FR2977433A1 - METHOD FOR FILTERING EARLY MEDIA FLOW IN AN IMS NETWORK AND SERVER IMPLEMENTING SAID METHOD - Google Patents

Abstract

This filtering method implemented by a server (ORIG) in an IMS network comprises the steps of: - (E20) receiving a SIP invitation message sent by an upstream entity (UA); - (E30, E80) reservation of two resources (MRF1, MRF2) with a server; - (E40) sending a SIP response to the upstream entity (UA) having an SDP field identifying the first resource; - (E90) sending to at least one downstream entity, a SIP invitation message comprising an SDP field identifying the second resource; - (E140) for selecting a media stream based on (E120, F70) SIP responses of 1xx type; - (E150) control of the resource server to restore the media stream in the first resource.

Description

Background of the invention

The present invention is in the field of telecommunications and more specifically in that of IMS networks.

In an IP Multimedia Subsystem (IMS), services are provided by AS application servers. Among these application servers, it is known to distinguish the servers that provide a triggered service to the transmission of the call, otherwise known to those skilled in the art under the name "service originating". A server offering an originating service is usually referred to as an "originating server". In the same way, the term "server terminating" is commonly referred to as an application server AS (for Application Server) providing a service triggered upon receipt of the call, such service being referred to as "service terminating". Some originating and terminating services known to date generate media streams to the calling terminal during the call setup phase. These streams are often referred to as "early media streams". It is recalled that the call set-up phase designates the entire phase comprised between the sending of the invitation SIP INVITE message (SIP for Session Initiation Protocol) by the calling terminal and the reception by this calling terminal of a SIP response. type 2xx, 3xx, 4xx, 5xx, 6xx for example a positive acknowledgment response 200 OK or a negative acknowledgment response of type 403 Forbidden when the call is not authorized. As an example of early media flow known to those skilled in the art, mention may in particular be made of the voice announcement sent by an originating server when the caller uses the incoming call consultation service which has not been able to open with the possibility of callback or Colored Ring Back Tone returned by the terminating server of the called terminal or by the called terminal. With the deployment of increasingly complex services, it frequently happens that SIP equipment has to deal with several competing SIP dialogues during the call establishment phase, each of these dialogues being associated or not with an early media flow. To date, the standardization bodies (IETF, 3GPP, ...) do not provide a solution to deterministically restore a plurality of early media streams intended for the same calling terminal. 135 Object and summary of the invention

The invention relates to a filtering method implemented by a server in a first IMS network, during a phase of establishment of a call between a calling terminal and a called terminal. This method comprises: a step of receiving a SIP invitation message sent by an upstream entity in a flow cutoff between the calling terminal and this server; a step of reserving a first resource and a second resource with a resource server of the first network; a step of sending a SIP response to the aforementioned upstream entity, this response comprising a SDP (Session Description Protocol) field identifying the first resource; a step of sending a SIP invitation message to at least one downstream entity chosen according to the called terminal, this invitation message comprising an SDP field identifying the second resource; a step of receiving at least one ixx SIP response from at least one of these downstream entities; and a step of selecting a media stream as a function of the one or more of these received responses; a step of controlling the resource server to restore this media stream in the first resource to the calling terminal. Correlatively, the invention relates to a server belonging to a first IMS network this server comprising, means for implementing, during a phase of establishment of a call between a calling terminal and a called terminal: - means receiving a SIP invitation message sent by an upstream entity in a flow failure between the calling terminal and this server; means for reserving a first resource and a second resource with a resource server of said first network; means for sending a SIP response to said upstream entity comprising an SDP field identifying said first resource; means for sending a SIP invitation message to at least one downstream entity chosen according to the called terminal, this invitation message comprising an SDP field identifying the second resource; means for receiving at least one SIP response of the ixx type from at least one of said downstream entities; means for selecting a media stream as a function of the response or responses received; and 2 - means for controlling the resource server to restore this media stream in the first resource to the calling terminal. In this document, the term "upstream entity", the calling terminal, or any entity of the IMS network placed in flux between the calling terminal and the server according to the invention and "downstream entity", the called terminal, or any entity of the IMS network placed in a power failure between the server according to the invention and the called terminal. Those skilled in the art will understand that the originating servers of the calling party and the terminating server of the called party respectively constitute upstream and downstream entities within the meaning of the invention. The "1xx SIP reply" notation refers to any SIP response whose type begins with the digit "1" and in particular the SIP 180 Ringing and 183 In Progress responses. Thus, and very advantageously, the media streams sent to the upstream entity (for example to the calling terminal), during the call establishment phase, these media streams being known to those skilled in the art under the name of "early media flow", are systematically restored in the same resource of a resource server controlled by the server according to the invention. This characteristic advantageously makes it possible to define, in a perfectly predictable manner, the scenario and the scheduling of restitution of the early media flows. In a first variant embodiment of the invention, the server according to the invention is an originating server, the aforementioned resource server being an MRF (Multimedia Resource Function) server.

