FR2925805A1 - METHOD FOR MANAGING DATA TRANSMISSION IN MULTICAST MODE TO A PLURALITY OF NETWORK ELEMENTS, AND NETWORK ELEMENT FOR IMPLEMENTING THE METHOD - Google Patents

Abstract

To handle the multicast data transmission to a plurality of network elements constituting an initial set E, a subset ES of network elements of the initial set E is determined, a query is transmitted to receive data. to the network elements of the ES subset. On response of at least one network element of the subset ES allowing the transmission of the data subject of the interrogation, the data is transmitted in multicast mode to all the network elements of the initial set E, and without response of at least one network element of the subset ES allowing the transmission of the data object of the interrogation, it repeats a cycle by determining a subset different from the subset determined in the previous cycle.

Description

METHOD FOR MANAGING DATA TRANSMISSION IN MULTICAST MODE TO A PLURALITY OF NETWORK ELEMENTS, AND NETWORK ELEMENT FOR IMPLEMENTING THE METHOD The present invention relates to a method for managing multicast data transmission to a network. plurality of network elements, as well as to a network element for carrying out the method.

One distinguishes today, for the purposes of data transmission from a source to a plurality of users, three modes of transmission: the point-to-point transmission (in English unicast), in which a source transmits to a recipient identified, the multicast transmission, in which a source transmits data to a plurality of identified recipients, and the broadcast, in which a source transmits data to a set of recipients without identifying them. The IP (Internet Protocol) packet transmission (or datagram) protocol is also very widely used in communication networks, whether wired or wireless. To the packet transmission modes from one network element to another (point-to-point) or to all the other elements of the network (broadcast mode) initially specified for the IP transmission has been added the multicast mode, in which a network element transmits a packet to a group of network elements identified as members of the destination group. IGMP provides an example protocol for managing destination network element groups (multicast groups) for multicast IP packet transmission. It allows routers supporting multicast to know the existence of members of a multicast group on its network interfaces.

Version 3 of IGMP is specified by RFC 3376, Internet Group Management Protocol, Version3, published by the IETF in October 2002. It defines two main functions: on the one hand the 2 multicast router function (or router multicast) of a local area network and the host function. Each function can be implemented within a network element (ER), the two functions can be simultaneously present within the same network element. A host ER uses IGMP messages exchanged with an ER multicast router to join a multicast group, confirm its membership in a multicast group, or leave it. The IGMP messages are encapsulated in IP datagrams, the datagram header then having a field identifying the IGMP protocol. Three categories of IGMP messages have been defined: the polling message (in English Membership Query) allows a multicast router ER to determine matches between one or more multicast groups and the host ER members of these groups. The Membership Report allows a host ER to inform an ER multicast router of its participation in a multicast group. Finally, the Leave Leave message allows a host ER to inform a multicast Router ER that it does not wish to participate in a multicast group. IGMP is also a level 3 protocol in the OSI reference model.

The implementation of the IGMP protocol can be illustrated on an IP network of the type illustrated in FIG. 1. The IP network 10 comprises a plurality of ER 11-15 interconnected via an IP sub-network 20. and arranged to support the multicast function. The IP subnet 20 comprises a set of communication links between the multicast router ER 11 and the host ERs 12-15 capable of carrying IGMP messages. The multicast router ER 11 is furthermore connected via a network 21 to a server 16 of data (or contents). A host ER wishing to receive the corresponding data from a multicast group may request it from the multicast router ER 11 by a REPORT registration message indicating the multicast group to which it wishes to subscribe, ie that is, for which he wishes to receive the corresponding data. Conversely, a host ER wishing to no longer receive the corresponding data to a multicast group may request it from the multicast router ER 11 by a LEAVE drop message indicating the multicast group from which it wishes to unsubscribe. . In order to allow the correspondence between multicast groups and hosts members of these groups to be updated, the multicast router ER 11 can send to the host ERs 12-15 a QUERY query message carrying identification information of a or multiple multicast groups to generate in response the sending by the host ERs 12-15 of a REPORT registration message relating to the multicast groups identified in the QUERY query message. This updating can be carried out regularly, for example by sending periodic QUERY query messages to the host ERs 12-15. It is also of considerable importance when it comes to transmitting multimedia content, that is to say consuming resources (in terms of bandwidth, throughput, etc.), to mobile user equipment, when each host ER corresponds to a transmission area to mobile terminals via a wireless access network. In such a context, the number of hosts to which an ER multicast router must send polling messages may be particularly high, so that IGMP multicast group participation update may be costly in itself. resource terms if a method of periodically sending QUERY query messages to all host ERs is used. Indeed, in the case of a wireless network with an uplink on the air interface that is in point-to-point mode, the number of REPORT registration messages is proportional to the number of members of a group of multicast. In addition, this method of periodic sending to all the host ERs is not very effective in the situation where the host ERs form a broadcast set of the same content.

