FR2762738A1 - Connection system for linking together telephone exchange commutators - Google Patents

Abstract

At least one radio transmitter-receiver (9,10) is connected to each of the commutators (5,6), and these are able to communicate with each other by means of a cellular radio communication network (20). The following stages are executed at the establishment of a call from a first terminal (11) to a second terminal (12) by means of the commutators. Firstly a call initialisation message (IAM) is sent from the radio transmitter-receiver (9), comprising as the call number the directory number of the second terminal. Secondly a call establishment request (SETUP) is transmitted over the cellular network, using as the call number the number of the second transmitter-receiver (10), and as a sub-address the directory number of the second terminal (12). Finally when the call establishment request (SETUP) is received by the second transmitter-receiver (10), a call initialisation message (IAM) is sent to the second commutator (6) having as its call number the directory number of the second terminal (12) read in the sub-address field.

Description

INTERCONNECTION METHOD AND EQUIPMENT BETWEEN SWITCHES
USING A CELLULAR RADIO NETWORK
The present invention relates to a method for interconnecting telecommunication switches, as well as to equipment used for implementing this method.

 The invention aims in particular to produce long distance links between remote local switches when there is not, or not enough, wired link capacity between these local switches. These switches will most often be equipment of a public network. But they could also belong to a private installation (PABX).

 In many geographic areas, particularly in developing countries, the infrastructure of the public switched telephone network (PSTN) is insufficient, while cellular radiocommunication networks are deployed or in the process of deployment.

It can be terrestrial cellular networks (PLMN) or satellite (for example GLOBALSTAR). A type of cellular network frequently used obeys the specifications of the GSM system or its derivatives (DCS, PCS ...). In particular, global satellite networks will make it possible to offer telecommunications services in isolated and underserved areas.

 It is known to provide a private branch exchange (PABX) with a radio transceiver, allowing access to terminals connected to this branch exchange from a cellular network. However, this technique has one constraint, which is to require that each terminal thus accessible be subject to a subscription with the cellular operator.

Cellular network protocols have been designed to establish point-to-point connections between a PSTN (or PLMN) subscriber and a PLMN subscriber with a cellular terminal. The addressing techniques have been designed to directly address a PLMN subscriber uniquely identified by their mobile identity and their subscription number. The mobile identity (International Mobile
Subscriber Identity: IMSI) establishes a radio connection with the PLMN subscriber. The subscription number, or directory number (Mobile Subscriber Integrated Services
Directory Number MS: MSISDN), allows the call to be routed to the mobile subscriber. The use of cellular radio network protocols requires associating with each cellular subscriber a radio transceiver and therefore a pair (IMSI, MSISDN).

It is not possible to share the same IMSI among a group of users of more than ten people (this limit is even three people in the case of GSM). A cellular terminal therefore makes it possible to connect a maximum of ten subscribers each having their own MSISDN and sharing the same
IMSI.

 Consequently, to connect together two switches of a PSTN by means of a PLMN, the current solution consists in providing each of these switches with operator type equipment corresponding to a pair (IMSI, MSISDN). A solution of this type is described in WO95 / 06995. Any call from one of the two switches reaches the cellular network, which routes it to the operator of the destination switch. It is this operator who then sets up the call. Communication is therefore not automatic since it requires an operator. A major drawback of this technique is that, if the called subscriber does not answer or if his terminal is busy, a radio traffic resource is however used, so that a billing of the caller occurs for the establishment of the communication with the operator.

 An object of the present invention is to propose a simple and economical method making it possible to connect several switches to each other by means of a cellular network so that the establishment of a communication is automated from end to end and does not entail no caller billing if the called party cannot answer the call.

The invention thus proposes a method of interconnecting switches, in which at least one radio transceiver is connected to a respective trunk line of each of the switches, these radio transceivers being able to communicate with each other by means of a cellular radio network. According to 1 invention, the following steps are performed when establishing a call from a first terminal to a second terminal via a link between a first and a second switch
- route to a radio transceiver connected to the first switch a call initialization message comprising as called number a directory number of the second terminal;
- send on the cellular network a call establishment request comprising as number called a number of a radio transceiver connected to the second switch and as sub-address the directory number of the second terminal; and
- upon receipt of the call establishment request by said radio transceiver connected to the second switch, deliver to the second switch a call initialization message comprising as number called the directory number of the second terminal read in the sub-address field.