In this first variant embodiment, the filtering method according to the invention comprises a step of blocking the ixx type SIP response received from the downstream entity. By "blocking" is meant that this SIP response is not transmitted to the upstream entity. This feature advantageously prevents the calling terminal from generating itself a call back. In a particular embodiment of this first variant embodiment of the invention, the filtering method according to the invention comprises a step of controlling the MRF server to restore, in the first resource, a first media stream generated by it. even. The SIP response sent to the upstream entity is in this case a 183 In Progress SIP response. It is preferably sent just after the reservation of the first resource so that this first early media stream is received as soon as possible by the calling terminal. In a particular embodiment of this first variant embodiment of the invention, the step of reserving the second resource is performed upon receipt of a notification from the MRF server. In practice, this notification can be sent by the MRF server on detection of an explicit action by the user of the calling terminal (entering a DTMF key sequence, voice command, etc.) to trigger a new service likely to generate a new early media feed, still in the call setup phase, competing with the first early media feed triggered by the originating server itself. In a second variant embodiment of the invention, the server according to the invention is an interconnection server between the first IMS network (to which it belongs) and a second IMS network (to which the called terminal and the downstream entity belong). , the resource server being an I-BGF entity (for Interconnection Border Gateway Function) In a particular embodiment of this second variant, the SIP response sent to the upstream entity is of the type of the SIP response received from the entity. downstream. In one embodiment of the first or second embodiment of the invention, the media stream selected and restored on the first resource may be: a media stream received in the second resource if the 1xx SIP response received the downstream entity has an SDP field; or - a call back signal generated by the resource server in the other cases.

In a particular embodiment of the first or second variant of the invention, the media stream is selected according to a priority parameter of the downstream entity at the origin of the response received, in the sense of a forking service as defined in the RFC SIP 3261 standard. In a particular embodiment, the server according to the invention comprises means for triggering a ringing return media stream on the first receiving resource, from said downstream entity, a SIP 180-type Ringing response without SDP.

In a particular embodiment, the various steps of the filtering method according to the invention are determined by instructions of computer programs.

Consequently, the invention also relates to a computer program on an information carrier, this program being capable of being implemented by a server, this program comprising instructions adapted to the implementation of the steps of a filtering process as mentioned above.

This program can use any programming language, and be in the form of source code, object code, or intermediate code between source code and object code, such as in a partially compiled form, or in any other form desirable shape. The invention also relates to a computer-readable information medium, comprising instructions of a computer program as mentioned above. The information carrier may be any entity or device capable of storing the program. For example, the medium may comprise storage means, such as a ROM, for example a CD ROM or a microelectronic circuit ROM, or a magnetic recording medium, for example a diskette (floppy disc) or a disk hard. On the other hand, the information medium may be a transmissible medium such as an electrical or optical signal, which may be conveyed via an electrical or optical cable, by radio or by other means. The program according to the invention can be downloaded in particular on an Internet type network.

Alternatively, the information carrier may be an integrated circuit in which the program is incorporated, the circuit being adapted to execute or to be used in the execution of the method in question.

BRIEF DESCRIPTION OF THE DRAWINGS Other features and advantages of the present invention will emerge from the description given below with reference to the drawings which illustrate an embodiment having no limiting character. In the figures: - Figures 1 and 2 show in the form of a timing diagram and flowchart a filtering method according to a first embodiment of the invention; FIGS. 3 and 4 represent, in the form of a timing diagram and a flowchart, a filtering method according to a second variant embodiment of the invention; and FIG. 5 represents the hardware architecture of a server conforming to a particular mode of the first or second variant of the invention.