An object of the present invention is to propose a method for updating correspondence between multicast groups and members of these groups improved in terms of the signaling cost exchanged between a management ER and the ER members. Another object of the present invention is to propose a method of updating correspondence between multicast groups and members of these groups improved in terms of efficiency in the situation where the ER members form a diffusion set of the same content, for example to mobile user devices.

The invention thus proposes a method for managing multicast data transmission to a plurality of network elements constituting an initial set E. The method comprises a cycle iteration, a plurality of said cycles comprising steps of determining a number of network elements. subset ES of network elements of the initial set E, of transmission of a query to receive data to the network elements of the subset ES, of multicast transmission of data to the set of elements of network of the initial set E on response of at least one network element of the subset ES allowing the transmission of the object data of the interrogation, and of reiteration of the cycle by determining a subset different from the determined subset in the previous unanswered cycle of at least one network element of the subset ES allowing the transmission of the data object of the interrogation. The subset determined during a reiteration of the cycle can be chosen so as to have no common element with the subset determined in the previous cycle. It can also be chosen so as to have a predetermined maximum number of elements common to the subset determined in the previous cycle. Other variants of determination of the subset during a reiteration of the cycle can be considered without departing from the scope of the invention. It is also possible to provide a maximum number of cycles of the iterative process, a parameter that can also influence the strategy for determining the successive subsets, so as to have a predetermined maximum number of subsets whose meeting covers the initial set E Finally, the process can be initialized by determining a first subset (at the first iteration of the cycle) of network elements of the initial set E randomly. Another possibility is to determine a first subset of network elements of the initial set E taking into account a network element having authorized the transmission of data object of the interrogation in a previous cycle. The invention further provides a network element that includes an ES subset definition module, an interrogation generation module, an interrogation sending module, a data transmission module, and a controller. multicast transmission. The network element is adapted to handle the multicast data transmission to a plurality of network elements constituting an initial set E according to a method comprising a cycle iteration, a plurality of said cycles including determination steps, by the ES subset definition module, an ES subset of network elements of the initial set E, generation, by the query generation module, of a query to receive data, transmission, by the interrogation sending module, the interrogation to the network elements of the subset ES, transmission, by the data transmission module of said network element, in multicast mode of the data to the set of network elements of the initial set E on response of at least one network element of the subset ES allowing the transmission of the data object of the interrogation. The network element further comprises a multicast transmission controller arranged to reiterate a cycle by determining a subset different from the subset determined in the previous cycle without response from at least one network element of the subset ES allowing the transmission of the data object of the interrogation. The multicast transmission controller may further be arranged for, without the response of at least one network element of the subset ES allowing the transmission of the data object of the interrogation, to repeat the cycle by determining a subset no common element with the subset determined in the previous cycle. It can also be arranged to reiterate the cycle by choosing a subset having a predetermined maximum number of elements common to the subset determined in the previous cycle. Other variants of determination of the subset during a reiteration of the cycle can be considered without departing from the scope of the invention. The multicast transmission controller may further be arranged to repeat the cycle until a predetermined maximum number of cycles is reached. The subset definition module ES may furthermore be arranged to determine the subset ES of network elements of the initial set E in a random manner. Another possibility is to arrange the subset definition module ES so as to determine the subset ES of network elements of the initial set E taking into account a network element having authorized transmission of the subset ES. data object of the query during a previous cycle. This network element according to the invention can advantageously be integrated in a WiMAX access network gateway. The invention finally proposes a computer program loadable in a memory associated with a processor, and comprising instructions for the implementation of a method as defined above during the execution of said program by the processor, as well as a computer medium on which said program is recorded. Other features and advantages of the present invention will become apparent in the following description of nonlimiting exemplary embodiments, with reference to the appended drawings, in which: FIG. 1, already commented on, shows the architecture of a IP data transmission network to which the invention can advantageously be applied; FIG. 2 schematically represents an exemplary algorithm for implementing the invention according to a particular embodiment; FIG. 3 is a block diagram of a multicast router 30 to which the invention may advantageously be applied; FIG. 4 shows the architecture of a WiMAX content broadcasting network to which the invention can advantageously be applied. The invention is particularly well suited, although not exclusively, to the management protocol for participation in IGMP type multicast groups, and is described below in the context of this non-limiting example. It is pointed out that the invention applies to all types of multicast data transmission management protocols including the sending of interrogations relating to data transmitted in multicast mode in order to determine whether the receiving ER wishes or not receive this data, as well as a polling response to indicate whether and / or confirm that the receiving ER wishes or not to receive this data.

