Abstract:
The invention relates to a method and a system of communication between at least one first communications terminal ( 13 ) of a first communications network ( 10 ) and at least one second communications terminal ( 14 ) of a second communications network ( 11 ), said first network ( 10 ) and said second network ( 11 ) being interconnected via an interface device ( 12 ). According to the invention the method includes:
       a step of dynamically calculating a maximum bit rate of data streams in transit between the first and second terminals ( 13, 14 ) taking account of bit rates observed in the first network ( 10 ) and the second network ( 11 ); and   a step of processing a request by said first and second terminals ( 13, 14 ) for admission of a new data stream, said interface device ( 12 ) being adapted to execute said processing taking account of the result produced in said calculation step.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is the U.S. national phase of the International Patent Application No. PCT/FR2008/050343 filed Feb. 29, 2008, which claims the benefit of French Application No. 07 53803 filed Mar. 13, 2007, the entire content of which is incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The present invention relates to communication between terminals of different communications networks, for example between a terminal of a private mobile local area communications network and a terminal of a wide area communications network administered by an operator. 
     To be more precise, the invention relates to controlling the setting up of a new call between such terminals as a function of the quality of service level offered by the networks hosting the terminals at a given time. 
     BACKGROUND OF THE INVENTION 
     The invention applies particularly, but not exclusively, to a UMA (Unlicensed Mobile Access) network architecture. A UMA architecture enables users to use their mobile telephones on an IP (Internet Protocol) local area network when at home, at work or, more generally, in an area covered by an access point of an IP private mobile local area network, rather than using the chargeable fixed or mobile network of an operator. 
     Under the UMA standard, an approach of this kind has the advantage of significantly reducing call costs, especially for businesses, where the number of calls made by its staff is often high. 
     A simple illustrative and non-limiting example is the use a network architecture conforming to the UMA standard to extend GSM/GPRS mobile services in free access IP wireless networks (Wi-Fi™ or Bluetooth™ networks in particular) by creating a direct communication tunnel between a client communications terminal and the core network of the operator, with the following objectives:
         enabling the user to use mobile voice and data services via private networks, retaining the same telephone number;   supporting handover between a private network and a GSM network;   remaining independent of the technology of the private network, for example the Wi-Fi™ or Bluetooth™ technology, while being transparent for existing units in the private network;   ensuring security equivalent to that of a GSM network.       

