Method for the transfer of a telephone call from one wireless network to another; and dual-mode mobile telephone terminal for the implementation of the method
The invention concerns a method for the transfer of a telephone call from a wireless telecommunication global network, of the GSM type for example, to a wireless telecommunication local area network, of the Wi-Fi or Bluetooth type for example, and vice versa. It also concerns a dual-mode terminal including means of implementing this method. More specifically, it concerns wireless local area networks which include at least one automatic exchange capable of setting up a telephone conference.
A dual-mode mobile telephony terminal has two radio interfaces, used respectively to communicate via a wireless telecommunication global network, in accordance with the GSM standard for example, and/or via a wireless telecommunication local area network, in accordance with the Wi-Fi standard for example, the latter being connected to a fixed public telecommunication network by a permanent link using the Internet protocol (called an IP link in the remainder of this document), which allows Voice Over Internet Protocol (Voice Over IP, VOIP) type telephone calls.
The global network provides coverage on a national level, but calls are expensive. The local area network offers a more restricted coverage, included in the previous coverage, but voice over Internet protocol (VoIP) type calls are inexpensive. A shopping center or an airport may be equipped with a wireless local area network, accessible to the public.
When a wireless local area network covers the site of a company, or the apartment of an individual, the use of this local area network must be preferred for telephone calls given its low cost. Since the terminal is mobile, the problem arises of transferring a. telephone call between the two networks, without interrupting this telephone call, and without an annoying delay for the users.
A method called UMA (Unlicensed Mobile Access) is known to carry out such a transfer between a global network, of the GSM type or a derivative, and a local area network, of the Wi-Fi or Bluetooth type for example. This known method requires modification of the GSM network, by installing specific servers called UMA controllers which automatically control the transfer of a calf from the global network to the local area network, and vice versa. According to the UMA method, when a mobile telephone terminal is communicating via a GSM base station and it detects the possibility of connecting to a wireless local area network, the UMA controUer orders the transfer of the call to the IP link which connects the local area network to a fixed public network. The UMA controller converts the signal from the mobile telephone terminal, via the IP link, so that it appears to come from another virtual GSM base station. As a result, when a mobile telephone

terminal enters the coverage area of a wireless local area network (where it has access rights), the GSM telephone core network simply considers that this terminal has changed GSM base station, as with a traditional inter-cellular transfer. The call is not therefore cut off while this telephone terminal passes from a GSM radio link to a Wi-Fi or Bluetooth radio link, or vice versa.
The disadvantage of this method is that it requires a modification of the global network concerned, involving installation costs and operating costs which the global network operator passes on to the user.
The aim of the invention is to propose a method which requires no modification to the global network.
A first objective of the invention is a method for the transfer of a telephone call, set up between a dual-mode wireless terminal and another terminal, said dual-mode terminal having simultaneous access to a wireless global network and to a wireless local area network, the latter containing an automatic exchange capable of holding a three party conference, and with a permanent link to the global network; this call being supported firstly by the local area network without passing via the global network, and then having to be supported by the local area network and the global network; characterized in that it involves asking this automatic exchange:
- to set up a second call between the dual-mode terminal and the second
terminal via the local area network and the global network;
- then to set up a telephone conference joining the first and the second call;
- then to release the first call, leaving the second call remaining.
A second objective of the invention is a method for the transfer of a telephone call, set up between a dual-mode wireless terminal and another terminal, said dual-mode terminal having simultaneous access to a wireless global network and to a wireless local area network, the latter containing an automatic exchange capable of holding a three party conference, and with a permanent link to the global network; this call being supported firstly by the local area network and the global network, and then having to be supported by the local area network without passing via the global network; characterized in that it involves asking this automatic exchange:
- to set up a second call between this dual-mode terminal and the second
terminal via the local area network without passing via the global network;
- then to set up a telephone conference joining the first and the second call;
- then to release the first call, leaving the second call remaining.
The invention also has the objective of a dual-mode wireless telephone terminal, including two radio interfaces respectively capable of setting up a first and second call