In FIG. 1, the ERs 12-15 form a set E of ER members managed from the point of view of IGMP signaling by the ER multicast router 11, and for which a method is proposed allowing the router ER to multicast 11 to quickly identify the existence of a member ER among this set interested in receiving data relating to a specific group GT. According to this method, the ER multicast router 11 sends a query QUERY to a first subset of the set of ER members 12-15, for example constituted by ER routers 12 and 14. Depending on the addressing mode QUERY interrogation messages, the multicast router ER 11 can send a QUERY interrogation message to each of the ER members of the first selected subset, or use a group address corresponding to this first subset, of to send only one polling message to the first subset. Two situations can then occur: either a non-response and / or a negative response of the subset ERs 12 and 14, or a positive response, ie a first request or confirmation of a request. previous request to receive the data relating to the GT multicast group, is received from at least one of the ER members 12 or 14 of the first subset. In the latter case, the multicast router ER 11 organizes the transmission of the data relating to the GT group to all the ER members 12-15 of the initial set E. The sending of the query QUERY by the ER router of multicast triggers the ER multicast router 11 a timer that expires at the end of a time window 8 response ER members. This makes it possible to decide on a nonresponse of a destination member ER of the QUERY query for the case where the ER multicast router has not received a response from this member ER within the time period defined by the window temporal response. In the situation of non-response and / or negative response, ie an indication that a member ER is not requesting to receive GT group data, member ERs 12 and 14 of the first subset, the ER multicast router 11 defines a second subset of ER members from the initial set E, the second subset being different from the first subset, and transmits to that second sub-set. together a query for GT group data. This second subset may for example consist of the ER members 14 and 15 of FIG. 1. On a positive response of one of the ER members 14 or 15 of the second subset, the ER multicast router 11 organizes the transmission of the data relating to the GT group to all ER members 12-15 of the initial set E. On nonresponse and / or negative response of the ER members 14 and 15 of the second subset, the ER multicast router defines a third subset of ER members among the initial set E, the third subset being different from the first and second subsets, and transmits to this third subset a query to receive the data relating to the group GT. This third subset may for example consist of the ER members 15 and 13 of FIG. 1. On a positive response from one of the ER members 13 or 15 of the third subset, the ER multicast router 11 organizes the transmission of the data relating to the GT group to all the ER members 12-15 of the initial set E. On non-response and / or negative response of the ER members 13 and 15 of the third subset, the ER multicast router resumes the initial step of defining a first subset as described above, since the meeting of the three subsets comprises all the elements of the initial set.

The proposed method may in some cases be iterative, in that the step of defining a subset of the initial set and that of testing that subset may be repeated in certain situations, such as illustrates FIG. 2 which shows the steps of an exemplary algorithm according to the invention. The iteration index of each loop is denoted i, and the algorithm is initialized at step 30, where the index i is equal to 0. For each iteration i, a subset ESi of ER is defined. an initial set of ER E (step 31). An interrogation is then sent to receive data (for example an IGMP QUERY query message to participate in one or more groups Gk, without this example being limiting) to the ERs of the subset ESi (step 32). On a positive response, that is to say requiring the reception of data subject to the interrogation of the ER router, of one of the ERs of the subset ESi, the ER router organizes the transmission of the data object of the interrogating all the ERs of the initial set E (step 34). Then the algorithm loops back to the initialization step 30 to start a new cycle. On non-response or negative response of the ERs of the subset ESi, the index i is incremented (step 35) and then compared with a maximum number of iterations N of the loop (step 36). If the maximum number of iterations N is reached, the algorithm loops back to the initialization step 30 to start a new cycle. In the opposite case, it loops back to the step of defining a subset ESi (step 31).