Detailed description of the invention

Referring to Figures 1 and 2, we will now describe a first embodiment of the invention in three scenarios. In this first variant, the server according to the invention is an AS originating application server (referenced ORIG in the figures), this server offering, in a first IMS network, an originating service of "callback of the last caller". In this service known to those skilled in the art, the server ORIG is able to broadcast, to a terminal UA calling this server, a voice announcement mentioning the date and time of incoming calls for this terminal UA and not having opportunity. Such an originating ORIG server then allows the caller via the terminal UA to request to be put in touch with one of these callers. In a first scenario of this first variant embodiment, one places oneself in the context in which a calling SIP terminal UA has dialed the service callback code of the last caller, service hosted by the ORIG server, and then asked to be put in contact with a called SIP subscriber belonging to the same IMS network. We will also assume that two SIP terminals with the same public IMPU identity of the called subscriber are registered in the heart of the IMS network.

When the calling terminal UA dials the callback service number of the last caller, the terminal UA sends, to the server ORIG through the IMS network, a SIP message of the INVITE type whose session description SDP field contains in particular the information related to to the codec (s) supported by the calling terminal UA as well as the IP address and port number on which it wishes to receive the media stream (s). This INVITE message is received by the originating ORIG server during a step E20. In this scenario, the originating ORIG server determines, during an E21 test, whether to play an early media stream. In the embodiment described here is the case, and the result of the test E21 is positive. The test E21 is then followed by a step E30 during which the AS originating ORIG server establishes a SIP dialogue with an MRF server of the first IMS network to reserve a first resource MRF1. During a step E40, the originating AS server ORIG sends a 183 In Progress SIP response to the calling terminal UA, in response to the invitation message received in step E20, the SDP field of this response comprising the SDP identifier MRF1 of the resource (codec chosen following the offer of the terminal UA, IP address and port number on which the resource MRF1 wishes to receive the media stream and / or DTMF sent by the terminal UA).

During a step E50, the original originating AS server controls the MRF server to restore, in the first resource MRF1, the media stream associated with the callback service of the last caller. According to this scenario, the user of the calling terminal UA requests to be connected, via the input of a DTMF key sequence, to a called SIP (terminal UB) having tried to join it before without success. In the exemplary embodiment described here, the originating server is notified of this operation by receiving a notification message from the server MRF, during a step E70. In the embodiment described here, if the originating ORIG server does not receive such notification during a predetermined TIM time, it releases the call during a step E71. Upon receipt of this notification, the AS server according to the invention requests, in this embodiment, the reservation of a second MRF2 resource from the aforementioned MRF server, during a step E80.

Then, in accordance with the invention, the originating server AS sends, during a step E90, a SIP invitation message destined for a downstream entity, chosen according to the terminal called UB, this invitation message comprising an SDP field identifying this second resource MRF2. In the scenario described here, this downstream entity is the AS terminating application server, referenced TERM, of the terminal called UB. It is clear to those skilled in the art that the timing diagram of FIG. 1 is simplified and that the SIP invitation message is not sent directly to the downstream entity but to the S-CSCF server which extends it to this downstream entity. . In the scenario described here, the terminal terminating server TERM of the terminal called UB reserves, with the MRF server, a third resource MRF3 and a fourth resource MRF4. The terminating AS server then establishes a SIP dialogue with the SCSCF server of the called terminal UB by sending it an invitation message SIP INVITE type whose SDP field includes the identifier MRF4 of the fourth resource.

The S-CSCF server splits the call to the two terminals UB1, UB2, public identity corresponding to that of the terminal called UB, and registered in the core network IMS. This step known to those skilled in the art under the name of forking consists, for the S-CSCF server, in extending the INVITE message to each of these two terminals UB1, UB2. We will assume in this example that the terminal UB1 responds to this invitation message with a RINGING response 180 including an SDP field (selected codec, IP address and port on which the called terminal expects to receive traffic) associated with the terminal. B1 and identified UB1. This SIP response is received by the AS terminating TERM server from the S-CSCF server. The terminal UB1 starts the restitution of an early media stream on the MRF4 resource of the MRF server.