     In the context of a residential UMA architecture (for example that deployed in the context of the Unik™ offer from Orange™), when an outgoing call request is generated by a GSM/GPRS-type communications network on a Wi-Fi™ network, a decision is taken relatively simply, in accordance with criteria defined beforehand by the mobile telephone operator. 
     If at the time of the call request the terminal is located in the coverage area of its Wi-Fi gateway, forming a unique access point to the network of the operator, then the Wi-Fi call is extended via the core network of the operator to the remote terminal for which the call is intended. 
     Otherwise, the outgoing call is switched to the chargeable GSM network of the operator automatically and without interruption (this procedure is usually known as “handover”). 
     Making such decisions proves incompatible with the use of a business-type UMA network architecture because, in such an architecture, the communication gateway serves as an interface between the private mobile local area network (of IP type, for example) of the business and the communications network of the operator. The gateway therefore needs to supervise and manage simultaneously a plurality of access points to the mobile local network, distributed over the site of the business, and must additionally manage a large number of calls by a large installed base of mobile communications terminals associated with the business&#39;s staff. 
     The maximum number of VoIP (Voice over IP) streams that a communications network can transmit at a given time, especially in a UMA architecture, proves to be an important parameter, representative of the level of quality of service (QoS) perceived by users. 
     If the number of VoIP streams in transit on the network at a given time exceeds this maximum number, the QoS offered to users is significantly degraded. 
     Also, one drawback of a UMA architecture comprising a plurality of mobile terminals (for example of the GSM, GPRS type) able to connect to a plurality of access points (AP) managed by a unique communication gateway (UMA gateway) enabling the IP private local area network of the business to interface with the core network of an operator, is primarily linked to the difficulty of being able to guarantee users and maintain a constant QoS, especially if there is a high influx of calls at a given time or over a given period. 
     Because of the large number of calls generated in the business on a daily basis, the maximum number of VoIP streams authorized by the operator will often be reached, with the consequence of degrading the quality of calls in transit via the UMA communication gateway. 
     In an attempt to address this problem, some technical approaches evaluate the maximum number of VoIP streams in transit at a given time in the WAN (Wide Area Network) of the UMA architecture, i.e. only on the operator core network side. 
     Such a prior art approach proves limited, however, in that it only partially addresses the above-mentioned drawbacks. 
     Although it is now possible for an operator to determine the maximum number of VoIP streams in transit on the WAN part of the network, either statically by recovering a predefined parameter value (maximum authorized VoIP streams), or dynamically, by means of a CAC (Call Admission Control) algorithm, with the objective of evaluating the possibility of adding a new stream, it is nevertheless impossible to evaluate the impact of adding any such new stream, end-to-end, both on the WAN part and, more importantly, on the WLAN part of a private mobile network. The degraded QoS of calls in a business-type UMA architecture stems most often from overloading said business&#39;s private mobile network. 
     What is more, in UMA architectures, there is no dynamic calculation of the number of VoIP streams, the WMM™ (Wireless MultiMedia) standard that aims to define criteria and methods of improving the quality of service rendered to wireless communication users specifying in its paragraph 3.5.2 (Version 1.1—Wi-Fi Alliance Technical Committee Quality of Service Task Group) that a method of determining the maximum VoIP stream value in a UMA architecture must be defined by each equipment manufacturer. 
     This situation contributes to the difficulty now encountered by operators and equipment manufacturers of being able to offer to businesses attractive and performing telephony solutions based on a UMA architecture. 
     SUMMARY OF THE INVENTION 
     The present invention offers a solution that is free of the drawbacks mentioned above. 
     The invention aims to eliminate the above-mentioned drawbacks by proposing a method of calculating and controlling the maximum number of VoIP streams in transit at a given time and/or over a given period in both the WLAN portion and the WAN portion (GSM/GPRS network of a mobile telephony operator) and thus from end-to-end in a network architecture conforming to the UMA standard. 
     A particular aim of the present invention is to be able to control admission or refusal of any new data stream representing a new call (a VoIP stream in particular), with the objective of assuring users a constant quality of service level during calls via a UMA architecture, in particular as implemented within a business. 
     To this end, the invention consists in a method of communication between at least one first communications terminal of a first communications network and at least one second communications terminal of a second communications network, said first network and said second network being interconnected via an interface device, characterized in that it includes:
         a step of dynamically calculating a maximum bit rate value of data streams in transit between the first and second terminals taking account of bit rates observed in the first and second networks; and   a step of processing a request by said first and second terminals for admission of a new data stream, said interface device being adapted to execute said processing taking account of the result produced in said calculation step.       

     The present invention proposes a solution that has the advantage that when, in a UMA network architecture, a first terminal of a first communications network, for example an IP private mobile wireless local area network (WLAN), is seeking to set up a new call with a second terminal of a second communications network, for example a wide area network (WAN), it evaluates the end-to-end availability of resources in the first and second communications networks. Accordingly, as a function of the result of this evaluation, if at least one of the two networks does not have resources available, especially in terms of bandwidth, to carry the data streams (for example voice data streams) necessary to support the new call, then said call is handed over to the mobile network of an operator. 
     Said processing step is preferably a step of dynamically analyzing at least one availability criterion associated with each of said networks such that:
         if each network is available to receive said new data stream, a communication tunnel is reserved via said interface device rated as a function of the bit rate necessary to support said new data stream between said first terminal and said second terminal; and   if at least one of said networks is not available, said new data stream is handed over to a mobile communications network.       

