EP1752013A1

EP1752013A1 - Method for switching between two telephony services

Info

Publication number: EP1752013A1
Application number: EP05772969A
Authority: EP
Inventors: Sophie Aveline; Nathalie Beziot
Original assignee: France Telecom SA
Current assignee: Orange SA
Priority date: 2004-06-01
Filing date: 2005-05-24
Publication date: 2007-02-14
Also published as: JP2008501267A; CN1973568A; WO2005120115A1; KR20070026692A; US20070249357A1; FR2871015A1

Abstract

When two terminals (T1, T2) communicate with each other by a first service via a network, the first telephony service is switched to a second telephony service in such a way that the two terminals (T1, T2) can continue to communicate with each other via the second telephony service. In order to carry out said switching, a telephony service transformer (6) interrupts the communication using the first telephony service between the terminals (T1, T2), calls the two terminals (T1, T2) in parallel via the second telephony service, connects the two terminals (T1, T2) by joining the two communications established between the transformer (6) and the two terminals (T1, T2) respectively in order to set up a communication between the two terminals (T1, T2) via the second telephony service.

Description

Switching method between two telephony services

The invention relates to a method for switching between two telephony services, for example a video telephony service and a voice telephony service. In a cellular telephone system, for example the third generation UMTS network, the radio coverage area of the UMTS access network is divided into a plurality of cells, each provided with its own "NodeB" (node B) provided with 'a transceiver and intended to communicate with the mobile terminals located in its cell. When during a communication, a mobile terminal moves, the network detects the moment from which the transmission signal between the terminal and the "NodeB" to which it is connected is weakened and finds another available "NodeB" so to change the cell terminal. Thus, the terminal changes the "NodeB" to which it is connected while maintaining the communication in progress, without interrupting it. This inter-cellular transfer mechanism is commonly called the "handover". To ensure handover over the entire territory covered, a mobile at each point of the territory must receive signals from at least two "NodeB" in order to be able to change cells (to a cell for which the received signal has better quality) . Third generation cell phone systems, including UMTS

(Universal system of communication with mobiles), aim to increase data rates in particular to make multimedia communications, for example video, between mobile terminals faster. Third generation cell phone operators are therefore considering offering video phone services to their users. However, to ensure handover for video communications, the radio coverage of the UMTS access network must be made more dense than to ensure handover for simply voice communications, for two reasons specific to the UMTS system. First, in a UMTS C cell (Figure 1), the video telephone service and the voice telephone service are both operational in the central part C1 of cell C, but only the voice telephone service is operational. in the peripheral part C2 of the cell. As can be seen in Figure 1, in the territories currently covered by a UMTS system, the neighboring cells partially overlap but only at the periphery: As a result, voice telephony service is provided over the entire territory covered but not the video phone service. Rule 26 the greater the number of cells in a given cell, the smaller the area of that cell. For the two reasons just mentioned, it is difficult to guarantee the continuity of the video telephony service when a mobile user moves from one cell to another cell. In this case, the user's video communication may be interrupted. This problem of radio coverage of the video telephony service has been raised in standardization within the framework of the 3GPP (Third Generation Partnership Project - standardization body of UMTS) in particular in document SP-030724. To solve this problem, it is envisaged to develop a switching service aimed at automatically switching from the video telephony service to the voice telephony service when the video communication between two terminals, one of which is a mobile terminal, degrades, so as to continue the communication by the voice telephony service and thus avoid interruption thereof. Two solutions have been envisaged for implementing this switching service. The first solution is based on the "multicall" service, defined in Release 99 of UMTS. This functionality allows a user to establish and maintain two parallel telephone calls simultaneously through circuit switched sections of the third generation cellular telephone network. A mobile terminal with the "multicall" service can thus establish parallel video and voice communication simultaneously. It is planned to use the "multicall" service to implement the switching service previously mentioned:

When the video telephone communication between two mobile terminals deteriorates, owing to the fact that one of the terminals, in movement, passes in an area not covered by the video telephony service, the terminal can make a call by the voice telephony service to the terminal with which it previously communicated in video mode. After establishing voice communication, video communication can be released. This solution has various drawbacks. The first drawback results from the fact that voice communication requires a certain establishment time, perceptible by the user, during which video communication, maintained in parallel, is degraded. During the delay in establishing voice communication, the quality of the communication may therefore not be satisfactory from the point of view of the user. The second drawback is linked to the fact that this solution can only be implemented between terminals supporting and having the "multicall" service. However, this service is not defined for the terminals of the fixed telephone network. In addition, although it is defined for the terminals of the cellular telephone network, it is not yet currently supported by the mobile terminals offered to users and does not appear in the forecasts of the manufacturers of mobile terminals either. A third drawback lies in the fact that the implementation of the "multicall" service requires an adaptation of the MSCs (switches of the mobile service) of the core network of the UMTS system, which is not yet provided by the manufacturers. The second solution is based on the "service change", defined in Release 5 of UMTS. This service makes it possible, during a telephone call between two terminals, to switch from video mode to voice mode, and vice versa, insofar as the characteristics of the two communication modes were specified during the establishment of the call and where they are supported by the two terminals. It is envisaged to use this service to implement the switching service previously mentioned as follows: When the telephone communication in video mode between two terminals deteriorates, one of the terminals can request that the communication be switched voice mode. This solution nevertheless has certain drawbacks. On the one hand, the implementation of the "service change" requires modifications in all the elements of the UMTS communication chain: mobile terminals, RNC (access network controllers) and MSC (switches in the mobile service). However, these modifications are not yet envisaged by the equipment manufacturers. On the other hand, a terminal with the "service change" offered by its operator will not be able to benefit from this service when it is roaming abroad, or "roaming", if the partner operator of "roaming" has not adapted its network for the "change" service, or when it communicates with a terminal of another mobile operator, if this interconnection partner operator has not adapted its network for the "service change". The "multicall" service and the "service change" are certainly defined in Release 99 and in Release 5 of UMTS respectively, but neither the mobile terminals nor the current infrastructure of the mobile network are adapted to be able to implement provides these services. The two solutions explained above for carrying out a switching service from the video telephony service to the voice telephony service, aiming to avoid a total interruption of the communication between the two terminals during video communication due to a degradation of that therefore, cannot be made operational in the short term. The problem of the invention therefore consists in proposing an alternative to these two solutions, the implementation of which would require only limited modifications to the communication chain. It will be noted here that the invention originates from a switching problem between the video telephone service and the voice telephone service. However, the invention is not limited to this particular case but more generally applies to a switching problem between two telephony services. To this end, the invention relates to a method of switching from a first telephone service to a second telephone service, in which, two terminals communicating with each other by the first service through a network, the procedure is carried out. switching from the first to the second telephone service so that the two terminals continue to communicate with each other via the second telephone service, characterized in that, in order to switch from the first to the second telephone service, a transformer of telephony service - interrupts the communication using the first telephony service between the two terminals, - calls in parallel the two terminals by the second telephony service then - connects the two terminals by abutment of the two communications established between, on the one hand, the transformer and, on the other hand, the two terminals respectively, in order to establish communication between the two termi by the second telephone service. The invention therefore consists in voluntarily cutting off the communication by the first telephony service between the two terminals, in initiating two communications between a transformer and the two terminals respectively by the second telephony service, then in abutting these in order to link the two terminals so that they communicate with each other by the second telephone service. It will be noted that the implementation of this process requires only a few modifications of the communication chain. Indeed, it suffices to add a transformer, placed in call cutoff between the two terminals. In a preferred embodiment, the first telephone service is a video telephone service and the second service is a voice telephone service. In this case, the method of the invention makes it possible to pass from a video communication to a communication by voice between two terminals, for example when one of them moves and passes in an area not covered by the service of video telephony. Advantageously, during the establishment of the communication using the first telephony service between the two terminals, one of the two terminals, the "calling terminal", having made a call to