Abstract:
The invention provides an apparatus for transmitting calls generated by a first communication network over a second communication network to a third communication network, the first communication network operating in accordance with a first communication protocol which enables call services, the third communication network operating in accordance with a communication protocol which enables at least one of the call services enabled by the first communication protocol and the second communication network operating according to a second communication protocol different to the first. The apparatus includes means to produce a data message compatible with the second communication protocol indicative of the required service and means to transmit the call and data message over the second communication network to the third communication network, where the data message is read by further apparatus and an appropriate call service established to the third communication network. By providing means to produce a data message compatible with the second protocol to carry the call service information it is possible to establish a call service over a communications network even though that network does not explicitly support the call service.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation (under 35 USC §120/365) of PCT/GB95/01639 filed on Jul. 12, 1995 designating the U.S. and filed as, in turn, a continuation-in-part (under 35 USC §120/365) of U.S. application Ser. No. 08/313,602, filed Sep. 29, 1994, now U.S. Pat. No. 5,517,564 granted May 14, 1996. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to a communications network arrangement and to a method of processing a virtual private network call in a communications network arrangement. It is particularly applicable to the provision of call services over communication networks. 
     BACKGROUND OF THE INVENTION 
     In today&#39;s commerce, companies and organisations often have departments at widely separated geographical locations. Each geographical location is usually provided with a private branch exchange, PBX. The PBX connects telephones at that site to a number of outgoing lines connected to a public network or private circuit routes. The private circuit routes allow employees of the company to communicate with employees at other sites and to use call services as desired. It has been found desirable for personnel in the different departments to have available to them a range of call services on their telephone networks. These services may include “call-back-when-free”, “call-back-when-next-used”, “divert-on-busy” and “centralised-operator”. 
     The call service “call-back-when-free” is a service which caters for the situation where a first telephone user wished to contact a second telephone user but is unable to do so because the second telephone user is engaged on another call. The first telephone user dials a code number for the “call-back-when-free” call service and replaces his handset. This causes his telephone to ring when the second telephone user finishes his present call and replaces his handset. By lifting his handset, the first telephone user can then cause the second user&#39;s telephone to ring and the call can then be made. 
     The call service “call-back-when-next-used” is a call service which caters for the situation where the first telephone user attempts to contact the second user but although a ringing tone is heard the second user does not answer the telephone. The first user then enters a code for this service and replaces his handset. This causes a “registration” of the request for call-back-when-next-used at the PBX serving the second user&#39;s telephone. The PBX monitors the second user&#39;s telephone for activity. When a call is made by the second user and then cleared from his telephone, the PBX notifies the PBX serving the first user&#39;s telephone. The PBX serving the first user&#39;s telephone then sets up a “no-ring” call which establishes a speech path between the two telephones. Only the first person&#39;s telephone is made to ring. When the first user answers his telephone, the second user&#39;s telephone is made to ring and the call is established. 
     The call service “divert-on-busy” caters for the situation where the first telephone user wishes to contact the second telephone user but that user is engaged. The second telephone user may have programmed his telephone system to divert calls to a colleague&#39;s extension when he is engaged on a call. 
     The call service “centralised-operator” allows one telephone extension of a local telephone network to act as the operator on behalf of other extensions of the entire private telephone network. 
     FIG. 1 shows a conventional communications network topology in which the network comprises four private branch exchanges  1  to  4 , a public switched telephony network  5  and communication paths  6  to  12 . 
     The private branch exchanges (PBX)  1  to  4  serve local telephone networks at various sites of the same company. It will be readily appreciated from the figure that there are a number of possible routes for calls routed from one local telephone network to another along communication paths  6  to  12 . 
     For example, suppose a call originating from a local telephone network served by PBX 2  is to be routed to a local telephone network served by PBX 3 . It may be routed via communication path  8 , the public switched network  5  and communication path  10  to arrive at PBX 3 . Alternatively, the call may be routed via communication path  9  to arrive at PBX 3 . This route is called a private circuit because it does not involve the call being carried over the public switched network  5 . 
     In a similar manner, calls may be routed from PBX 4  to PBX 2  by means of communication path  11 , the public switched network  5 , and communication path  8 . Alternatively, a private circuit may be used, comprising communication path  12 , PBX 3  and communication path  9 , to arrive at PBX 2 . 
     Current PBX designs accept calls from an associated network of telephones at that site governed by a first communication protocol and pass that call to the public network by means of a second protocol. An example of the first protocol is digital private network signalling system (DPNSS) and an example of the second is digital access signalling system No2 (DASS2). DPNSS is a protocol which has been designed to support call services but the DASS 2 protocol does not support call services. 