     Said step of reserving said communication tunnel advantageously includes at least one preliminary step of dynamically calculating a maximum bit rate of said new data stream as a function of at least one parameter representing a coding-decoding operation applied to said stream. 
     In one particular implementation of the invention said at least one preliminary dynamic calculation step is a function of at least one parameter representing a type of coder-decoder used by said terminal to process said stream. 
     Such an approach advantageously calculates a maximum bit rate necessary for transmitting the new data stream, especially a UMA voice stream, whatever type of coder-decoder is associated with the terminal of the first communications network. 
     In another implementation of the invention, said at least one preliminary dynamic calculation step is a function of at least one parameter representing a mode of operation of a coder-decoder used by said first terminal to process said stream. 
     In a preferred implementation of the method of the invention, said second communications network is a wide area network (WAN) including a virtual communications channel a first end of which is connected to said interface device and a second end of which is connected to a collection and routing unit, and said dynamic analysis step is a step of said interface device verifying a possibility of adding said new data stream to said virtual channel. 
     Still in the preferred implementation of the method of the invention, said first communications network ( 10 ) is a free access IP wireless local area network (WLAN) comprising a plurality of access points ( 15   i , i=1, . . . , N) and said dynamic analysis step is a step of at least one access point ( 15   i , i=1, . . . , N) verifying a possibility of adding said new data stream to said first network between said interface device and said access point. 
     When said first communications terminal sends an outgoing call request, it preferably sends said interface device via an access point a solicitation message carrying a request for reservation of resources, so that if said request for reservation of resources for the sending a data stream via said access point cannot be met, a step of at least one other access point to which said communications terminal ( 13 ) can be connected automatically sending a resource solicitation message is initiated. 
     If reservation of resources on the first network has been possible with a view to adding the new stream (for example a UMA voice data stream), the method preferably includes a step of adding said new data stream to a management table for the streams of said first network (located in the interface device or at least one access point) and a step of said interface device verifying a possibility of adding said data stream to said second network, so that if this possibility is established, a step is executed of adding said new data stream to a management table for the streams of said second network, said table being contained in said interface device. 
     Thus, to go into more detail, if an outgoing call request is sent by said first communications terminal to set up a call with said second communications terminal, the following steps are executed:
         step 1: transmission by said first terminal of a message requesting reservation of resources for said new data stream going to at least one first available access point;   step 2: verification by said first access point of the possibility of adding said new stream to said first network, so that if this possibility is established, the following additional steps are executed:
           said first access point sending said interface device a message requesting reservation of resources for said new stream on said second network;   said interface device verifying a possibility of adding said new stream to said second network, so that if said possibility is established the following additional steps are executed;
               addition of said new data stream to a management table of the streams of said interface device;   said interface device sending said first access point a message accepting said request for reservation of resources for said new data stream on said second network;   adding said new data stream to a management table of the streams of said first network;   said first access point sending said first terminal a message accepting said request for reservation of resources for said new data stream on said first network; and   setting up said call between said first terminal and said second terminal;
 
and if at least one possibility of adding said new data stream to the first or second network is not established, at least one of the following steps is executed:
   
               
           for any other available access point of said first network different from said first access point, iteration of said steps 1 and 2 mentioned above; and   handing over said outgoing call to a mobile communications network.       

     With an incoming call request coming from said second communications terminal to set up a call with said first communications terminal, the following steps are preferably executed:
         step A: said second terminal sending said interface device a message requesting reservation of resources for said new data stream on said second network;   step B: said interface device verifying a possibility of adding said new stream to said second network, so that if said possibility is established, the following additional steps are executed:
           said interface device sending at least one first available access point to which said first terminal is connected a message requesting reservation of resources for said new data stream on said first network;   said available first access point verifying a possibility of adding said new stream to said first network, so that if that possibility is proved, the following additional steps are executed:
               addition of said new data stream to a management table of the streams of said first network;   said available access point sending said interface device a message accepting said request for said new stream on said first network;   adding said new data stream to a management table of the data streams of said second network; and   setting up said call between said first terminal and said second terminal;
 
and if at least one possibility of adding said new data stream to the first or second network is not established, at least one of the following steps is executed:
   
               
           for any other access point of said first network, available and different from said first access point, iteration of said steps A and B mentioned above; and   handing over said outgoing call request to a mobile communications network.       