the other terminal, the "called terminal", i) the network routes this call to the transformer so as to establish a communication by the first telephone service between the calling terminal and the transformer, ii) on reception of the call from the calling terminal, the transformer calls the called terminal in order to '' establish a communication with it by the first telephony service, then connects the two terminals by abutting the two respective communications between the calling terminal and the transformer, on the one hand, and between the transformer and the called terminal, on the other hand, in order to establish communication by the first telephone service between the two terminals. Thus, when the communication between the two terminals using the first telephone service is established, the network routes the call through the transformer. The invention also relates to a telephone service transformer for a network comprising i) call interruption means intended to interrupt a communication using the first telephony service between two terminals, ii) call means arranged to call in parallel the two terminals in order to establish with them two respective communications by the second telephone service and iii) abutment means arranged to connect the two terminals by abutting the two communications between the transformer, on the one hand, and the two terminals respectively, on the other hand, in order to establish communication between them by the second telephone service. The invention finally relates to a call routing device characterized in that it comprises means for identifying the telephone service used for communication between two terminals during the establishment of the latter and switching means intended at direct the call sent by one of the terminals, the "calling terminal", and intended for the other terminal, the "called terminal", to a telephony service transformer previously defined, in the case where it is a predefined telephone service. The invention will be better understood with the aid of the following description of a particular embodiment of the method for switching between two telephony services, of a particular embodiment of the telephony service transformer and of the routing device of call, with reference to the accompanying drawings in which: - Figure 1 shows a schematic view of UMTS cells allowing voice telephony service on all their covers and video telephone service on only part of their covers, - the FIG. 2A represents the elements of a communication chain of a cellular telephone system of FIG. 1, in the case of an outgoing call, according to a first alternative embodiment; FIG. 2B represents the elements of a communication chain of a cellular telephone system of FIG. 1, in the case of an incoming call, according to a first alternative embodiment; - Figure 3 shows a functional block diagram of the call router of Figures 2A and 2B and Figure 4 shows a functional block diagram of the telephone service transformer of Figures 2 A and 2B. FIGS. 2A and 2B represent a third generation mobile telephony system, in this case UMTS (universal system for communication with mobiles), integrating a video telephony service, or "videophone", allowing users to communicate between them by video (ie voice and moving pictures) and a voice telephone service, allowing users to communicate with each other only by voice. This system comprises, in a known manner, a plurality of "NodeB" (nodes B) 1, RNC (access network controllers) 2, MSC 3 (switches for the mobile service) and G-MSC 4 playing the role of gateways between the UMTS network and the external switched telephone network, which includes the ISDN network (Digital Service Integration Network) 9. Each NodeB 1 is provided with a transceiver ensuring radio coverage of a cell, which includes a central part C1 and a peripheral part C2 (Figure 1). The video telephone service and the voice telephone service are operational in the central part C1 of the cell, while only the voice telephone service is operational in the peripheral part C2 of the cell. As can be seen in FIG. 1, the neighboring cells partially overlap at the periphery so that the voice telephony service is operational over the entire region R covered by the cells, extending inside the periphery P shown in dotted lines in FIG. 1, while the video telephone service is not operational over the whole of this region R owing to the fact that certain peripheral zones C2 of cells C are not covered by the central zones C1 of neighboring cells . The system further comprises call routers 5 and telephone service transformers 6. In the particular example of the description, each transformer 6 is connected to several call routers 5 and, conversely, each router 5 is connected to several transformers 6. Alternatively, each transformer could be connected to a single router. The number of call routers 5 and the number of transformers 6 depend on the importance of video telephony traffic, on the one hand, and on the call processing capacities of the equipment 5 and 6, on the other hand. From the outset, it will be noted that: - an "outgoing call" designates a call on the initiative of a mobile terminal (to another mobile terminal or a fixed terminal) and - an "incoming call" designates a call to a mobile terminal (on the initiative of another mobile terminal or a fixed terminal). In the case of an outgoing call, the UMTS communication chain comprises, in order, with reference to FIG. 2A: the mobile terminal calling T1, a "NodeB" 1, an RNC 2, an MSC 3, a call router 5 and a transformer 6. In the case of an incoming call, the UMTS communication chain comprises, in order, with reference to FIG. 2B: a G-MSC 4, a call router 5 , a transformer 6, an MSC 3, an RNC 2, a "NodeB" 1 and the mobile terminal called T2. A call router 5 is a device intended to route, in other words route, to the transformer 6 the calls coming from a "calling" terminal intended for a "called" terminal, when they are calls using video phone service. Referring to Figure 3, it includes the following: a module 50 for identifying the nature of the call, intended to determine the telephony service used for the identified