers any type of service which comprises a step of transmitting digital information from one terminal to another terminal of a communication network, this transmission possibly being based on a point-to-point link or else on a point-to-multipoint link. 
     In one embodiment of the present invention, at least one other service is offered in the network by at least one other terminal to which a maximum value of upload bitrate is allocated. The second terminal is already subscribed to this other service. The maximum value of upload bitrate allocated to this other terminal is stored as service bitrate value associated with the second terminal. In this context, provision may be made to perform step /2/ on the basis furthermore of the service bitrate value. 
     Next, if the requested subscription is admitted in this step /2/, then the maximum value of upload bitrate allocated to the first terminal is added to the service bitrate value associated with the second terminal. 
     Stated otherwise, here, the decision-taking relating to the admission of the requested subscription to the service takes into account another bitrate value which corresponds to the sum of bitrate values, if any, corresponding to the services, if any, to which the terminal concerned is already subscribed. Consequently, this decision-taking may be finer and can allow better anticipation of forthcoming congestion on the communication network access interfaces. On admission of the requested subscription, the service bitrate value associated with the second terminal is updated by adding thereto the maximum value of upload bitrate allocated to the first terminal, that is to say to the new subscription which has just been admitted. 
     The service bitrate value associated with the second terminal therefore in fact represents the sum of the maximum values of upload bitrates respectively allocated to the terminals offering the services to which the second terminal is subscribed. 
     In one embodiment of the present invention, the method of managing subscription furthermore comprises the following steps: 
     /a/ receiving an information item relating to an unsubscription of the second terminal from the service offered by the first terminal; 
     /b/ updating the service bitrate value associated with the second terminal by subtracting the maximum value of upload bitrate allocated to the first terminal; 
     /c/ acknowledging receipt of this information item relating to the unsubscription. 
     By virtue of these provisions, when the second terminal subscribed to the service offered by the first terminal unsubscribes, the service bitrate value is also updated by subtracting the maximum value of upload bitrate allocated to the terminal offering the service considered. Consequently, the admission decision-taking is taken on the basis of information updated over time and can therefore remain relevant despite certain alterations. 
     In order to follow the alterations of certain characteristics of the network, provision is also made that, in one embodiment, when the maximum value of upload bitrate allocated to the first terminal is increased, steps /2/ and /3/ are performed again. 
     It may be relevant to take into account alterations, if any, as regards the maximum values of upload bitrate allocated to the terminals offering services in the communication network. Thus, when a variation in maximum value is detected, the admission decision-taking step is here performed again so as to monitor whether despite this change the admission criteria are still satisfied. If such is not the case, then the subscribed terminal is informed thereof. 
     These steps may be implemented at the level of an admission entity in charge of taking the decision regarding admission or denial of subscription. Provision may then be made for either the terminal to emit a request for unsubscription from the service on receipt of this new information received from the admission entity, or the admission entity itself requests unsubscription from the service in respect of this terminal. 
     Whatever the mechanism used here in the case of alteration of the maximum value of upload bitrate allocated to a terminal offering a service, when an unsubscription is decided, it may be advantageous to select, from among the subscriptions admitted, if any, in respect of the second terminal, that one which represents the largest bitrate, that is to say the largest maximum value of upload bitrate. 
     It is advantageous for provision to be made to store per service offered an information item relating to the terminals subscribed to this service. Thus, when this characteristic alters, it is then possible to perform steps /2/ and /3/ again for each of the terminals subscribed to the service for which this upload bitrate characteristic has altered. 
     It may also be relevant to make provision to adapt to other alterations. Thus, when the value of download bitrate allocated to the second terminal is reduced, steps /2/ and /3/ can be performed again. 
     Here also, if this alteration entails the fact that the admission criteria of step /2/ are no longer satisfied, then it may be relevant to trigger an unsubscription from a service for the terminal concerned, as described hereinabove. 
     In one embodiment, the service offered is a multibroadcast service. A first individual send address and a second multibroadcast address being allocated to the first terminal, and a third individual send address being allocated to the second terminal; the maximum value of upload bitrate allocated to the first terminal is stored in association with the first individual send address and the value of download bitrate allocated to the second terminal is stored in association with the third individual send address; and 
     the admission request indicates the first address relating to the first terminal and the third address relating to the second terminal. 
     The term ‘individual send address’ is intended to mean a network address allocated to a terminal which allows the latter to establish a point-to-point communication in the network. The term ‘multibroadcast address’ is intended to mean an address allocated to a terminal in the network which allows the latter to broadcast to a set of addresses a stream of data in the network. This set of addresses can correspond to a set of terminals which have subscribed to the multibroadcast address considered. 
     Here, the communication network may for example be a network of IP type, the IP layer allowing subscription of terminals to a multibroadcasting address of another terminal. 
     By making provision to structure the storage of information in this way, that is to say on the basis of the unicast addresses allocated to the terminals of the network, it is thereafter easy to implement the control of admission parameters according to one embodiment of the present invention. 
     The architecture of the network considered may be such that the terminals have access to the network via an access entity. 
     Thus, the second terminal has access to the network via an access entity; and the method comprises the following steps at the level of said access entity: 
     /i/ receiving a request for subscription of the second terminal to the service offered by the first terminal; 
     /ii/ transmitting an admission request in respect of this subscription to an admission entity; and 
     /iii/ on receipt of a response indicating the admission of the second terminal to the service, transmitting the digital information received to the second terminal, 
     in which in step /ii/, if said service is not already received, the following steps are performed: 
     