     Said new data stream is preferably a UMA voice type stream. 
     The invention also consists in a system of communication between at least one first communications terminal of a first communications network and at least one second communications terminal of a second communications terminal, said first network and said second network being interconnected via an interface device, characterized in that it includes:
         means for dynamically calculating a maximum bit rate of data streams in transit between said first and second terminals taking account of at least one bit rate observed in the first communications network and/or the second communications network; and   means for processing a request by said first and second terminals for admission of a new data stream, said interface device being adapted to execute said processing taking account of the result produced by said calculation means.       

     In the system of the invention, said first communications terminal preferably conforms to the UMA standard. 
     The invention also consists in a computer program product downloadable from a communications network and/or stored on a computer-readable information medium and/or adapted to be executed by a microprocessor, said program including code instructions for executing the above communications method. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The features and technical advantages of the present invention emerge more clearly from the following description, given by way of non-limiting illustration and with reference to the appended drawings, in which: 
         FIG. 1  is a diagrammatic view of an architecture in which a communications method and system of the invention are used; 
         FIG. 2  is a flowchart of one implementation of the communications method of the invention, in relation to an incoming call; and 
         FIG. 3  is a flowchart of one implementation of the communications method of the invention, in relation to an outgoing call. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention proposes a solution which, in a UMA network architecture, when a first terminal of a first communications network, for example an IP private mobile wireless local area network (WLAN), is seeking to set up a new call to a second terminal of a second communications network, for example a wide area network (WAN), evaluates the end-to-end availability of the resources available in the first and second communications networks. Accordingly, as a function of the result of this evaluation, if either or both of the two networks does not have resources available, particularly in terms of bandwidth, to carry the data stream (for example a voice data stream) necessary to support the new call, then the call is handed over to the mobile network of an operator. 
       FIG. 1  illustrates a UMA network architecture in which the invention can be used. This architecture includes a first part  10  corresponding to a wireless local area network (WLAN)  10 , for example an IP private mobile local area network, and a second part  11  corresponding to the core network part of a wide area network (WAN) of an operator. A hardware interface device  12  extends communications services between the WLAN  10  and the WAN  11 , in particular for calls set up between a first radio communications terminal  13  of the WLAN and a second remote communications terminal  14  reached via at least one collection and/or routing unit  18  and/or at least one secure gateway  110  of the WAN. 
     The WLAN  10  comprises access points  15   1 ,  15   2 ,  15   3 ,  15   4  connected to the interface device  12  via an Ethernet switch  16 , all the access points, including the interface device  12 , defining a coverage area  19  (or business zone) for the IP mobile local area network  10  within that the communications terminals  13  are recognized. 
     The terminals  13  conform to the UMA standard (Technical Specification Group GSM/EDGE Radio Access Network; Generic Access to the A/Gb interface; Stage 2—version 6—January 2006) and the business zone  19  conforms to the WMM™ standard (WMM™—Version 1.1—Wi-Fi Alliance Technical Committee Quality of Service Task Group). 
     The business zone  19  includes a DHCP server on the interface device  12 . The WAN interface of the interface device  12  executes the network address translation (NAT) function ( 1  to N). 
     The source IP address of an IPsec (secure IP) packet sent by a UMA terminal  13  to a secure gateway  110  of the WAN  11  comes from the IP address range administered by the DHCP server of the business zone  19 . 
     A conversational virtual channel (VC)  17  is set up in the WAN part  11  of the UMA architecture between the interface device  12  and a collection and routing unit  18  of the WAN  11 , said unit forming an access point of the multiservice point of presence (PoP), including the Voice over IP service. 
     Setting up a secure connection between a terminal  13  and the secure gateway  110  requires (i) detecting the presence of the NAT function in the WAN interface of the interface device, and (ii) authenticating the two ends of the connection. The secure connection is set up under the IKEv2 protocol. 
     The steps of setting up a secure connection between the terminal  13  and the secure gateway  110  of the WAN are not described in detail here. 
     Referring to the  FIG. 1  architecture, the maximum number of UMA voice streams that can be in transit end-to-end on the first communications network  10  and the second communications network  11  substantially corresponds to whichever is the lower of (i) the maximum number of UMA voice streams that the conversational VC  17  can transmit, and (ii) the maximum number of UMA voice streams that the business zone  19  can handle. 
     The conversational VC  17  transmits fixed telephony streams, UMA voice streams, secure connection set-up and maintenance streams, and other UMA data streams in transit on the part  12  of the WAN. 
     At a time  t , the maximum number of UMA voice streams that the conversational VC can transmit is called the UMA VC CAC. Because it is bidirectional, it is calculated for the uplink or downlink direction. 
     Furthermore, each mobile local area network, for example each Wi-Fi network, present in the business zone  19  can transmit UMA voice streams, secure connection set-up and maintenance streams, and various other UMA and/or data streams. 
     At the same time  t , the maximum number of UMA voice streams that a Wi-Fi network of the business zone  19  can transmit is called the UMA WLAN CAC. Because it is also bidirectional, is also calculated for the uplink or downlink direction. 
     The maximum number of UMA voice streams in a UMA architecture, such as that represented in  FIG. 1 , therefore depends on the UMA VC CAC and the UMA WLAN CAC. 
     a) Calculation of the UMA VC CAC 
     Calculating the UMA VC CAC requires (i) the bit rate of the conversational VC, and (ii) the bit rate of a UMA voice stream. The bit rates are calculated at the ATM (Asynchronous Transfer Mode) level. 
     At time  t , the maximum number of streams transmitted on the conversational VC  17  is such that:
 