Several variants of the definition mode in step 31 of the subset ESi can be envisaged, without departing from the scope of the invention. For example, when i = 0, the first subset ESO can be determined by random selection of a given number of elements of the initial set E. Alternatively, it is also possible to determine the first subset ESO in the first set. constituting last ER that responded positively to the sending of a query (step 32), that is to say those who have most recently responded positively to the sending of a query. The following subsets ESi, for i ≠ 0 can be determined by ensuring that they are different from all previous iterations of the current cycle, so as to ensure that it is transmitted as and when as iterations proceed, an interrogation to the set of elements of E. For example, it may be decided that each subset consists of a single ER, denoted ERj, determined by an index number j in the set E , the set of indices j being obtained by permutation on indices i of subsets ESi. It is also possible, in a variant, to set the maximum number of iterations (as is the case in the example illustrated in FIG. 2), and to determine a priori, statically or dynamically, at each beginning of the cycle, a partition of the set E in N subsets (Esi) o <i <N.

The signaling relating to the management of the multicast data transmission used may be of the IGMP type, without this example being limiting. In this case, according to IGMP terminology, the network element 40 in Figure 3 will include the functions of a multicast router. The ER 40 comprises a multicast transmission controller, which communicates with an ES subset definition module 47, an interrogation generation module 46, an interrogation sending module 45, an interrogation response receiving module 44 , and a data transmission / reception module 42, and controls the operations of these modules. The iterative cycle described above is in particular managed within the controller 41. The module for defining a subset comprises means for generating a subset ESi of a set E, from an index value i it sends the controller 41. It returns to the controller 41 a subset ESi, the subsystem network elements ESi being for example identified by their respective IP addresses. The controller then commands the interrogation generation module to generate one or more interrogations to receive data, by identifying in the command the data in question (it can be for example a data flow identifier, a a service identifier, a session identifier or a group identifier) as well as the recipients of the queries. In the case where the ER 40 uses IGMP signaling, the interrogation generation module 46 is designed to generate IGMP messages of the QUERY type on the basis of parameters transmitted by the controller 41. The interrogations thus generated are transmitted to the module. 45 sending, which is responsible for their transmission through the interface 43 receiving / transmission. The interrogation response reception module 44 monitors the receipt of any interrogation responses transmitted by the sending module 45, and if necessary informs the controller 41. In the case where the ER 40 uses the IGMP signaling , the reception module 44 is arranged to receive and process IGMP messages of the REPORT type. The controller 41 may be arranged to trigger a timer following the command sending interrogations transmitted to the module 45, to evaluate what has been received in response to the interrogations after a predetermined period of time. Alternatively, the controller 41 can be arranged to control the sending of polls periodically, so that the evaluation can be synchronized with the sending of interrogations and also made periodically. When the controller receives from the reception module 44 a positive interrogation response indication sent by the module 45, it commands the data transmission / reception module 42 to carry out the transmission of the data object of the interrogation to the all of the ERs of the set E. The transmission / reception interface module 43 is furthermore arranged to perform the data transmission and reception interface, the ER 40 being connected to a data server as well as to the ERs of the set E by means of this interface. The interface module 43 is also designed to perform the multicast transmission management signaling transmission and reception interface (in the case where the ER 40 uses IGMP signaling, the interface 43 includes an IGMP signaling interface). 40 being connected to the ERs of the set E for signaling purposes by means of this interface.

The invention is particularly well suited, although not exclusively, to a multimedia content broadcasting system comprising a WiMAX type radio communication network, and is described hereinafter in its application to such a system, illustrated by FIG. figure 4.

FIG. 4 shows a WiMAX type wireless local area network, implementing an MBS service, comprising at least one access network, generally called ASN (Access Service Network), to which mobile stations can connect. In the nonlimiting example illustrated, only an access network of the ASN type has been represented. But, the wireless LAN could include at least two access networks.