     Private circuit routes allow the PBXs to communicate using the first protocol DPNSS and hence allow supplementary services. For geographically distant sites, it is usual for the organisation to rent a dedicated communication path from a telephone service provider. In this case the dedicated communication path is path  9 . However, generally speaking, these paths are often underutilised and this is especially the case for primary rate 2.048 Mbit/s paths. This results in the company or organisation having to pay for dedicated communication paths which can carry a greater volume of call traffic than the company generates. 
     International Patent Application WO93/15583 discloses a communications network arrangement comprising a first communications network (“public network”) which operates in accordance with a first signalling protocol, and a plurality of further communications networks (“PABXs”) which are connected to the first communications network and which belong to the same virtual private network. The further communications networks operate in accordance with a second signalling protocol (which may be different for different ones of the further communications networks) which supports a call service which is not supported by the first protocol. Each PABX is provided with an “interworking unit” which sits longside the PABX providing the virtual network to the PABX. All private network calls route through the interworking unit which uses a database to translate the incoming private network number to a public network number and establishes an outgoing call to that public network address across another private interface. This is routed back through the PABX so that the public network interfaces that are required on the PABX can be used. The address provided on the secondary call identifies the call, but need not route the call to the final extension across the public network. The signalling necessary to establish the final part of the call on the other side of the network is sent through an overlay network, for example an X.25 network. Calls are established within the X.25 network as required to support the private network. Thus dedicated private circuit functionality is provided using separate signalling and transmission network bearers. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the invention there is provided a communications network arrangement comprising 
     a first communications network which operates in accordance with a first signalling protocol which supports user to user data message signalling, and 
     a plurality of further communications networks which are connected to the first communications network and which belong to the same virtual private network, said further communications networks operating in accordance with a second signalling protocol which supports a call service which is not supported by the first protocol, 
     each said further communications network being provided with an interface for interfacing between its signalling in accordance with the second protocol and the signalling of the first network in accordance with the first protocol, 
     each interface being arranged to operate as a converter between the first and second protocols for signalling messages to/from its corresponding further communications network which relate to calls which are not virtual private network calls, 
     each interface being furthermore arranged to respond to a request message from its corresponding further communications network for a virtual private network call to a destination served by another one of said further communications networks by originating an auxiliary call across the first network, the auxiliary call providing for user to user data message signalling and constituting a segment of the requested virtual private network call in that it is to the interface corresponding to said another of said further communications networks, 
     each interface being furthermore arranged, each time it is a party to such an auxiliary call, to pass on messages relating to the corresponding requested virtual private network call which are received from its corresponding further network to the other party to the auxiliary call via the user to user data message signalling, and to pass on to its corresponding further network signalling messages relating to the corresponding requested virtual private network call which are received from the other party via the user to user data message signalling. 
     The invention thus enables the establishment of call services across a communication network that need not itself operate according to a communication protocol that provides call services. All that is required is that the protocol allows the transmission of data representing the service required which can then be read by an element of the destination network. Thus, it is not necessary for a dedicated communications network to be provided between two geographically remote sites of the same organisation. A public switched network, for example, can be used to carry call service information in a transparent way. This is less expensive to the user than using private circuits. In accordance with the invention only one protocol is required to carry the call and information required to set up a call service. 
     The signalling protocols may include DPNSS for the second protocol and DASS2 for the protocol operating on the first network. 
     According to a second aspect of the invention there is provided, in a communications network arrangement which comprises 
     a first communications network which operates in accordance with a first signalling protocol which supports user to user data message signalling, and 
     a plurality of further communications networks which are connected to the first communications network and which belong to the same virtual private network, said further communications networks operating in accordance with a second signalling protocol which supports a call service which is not supported by the first signalling protocol, 
     each said further communications network being provided with an interface for interfacing between its signalling in accordance with the second protocol and the signalling of the first network in accordance with the first protocol, 
     a method of processing a virtual private network call from a source served by a first said further communications network to a destination served by a second said further communications network, 
     the method comprising setting up a segment of the call across the first network from the interface corresponding to the first further network to the interface corresponding to the second further network, said segment providing for user to user data message signalling, and 
     passing signalling messages relating to the call between the first and second further networks via the user to user data message signalling. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     A specific embodiment of the invention will now be described, by way of example only, with reference to the drawings in which, 
     FIG. 1 shows in schematic form a prior art communications network of a number of private branch exchanges interlinked by dedicated lines and a public switched network; 
     FIG. 2 shows in schematic form a communications network arrangement in accordance with the invention; 
     FIG. 3 shows a Virtual Private Network Server which is included in the network arrangement of FIG. 2; 
     FIG. 4 shows a memory structure stored in a memory of the VPN server shown in FIG. 3; 
     FIG. 5 shows a User to User Data Message (UUD) compatible with a protocol used in the communications network shown in FIG. 2; and 
     FIGS. 6 and 7 are explanatory diagrams. 