respectively and simultaneously in a local area and global wireless telecommunication network; characterized in that it also includes control means coupled with the two radio interfaces, capable of controlling the radio interfaces to send messages to an automatic exchange located in the local area network to ask this automatic exchange:
- during the transfer of a first call akeady set up in the local area network without passing via the global network, and which has to be transferred to a global network,
— to set up a second call between the dual-mode terminal and a
second terminal via the local area network and the global network;
— then to set up a telephone conference joining the first and the second call;
— then to release the first call, leaving the second call remaining;
-- during the transfer of a first call, already set up in a global network and a local area network, and which has to be transferred to this local area network without passing via this global network, to send messages to an automatic exchange located in the local area network, to ask this automatic exchange:
— to set up a second call between this dual-mode terminal and a
second terminal via the local area network without passing via the
global network;
— then to set up a telephone conference joining the first and the second call;
— then to release the first call, leaving the second call remaining.
The invention will be better understood and other characteristics will appear with the help of the description below and the accompanying figures:
- Figure 1 illustrates a first stage in the implementation of the method according to the invention.
- Figure 2 illustrates a second stage in the implementation of the method according to the invention.
- Figure 3 illustrates a third stage in the implementation of the method according to the invention.
- Figures 4, 5, 6 illustrate the transfer in the opposite direction.
- Figure 7 shows the block diagram of an embodiment example of the dual-mode terminal according to the invention.
Figure 1 shows a first stage of the implementation of the method according to the invention, in an example in which a Wi-Fi/GSM dual-mode mobile telephone terminal T1 is communicating with a telephone terminal T2 of a fixed public network PSTN, via a

Wi-Fi type wireless local area network, WLAN. The local area network WLAN is connected to the fixed public network PSTN by an IP link with the reference IPL on the figures. The local area network WLAN includes an automatic exchange PBX connected to radio access points AP1, AP2, AP3, by a wireline Ethernet network E; and connected to the fixed public network PSTN by the IPL link. This automatic exchange PBX is capable of switching voice over Internet protocol type calls, inside the local area network WLAN, by supplying'additional services, in particular allowing three party conferences.
At the moment the call is considered to be set up, the terminal T1 is fully within the coverage area of the local area network WLAN. The terminal T1 is also within the coverage area of a wireless global network PLMN, of the GSM type in this example. The call concerned is set up via the local area network WLAN rather than via the global network PLMN, to take advantage of a much better tariff. The network is selected manually by the user, or by a known automatic method. The automatic exchange PBX sets up a connection which includes a branch B1 connecting the automatic exchange PBX to the fixed public network PSTN through the IPL link, and a branch B2 connecting the automatic exchange PBX to the terminal T1 through the radio access point AP2.
Figure 2 illustrates a second stage in the implementation of the method according to the invention. The mobile terminal T1 is moving and approaches the boundary of the radio coverage of the local area network WUN. A procedure for the transfer of the call B1-B2 from the local area network WLAN to the global network PLMN is triggered, either automatically by the terminal T1 when it detects a drop in the amplitude of the signal received from the access point AP2, and it cannot use one of the other radio access points AP1 and AP3; or manually by the user of the terminal T1.
The GSM radio interface of the dual-mode terminal T1 periodically listens to the GSM base stations that it can detect. It knows that it can set up a call via the base station BS in this example. Through its GSM interface, the dual-mode terminal T1 requests the setting up of a second call, sending the switch of the mobile service, MSC, a message containing a called number and a calling number which are both the number of the terminal T1 itself in the public numbering plan which is common to the wireless global network PLMN and to the fixed public network PSTN. This number includes a first field which designates the automatic exchange PBX and a second field which designates the extension constituted by the terminal T1 when it is connected to the local area network WLAN. The switch of the mobile service MSC communicates with the automatic exchange PBX via the fixed network PSTN, a fixed link with the reference FL, connecting the switch of the mobile service MSC to the fixed public network PSTN.

This second call is set up in the traditional manner, in the local area network WLAN, using the method known as automatic incoming selection: the automatic exchange PBX is designated by the first field of the number; ii receives the called number and extracts from it the second field, then sends a call signal to the extension for which the number is contained in this second field. In this case, the automatic exchange PBX knows that this extension is the dual-mode mobile terminal T1, which is associated with the radio access point AP2. The automatic exchange PBX produces a connection between the branch B2 and a new branch B3 which connects the automatic exchange PBX to the global network PLMN.
The mobile terminal T1 therefore receives a call notification, on its Wi-Fi interface, in the traditional manner. It receives signaling data containing the called party number and the caller number. According to the invention, the terminal T1 includes means to:
- analyze this caller number,
- detect whether the caller number is identical to the called party number (its own number),
- and if this is the case: accept a second call, then request that this second call B2-B3 be placed in conference with the first call B1-B2, in the traditional manner for a three party conference (in cases in which the terminal receives a second call on the Wi-Fi radio interface while it is communicating via the GSM radio interface, it sends a signal refusing the second call).
The dual-mode terminal T1 is then in communication with the terminal T2 simultaneously through two telephone calls, B1-B2 and B1-B3, placed in a conference.
Figure 3 illustrates a third stage in the implementation of the method according to the invention. Terminal T1 receives on its Wi-Fi radio interface a signaling message informing it that the automatic exchange PBX has carried out the conferencing that it requested. The terminal T1 then automatically requests that the automatic exchange PBX release the first call. The automatic exchange PBX releases branch B2. It maintains the connection between the branch B3 and the branch B1. The second call B1-B3 replaces the first call without any disruption for the users: There is no interruption since the two calls co-exist for a certain time.
For the users, everything occurs as if the radio segment of the call had been transferred instantly from the local area network WLAN to the global network PLMN. It should be noted that the call continues to pass via the automatic exchange PBX, and therefore the user of the terminal T1 continues to benefit from the services that this automatic exchange can provide.