 In most cellular networks, in particular in GSM networks, signaling protocols provide in call establishment requests (SETUP messages) a sub-address field allowing the mobile subscriber identified by his MSISDN to connect several types of equipment to its radio transceiver (telephone, fax, modem, etc.). The sub-address allows incoming calls to be routed to either of these devices.

 In the context of the present invention, the sub-address field allows the transceiver connected to the destination switch to transmit to this switch the directory number of the called terminal. The communication will therefore be established by respecting the signaling protocols, and on the condition that the called terminal responds effectively.

 In a particularly simple embodiment of the method, a semi-permanent link is established between a first radio transceiver connected to the first switch and a second radio transceiver connected to the second switch: when the first radio transceiver receives a message of call initialization comprising as number called a directory number of a second terminal, it automatically transmits on the cellular network a call establishment request comprising as number called the number of the second radio transceiver and as subaddress the directory number of the second terminal.

 Thus, all calls from the first switch and routed to the first transceiver are destined for a subscriber whose terminal is connected to the second switch. This mechanism works in both directions.

 This embodiment offers great simplicity of implementation since it suffices to equip each switch with a radio transceiver arranged to perform the required address translation.

In a more sophisticated embodiment, the cellular network is equipped with at least one point-to-multipoint interconnection server. The method then comprises the following steps, after reception by the radio transceiver connected to the first switch of a call initialization message comprising as number called a directory number of a second terminal
- Said radio transceiver connected to the first switch transmits on the cellular network a call establishment request comprising as number called the directory number of the second terminal
a cellular network switching equipment receiving the call establishment request sent by said radio transceiver connected to the first switch interrogates the server which supplies it with a number of a second radio transceiver connected to a second switch by which the second terminal is accessible; and
- said switching equipment performs an address translation to re-issue on the cellular network a call establishment request comprising as number called the number of said second radio transceiver and as sub-address the directory number of the second terminal .

 This solution offers greater flexibility of use since any switch provided with access to the cellular network can then establish on demand a link with any other switch having such access. In return, it requires the creation of a server center and corresponding adaptations in the equipment of the cellular network.

Other features and advantages of the present invention will appear in the description below of nonlimiting exemplary embodiments, with reference to the accompanying drawings, in which
- Figure l is a diagram illustrating a method according to the invention in its point-to-point version; and
- Figure 2 is a diagram illustrating the method according to the invention in its point-to-multipoint version.

 In the description below, the interconnection of public switches supporting the SS7 signaling protocol by means of a cellular radio network (PLMN) of GSM type is considered by way of example.

It will be understood that this example is only illustrative, and that in particular other types of signaling protocols (for example R2) could be used in the PSTN, and that the PLMN could be of a type other than GSM. .

 In the embodiment illustrated in FIG. 1, a semi-permanent link is established between the two switches 5,6 by means of the cellular network 20.

 To this end, each of the two switches has a trunk line 7, 8 connected to a radio transceiver 9, 10. Each of these two transceivers is considered in PLMN 20 as a mobile station, and is notably identified there. by a couple (IMSI, MSISDN).

When the user of a terminal connected directly or indirectly to the switch 5 picks up his handset, after exchange of the line seizure signaling with the switch 5, the latter conventionally forms a message SS7 for initializing a call IAM ("Initial Address
Message "), in which the called number mentioned is, as usual, the directory number of the called terminal 12.

 When the switch 5 detects that the called terminal must be reached through the switch 6, it routes the IAM message to the transceiver 9 connected to its trunk line 7.

 This transceiver 9 responds to the reception of the IAM message by transmitting on the cellular network 20 a call establishment request (SETUP) in accordance with the GSM signaling protocol. In this SETUP request constructed by the transceiver 9, the number field called contains the MSISDN number of the transceiver 10 connected to the switch 6 (the transceiver 9 knows unambiguously this transceiver 10 given that it is arranged to provide a semi-permanent link with it), and the sub-address field contains the directory number of the called terminal 12, which the transceiver 9 has read in the call initialization message I AM.