call, and a routing module 51 intended to direct the call either to the transformer 6 or directly to the called terminal, depending on whether it is a call by the video telephony service or a call by another telephony service, for example by the voice telephony service. The identification module 50 is intended to identify the type of service used by each call from a calling terminal to a called terminal, when at least one of the two terminals is a mobile terminal of the UMTS network. In the event of an outgoing call, the identification module 50 analyzes the content of the 'Other Rate Adaptation' and 'FNUR' fields, appearing in the description of the support capabilities ("ISDN Bearer Capability") contained in the IAM message ( Initial Address Message) transmitted by the MSC 3 to the called terminal on receipt of the call setup CC-Setup message from the calling mobile terminal and initiating the communication. In the event of an incoming call on the initiative of a fixed terminal of the ISDN network 9, the identification module 50 analyzes the content of the 'Other Rate Adaptation' and 'FNUR' fields, appearing in the description of the support capacities ("ISDN Bearer Capability") contained in the IAM (Initial Address Message) message transmitted by the ISDN network 9 to the G-MSC 5. The module 50 identifies that it is a video telephony call when: - ' Other Rate Adaptation '= "H223 &H245"-' FNUR '= 64 kb / s. The referral module 51 is intended to direct the call identified by the module 50 to the transformer 6, if it is a video telephony call, or directly to the called terminal, if not. The transformer 6 is intended to transform the telephony service used between two terminals during communication. In the mobile telephone system represented in FIGS. 2A and 2B, it is located between, on the one hand, the MSC 3s of the UMTS network and, on the other hand, the gateways between the UMTS network and the networks outside the UMTS network , in particular the G-MSC 4. With reference to FIG. 4, the transformer 6 comprises - a module 60 for receiving a call set-up message containing the calling number and the called number, a module 61 for detecting a degradation in the quality of the communication between two terminals, a call cut module 62 arranged to interrupt a video communication in progress between two terminals, a call module 63 arranged for call terminals, - an abutment module 64 arranged to connect two terminals by abutting two communications between the transformer 6, on the one hand, and the two terminals respectively, on the other hand. a memory 65 for storing the calling and called numbers. The reception module 60 is arranged to - receive a call set-up message containing the calling number and the called number, in this case an IAM (Initial Address Message) message, originating either from an MSC 3, in the in the case of an outgoing video call (FIG. 2A), that is to say from a G-MSC 4, in the case of an incoming video call (FIG. 2B), and transmitted via a call router 5, and extract the two numbers, calling and called, from the IAM message and save them in memory 65 by associating them with one another. The detection module 61 is arranged to detect a degradation in the quality of the video communication between two terminals, here by analysis of the H.263 video coding protocol frames which transport the packets of coded video data between the two terminals. Based on an indicator of the quality of the communication, for example the number of missing or erroneous data packets, the detection module 61 determines the moment from which the quality of the video communication between two terminals becomes insufficient to be able to be continued properly. The quality of the video communication can become unsatisfactory for example because one of the two terminals, mobile, moves and passes inside an area located on the periphery of a "NodeB" cell, in which the video phone service is not operational. The call interruption module 62 is arranged to interrupt a video communication in progress between two terminals, on command of the detection module 61, when the quality of the video commumcation becomes insufficient, and to signal to the two terminals that a communication using voice telephony service will be established. Each terminal is thus warned of the temporary nature of the interruption of communication and can signal to its user that a communication by voice will be established, as will be explained in the description of the process. The call module 63 is arranged to make calls, on the one hand, during the establishment of a video call between two terminals respectively "calling" and "called" and, on the other hand, after interruption by the call cut-off module 62 of a video commumcation in progress between two terminals: - When establishing a video call on the initiative of a "calling" terminal and intended for a "called" terminal, the router 5 routes the call to the transformer 6. The call module 62 is arranged to, on receipt of this video call, call the terminal called by the video telephony service by inserting the identity of the calling terminal, by cash his call number, in a caller identification field transmitted to the called terminal. - After interruption of a video communication between two terminals, the call module 63 is arranged to call the two terminals in parallel in order to establish with them two respective communications by the voice telephony service. The abutment module 64 is arranged for, after interruption of a video communication between two terminals and establishment of two voice communications between the transformer 6 and the two terminals respectively, to terminate the communication between the transformer 6 and one of the terminals and the communication between the transformer 6 and the other termmal, that is to say end to end these two communications, in order to put the two terminals in communication by the voice telephony service. The method of switching from the video telephone service to the voice telephone service, corresponding to the operation of the call routers 5 and of the transformer 6, will now be described. In the case of an outgoing video call (Figure 