         
         
           
             emitting a request for subscription to the service destined for the network; 
             receiving the digital information. 
           
         
       
    
     Thus, in this case, the access entity is in charge of triggering the transmission of the digital information to it as soon as the first of the terminals under its charge requests same. 
     The method can furthermore comprise the following steps at the level of the access entity:
         receiving a request for unsubscription of the second terminal from the service offered by the first terminal;   transmitting an information item relating to an unsubscription to the admission entity; and   halting the transmission of the digital information destined for the second terminal,
 
in which, when the second terminal is the last terminal subscribed to the service for the access entity, said access entity transmits the request for unsubscription to the network.
       

     A second aspect of the present invention proposes an admission entity suitable for implementing a method according to the first aspect of the present invention. 
     A third aspect of the present invention proposes a system for managing subscription comprising an admission entity according to the second aspect of the present invention and comprising an access entity suitable for implementing certain aspects of the management method according to the first aspect of the present invention. 
     In such a system for managing subscription, the access entity may be suitable for limiting the upload bitrate of the first terminal short of the maximum value of upload bitrate which is allocated to it. 
     A fourth aspect of the present invention proposes an item of global network switching equipment suitable for implementing a method according to the first aspect of the present invention. 
     A fifth aspect of the present invention proposes a computer program comprising instructions for the implementation of the method according to the first aspect of the present invention, when this program is executed by a processor. 
     A sixth aspect of the present invention proposes a recording medium on which the computer program according to the fifth aspect of the present invention is stored. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other aspects, aims and advantages of the invention will become apparent on reading the description of one of its embodiments. 
       The invention will also be better understood with the aid of the drawings, in which: 
         FIG. 1  illustrates an architecture of a communication system according to one embodiment of the present invention; 
         FIG. 2  illustrates the main steps of a method for managing a service according to one embodiment of the present invention; 
         FIG. 3  illustrates an exemplary implementation of the method of managing subscription to a service according to one embodiment of the present invention in a system such as illustrated in  FIG. 1 ; 
         FIG. 4  illustrates a step of unsubscription of a terminal from a multibroadcast service offered by another terminal according to one embodiment of the present invention; 
         FIG. 5  illustrates steps of subscription and of unsubscription of an access entity to and from the service according to one embodiment of the present invention; and 
         FIG. 6  illustrates an architecture of an admission entity and of an access entity according to one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates an architecture of a communication system according to one embodiment of the present invention. 
     Such a communication system comprises a communication network  101  which is accessible to a first terminal  301  via an access entity  501  and to second and third terminals  302  and  303  via an access entity  502 . This system furthermore comprises an admission entity  601  which is in charge of deciding to accept or to deny a subscription request of a terminal according to one embodiment of the present invention. 
     In this network, the terminal  301  is suitable for offering a service to which other terminals of the network, such as for example the terminal  302  and/or the terminal  303 , can subscribe. 
     The maximum values of upload bitrate allocated respectively to the terminals  301 ,  302  and  303  are denoted D up1 , D up2  and D up3 . 
     The values of download bitrate allocated respectively to the terminals  301 ,  302  and  303  are denoted D down1 , D down2  and D down3 . The download bitrate value D down  corresponds to a guaranteed value of download bitrate. 
     In this embodiment, these access entities  501  and  502  are distinct and are suitable for managing the transfer of the multicast packet streams through the network according to a transmission protocol. No limitation is attached to the protocol used between these access entities. 
     If the access entities  501  and  502  are IP routers, the multicast protocol used can for example be one of the variants of the protocol of PIM type (for ‘Protocol Independent Multicast’), for example PIM-SM (for ‘Protocol Independent Multicast Sparse Mode’), described in RFC 4601. If the access entities  501  and  502  are nodes relaying the IP packets without routing them, the multicast protocol used can then be for example a protocol of the IGMP type (for ‘Internet Group Management Protocol’) for IPv4 (for IP version 4) or MLD (for “Multicast Listener Discovery”) for IPv6 (for IP version 6). 
       FIG. 2  illustrates the main steps of a method of admission of subscription to a service according to one embodiment of the present invention. 
     In a step  201 , a request for subscription of the second terminal to the service offered by the first terminal is received. 
     In a step  202 , a decision is made to admit or to deny this subscription of the second terminal on the basis of the maximum value of upload bitrate allocated to the first terminal D up  and of the value of download bitrate allocated to the second terminal D down . 
     Next, in a step  203 , a response indicating the decision taken in step  202  is emitted. 