In the scenario described here, the AS terminating server TERM controls the MRF server to replicate, on the MRF3 resource of the MRF server, the stream received on the resource MRF4. The AS terminating server TERM then sends a SIP 180 RINGING response whose SDP field identifies the resource MRF3, in response to the INVITE invitation message sent to step E90. This SIP response type lxx is received by the originating ORIG server according to the invention during a step E120. In this first variant embodiment of the invention, the originating ORIGINAL AS server blocks this SIP response, in other words does not transmit it to the calling terminal UA, during a step E130.

According to the invention, the originating ORIG server selects, during a general step E140, based on the SIP responses lxx received in step E120, a media stream to be restored to the calling terminal UA. In the embodiment described here, the AS server ORIG origin selects: the stream received on the MRF2 resource if the first lxx SIP response received in step E120 includes an SDP field; or a ringback return media stream, generated by the MRF server, if the first 1xx SIP response received in step E120 is 180 RINGING without SDP.

The AS originating server according to the invention therefore selects in this example the stream received on the resource MRF2. According to the invention, the AS Originating server according to the invention controls, during a step E150, the MRF server for restoring the selected media stream on the first resource MRF1. This operation consists, in this example, in replicating the stream received on the resource MRF2 on the resource MRF1.

Thus, the calling terminal UA receives the media stream generated by the terminal UB1 from the same resource MRF1 that used at the beginning of the callback service of the last caller. In the embodiment described here, if the result of the test E21 is negative, the invitation message SIP is extended directly to the called terminal UB, and the first answer type SX answered lxx is transmitted to the calling terminal UA , the other SIP replies of type 1xx being blocked. In the first scenario of the first embodiment which has just been described, the terminals UB1 and UB2 were SIP terminals of the same IMS network as the calling terminal UA. In a second scenario of this first variant embodiment of the invention, we assume that the calling SIP terminal having dialed the callback service number of the last caller hosted by the originating ORIG server, requests by entering a DTMF key sequence. , to be connected to a subscriber of the PSTN or GSM network and that this connection triggers a service of the intelligent network generating, still during the call establishment phase, an early media flow to the calling terminal. This second scenario differs from the first scenario in the choice of the downstream entity to which the originating AS sends the SIP invitation message to step E90.

More precisely, in this second scenario, the SIP message whose SDP field identifies the second resource MRF2 is sent to an MGCF server able to transform the SIP call signaling into ISUP call signaling and to route the call on the network. RTC or GSM circuit. In this second scenario, the called number triggers an intelligent network service generating a circuit-mode early media flow to the T-MGF media gateway which itself transmits this early media stream, using the RTP / UDP / IP protocol. , to the second resource MRF2. The MGCF server sends a 183 In Progress SIP response, the SDP field of this response including the identifier of the resource used by the T-MGF gateway for this call. This SIP message type 1xx is received by the server AS ORIG ORIG according to the invention during the step E120 already described. In accordance with this first embodiment of the invention, the response 183 In Progress is blocked by the server AS originating ORIG during the step [130 already described.

The 183 In Progress response comprising an SDP field, the originating AS controls, during the step E150, the MRF server to restore, in the first resource MRF1, the early media stream generated by the smart network. Still in this second scenario, the user of the calling terminal UA can request call setup to the called party UB. In this case, where this terminal called UB itself generates an early media stream, for example a Colored Ring Back Tone (CRBT) stream, this stream would be transmitted via the T-MGF media gateway and the originating AS server ORIG would receive a response from 180 Ringing type with the SDP field containing the identifier of the same resource used by the T-MGF gateway for this call. On the contrary, if the terminal called UB does not generate an early media stream, the called party's switch itself would generate an RBT ringback in early media mode. Again, the originating ORIG server would receive, in step 120, a 180 Ringing type response, the SDP field of this response having the same identifier of the resource used by the T-MGF gateway. This response would be blocked by the server ORIG according to this first embodiment of the invention. In a third scenario of this first variant embodiment, it is assumed that the calling terminal UA having dialed the callback service code of the last caller hosted by the server ORIG, requests to be put in contact with a SIP subscriber belonging to another IMS network. . This third scenario differs from the first two scenarios in the choice of the downstream entity to which the SIP invitation message is sent in step E90. More precisely, in this third scenario, the originating AS server establishes a SIP dialogue with the IBCF interconnection server between the two networks, by sending a SIP INVITE message whose SDP field identifies the second resource MRF2. Referring to Figures 3 and 4, we will now describe a second embodiment of the invention.