2A) In the case of an outgoing video call, on the initiative of a UMTS Tl mobile terminal "calling" and intended for a terminal "called" T2, for example a ISDN 9 fixed network terminal, the communication is initiated by a CC-Setup (El) message, defined by specification TS24.008 of the 3GPP telecommunications standard, originating from the calling terminal T1 and transmitted to the MSC 3 to which the calling terminal T1 is connected, via UTRAN consisting of NodeB 1 and RNC 2 to which the terminal Tl is connected. After reception of the CC-Setup message by the MSC 3, the UMTS network allocates the resources necessary in UTRAN by an RAB assignment procedure (E2). Once the resources have been allocated, the MSC 3 sends an IAM message (E3) to the terminal T2. This message includes a caller identification field, containing the number of the calling terminal T1, a called party identification field, containing the number of the called terminal T2, and a description part of the capabilities of the "ISDN Bearer" medium. Capability "containing the following fields: - 'Other Rate Adaptation'=" H223 & H245 "- 'FNUR' = 64kb / s The call router 5 intercepts the IAM message and identifies that it is a video call (c (i.e. using the video telephony service), by analyzing the content of the 'Other Rate Adaptation' and 'FNUR' fields, and then routes the call to transformer 6 to which it is connected by transmitting the IAM message to it received (E4). The transformer 6 receives the IAM message, extracts the calling and called numbers therefrom and stores them in memory by associating them with one another. Then, the transformer 6 calls on the one hand the terminal called T2 and on the other hand finalizes the establishment of the communication with the calling terminal Tl, in the following manner: To call the terminal T2, the transformer 6 transmits to it ci a new IAM message (E5) by inserting the identity of the calling terminal Tl, constituted here by the call number of Tl, in the identification field of the caller. Thus, the call is routed from the calling terminal T1 to the terminal called T2, via the transformer 6, by means of two IAM messages originating respectively from the MSC 3 (E3) and from the transformer 6 (E5) and both containing the number of the calling terminal Tl in the caller identification field. Thus, from the point of view of the terminal called T2, the origin of the call received is the terminal calling T1 and not the transformer 6. When the called party T2 has been alerted by a ringing signal on receipt of the call, an ACM (Address Complete Message) message from the ISDN User Part (E6) layer is transmitted from the terminal called T2 to the transformer 6. When the called party T2 picks up, an ANM (Answer Message) message (E8) is transmitted to the transformer 6 and a video telephone communication is then established between the terminal T2 and the transformer 6. The process of establishing communication between the calling terminal Tl and the transformer 6 ends as follows: The message ACM (E6) indicating that the terminal called T2 has been alerted by a ring tone and is retransmitted to MSC 3 by transformer 6 _E6). This information is transmitted to the calling terminal T1 by the MSC 3 by means of a message. CC Alerting (E7). Likewise, the message ANM (E8) indicating that the called terminal T2 has picked up is retransmitted to the MSC 3 by the transformer 6 C 38) and this information is transmitted to the calling terminal by the MSC 3 by means of a CC Connect message ( E9). The calling terminal Tl then sends to the MSC 3 a CC Connect Ack message (E10) to confirm the establishment of the communication. A video telephone communication between the calling terminal T1 and the transformer 6 is then established. After establishment of the two video communications between, on the one hand, the transformer 6 and, on the other hand, the two terminals T1 and T2 respectively, the transformer 6 abuts the two communications in order to put the two terminals T1 and T2 in contact by video phone service. The two terminals T1 and T2 are thus put into communication by the video telephone service, via the transformer 6. However, the intervention of the transformer 6 is visible neither by the calling terminal and nor by the called terminal. After connection, the terminals T1 and T2 communicate by the video telephone service. In the case of an incoming video call (Figure 2B) In the case of an incoming video call, intended for a mobile terminal T2 of the UMTS network, on the initiative for example of a fixed terminal Tl of the ISDN network 9, the call is routed by the ISDN network 9 to a G-MSC 4, by means of an IAM message (El ') comprising a caller identification field, containing the number of the calling terminal Tl, a called party identification field, containing the number of the terminal called T2, and a description part of the capabilities of the "ISDN Bearer Capability" support containing the following fields: - 'Other Rate Adaptation' = "H223 &H245" - 'FNUR' ≈ 64kb / s Since the G-MSC 4 does not have any information on the location of the mobile terminal called T2, it interrogates (E2 ') the HLR (Home Location Register) S, nominal location register, which interrogates the VLR register (Visitor Location Visitor), not shown, with which terminal 72 is located to obtain an MSRN number ( Mobile Station Roaming Number). This number is used to route the IAM message from the G-MSC 5 to the recipient MSC 3 (E3 '). The IAM message is intercepted by a call router 5 which identifies that it is a video call by analyzing the content of the 'Other Rate Adaptation' and 'FNUR' fields. The router 5 then routes the call to the transformer 6 with which it is associated, by retransmitting to it the received IAM message (E4 '). The transformer 6 receives the IAM message, extracts the calling and called numbers therefrom and stores them in memory by associating them with one another. Then, it calls the mobile terminal called 72 and finalizes the establishment of the communication with the fixed terminal calling T1, as follows: To call the terminal T2, the transformer 6 transmits to the MSC 3 to which the terminal