(Bit rate of conversational VC)( t )=(Bit rate of fixed telephony streams)( t )+(Bit rate of UMA voice streams)( t )+(Bit rate of UMA data streams)( t )+(Bit rate of IPsec tunnel set-up streams)( t )+(IPsec tunnel maintenance bit rate)( t ).
 
     At time  t , the maximum number of UMA streams is such that:
 
(Bit rate of UMA streams)( t )=(Bit rate of conversational VC)( t )−(Bit rate of fixed telephony stream)( t ),for simplicity denoted  Dvc−Dtoip ( t ).
 
     At time t k , the conversational VC  17  is transmitting (k−1) UMA voice streams. 
     Accordingly, the possibility of adding UMA voice stream No. k therefore makes it necessary to know that maximum bit rate this stream  k  will occupy on the conversational VC  17  on the WAN side. 
     The bit rate at the ATM level of the UMA voice stream No. k, denoted “bit rate of UMA voice stream  k ”, is equal to the number of packets per second sent by the codec multiplied by the length of a packet at the ATM level, the codec of the UMA voice stream No. k being itself characterized by the following two parameters:
         R k : number of packets per second sent by codec;   (codec bit rate) k : maximum codec bit rate in bps.       

     There is obtained in this way the following formula for calculating the maximum bit rate of the UMA voice stream No. k, denoted (bit rate of UMA voice stream No. k) max , if the codec performs no voice activity detection (VAD) function, i.e. when (bit rate of codec(t)) k =(bit rate of codec) k :
 
(bit rate of UMA voice stream No.  k ) max   =R *((bit rate of codec) k   /R   k +length(header( PLCP - MAC - CCMP - LLC -IPsec-IP))+length(acknowledgement frame)+ SIFS*Dwlan )+( Rk ( t )−1)* SIFS*Dwlan +(TIME_FOR_THE_CLASS_TO_TRANSMIT) k   *Dwlan;  
 
where:
         “header(PLCP-MAC-CCMP-LLC-IPsec-IP)” denotes bits that are not part of the VoIP field;   Dwlan is the bit rate on the WLAN;   according to the paragraph “Obtaining an EDCA TXOP” of the WMM™ specification (WMM™—Version 1.1, Wi-Fi Alliance Technical Committee Quality of Service Task Group), authorization to transmit a new stream corresponds to the time that the class AC_VO that corresponds to the voice access category must wait before it can transmit its PLCP frames over the Wi-Fi network. This time is denoted (TIME_FOR_THE_CLASS_TO_TRANSMIT) k .       

     The characteristics of the codec of a terminal are required to calculate the maximum bit rate of a UMA voice stream. 
     As recommended by the 3GPP™ consortium, the values of the bit rate and the number of packets per second sent by the AMR (Adaptive Multi-Rate) codec (cf. “Technical Specification Group GSM/EDGE Radio Access Network; Generic Access to the A/Gb interface; step 2, Version 6, January 2006) used in third-generation mobile telephones can be considered here in the above equation. 
     b) Addition of Stream No. k to Conversational VC  17   
     It is assumed that UMA voice stream No.  i  is transmitted on the conversational VC  17  from a time t i . 
     The bit rate of the UMA voice stream No.  i , measured between the times t i  and t k , is then equal to:
 
(Bit rate of UMA voice stream No.  i ) measured in [ti,tk] =(number of bits of UMA voice stream No.  i ) measured in [ti,tk] /( t   1   −t   i ).
 
     The summation of the (k−1) streams is denoted Σ i =SIGMA(i=1, i=k−1). 
     At time t k  the addition of the (uniquely identified) UMA voice stream No. k to the conversational VC  17  is possible if the following equation EQ1 is satisfied:
 
Σ i (Bit rate of UMA voice stream No.  i ) measured uplink in [ti,tk] +Σ i (bit rate of UMA voice stream No.  i ) measured downlink in [ti,tk] +2*(Bit rate of UMA voice stream No.  i ) max &lt;( Dvc−Dtoip ( t )) uplink +( Dvc−Dtoip ( t )) downlink .
 
c) Calculation of UMA WLAN CAC
 
     Calculating the UMA WLAN CAC requires (i) the bit rate of the private mobile network, which is a Wi-Fi network in the embodiment described here, and (ii) the bit rate of a UMA voice stream. The bit rates are therefore calculated at the radio level. 
     The business zone  19  has one or more networks defined by their respective access points  15   1 ,  15   2 ,  15   3 ,  15   4  according to the 802.11b/g standard. 
     The nominal bit rate of a Wi-Fi network is denoted Dwlan below. 
     The bit rate of a Wi-Fi network at time  t  is denoted D(t) and the maximum number of streams transmitted at this time  t  in the Wi-Fi network is calculated as follows:
 
 D ( t )=(Bit rate of UMA voice streams)( t )+(Bit rate of data UMA steams)( t )+(Bit rate of IPsec tunnel set-up streams)( t )+(IPsec tunnel maintenance bit rate)( t )+(Bit rate of diverse streams)( t ).
 
     At time  t , the maximum number of UMA streams is therefore such that:
 
(Bit rate of UMA steams)( t )= D ( t ).
 