Conversely, figures 4, 5 and 6 illustrate the transfer in the opposite direction,
in other words from the global network PLMN to the local area network WLAN. These networks are the same as in figures 1 to 3, they will not therefore be described again.
Figure 4 illustrates a first stage of the transfer in the opposite direction. Initially, a dual-mode Wi-Fi/GSM mobile telephone terminal T3 is communicating with a telephone terminal T4 of the fixed public network PSTN, via the wireless global network, PLMN. It should be noted that this call also passes via the automatic exchange PBX of the wireless local area network, WLAN, for the following reasons. This terminal T3 and the local area network WLAN usually belong to a company. The outgoing calls from this automatic exchange PBX which need to use the global network PLMN enjoy a particularly favorable tariff, from the operator of the global network PLMN, due to the high number of calls billed to the company. On the other hand, it is in the interests of the user of the terminal T3 to request that GSM calls be set up using the automatic exchange PBX of the company which uses this automatic exchange, since it then has available additional services provided by this automatic exchange, in particular a company directory.
At the moment the call is set up, the terminal T3 is far from the coverage area of the local area network WLAN. It is however in the coverage area of the wireless global network PLMN. The network is selected manually by the user, or by a known automatic method. The automatic exchange PBX sets up a connection between a branch B4 connecting the automatic exchange PBX to the global network PLMN via the link lPL and the fixed public network PSTN, and a branch B5 connecting the automatic exchange PBX to the terminal T4 using a permanent link IPL.
Figure 5 illustrates a second stage in the implementation of the method according to the invention. The mobile terminal T3 is moving and enters the coverage area of the local area network WLAN. The Wi-Fi interface of the dual-mode terminal T3 periodically listens to Wi-Fi access points. A procedure for the transfer of the call from the local area network WLAN to the global network PLMN is triggered, either automatically by the terminal T3 when it detects that it is in the coverage area of the local area network WLAN, and when it has checked that it has the right to access this network, or manually by the user of terminal T3.
The Wi-Fi interface of the dual-mode terminal T3 is associated with the access point AP3, for example, and is then authenticated by the local area network WLAN, according to a known method. Terminal T3 signals its presence to the automatic exchange PBX. The automatic exchange PBX knows that the terminal T3 is associated with the access point AP3, through a traditional method. It is then able to route a call to this terminal T3, if it is called.

The terminal T3 then asks the automatic exchange PBX to set up a second call, by calling its own number in the numbering plan common to the fixed public network PSTN and the global network PLMN. This number includes a first field which designates the automatic exchange PBX and a second field which designates the extension constituted by the terminal T3 when it is connected to the local area network WLAN.
During this time, the terminal T3 continues to communicate with the terminal T4 via the base station BS, the switch of the mobile service MSC, and the fixed link FL.
The automatic exchange PBX receives the message requesting the setup of a second call. It sets up this second call in an absolutely traditional manner:
- It sets up a new branch B6 which connects the automatic exchange PBX to the access point AP3 since the terminal T3 is associated with this access point AP3;
- it connects this new branch B6 to the branch B5 which goes to the terminal T3 designated by the requested number.
The mobile terminal T3 therefore receives a call notification, on its GSM interface, in the traditional manner. It receives signaling data containing the called party number and the caller number. According to the invention, the terminal T3 includes means to:
- analyze this caller number,
- detect whether the caller number is identical to the called party number (its own number),
- and if this is the case: accept the second call, then request that this second call B4-B5 be placed in conference with the first call B3-B4, in the traditional manner for a three party conference (in cases in which the terminal receives a second call on the GSM radio interface while it is communicating via the Wi-Fi radio interface, it sends a signal refusing the second call).
The dual-mode terminal T3 is then in communication with the terminal T4 simultaneously through two telephone calls, B1-B2 and B1-B3, placed in a conference, via its Wi-Fi interface and its GSM interface simultaneously.
Figure 6 illustrates a third stage in the implementation of the method according to the invention. Once the automatic exchange PBX has indicated to the terminal T3 that it has carried out the conferencing, the terminal T3 orders a release of the first call B4-B5, in the traditional manner. The automatic exchange PBX releases branch B4. It maintains the connection between the branch B5 and the branch B6. The second call B5-B6 replaces the first call B4-B5 without any disruption for the users: There is no interruption since there is a short time in which the'two calls co-exist. For the users, everything occurs as if the radio segment of the call had been transferred instantly from the global network PLMN to the local area network WLAN.