     Even if no limitation is attached to the type of service considered, in the following sections the service offered in the network is a multicast service or multibroadcast service. Generally, the terms ‘multibroadcast’ and ‘multicast’ are employed here in a similar manner. 
       FIG. 3  illustrates an exemplary implementation of the method of managing subscription to a service according to one embodiment of the present invention in a system such as illustrated in  FIG. 1 . 
     The communication network  101  may be a network of IP type in which the terminals can establish links of various types between themselves, and in particular point-to-point (or ‘unicast’) links and point-to-multipoint (or ‘multicast’ or ‘multibroadcast’) links. By using the multicast links, it is easy to offer in the network multibroadcast services such as for example a video broadcasting service to subscribed terminals. 
     The communication network  101  makes it possible to establish a multicast link on the basis of a multicast IP address. 
     Hereinafter, the unicast addresses or individual send addresses are denoted @ uni  and the multicast addresses or multibroadcast addresses are denoted @ multi . 
     It is possible to provide for a first terminal  301  to offer a multibroadcast service to other terminals of the network. In this case, a unicast source IP address @ uni1  and a multicast IP address @ multi1  are allocated to the terminal  301 . Next, in order to be subscribed to this service, a terminal subscribes in respect of the multibroadcast address of the terminal. 
     To the other terminals  302  and  303  are allocated the respective unicast IP addresses @ uni2  and @ uni3 . 
     In the example illustrated in  FIG. 1 , the access entity  501  is in charge of managing access to the network of the terminal  301 , and in particular in this regard it is in charge of limiting the upload bitrate short of a maximum bitrate value D up1  and of guaranteeing the download bitrate of the terminal  301  as far as a value of download bitrate allocated to this terminal D down1 . The access entity  502  manages the network access of the terminals  302  and  303 , and in this regard limits the upload bitrate short of a maximum bitrate value D up2 , D up3  and guarantees the download bitrate as far as a value of download bitrate allocated both in respect of the terminal  302  D up2  and in respect of the terminal  303  D up3 . 
     Thus, the terminal  301  is availed of a unicast address @ uni1  and of a multicast address @ mult1  which allows it to offer a multibroadcast service. 
     In one embodiment of the present invention, provision is made to take a decision regarding admission of a subscription of a terminal to the service offered by the terminal  301  on the basis on the one hand of the maximum value of upload bitrate allocated to the terminal  301  and on the other hand of the value of download bitrate allocated to the terminal which wishes to subscribe. By comparing these two values it is possible to avoid, in a relevant manner, congestions of the network access link. It is in particular possible to deny a request for subscription to a service of a terminal to which is allocated a lower value of download bitrate than the maximum value of upload bitrate of the terminal which is the one which is offering the service. 
     In one embodiment, provision is made to take the decision to accept or to deny the subscription to a service by furthermore taking into account one or more subscriptions to services previously accepted in respect of the terminal which is currently requesting a new subscription. In this case, it is advantageously possible to take a subscription admission or denial decision in a way which is still more suited to certain conditions. 
     In the following sections, the previously accepted subscriptions are taken into account. 
     For this purpose, the respective values of service bitrate already permitted respectively for the terminals  302  and  303  are denoted D serv2  and D serv3 . More precisely, for a given terminal, the service bitrate value corresponds to the sum of the maximum values of bitrate of each service to which the terminal considered is already subscribed. Thus, in the case illustrated, if the terminal  302  is already subscribed to a service offered by the terminal  303  for example, the service bitrate value D serv2  associated with the terminal  302  can indicate a bitrate value representing this service. Provision may in particular be made in this case for the service bitrate value D serv2  to represent the maximum value of upload bitrate allocated to the terminal  303 . 
     Thus, in this embodiment, the decision to accept the subscription to the service of the terminal  301  will be performed by advantageously taking into account the services to which the terminal requesting a new subscription is already subscribed. 
     No limitation is attached to the type of service bitrate D serv  already permitted for a given terminal. Indeed, the value of this service bitrate can correspond to any type of service that the terminal considered is apt to receive, whether these services are offered by a server or else by an actual terminal of the network. 
     The subscription admission decision may be taken at the level of any network entity which is suitable for receiving a message indicating a subscription request of a terminal and for deciding to admit it or not as a function of admission parameters relating to the various bitrates such as those stated hereinabove. This network entity is suitable for storing, or at the very least recovering, the bitrate information on the basis of which it decides to admit or to deny the requested subscription, according to one embodiment of the present invention. Indeed, the bitrate information may be stored in a database to which the admission entity has access. 
     The subscription admission decision may be taken at the level of an admission entity  601 . In this case, provision may be made for the admission entity to obtain and store the following information in association with each terminal of the network for which it is in charge of managing admission to the network:
         a maximum value of upload bitrate D up ;   an allocated value of download bitrate D down ; and   an already allocated value of service bitrate D serv .       