72 is connected to a new IAM message (E5 ′) by inserting therein the identity of the calling terminal Tl, constituted here by its number, in the identification field of the caller. Thus, the call is routed to the called terminal 72, via the transformer 6, by means of two IAM messages originating respectively from the ISDN network 9 (E1 ') and from the transformer 6 (E5') and both containing the number of the calling terminal Tl in the caller identification field. Thus, from the point of view of the called terminal 72, the origin of the received call is indeed the calling terminal T1 and not the transformer 6. The call establishment process between the transformer 6 and the mobile terminal T2 is continues in a known manner: on receipt of the IAM message, the MSC 3 sends a CC-Setup message (E6 ') to the terminal 72 which, on receipt of this message, sends it a CC-Call Confirmed (ET) message. The UMTS network then allocates the necessary resources in the UTRAN, consisting of the NodeB 1 and the RNC 2 to which the terminal 72 is connected, by an assignment RAB procedure (E8 ′). When the terminal called T2 rings, a CC-Alerting message (E9 ') is sent to the MSC 3 and relayed by an ACM (Address Complete Message) message (E10') to the transformer 6. When the terminal called 72 picks up, a message CC-Connect (El l ') is transmitted from terminal 72 to MSC 3 then relayed by an ANM (Answer Message) message (E12') indicating that called party 72 has picked up, sent by MSC 3 to transformer 6. A video telephone communication is then established between the terminal called 72 and the transformer 6. The process of establishing communication between the calling terminal Tl and the transformer 6 ends as follows: the transformer 6 retransmits to the calling terminal 77 of the ISDN fixed network 9 the ACM message (_E10 ') ₅ coming from the terminal called T2 and indicating that the latter has been alerted to the call by ringing, then the ANM message JB12'), coming from the term called 72 and indicating that the termmal called 72 has dropped . The video communication between the transformer 6 and the term called T1 is then established. After establishment of the two video communications between, on the one hand, the transformer 6 and, on the other hand, the two terminals T1 and T2 respectively, the transformer 6 connects the two communications in order to connect the two terminals Tl and 72 by the video telephone service. It will be emphasized here that the intervention of the transformer 6 is visible neither by the calling terminal Tl, nor by the called terminal 72. Process for switching from the video telephone service to the voice telephone service During video communication between the two terminals T1 and 72, at least one of the two terminals belonging to the UMTS network (incoming or outgoing call), the transformer 6 analyzes the H.263 video coding protocol frames which transport coded video data packets transmitted between the two terminals 77 and 72. In the particular example of the description, the transformer 6 monitors the quality of the video communication, by regularly evaluating the number of missing or erroneous data packets. If one of the two terminals, Tl or 72, mobile, moves and finds itself in an area located on the outskirts of a UMTS cell, in which the quality of the video telephony service is insufficient, the video communication between the two terminals Tl and 72 degrades. The transformer 6 detects the degradation of the quality of the video communication and then interrupts it by transmitting in parallel to each of the two terminals T1 and 72 a warning message, in this case a Userlnputlndication message of the H.245 negotiation protocol. , signaling that a voice communication will be established. On receipt of this message, each terminal T1, T2 signals this information to its user, here by displaying a message on the terminal. As a variant, or in addition, on receipt of the warning message, each terminal T1, T2 could stop on the last image displayed for the duration of the interruption of the communication, or send a voice warning message. After having interrupted the communication between the two terminals T1 and 72, the transformer 6 calls in parallel the two terminals T1 and 72 by the voice telephony service, thus establishing a first voice communication with the terminal T1 and a second voice communication with the terminal 72 then connects the two terminals T1 and 72 by joining the two voice communications. A communication between the two terminals Tl and 72 is thus established via the transformer 6 by the voice telephony service. After the momentary interruption of the call, the two terminals T1 and 72 continue their communication by the voice telephony service. In a variant embodiment of the invention, the function of routing video calls to the transformer 6 is integrated into the MSC 3 and the G-MSC 5 of the UMTS network, these devices thus playing the role of call routing device. . In this case, in the event of an outgoing call, the video call identification module, integrated into the MSC 3, is arranged to analyze the content of the 'Other Rate Adaptation' and 'FNUR' fields, appearing in the description part of the support capabilities ("TE Bearer Capability") contained in the CC-Setup call establishment message from the calling terminal and initiating communication. In the event of an incoming call, the video call identification module integrated in the G-MSC 5 is designed to analyze the content of the 'Other Rate Adaptation' and 'FNUR' fields, shown in the description of the support capabilities ("ISDN Bearer Capability") contained in the IAM message from the ISDN network 9. In the example embodiment which has just been described, the transformer 6 makes it possible to switch from the video telephony service to the telephony service by the voice. Of course, the invention could be applied to switching between two telephony services other than those described.