In this second embodiment, the server according to the invention is an IBCF interconnection server between two IMS networks. In this embodiment, it is assumed that the IBCF server receives, during a step F20, an INVITE message sent by an upstream entity, which may be constituted by a calling terminal UA or by a server placed in an outage. flow between the calling terminal UA and the IBCF server.

In this second variant embodiment of the invention, the IBCF interconnection server according to the invention does not respond directly to the INVITE invitation message. In accordance with this second embodiment of the invention, the IBCF server reserves a first resource MRF3 (step F30) and a second resource MRF4 (step F40) with an I-BGF server of the first IMS network. During a step F60, the IBCF interconnection server sends a SIP INVITE invitation message to a downstream entity chosen according to the terminal called UB. In accordance with the invention, this invitation message comprises an SDP field identifying the second resource MRF4 reserved for step F40. In this second variant embodiment of the invention, this downstream entity is an entity of the second IMS network. In this example, it is assumed that this downstream entity is an AS terminating TERM server associated with the terminal called UB. The IBCF interconnection server according to the invention is capable of receiving, during a step F70, several Ixx type SIP responses in response to the INVITE message sent to the step F60. In this example, it is assumed that the terminating server of the terminal called UB is driving an MRF belonging to the second IMS network to play an early media stream, still during the call establishment phase, this stream being received by the IBGF server on the second resource MRF4. In accordance with the invention, the IBCF server selects, during a step F90, a media stream to be restored to the calling terminal UA as a function of the SIP responses received at the step F70. In the exemplary embodiment described here, this selection is made on the basis of the first 1xx SIP response received in step F70. If this first response received is 180 Ringing type without SDP, the IBCF server according to the invention responds to the INVITE message received in step F20 by a 180 Ringing response whose SDP field includes the identifier MRF3 of the first resource .

If the first response received from the second network is a 180 Ringing response with SDP, the IBCF interconnection server according to the invention responds to the INVITE invitation message received in step F20 by sending a response 180 Ringing whose SDP field identifies the first MRF3 resource. Finally, if the first response received from the second network is a 183 In Progress response with SDP, the IBCF server according to the invention responds to the INVITE message received at step F20, by sending a 183 Progress response with the SDP field identifying the first resource MRF3. In other words, in this embodiment of the invention, the IBCF server sends, to the upstream entity, a single response of the type of the first response received from the downstream entity. According to another exemplary embodiment, the selection of step F90 can be performed according to the type ixx and the presence or absence of an SDP field in the response. An order of priority of the responses received can be defined such that the highest priority response is a 183 In Progress response, then 180 Ringing with SDP and finally 180 Ringing without SDP. Then, during a step F100, the IBCF server according to the invention controls, through the SPDF equipment, the interconnection media server IBGF to restore, in the first resource MRF3, the media stream selected at the step F90, either to play a ringback on the MRF3 resource in the case of a 180 Ringing response without SDP, or to replicate the early media stream received on the MRF4 resource to the MRF3 resource in the case of a 180 Ringing response or 183 In Progress with SDP. FIG. 5 represents the architecture of an AS application server according to the invention. In the embodiment described here, this server has the hardware architecture of a computer. It comprises a processor 10, a read-only memory ROM 11, a random access memory RAM 13 and communication means 14 able to implement the SIP protocol to communicate with other entities implementing the same protocol in a IMS network. ROM type ROM 11 is a medium in the sense of the invention. More precisely, when the application server AS is an originating server, this read-only memory comprises a PGO computer program whose instructions, when they are executed by the processor 10, implement the filtering method whose main steps have 2. Similarly, when the application server AS is an interconnection server, this read-only memory comprises an PGI computer program whose instructions, when executed by the processor 10, the filtering method whose main steps have been described with reference to FIG. 4. The communication means 14 of the server AS are able to receive, transmit and interpret a SIP invitation message, to create a SIP response comprising or not an SDP field, to send such a response on the IMS network and to interpret it.

The processor 10, when executing the PGO or PGI computer program, allows the AS server to reserve one or more resources with an MRF or I-BGF server and to control an MRF or I-BGF server so that it renders a media stream on a given resource and replicates a second resource on a first resource.

The processor 10, when executing the PGO or PGI computer program, is able to select a media stream to be restored to a SIP terminal, as a function of a SIP response received by the communication means 14.