     The information regarding allocated maximum value of upload bitrate and allocated value of download bitrate is for example obtained from an access nodes control facility, not represented. 
     No limitation is attached to the storage of this information in association with a terminal of the network. 
     Provision may in particular be made for this information to be stored in association with a unicast address @ uni  of the terminal considered. 
     In the architecture illustrated in  FIG. 1 , the messages emitted and received by a terminal travel through an access entity. Thus, the service subscription requests emitted by a terminal travel through the access entity which is in charge of managing this terminal&#39;s network access. However no limitation is attached to this aspect. Indeed, whatever the architecture considered, it suffices that the terminals can emit requests destined for the admission entity. 
       FIG. 3  illustrates the various exchanges of messages implemented according to one embodiment of the present invention. 
     The terminal  301  is suitable for offering a multibroadcast service in the course of which it transmits from its source IP address @ uni1  to terminals which are subscribed to its multicast IP address @ multi1  a service stream  21  via the access entity  501  and the network  101 . 
     Here, when the terminal  302  wishes to receive the content multi-broadcasted by the terminal  301 , it subscribes by emitting destined for the access entity  502  a request  901  for subscription to the service multibroadcasted from the multicast IP address @ multi1 . The subscription request  901  furthermore comprises the unicast IP address @ uni1  of the terminal  301 . Such a service can correspond to a service stream originating from the unicast IP address @ uni1  of the terminal  301 . 
     Next, the access entity  502  sends an admission request  902  to the admission entity  601  to ask to it to decide to accept or to deny the subscription requested by the terminal  302 . 
     The admission request  902  indicates on the one hand the unicast IP address @ uni1  of the multibroadcast source of the terminal  301  and on the other hand the unicast IP address @ uni2  of the terminal  302  corresponding to the subscription request  901 . 
     On receipt of this admission request  902 , the admission entity  601  performs an admission control according to one embodiment of the present invention. 
     The admission entity  601  storing the bitrate values as a function of the respective unicast addresses of the terminals, it recovers in memory the maximum value of the upload bitrate D up1  of the terminal  301  on the basis of the unicast address @ uni1  indicated in the admission request  902  and, on the basis of the unicast address @ uni2  of the terminal  302 , indicated in the admission request  902  also, the download bitrate value D down2  allocated to the terminal  302  as well as the service bitrate value D serv2  already allocated to the terminal  302 . 
     Next, the admission entity  601  adds the maximum value of the upload bitrate D up1  allocated to the terminal  301  to the already permitted value of the service bitrate D serv2  for the terminal  302 . It compares the result of this addition with the download bitrate value D down2  allocated to the terminal  302 , subsequent to this comparison, it decides whether or not to accept this subscription in respect of the terminal  302 . 
     This comparison step can for example consist in comparing these two values directly, as follows:
 