     At time t k , corresponding to an attempt to set up a new connection, the Wi-Fi network is already transmitting (k−1) UMA voice streams and the possibility of adding the UMA voice stream No. k therefore requires the maximum bit rate that this stream would occupy on the Wi-Fi network. 
     d) Calculation of Maximum Bit Rate of UMA Voice Stream No. k 
     The bit rate at the radio level of the UMA voice stream No. k, denoted “bit rate of UMA voice stream No. k”, is equal to the number of packets per second sent by the codec multiplied by the length of a packet at the radio level. Sending a Wi-Fi frame containing a Voice over IP (VoIP) sample is equivalent to sending a virtual Wi-Fi frame containing that sample, which implies that at the radio level the bit rate of UMA voice stream No. k is equal to:
 
(bit rate of UMA voice stream No.  k )( t )= R   k ( t )*(length of Wi-Fi frame No.  k )+length(frame acknowledgement)+ t   sifs   *Dwlan );
         where t sifs  is the duration of the short inter-frame space.
 
e) Addition of UMA Voice Stream No. k to Wi-Fi Network
       

     It is assumed that the UMA voice stream No. i is transmitted over the Wi-Fi network from time t i . 
     At time t k  adding UMA voice stream No. k to the WLAN-Wi-Fi network  11  is therefore possible if the following equation EQ2 is satisfied:
 
Σ i (Bit rate of UMA voice stream No.  i ) measured uplink in [ti,tk] +Σ i (bit rate of UMA voice stream No.  i ) measured downlink in [ti,tk] +(Bit rate of uplink UMA voice stream No.  k ) max +(Bit rate of downlink UMA voice stream No.  k ) max   &lt;D ( tk ).
 
f) Setting Up New Call Between a Terminal  13  of the WLAN  10  and a Recognized Terminal  14  of the WAN  11 
 