    
    
     DETAILED DESCRIPTION 
     With reference to FIG. 2, a communication network  21  in accordance with the invention comprises a public switched telephony network  22 , four PBXs  23 ,  24 ,  25  and  26  each PBX serving an associated local network (not shown) and three virtual private network servers VPN server  27 , VPN server  28  and VPN server  29 . 
     The communication network  21  also includes a number of communication paths. Communication path  30  links PBX 25  to the VPN server  27  whilst communication path  31  links the VPN server  27  to the PSTN  22 . Similarly, communication path  32  links PBX 26  to VPN server  28  and communication path  33  links VPN server  28  to the PSTN  22 . It will be seen from FIG. 2 that VPN server  29  is linked to two PBXs. Communication path  34  links the VPN server  29  to PBX  23  whilst communication path  35  links the VPN server  29  to PBX  24 . A further communication path  36  links VPN server  29  to PSTN  22 . 
     Each of the local networks includes equipment of a well known type such as telephones, facsimile machines, computers and computer modems and these will not be described in greater detail. 
     The communication paths comprise optical fibre but may comprise other communication cable. 
     The PBXs are Meridian 1 private branch exchanges, made by Northern Telecom, and available from British Telecommunications pic of 81 Newgate Street, London. 
     Each VPN server  27 ,  28 ,  29  is an intelligent switching unit capable of signalling protocol conversion and signalling protocol manipulation in a way which will be later described. The VPN servers  27  and  28  are nominally identical, the structure is as shown schematically in FIG.  3  and it comprises four major components, a processor  40 , a call transceiver  41 , a memory  42 , and a processor instruction means  45 . 
     For VPN server  28 , the call transceiver  41  receives calls on communication path  32  originating from PBX  26  and on path  33  calls arriving by way of the PSTN  22 . Calls are also redirected by the call transceiver  41  onto these paths under the control of the processor  40 . Similarly, for VPN server  27  calls are received by its transceiver  41  on paths  30 ,  31  and calls can also be redirected onto these paths. 
     VPN server  29  differs from VPN servers  27 , 28  only in the connection of a further communication path, that is to say, communication paths  34 ,  35  and  36  are connected to its call transceiver  41 . 
     The call transceiver  41  is connected to the processor  40  by a control and data bus  44 . The processor  40  is able to interpret call information received by the call transceiver  41  and to instruct the call transceiver  41  to retransmit the call onto an appropriate one of the paths. The call information is passed by the call transceiver  41  along the control and databus  44  to the processor  40 . The same control and databus  44  carries the instructions from the processor  40  to call transceiver  41 . 
     The memory  42  stores a database of routing data and corresponding dialled digit strings arranged as a set of look-up tables. The processor  40  can access the data stored in the look-up table by means of a databus  43  in a way that will be later described. The processor instruction means  45  is a data storage area which stores the instructions which the processor  40  follows to perform the required operations. It takes the form of a ROM (read only memory), but it could take the form of a floppy disk, hard disk or other data storage device. 
     FIG. 4 shows a table  46  which is held in the memory  42  of each VPN server  27 ,  28  and  29 . The table  46  comprises a first field  46   a  within which is stored lead digits of digit strings that may be dialled by a telephone user. The lead digits include “9”, “22”, “33”, “34” and “44”. 
     A second field  46   b  contains full bearer call numbers to be associated with the dialled lead digits. Lead digit “9” is associated with null bearer call number. Lead digits “22” are associated with a bearer call number “0642-224694”. Lead digits “33” are associated with a bearer call number “0798-332040”. Lead digits “34” are associated with a bearer call “0798-342041”. Lead digits “44” are associated with a bearer call “0402-440103”. 
     Table  46  is configured as a look-up table such that inputting a lead digit or digits returns a bearer call number. Thus, if the lead digits “33” are input into the table  46  the bearer call number “0798-332040” is returned. 
     As can be seen from FIG. 2, numbers are allocated to parts of the communications network  21  as follows. 
     VPN server  27  is allocated the number “0642 224694” and PBX 25  is allocated the number “224” the extensions supported by the PBX  25  being represented by “XXX” in the figure. 
     VPN server  28  is allocated the number “0402 440103” with the PBX it serves PBX  26  being allocated “440”. Again, extensions being supported by the PBX are indicated as allocated to the number range “XXX”. 