This method has the considerable advantage that it does not require any modification to the equipment of the local area network WLAN and of the global network PLMN.
It should be noted that the terminals T2 and T4 are any terminals of the fixed network PSTN, but that they could be any terminals of the wireless global network PLMN or of a local area network, for example the local area network WLAN itself.
According to one embodiment variant, instead of calling its own number to request the setup of a second call, the dual-mode mobile terminal, T1 or T3, could use a method known as Direct Inward System Access (DISA): The terminal would call a pre¬determined number which would allow it to access the automatic exchange PBX and to run a specific application. In this case, the specific application would set up a second call. The terminal would then ask this application to place the first and the second call in conference, then would ask it to release the first call.
Figure 7 represents the block diagram of an embodiment example of the dual-mode term'ma) according to the invention, for example the block diagram of the terminals T1 and T3 mentioned previously in reference to figures 1-6. This embodiment example includes:
- A traditional radio interface Gl, for communicating with a global mobile network, of the GSM type for example, such as the network PLMN described previously;
- A traditional radio interface Wl, for communicating with a local mobile network, of the Wi-Fi type for example, such as the network WLAN described previously;
- control means CM coupled with the two radio interfaces, Gl and Wl, and consisting of a microprocessor running a program carrying out the following operations:
-- To transfer from a local area network such as WLAN to a global network, such as PLMN, a first call already set up in the local area network without the global network, the control means CM send messages to an automatic exchange, such as PBX, located in the local area network to ask this automatic exchange:
— to set up a second call, such as B1-B3, between the dual-mode
terminal and a second terminal via the local area network WLAN and the
global network PLMN;
— then to set up a telephone conference, such as B1-B2-B3, joining the
first and the second call;
— then to release the first call, leaving the second call B1-B3 remaining.
- To transfer from a global network such as PLMN to a local area network
such as WLAN, without passing via the global network PLMN, a first call,

such as B4-B5, already set up in this global network and this local area network, the control means CM send messages to an automatic exchange, such as PBX, located in the local area network to ask this automatic exchange:
— to set up a second call, such as B5-B6, between this dual-mode
terminal and a second terminal via the local area network without the
global network;
— then to set up a telephone conference, such as B4-B5-B6, joining the first and the second call;
— then to release the first call, leaving the second call remaining.
According to a first embodiment method, the control means CM are capable of controlling the radio interfaces Gl and Wl to send to an automatic exchange of the local area network messages containing a call setup request containing a calling number and a called number which both consist of the number of the dual-mode terminal in the public numbering plan common to the global network PLMN and to the fixed public network PSTN.
According to a second embodiment method, the control means CM are capable of controlling the radio interfaces Gl and Wl to send an automatic exchange messages containing a call setup request containing a specific pre-determined number for this call transfer request.

CLAIMS:
1) Method for the transfer of a telephone call (B1-B2), set up between a dual-
mode wireless terminal (T1) and another terminal (T2), said dual-mode terminal having
simultaneous access to a wireless global network (PLMN) and to a wireless local area
network (WLAN), the latter containing an automatic exchange (PBX) capable of holding a
three party conference, and having a permanent link (IPL) to the global network (PLMN);
this call (B1.B2) being supported firstly by the local area network (WLAN) without passing
via the global network (PLMN), and then having to be supported by the local area network
(WLAN) and the global network (PLMN); characterized in that it involves asking this
automatic exchange (PBX):
- to set up a second call (B3-B2) between the dual-mode terminal (T1) and the second terminal (T2) via the local area network (WLAN) and the global network (PLMN);
- then to set up a telephone conference (B1-B2-B3) joining the first and the second call;
- then to release the first call, leaving the second call (B1-B3) remaining.