The ASN type access network comprises in particular at least one base station (BS) which can be considered as an equivalent of a DSLAM multiplexer in an xDSL network, and by which the mobile stations can connect to the wireless LAN, as well as a PS service gateway, usually called ASN-GW (ASN-GateWay). The WiMAX network 50 makes it possible to offer mobile stations that are connected to the access network a data broadcasting service (or content) called MBS (Multicast Broadcast Service). This MBS service can be implemented on an access network comprising a plurality of base stations (in English Multi-BS MBS access mode) by grouping base stations (or the like) within geographical zones defining so-called zones. MBS zones (or MBS zones). It is thus possible to define at least one MBS zone ZM in a wireless local area network. Each MBS zone ZM groups together at least one base station BSi (i = 1 to N, with N 1) which belongs to at least one access network. In the nonlimiting example illustrated, the MBS zone ZM groups together 3 base stations BS1, BS2 and BS3 belonging to the single access network 51. Each MBS zone ZM is associated with a zone identifier (MBS_Zone_id) and controlled by An MBS server 55. The base stations BSi of an MBS zone ZM are in particular responsible for broadcasting to the mobile stations the zone identifier (MBS_Zone_id) of their MBS zone ZM. An MBS server is a network element through which all the traffic data (in particular the contents to be broadcast) which are intended for the MBS zone ZM that it controls, and which is coupled to the BSi base stations which are part of the MBS ZM zone that he controls. In the nonlimiting example illustrated, the MBS server function 55 is implemented within the access network gateway PS of the access network. But, this is not mandatory. The MBS server of a MBS ZM zone may indeed be a network element that is or is part of a piece of equipment or network element that is not necessarily a PS service gateway. Thus, in the aforementioned mode a same MBS service stream can be transmitted to mobile stations 52a, 52b and 52c by a set of base stations 50a, 50b, 50c which use the same pair of information CID 13 (Connection Identifier) and SA (Security Association), which together constitute an MBS zone associated with an MBS service server and a zone identifier (MBS_Zone_id) that they broadcast. When mobile stations are under the coverage of base stations of an MBS area 56, they do not need to register with each of the base stations 50a, 50b, 50c of this MBS area 56. It is sufficient that they are registered when they access the network by one of the base stations of the MBS zone in order to obtain the information pairs <C1D, SA> of this MBS zone which then allow them to receive flows of service defining at least some MBS content that is broadcast locally by all base stations in that MBS area. A base station can then remain in idle mode while listening to the downstream traffic to receive broadcast content, thereby saving power.

The MBS server 55 is in particular responsible for assigning to all BSi base stations 50a-50c of the MBS zone ZM that it controls information pairs CIDj (Connection Identifier) and SAj (Security Association) (<CIDi, SAi >) respectively associated with the service flows (j) of the different MBS contents (j) to be broadcast. Indeed, when the MBS service is implemented thanks to a multiple base station access mode (or Multi-BS access mode), the same MBS service stream can be transmitted (broadcast) to the mobile stations by the stations. BSi base stations of an MBS area that use the same information pair <CIDj, SAj>. The MBS server MBS ZM zone transmits to the mobile stations 52a-52c which are connected to the base stations 50a-50c of this zone MBS ZM, via the latter (BSi), interrogation messages each intended to ask if at least one of the mobile stations is interested in a content group (s), or multicast group, which is designated by a group identifier. The protocol that is used to transmit the MI polling messages is for example IGMP. In this case, each MI interrogation message is for example of type Query message.

In the presence of such an IGMP protocol, the MBS server 55 constitutes a local IGMP router, the BSi base stations 50a-50c constitute intermediate IGMP type of proxy equipment, and the mobile stations constitute ER members (or hosts, in english hosts). More specifically, a base station of the MBS zone ZM 56 incorporates an IGMP proxy function which comprises at least two components: on the one hand a role of ER member (or host) seen from the function MBS server 55 which is then the Corresponding multicast server, and on the other hand a role of ER multicast server seen ER members that are mobile stations under its coverage.

The transmission of the interrogation messages MI relating to the same multicast group is for example periodic, typically every 125 seconds. An interrogation message MI, designating a multicast group (for example GMI) and generated by the MBS server 55, is therefore addressed to each base station BSi of the MBS zone ZM 56, each base station taking in turn the ER multicast router function to transmit the interrogation message (here by wave) to MS mobile stations that are connected to it.

Upon receipt of such an interrogation message MI, a mobile station MS is free to answer or not to answer, depending on whether it is interested or not interested in the content (s) of the group of multicast (here GM1) that it designates.