     Adding UMA voice stream No. k to the conversational virtual channel  17  is then possible if the equations EQ1 and EQ2 are satisfied at time t k  corresponding to a request to set up a new voice call between a communications terminal  13  of the local wireless area network  10  (for example a Wi-Fi network) and a communications terminal  14  recognized by the WAN  11 . There are therefore two scenarios for adding a stream, according to whether the call request is an outgoing call request or an incoming call request. 
     If either or both of the equations EQ1 and EQ2 is not satisfied, it is not possible to add a UMA voice stream No. k to support a new call (independent incoming or outgoing call request) the setting up of which has been requested at time t k  without significantly degrading the quality of service of the VoIP services rendered to the users of the  FIG. 1  UMA architecture who are already communicating. 
     In this situation, the call being set up is handed over to the mobile network of an operator. 
     Incoming Call Request, Initiated from a Communications Terminal  14 , Going to a Communications Terminal  13   
     As shown in  FIG. 2 , if the interface unit  12  receives over its WAN interface a UMA CAC request message concerning the conversational virtual channel  17 , it verifies the above-mentioned equation EQ1 (step S 1 ). 
     If the equation EQ1 is not satisfied, the interface unit  12  sends a UMA CAC response concerning the conversational virtual channel  17 , the Status Code field of which message is set to Deny (refusal to add a new UMA voice stream), to the access point  15   1 , in the coverage area of which is located a terminal  13  with which a terminal  14  is seeking to communicate (step S 2 ). Consequently, UMA voice stream No. k is then not transmitted via the LAN interface of the interface device  12 . 
     The call must then be set up using the GSM network of an operator, not the UMA architecture (step S 3 ). 
     In the converse situation, if the equation EQ1 is satisfied, the interface unit  12  adds UMA voice stream No. k to a management and reference table for UMA voice streams in transit on the conversational virtual channel  17  and then sends a UMA CAC response message concerning the conversational virtual channel  17 , the Status Code field of which is set to Accept (addition of field accepted) to the access point (AP)  15   1 , in the coverage area of which is located a terminal  13  with which a terminal  14  is seeking to communicate (step S 4 ). 
     If the AP  15   1  receives a UMA CAC response message concerning the conversational virtual channel  17 , it verifies the value of the Status Code field. 
     If that field is set to Deny, the AP  15   1  sends the terminal  13  an ADDTS response message conforming to the WMM standard in which the Status Code field is set to Deny (step S 5 ). 
     Conversely, if the field Status Code is set to Accept, the AP  15   1  verifies the equation EQ2. 
     If the equation EQ2 is satisfied, the access point  15   1  adds UMA voice stream No. k to the table of UMA voice streams of the Wi-Fi network  10  and then sends the terminal  13  an ADDTS response message conforming to the WMM standard having the Status Code field set to Accept (step S 6 ). 
     The call  100  between the communications terminal  13  of the WLAN  10  and the communications terminal  14  attached to the WAN  11  is then set up end-to-end via the UMA architecture of  FIG. 1 . 