( D   up1   +D   serv2 )&lt; D   down2  
 
     If this inequality is satisfied, the admission entity can accept the subscription and deny it in the converse case. 
     It is also possible to envisage performing a slightly more complex comparison based for example on the following inequality:
 
( D   up1   +D   serv2   D   sec )&lt; D   down2 
 
     where D sec  is a constant value which is a safety margin making it possible to take into account in a coarse manner other receptions, if any, of data intended for the terminal  302 , without however knowing the quantity thereof. 
     No limitation is attached to the comparison implemented at this level on the basis of these three types of bitrate. 
     Next, if the subscription is denied, the admission entity  601  returns to the access entity  502  a negative response to its admission request  902  and the access entity  502  returns to the terminal  302  a negative response to its subscription request  901  (not illustrated). 
     In the case where the subscription is accepted on the basis of a comparison of the bitrate values stated hereinabove, the admission entity  601  updates the service bitrate value associated with the terminal  302  by adding to the previous value the maximum value of upload bitrate of the terminal  301 . Next, it returns to the access entity  502  a positive response  903  to its admission request  902 . Provision may be made, optionally, for the maximum value of the upload bitrate D up1  allocated to the terminal  301  to be indicated in the positive admission response  903 . Thus, not only the access entity  502  but also the terminal  302  can be informed thereof. 
     The access entity  502  thereafter returns to the terminal  302  a positive response  904  to its subscription request  902 . This response  904  can indicate the maximum upload bitrate D up1  allocated to the terminal  301 . 
     The terminal  302  knowing the maximum upload bitrate D up1  allocated to the terminal  301  for the associated service, it can advantageously unsubscribe from this service if it deems that the associated bitrate is too large, in regard to the download bitrate guaranteed on its access interface and of the bitrate associated with the other services to which it has subscribed. 
     On completion of these exchanges of messages, either the access entity is already receiving the service stream  21 , for example because another terminal under its charge is already subscribed to this service, or the access entity is not yet receiving this service stream  21 . 
     In the first case, the access entity is permitted to relay to the terminal  302  the multicast service stream  21  received from the address @ uni1  of the terminal  301 . 
     In the second case, that is to say when the terminal  302  is the first terminal to be subscribed to the multibroadcast service offered by the terminal  301 , the access entity  502  emits to the network  101  a subscription request  1004  in respect of the service offered by the terminal  301 . Subsequent to this subscription request, the multibroadcast service stream  21  emitted by the terminal  301  is transmitted up to the access entity  502 . These steps are more precisely described in conjunction with  FIG. 5 . 
       FIG. 4  illustrates an step of unsubscription of the terminal  302  from the multibroadcast service offered by the terminal  301 . 
     When the terminal  302  no longer wishes to be subscribed to the service offered by the terminal  301 , that is to say here when it no longer wishes to receive the service stream  21  from the source with address @ uni1  of the terminal  301 , it sends to the access entity  502  an request  41  for unsubscription to the multicast IP address @ multi1  for the service stream  21  originating from the unicast IP address @ uni1 . 