Thus, when the method described above is implemented between the MBS server 55 and the base stations 50a-50c of the MBS zone ZM, the MBS server 55 sends in a first interrogation message step that to a subset of the base station set consisting of the base stations of the considered MBS area. As indicated above, this first subset may, for example, consist of the base station that has responded positively to the interrogation message during the previous cycle of sending interrogation messages. For example, it may be base station BS2 50b in FIG. 4. Base station BS2 50b retransmits the interrogation message to mobile stations 52b under its cover, collects the response or non-response of these stations. mobile stations, and consolidates the responses received in a response sent to the MBS server 55. If the base station BS2 50b has not received a response, the following subset may consist for example of a set of base stations surrounding the BS2 50b because the probability is significant that a user previously interested in the multicast group has made mobility to one of the neighboring base stations. This method therefore makes it possible to avoid sending interrogation messages to all the base stations of an MBS zone, interrogation messages which would then be retransmitted on the air interface to the mobile stations under the coverage of the base stations. of the MBS zone. However, the transmission of data in the MBS zone being carried out in broadcast mode from all the base stations of the MBS zone, the interrogation of all the base stations of the zone with regard to the transmission of given contents. is inefficient, and consumer of resources on the air interface. The sending of interrogations to subsets of base stations of the MBS zone by successive iterations makes it possible to limit the consumption of resources on the interface air.

Claims (13)

A method for managing the multicast data transmission to a plurality of network elements constituting an initial set E, the method comprising a cycle iteration, a plurality of said cycles comprising the following steps: determining a subset ES network elements of the initial set E; - transmit a query to receive data from the network elements of the ES subset; - upon response of at least one network element of the subset ES allowing the transmission of the data object of the interrogation, transmitting in multicast mode the data to all the network elements of the initial set E; - without response from at least one network element of the subset ES allowing the transmission of the data object of the interrogation, repeat the cycle by determining a subset different from the subset determined in the previous cycle. 2. Method according to claim 1, wherein, without response of at least one network element of the subset ES allowing the transmission of the data object of the interrogation, the cycle is reiterated by determining a subset no common element with the subset determined in the previous cycle. 3. The method of claim 1 or 2, wherein the cycle is repeated until a predetermined maximum number of cycles. A method according to any one of the preceding claims, wherein the subset ES of network elements of the initial set E is determined randomly. 17 5. Method according to claim 1, wherein the subset ES of network elements of the initial set E is determined by taking into account a network element having authorized the transmission of data. subject of the query in a previous cycle. A network element adapted to handle the multicast data transmission to a plurality of network elements constituting an initial set E according to a method comprising a cycle iteration, a plurality of said cycles comprising steps of: - determination, by an ES subassembly definition module of said network element, an ES subset of network elements of the initial set E; - generating, by an interrogation generation module of said network element, a query to receive data; transmitting, by an interrogation sending module of said network element, the interrogation to the network elements of the subset ES; transmitting, by a data transmission module of said network element, multicast mode of the data to all the network elements of the initial set E on response of at least one network element of the subset ES allowing the transmission of the interrogated data the network element further comprising a multicast transmission controller arranged to reiterate a cycle by determining a subset different from the subset determined in the previous cycle without response from at least one network element of the subset ES allowing the transmission of the data object of the interrogation. The network element according to claim 6, wherein the multicast transmission controller is further arranged for, without response from at least one network element of the subset ES allowing the transmission of the data object of the interrogation, to reiterate the cycle en18 determining a subset having no common element with the subset determined in the previous cycle. The network element of claim 6 or 7, wherein the multicast transmission controller is further arranged to reiterate the cycle until a predetermined maximum number of cycles is reached. The network element according to any one of claims 6, 7 or 8, wherein the subset definition module ES is arranged to determine the subset ES of network elements of the initial set E of random way. 10 The network element according to any one of claims 6, 7 or 8, wherein the subset definition module ES is arranged to determine the subset ES of network elements of the initial set E taking into account a network element that has authorized the transmission of data object of the interrogation during a previous cycle. 15 A WiMAX access network gateway comprising a network element according to any one of claims 6 to 11. 12. Computer program, loadable in a memory associated with a processor, and including instructions for the implementation of a method according to any one of claims 1 to 5 during the execution of said program by the processor. . Computer medium on which a program according to claim 12 is recorded.