Glossary This glossary presents the list of Anglo-Saxon acronyms used in this patent application. These acronyms are defined within the framework of the 3GPP telecommunications standard. ACM Address Complete Message ANM Answer Message CC Call Control ETSI European Telecommunications Standards Institute FNUR Fixed Network User Rate 3 GPP Third-Generation Partnership proj ect (of ETSI) G-MSC Gateway MSC HLR Home Location Register IAM Initial Address Message ISDN Integrated Service Digital Network ISUP ISDN User Part MSC Mobile Switching Center RAB Radio Access Bearer RNC Radio Network Controller UMTS Universal Mobile Telecommunications System

Claims

1) Method for switching from a first telephony service to a second telephony service, in which, two terminals (Tl, T2) communicating with each other by the first service through a network, a procedure is carried out. switching from the first to the second telephony service so that the two terminals (Tl, T2) continue to communicate with each other by the second telephony service, characterized in that, in order to switch from the first to the second telephony service , a telephony service transformer (6) - interrupts the communication using the first telephony service between the two terminals (Tl, T2) - calls the two terminals (Tl, T2) in parallel by the second telephony service then - activates relationship between the two terminals (Tl, T2) by joining the two communications established between, on the one hand, the transformer (6) and, on the other hand, the two terminals (Tl; 72) respectively, in order to establish communication cation between the two terminals (T1, T2) by the second telephone service.