     On receipt of this unsubscription request  41 , the access entity  502  relays the unsubscription request in the form of an information message  42  to the admission entity  601  so as to inform it of the request for unsubscription of the terminal  302  from the multibroadcast service offered by the terminal  301 . 
     The information message  42  indicates the unicast IP address @ uni1  of the multicast source of the terminal  301  and the unicast IP address @ uni2  of the terminal  302  which emitted the unsubscription request  41 . 
     On receipt of the information message  42 , the admission entity  601  subtracts the maximum value of the upload bitrate D up1  of the terminal  301  from the value of service bitrate D serv2  already permitted for the terminal  302 . 
     Next, the admission entity returns to the access entity  502  an acknowledgment response  43  to the information message  42 . The access entity  502  returns to the terminal  302  a positive response  44  to its unsubscription request  41  and ceases relaying  45  to the terminal  302  the IP packets of the service stream  21  originating from the terminal  301 . 
       FIG. 5  illustrates steps of subscription and of unsubscription of an access entity to and from the service according to one embodiment of the present invention. 
     When, among the terminals under the charge of the access entity  502 , the first receives a positive decision regarding acceptance of subscription of the admission entity, this access entity requests reception of the corresponding service stream  21 . For this purpose, it emits a subscription request  1004  destined for the multibroadcast network  101 . Next, subsequent to this request it receives the service stream  21 . 
     Thereafter, when, among the terminals under the charge of the access entity  502 , the last terminal unsubscribes from the service considered, the access entity  502  then emits a service unsubscription request  51  to the network. Next, subsequent to this message, the access entity  502  ceases receiving the service stream  21 . 
     It is possible that the maximum values of upload bitrate allocated to the terminals of the network alter over time. In this case, it may be advantageous to take these alterations into account. 
     In one embodiment of the present invention, when the maximum value of the upload bitrate allocated to the terminal  301  decreases, the admission entity  601  subtracts from the value of already permitted service bitrate D serv  of each terminal of the network whose source IP address @ uni  has been associated with the source IP address @ uni1  of the terminal  301  the difference between the previous maximum value of the upload bitrate and the new maximum value of the upload bitrate, allocated to the terminal  301 . 
     It is thus advantageously possible to have an admission control that can alter over time. 
     In one embodiment of the present invention, the admission entity  601  determines the access entity or the access entities which are in charge of the terminals subscribed to the service offered by the terminal  301 . Next, this information is relayed by the corresponding access entities and the subscribed terminals are informed of these changes of maximum value of upload bitrate. 
     The admission entity  601  can recover their respective addresses @uni which are associated with the address @ uni1  of the terminal  301 . 
     In one embodiment, when the maximum value of upload bitrate D up1  allocated to the terminal  301  increases, the admission entity  601  adds, to the value of already permitted service bitrate D serv  for each terminal whose source IP address @ uni  has been associated with the source IP address @ uni1  of the terminal  301 , the difference between the new maximum value of upload bitrate and the previous maximum value of upload bitrate allocated to the terminal  301 . 
     Thus, at the level of the admission entity, the following equation may be applied:
 