 On receipt of the SETUP request, the switching equipment MSC ("Mobile Switching Center") of the GSM network 20 interrogates its HLR database ("Home Location Register") to find out the TMST of the transceiver 10, including the MSISDN directory number is listed in the called number field. The IMSI thus obtained is used to establish a signaling radio link with the transceiver 10. The SETUP request sent by the transceiver 9 can then reach the destination transceiver 10, which responds to the reception of this request by retrieving the sub-address and using it as the number called in the IAM call initialization message which it sends to switch 6. This switch 6 then sends the appropriate signaling to the called terminal 12.

 If the holder of the called terminal 12 does not answer the call, the traffic resource will not be allocated to the communication within the PLMN.

If the subscriber accepts the call, switch 6 responds to the loop pickup signal by generating an SS7 message
CONNECT to the transceiver 10. This in turn generates a CONNECT message to the GSM network 20 which retransmits it to the transceiver 9. Once the transceiver 9 has generated an SS7 CONNECT message to the switch 5, communication is established between terminals ll and 12.

 When one of the two parties hangs up, the switch to which the terminal in question is linked sends a SS7 line release message (RELEASE) to its transceiver, which retransmits it to the other transceiver via PLMN 20 Communication is released when this other switch has produced an SS7 RELEASE message to its switch.

 The embodiment illustrated in FIG. 1 is very simple since, to establish a semi-permanent link between two switches 5, 6, it suffices to each provide them with a radio transceiver programmed to perform address translations. predetermined and associated treatments. If a single radio link is not sufficient to absorb the desired traffic capacity, it is of course possible to provide each switch with several mutually associated radio transceivers. If the radio link between the pair of transceivers 9, 10 is unavailable when initiating a call, the switch of the calling terminal will use the transceiver connected to another trunk line which will call its transceiver associated with the level of the other switch.

 The embodiment illustrated in FIG. 2 makes it possible to interconnect more than two switches. FIG. 2 shows by way of illustration three switches 25, 26, 36 of the PSTN each connected to a radio transceiver 29, 30, 40, these transmitters being able to communicate with each other via the PLMN 20.

 In the representation of the cellular network 20 in FIG. 2, an MSC 21 has been shown connected to two base station subsystems (BSS) 22, 23. Each BSS consists of a base station controller (BSC) and d one or more base stations (BTS) managed by this BSC, the BTS being able to communicate by radio with the mobile stations and in particular with the transceivers 29, 30, 40.

In the example shown, the transceiver 29 is within range of a BTS from BSS 22, and the transceivers 30 and 40 are within range of one or more BTS from
BSS 23. Of course, the path taken by the interconnection link within the PLMN 20 could pass through several MSCs.

The PLMN 20 is provided with a server 24 interrogable from the MSCs 21. This server 24 comprises tables which associate directory numbers of PSTN terminals with switches provided with radio transceivers and by which these terminals are accessible , these switches being in turn associated with the MSISDN numbers of their respective radio transceivers. Thus, the list of directory numbers callable via one of the switches 25, 26, 36 can be associated with the number
MSISDN of a radio transceiver 29, 30, 40 connected to this switch. It is by interrogating these tables that the MSC 21 can perform address translation in accordance with the invention.

 The list of numbers associated with a switch 25, 26, 36 can be an explicit list of directory numbers, or can simply correspond to the numbers starting with a given prefix identifying the coverage area of the switch or more generally a subset of the PSTN.

 When the user of a terminal 11 connected directly or indirectly to one of the switches, for example 25, picks up his handset, after exchange of the line seizure signaling with the switch 25, the latter forms a message SS7 d IAM call initialization in which the number called is conventionally the directory number of the called terminal 12.

 The switch 25 routes this IAM message to the transceiver 29. On receipt of this message, the transceiver 29 establishes a signaling radio link with the GSM network 20 and sends a SETUP call establishment request comprising as called number the directory number of the called terminal 12 read in the IAM message.

 The transceiver 29 can optionally insert at the end of the address field a specific character indicating to the PLMN equipment 20 that an interrogation of the server 24 and an address translation will be necessary.

 On receipt of this SETUP request, the MSC 21 connected to the BSS 22 with which the transceiver 29 communicates interrogates the server 24 by indicating the directory number of the called terminal 12.

 The server 24 then returns to the MSC 21 a new address consisting of a called number equal to the MSISDN number of a transceiver 30 connected to the switch 26 by which the called terminal 12 is accessible and a subaddress equal to the directory number of the terminal 12.