2) The method of claim 1, wherein the first telephone service is a video telephone service and the second service is a voice telephone service. 3) Method according to one of claims 1 and 2, wherein, during the establishment of the communication using the first telephony service between the two terminals (Tl, T2), one of the two terminals (Tl), the "calling terminal", having made a call to the other terminal (T2), the "called terminal", i) the network (5) routes this call to the transformer (6) so as to establish a communication by the first telephone service between the calling terminal (Tl) and the transformer (6), ii) on receiving the call from the calling terminal (77), the transformer (6) calls the called terminal (72) in order to establish with it a communication by the first telephone service, then connects the two terminals (Tl, T2) by abutment of the two respective communications between the calling terminal (Tl) and the transformer (6), on the one hand, and between the transformer (6) and the terminal called (72), on the other hand, in order to establish communication by the first telephone service between the two terminals (T1, T2).

4) Method according to claim 3, wherein, on receipt of the call from the calling terminal (77), the transformer (6) stores the numbers of the calling terminal (T1) and of the called terminal (T2) in order to call them later with a view to establishing communication by the second telephone service between the two terminals (Tl and 72).

5) Method according to one of claims 3 and 4, wherein, when calling the called terminal (Tl) by the second telephone service, the transformer inserts the identity of the calling terminal (72) in a field of identification of the caller, transmitted to the called terminal (72).

6) Method according to one of claims 1 to 5, wherein the transformer (6) monitors the quality of communication using the first telephone service between the two terminals (Tl, T2) and, upon detection of a degradation of the quality of the communication, interrupts it. 7) Method according to one of claims 1 to 6, wherein, after interrupting the communication using the first telephone service between the two terminals (Tl, T2), the transformer signals to each of the two terminals (Tl, T2) that a communication by the second telephone service will be established.

8) Telephone service transformer for a network, comprising i) call interruption means (62) intended to interrupt a communication using the first telephone service between two terminals (T1, T2), ii) means for call (63) arranged to call in parallel the two terminals (Tl, T2) in order to establish with them two respective communications by the second telephone service and iii) abutment means (64) arranged to connect the two terminals (Tl, T2) by abutment of the two communications between the transformer (6), on the one hand, and the two terminals (Tl, T2) respectively, on the other hand, in order to establish communication between them by the second service telephony. 9) A transformer according to claim 8, wherein the call means (63) are arranged for, on receipt of a call by the first telephone service issued by a terminal (77), the "calling terminal", to a other terminal (72), the "called terminal", said call having been routed by the network (5) to the transformer (6), call the called terminal (72) by the first telephone service, and the abutment means (64) are arranged to link the two terminals (T1, T2) by abutment of the two respective communications between the calling terminal (77) and the transformer (6), on the one hand, and between the transformer (6) and the called terminal (72), on the other hand, to establish communication by the first telephony service between the two terminals (77, 72).

10) A transformer according to claim 9, in which means are provided for inserting the identity of the calling terminal in a field for identifying the caller transmitted to the called terminal (T2) during the establishment of the call between the transformer (6) and the terminal called (72).

11) Transformer according to one of claims 8 to 10, wherein there is provided means (61) for detecting a degradation of a communication between two terminals (Tl, T2), intended to control a change in the service of telephony used for said communication.

12) Transformer according to one of claims 8 to 11, wherein there is provided means (62) for signaling to the two terminals (T1, T2), after interruption of their communication by the first telephone service, that a communication using the second mode of telephony will be established. 13) Call routing device, characterized in that it comprises means (50) for identifying the telephone service used for a communication between two terminals (Tl, T2) during the establishment of the latter and of the switching means (51) intended to direct the call sent by one of the terminals (77), the "calling terminal", and intended for the other terminal (72), the "called terminal", towards a transformer telephone service (6) according to one of claims 8 to 11, in the case where it is a predefined telephone service.

14) Call routing device according to claim 13, characterized in that it is integrated in a mobile service switch (3).

15) Telephony system capable of supporting first and second telephony services, comprising a telephony service transformer (6) according to one of claims 8 to 12 and a call routing device according to one of claims 13 and 14.