 D   servi ( t+ 1)= D   servi ( t )+Δ( t+ 1 ,t )
 
     where D servi (t) represents the value of service bitrate as a function of time for a terminal i; and 
     where Δ(t+1,t) represents the difference between the maximum value of upload bitrate at the instant t+1 and the previous maximum value of upload bitrate at the instant t, allocated to the terminal offering the service considered. 
     In one embodiment of the present invention, the admission entity  601  sends the new maximum value of upload bitrate allocated to the terminal  301  to the access entity  502  or to the access entities in charge of managing the access of the terminals which are subscribed to the service offered by this terminal  301 . The admission entity  601  can recover their respective addresses @ uni  which are associated with the address @ uni1  of the terminal  301 . 
     Provision may be made for the access entity  502  or the corresponding access entities to relay this information to all the terminals subscribed to the multicast service stream emitted by the terminal  301 . 
     In order to render this system suited to the various alterations, provision may be made for the admission entity  601  to perform admission controls in the course of time as a function of the alterations over time of the maximum values of upload bitrate allocated to the terminal offering the service. 
     In this context, provision may in particular be made that, when this maximum value increases, the admission entity monitors for each subscribed terminal whether the admission criteria are still satisfied. Thus, in the case where the following inequality is not satisfied:
 
( D   up1   +D   serv2   +D   sec )&lt; D   down2  
 
     where D up1  corresponds to the last value received, 
     then the admission entity can decide to send a request for rejection of subscription to the multicast service offered by the terminal  301  destined for the access entity or corresponding entities in charge of the subscribed terminals and for which the inequality hereinabove is no longer satisfied. 
     It may be advantageous to wait a period of time determined for example by configuration between the moment when the admission entity is informed of a change and the moment when it informs the subscribed terminals thereof, so as to avoid an instability related to too many modifications of this type in the network. 
     The access entity  502  can thereafter relay it to all the terminals subscribed to the service concerned. 
     In one embodiment of the present invention, when the value of download bitrate D down2  allocated to a subscribed terminal, for example to the terminal  302 , decreases and becomes less than the already permitted value of multicast service bitrate D serv2 , the admission entity  601  sends to the access entity  502  in charge of this terminal  302 , a request for rejection of subscription to the multicast service offered. 
     It may be advantageous to wait a period of time determined for example by configuration between the moment when the admission entity is informed of a change and the moment when it informs the subscribed terminals thereof, so as to avoid an instability related to too many modifications of this type in the network. 
     The access entity  502  can thereafter relay this information to the terminals concerned subscribed to the multicast service of the terminal  301 . 
     It is possible that the terminal  302  may already be subscribed to one or more other services. In this case, it may happen that despite the unsubscription from the service offered by the terminal  301 , the service bitrate value allocated to the terminal  302  is still greater than the download bitrate value allocated to it. Provision may then be made that, if this situation endures after a waiting time defined by default, and that may be configured for example, then the admission entity  601  sends the access entity  502  a request for rejection of subscription to the multicast service offered by this other terminal. 
     Next, the access entity  502  can relay this information to the terminal concerned  302 . 
     In the case where, for a terminal subscribed to several multicast services, the service bitrate value is greater than the value of download bitrate, the admission entity  601  can advantageously choose, from among the catalogued services to which the terminal considered is subscribed, the one offered by the terminal having the largest allocated upload bitrate value. 
     Whatever the protocol used by access entities  501  and  502  to control the transfer of the multicast service stream  21  from one to the other, the following steps may be implemented. 
     As long as the access entity  502  does not receive any positive response to the first emitted subscription request in respect of a terminal under its charge in relation to the service offered by the terminal  301 , it does not relay to the network  101  the request  901  for subscription to the multicast IP address @ uni1  in respect of the packets originating from the unicast IP address @ multi1 . 
     When the access entity  502  receives the first positive response to a subscription admission request in respect of a terminal under its charge, it relays to the network  101  according to a suitable multicast protocol the subscription request so that the network  101  sends the service stream  21  emitted by the terminal  301  to the multicast address @ multi1 . 
     The access entity  502  does not relay the request for unsubscription to the multicast IP address @ multi  in respect of the packets originating from the unicast IP address, except after it has received the last positive response to its request for information in respect of the subscribed terminals under its charge. 
     Then, the access entity  502  relays to the transport network  101  the request so that the network  101  ceases relaying to it the multicast packets of the service stream  21  emitted by the terminal  301  to the multicast address @ multi1 . 
     In the case of a non-multibroadcast service, provision may be made for the subscription request emitted by the receiver terminal  302  to indicate the unicast IP address @ uni1  of the terminal  301  offering a point-to-point service. 
     In one embodiment of the present invention, the method differs from the case of a multibroadcast service in respect of the following cases:
         when the admission entity  601  accepts the subscription to this service, and as long as the subscription to this service is effective, the access entity  502  records the unicast IP address @ uni1  of the terminal  301  so as to relay to the terminal  302  all the packets originating from the terminal  301  by priority,   when the admission entity  601  denies the subscription to this service, the access entity  502  relays to the terminal  302  the unicast packets originating from the terminal  301  only if the download bitrate on the access interface to the terminal  302  is lower than a maximum bitrate allocated to this interface, advantageously leaving the priority to the packets corresponding to the services which have been accepted by the admission entity  601 .       