     In the converse situation, if a communications terminal  13  receives an ADDTS response message conforming to the UMA specification and having its Status Code field set to Deny, then it can (i) search for another available AP among the APs  15   2  to  15   4  or (ii) initiate automatic handover to the GSM network of an operator (step S 3 ). 
     Outgoing Call Request, Initiated from a Communications Terminal  13 , Going to a Communications Terminal  14   
     As shown in the  FIG. 3  flowchart, at time t k  corresponding to the initiation of an outgoing call request by a terminal  13  of the WLAN, with a view to setting up a new call to a recognized remote terminal  14  of a WAN  11 , conforming to the WMM™ specification, a terminal sends a message containing an ADDTS request to reserve resources on the Wi-Fi network to which the terminal  13  is attached (step E 1 ) for UMA voice stream No. k and intended to support the new call. 
     On reception of this message, the AP  15   1  must verify the equation EQ2 (step E 2 ). 
     If the table of codecs of the terminals administered by the AP  15   1  does not contain the parameters of the codec of the terminal  13  necessary for verifying the equation EQ2, the AP  15   1  sends a message requesting recovery of the value of the codec corresponding to the terminal  13  to the interface unit  12  (step E 3 ). Said unit then sends the AP  15   1  a response message containing the required codec value, integrated into the table of codecs of the AP  15   1 . 
     As indicated above, the bit rates and numbers of packets per second sent by the AMR codec are preferably used in the equation EQ2 to be verified. 
     If the equation EQ2 is not satisfied, the AP  15   1  sends the communications terminal  13  an ADDTS response message conforming to the WMM™ specification with a Status Code field set to Deny (step E 4 ). 
     In contrast, if the equation EQ2 is satisfied, the AP  15   1  sends the interface unit  12  a UMA CAC request message concerning the conversational virtual channel  17  (step E 5 ). 
     When the unit  12  receives said message, it verifies the equation EQ1 (step E 6 ). 
     If the equation EQ1 is not satisfied, the interface unit  12  sends the AP  15   1  a UMA CAC response message concerning the virtual channel  17  with a Status Code field set to Deny (step E 7 ). 
     In contrast, if the equation EQ1 is satisfied, the unit  12  adds UMA voice stream No. k to its UMA voice stream table relating to UMA streams in transit on the virtual channel  17  and then sends the AP  15   1  a UMA CAC response message having the Status Code field set to Accept (step E 8 ). 
     When the AP  15   1  receives a UMA CAC response message concerning the virtual channel  17  with the Status Code field set to Deny, it sends the terminal  13  an ADDTS response message conforming to the WMM™ specification with a Status Code field set to Deny (step E 4 ). 
     If the Status Code field of said message is set to Accept, the AP  15   1  adds UMA voice stream No. k to the UMA voice streams table of the Wi-Fi network, and then sends the terminal an ADDTS response message conforming to the WMM™ specification having the status Code field set to Accept (step E 9 ). 
     The call  100  is then set up via the  FIG. 1  UMA architecture. 
     In contrast, if a UMA terminal  13  receives an ADDTS response message with a Status Code field set to Deny, then it can either (i) search for another AP among the APs  15   2 ,  15   3  or  15   4  if the first AP  15   1  was not available or (ii) initiate automatic handover to the GSM network of an operator (step E 10 ).