Claims (14)

REVENDICATIONS1. A filtering method implemented by a server (ORIG, IBCF) in a first IMS network, during a phase of establishing a call between a calling terminal (UA) and a called terminal (UB), this method comprising: - a step (E20, F20) for receiving a SIP invitation message sent by an upstream entity (UA) in a flow-off between the calling terminal and said server; a step (E30, E80, F30, F40) for reserving a first resource (MRF1, MRF3) and a second resource (MRF2, MRF4) with a resource server (MRF, I-BGF) of said resource first network; a step (E40, F90) of sending a SIP response to said upstream entity (UA) comprising an SDP field identifying said first resource (MRF1, MRF3); a step (E90, F60) of sending a SIP invitation message to at least one downstream entity (UB, TERM, S-CSCF, IBCF) chosen according to said called terminal (UB), said an invitation message comprising an SDP field identifying said second resource (MRF2, MRF4); a step (E120, F70) of receiving at least one 1xx SIP response from at least one of said downstream entities; a step (E140, F90) for selecting a media stream based on said at least one received response; a step (E150, F100) of controlling said resource server to restore said media stream in said first resource (MRF1, MRF3) to said calling terminal. 2. Filtering method according to claim 1, characterized in that said server is an originating server (ORIG), said resource server being an MRF server. 3. A filtering method according to claim 2, characterized in that it comprises a step (E130) of blocking said SIP response type 1xx. 4. A filtering method according to claim 2, characterized in that it comprises a step (E50) for controlling said MRF server to restore, in said first resource (MRF1), a first media stream triggered by said originating server. , said SIP response being 183 In Progress with SDP. 35 5. A filtering method according to claim 4, characterized in that said step (E40) of sending said SIP response to said upstream entity is performed just after the reservation (E30) of said first resource. 6. A filtering method according to claim 4, characterized in that said step (E80) for reserving said second resource (MRF2) is performed on reception (E70) of a notification of said MRF server. The filtering method according to claim 1, wherein said server is an interconnection server between said first network and a second IMS network to which said downstream entity belongs, said resource server being an I-BGF entity. 8. Filtering method according to claim 7, characterized in that said SIP response sent (F90) to said upstream entity is of the type of said SIP response received from said downstream entity. 9. A filtering method according to claim 1, characterized in that said selected media stream is: a media stream received in said second resource (MRF2, MRF4) if said received 1xx SIP response (E120, F70) of said entity downstream has an SDP field; or - a call back signal generated by said resource server in the other cases. 10. A filtering method according to claim 1, characterized in that said media stream is selected (F90) according to a priority parameter of said downstream entity at the origin of said response received in the sense of a forking service as defined in RFC SIP 3261. 11. Server (ORIG, IBCF) belonging to a first IMS network, characterized in that it comprises means for implementing, during a phase of establishment of a call between a calling terminal (UA) and a called terminal (UB): means (14) for receiving a SIP invitation message sent by an upstream entity (UA) in a flow-off between said calling terminal and said server; means (10, PGO, PGI) for reserving a first resource (MRF1, MRF3) and a second resource (MRF2, MRF4) with a resource server (MRF, IBGF) of said first network; means (14) for sending a SIP response to said upstream entity (UA) having an SDP field identifying said first resource (MRF1, MRF3); means (14) for sending a SIP invitation message to at least one downstream entity (UB, TERM, S-CSCF, IBCF) chosen according to said called terminal (UB), said message d an invitation comprising an SDP field identifying said second resource (MRF2, MRF4); means (14) for receiving at least one 1xx SIP response from at least one of said downstream entities; means (10, PGO, PGI) for selecting a media stream as a function of said at least one received response; and means (10, PGO, PGI) for controlling said resource server (MRF, IBGF) so that it renders said media stream in said first resource (MRF1, MRF3) to said calling terminal (UA). 12. Server (ORIG, IBCF) according to claim 11, characterized in that it comprises means for triggering a ringing return media stream on said first resource (MRF1, MRF3) on reception, from said downstream entity, a 180 Ringing SIP response without SDP. 13. Computer program (PGO, PGI) comprising instructions for performing the steps of the filtering method according to claim 1 when said program is executed by a computer (AS). 14. A computer-readable recording medium (11) on which is recorded a computer program comprising instructions for carrying out the steps of the filtering method according to claim 1.