       FIG. 6  illustrates an architecture of admission entity  601  and of an access entity according to one embodiment of the present invention. The admission entity  601  comprises:
         an obtaining unit  60  suitable for obtaining a maximum value of upload bitrate and a value of download bitrate respectively allocated to the first and second terminals;   a reception unit  61  suitable for receiving an admission request in respect of a subscription of the second terminal to the service offered by the first terminal;   a decision unit  62  suitable for taking a decision regarding admission or denial of subscription of the second terminal on the basis of the maximum value of upload bitrate allocated to the first terminal and of the value of download bitrate allocated to the second terminal; and   a transmission unit  63  suitable for emitting a response indicating the decision.       

     In one embodiment, at least one other service being offered in the network by at least one other terminal  303  to which a maximum value of upload bitrate is allocated, the second terminal is already subscribed to said other service; the maximum value of upload bitrate allocated to said other terminal being stored as service bitrate value associated with the second terminal; 
     the decision unit  62  taking a decision on the basis furthermore of said service bitrate value; 
     the admission entity furthermore comprising an update unit  67  suitable for updating the service bitrate value associated with the second terminal by adding thereto the maximum value of upload bitrate allocated to the first terminal if the requested subscription is admitted. 
     The access entity  502  comprises:
         a reception unit  64  suitable for receiving a request for subscription  901  of the second terminal to the service offered by the first terminal;   a transmission unit  65  suitable for transmitting an admission request in respect of a subscription to an admission entity  601  and for transmitting the digital information received to the second terminal if the subscription is admitted by the admission entity; and   a subscription unit  66  suitable for requesting the network for reception of the digital information if the latter is not already being received.       

     This invention may be carried out by implementing the IPv4 protocol such as defined in RFC 791 (for ‘Request For Comment’) or else the IPv6 protocol whose specification may be found in RFC 2460. 
     In the case where the IPv4 protocol is used, the protocol for subscription to a multicast IP address may be the IGMP protocol for example in its version V3, defined in RFC 3376. 
     In the case where the IPv6 protocol is used, the protocol for subscription to a multicast IP address may be the MLD protocol (for ‘Multicast Listener Discovery’) defined for example in RFC 3810. 
     The present invention can be applied equally well to fixed networks (the access entities can then be for example DSLAM (for ‘Digital Subscriber Line Access’), or MSANs (for ‘Multiservice access nodes’) entities, and to mobile networks (the access entities can then be for example RNC (for ‘Radio Network Controller’) entities, or eNodeB (for ‘evolved NodeB’) entities, or else ePDG (for ‘evolved Packet Data Gateway’) entities).