 The MSC 21 then interrogates the HLR database to know the TMST of the transceiver 30, which is used to establish a signaling radio link with this transceiver 30. A SETUP request comprising the composite address supplied by the server 24 is then sent in the GSM network 20 to the transceiver 30.

This retrieves the sub-address and uses it as the number called in the IAM message that it sends to the switch 26. The switch 26 then notifies the called terminal 12 of the arrival of a call. If he picks up, the connection will be established by a message transmission
CONNECT between the two switches via the transceivers 30, 29 and the PLMN 20.

 The release of the call takes place as in the case of FIG. 1.

Claims (6)

 1. Method for interconnecting switches (5,6;  - upon receipt of the call establishment request by said radio transceiver (10; 30) connected to the second switch, deliver to the second switch (6; 26) a call initialization message (IAM) comprising as number called the directory number of the second terminal (12) read in the sub-address field.  - send on the cellular network (20) a call establishment request (SETUP) comprising as number called a number of a radio transceiver (10; 30) connected to the second switch (6; 26) and as below -addresses the directory number of the second terminal; and  - routing to a radio transceiver (9; 29) connected to the first switch (5; 25) a call initialization message (IAM) comprising as number called a directory number of the second terminal; 25,26,36), in which at least one radio transceiver (9,10; 29,30,40) is connected to a respective junction line of each of the switches, these radio transceivers being able to communicate with each other by means of '' a cellular radio communication network (20), characterized by the following steps for establishing a call from a first terminal (11) to a second terminal (12) via a link between a first and a second switches  2. Method according to claim 1, in which a semi-permanent link is established between a first radio transceiver (9) connected to the first switch (5) and a second radio transceiver (10) connected to the second switch (6), and in which, when the first radio transceiver (9) receives a call initialization message comprising as called number a directory number of a second terminal (12), it transmits on the cellular network (20) a call establishment request comprising as number called the number of the second radio transceiver (10) and as sub-address the directory number of the second terminal (12).  3. Method according to claim 1, in which the cellular network (20) is equipped with at least one point-to-multipoint interconnection server (24), and comprising the following steps after reception by the radio transceiver connected to the first switch of a call initialization message comprising as number called a directory number of a second terminal  - said radio transceiver (29) connected to the first switch (25) transmits on the cellular network a call establishment request (SETUP) comprising as number called the directory number of the second terminal  - switching equipment (21) of the cellular network receiving the call establishment request sent by said radio transceiver connected to the first switch interrogates the server (24) which supplies it with a number of a second radio transceiver (30) connected to a second switch (26) by which the second terminal (12) is accessible; and  - said switching equipment (21) performs an address translation to re-transmit on the cellular network (20) a call establishment request comprising as number called the number of said second radio transceiver (30) and as sub- addresses the directory number of the second terminal (12).  4. The method of claim 3, wherein the radio transceiver (29) connected to the first switch (25) inserts in the call establishment request a character signaling to the switching equipment (21) of the network cell that a server query (24) and address translation are required.  5. A radio transceiver (9) for implementing an interconnection method according to claim 2, capable of communicating with other transceivers (10) by means of a cellular radiocommunication network (20 ), the radio transceiver (9) being intended to be connected to a junction line (7) of a switch (5) and arranged to respond to the reception from said junction line of an initialization message call (IAM) comprising as number called a directory number of a terminal by transmitting on the cellular network (20) a call establishment request (SETUP) comprising as number called a number of a transmitter- specific receiver connected to another switch and as sub-address the directory number of said terminal.  6. interconnection server (24) for implementing an interconnection method according to claim 3 or 4, comprising tables which associate with at least one list of terminal directory numbers a respective switch by which said terminals are accessible, this switch (25,26,36) being connected to at least one radio transceiver (29,30,40) capable of communicating with other transceivers by means of a cellular radiocommunication network (20 ), the server (24) being arranged to respond to interrogations originating from a switching equipment (21) of the cellular network and indicating a directory number belonging to said list by returning a call number of a transmitter- radio receiver (30) connected to the associated switch (26), so that the switching equipment (21) of the cellular network can transmit on the cellular network (20) a call establishment request (SETUP) comprising as call number the number of said radio transceiver (30) and as sub-address said directory number from the list.