2) Method according to claim 1, characterized in that to ask this automatic exchange (PBX) to set up a second call (B3-B2) between the dual-mode terminal (T1) and the second terminal (T2) via the local area network (WLAN) and the global network (PLMN), it involves sending the automatic exchange (PBX) a call setup request containing a calling number and a called number which both consist of the number of the dual-mode terminal (T1) in the numbering plan of the local area network.
3) Method according to claim 1, characterized in that to ask this automatic exchange (PBX) to set up a second call (B3-B2) between the dual-mode termina\ (T1) and the second terminal (T2) via the local area network (WLAN) and the global network (PLMN), it involves sending the automatic exchange (PBX) a call setup request containing a specific pre-determined number to request a call transfer.
4) Method for the transfer of a telephone call (B1-B2), set up between a dual-mode wireless terminal (T1) and another terminal (T2), said dual-mode terminal having simultaneous access to a wireless global network (PLMN) and to a wireless local area network (WLAN), the latter containing an automatic exchange (PBX) capable of holding a three party conference, and having a permanent link (IPL) to the global network (PLMN);

this call (B1.B2) being supported firstly by the local area network (WLAN) and the global network (PLMN), and then having to be supported by the local area network (WLAN) without passing via the global network (PLMN);
characterized in that it involves asking this automatic exchange (PBX):
- to set up a second call (B5-B6) between this dual-mode terminal (T3) and the second terminal (T4) via the local area network (WLAN) without passing via the global network (PLMN);
- then to set up a telephone conference (B4-B5-B6) joining the first and the second call;
- then to release the first call, leaving the second call (B5-B6) remaining.
5) Method according to claim 4, characterized in that to ask this automatic exchange (PBX) to set up a second call (B5-B6) between this dual-mode terminal (T3) and the second terminal (T4) via the local area network (WLAN) without the global network (PLMN), it involves sending this automatic exchange (PBX) a call setup request containing a calling number and a called number which both consist of the number of the dual-mode terminal (T3) in the numbering plan of the first network (WLAN).
6) Method according to claim 4, characterized in that to ask this automatic exchange (PBX) to set up a second call (B5-B6) between this dual-mode terminal (T3) and the second terminal (T4) via the local area network (WLAN) without the global network (PLMN), it involves sending this automatic exchange (PBX) a call setup request containing a specific pre-determined number to request a call transfer.
7) Dual-mode wireless telephone terminal (T1; T3), including two radio interfaces (Wl, Gl) capable respectively of setting up a first and second call respectively and simultaneously in a wireless local area network (WLAN) and telecommunication global network (PLMN); characterized in that it also includes control means (CM) coupled with the two radio interfaces, and capable of controlling the radio interfaces to send messages to an automatic exchange (PBX) located in the local area network to ask this automatic exchange:

-- during the transfer of a first call (B1-B2) already set up in the local area network (WLAN) without passing via the global network (PLMN), and having to be transferred to a global network (PLMN),
— to set up a second call (B1-B3) between the dual-mode terminal and a
second terminal, via the local area network (WLAN) and the global network
(PLMN);
— then to set up a telephone conference (B1-B2-B3) joining the first and the second call;
— then to release the first call (B1-B2), leaving the second call (B1-B3) remaining;
-- during the transfer of a first call (B4-B5), already set up in a global network (PLMN) and a local area network (WLAN), and having to be transferred to this local area network (WLAN) without passing via this global network (PLMN), to send messages to an automatic exchange located in the local area network (WLAN), to ask this automatic exchange:
— to set up a second call (B5-B6) between this dual-mode term'mal and a second terminal, via the local area network (WLAN) without passing via the global network (PLMN);
— then to set up a telephone conference (B4-B5-B6), joining the first and the second call;
— then to release the first call (B4-B5), leaving the second call (B5-B6) remaining.
8) Terminal according to claim 7, characterized in that said control means (CM) are capable of controlling the radio interfaces to send an automatic exchange (PBX) a request to set up a second call, containing a calling number and a called number which are identical to the number of this terminal in a public numbering plan;
and are capable of:
- receiving a call notification containing a calling number and a called number,
- analyzing this calling number,
- detecting whether the calling number is identical to the called number,
- and, if this is the case, accepting the setting up of a second call, then requesting conferencing of this second call with the first call.

9) Terminal according to cfaim 7, characterized in that said control means (CM) are capable of controlling the radio interfaces to send an automatic exchange (PBX) a call setup request containing a specific pre-determined called number to request a call transfer.