     VPN server  20  is allocated the numbers “0798 332040” and “0798 342041”. The PBX  23  is allocated the number “332” with its extensions being allocated numbers in a range represented by “XXX” in the figure. The PBX  24  is allocated the number “342” with the extensions it supports being allocated numbers in a range represented in the figure by “XXX”. 
     The local networks operate in accordance with a communications protocol called DPNSS-1 (digital, private network signalling system). This protocol is well known to those skilled in the art of telecommunications and it enables a number of call services such as “call-back-when-free”, “call-back-when-next-used”, “divert-on-busy” and “centralised-operator”. 
     The PSTN  22  operates in accordance with a communications protocol called DASS 2 (Digital Access Signalling System No.2). Again, this protocol is well known to those skilled in the art of telecommunications, but it is important to note that it does not support call services. It does, however, allow signalling between two PBX nodes in the form of user to user data messages. 
     The user to user data message has a format as shown in FIG.  5 . It comprises a thirty two byte structure  50 . A first one byte  51  is the MESSAGE_TYPE field of the message. It signifies whether the data message is complete or incomplete. This caters for the situation where a number of data messages are required to carry a particular set of data. A first to a penultimate data message will have a first byte  51  signifying the data message is incomplete and the last data message will have a first byte  51  signifying that the data message is complete. A second byte  52  signifies the length of the data carried in a data field  53 . The data field  53  is allocated thirty bytes of the message. 
     The communications network  21  operates in a manner as shown in FIG. 6 when a call is to be established between extensions served by different PBX&#39;s. In the figure a prefix P1 means that the message is configured according to the first protocol, DPNSS, and a prefix P2 means that the message is configured according to the second protocol DASS2. 
     Let us suppose that a first user is on an extension served by PBX  24  and wishes to call a second user on an extension served by PBX 25 . The first user dials a “9” for an outside line followed by “0642 224XXX” the PSTN number for a direct connection to the extension (direct in the sense that it does not go via a switchboard operator). 
     The dialling of a “9-0642 224XXX” results in a message P 1_CALL ( 9-0642 224XXX) being sent to VPN server  29  from PBX  24 . The VPN server  29  receives this along communication path  35 . The call transceiver  41  passes the message to the processor  40 . The processor  40  inputs the leading digit 9 into the look-up table  46  held in memory  42 . The look-up table returns the bearer call number, which in this case is null. The processor  40  thus instructs the call transceiver  41  to suppress the leading digit “9” and a call message according to the DASS2 protocol, P 2_CALL ( 0642 224XXX), is sent over the PSTN 22  to VPN server  27 . VPN server  27  converts this message to its equivalent in DPNSS P 1_CALL ( 0642 224XXX). 
     PBX 25  then sends an acknowledgement message P 1_ACK to the VPN server 27. VPN server 27 transmits an acknowledge message P   2_ACK over the PSTN22 to VPN server 29. VPN server 29 then sends a P   1_ACK to PBX24.    
     PBX 25  then sends a P 1_ANSWER message which results in VPN server 27 sending a P   2_ANSWER over PSTN22 to VPN server 29. VPN server 29 sends a P   1_ANSWER message to PBX24.    
     Speech communication between the two extensions can then be initiated. 
     FIG. 7 shows the messaging sequence that occurs when a call service is to be utilised over the PSTN 22  between extensions served by different PBXs. Suppose a call is to be made from a first extension served by PBX 25  to a second extension served by PBX 23 , and then is to utilise a call service. Again in this figure the prefix P1 is attached to a message which conforms to the protocol DPNSS and the prefix P2 is attached to a message which conforms to the protocol DASS2. 
     To set up the call, the PBX 25  receives a dialled digit string including a code for a call service from the first extension. PBX 25  then sends a message to VPN server  27 , P 1_CALL (SERVICE+ 332XXX), 332XXX being the second extension served by PBX 23 . SERVICE being a code for the particular call service required. 
     VPN server  27  receives the message and inputs the first two digits “33” of the extension number into its look-up table  46 . This returns the bearer call number 0798-332040. The processor  40  transmits from the call transceiver  41  a message P 2_CALL (UUD+ 0798-332040). The message is transmitted across the PSTN 22  to VPN server  29 . VPN server  29  sends an P 2_ACK acknowledgement message across the PSTN22 to VPN server 27. This is followed by a P   2_ANSWER message.    
     The VPN server  27  then sends the original call message P 1_CALL (SERVICE+ 232XXX) in a UUD in the DASS2 protocol. This is shown in FIG. 7 as the message P 2_UUD (P   1_CALL(SERVICE+ 332XXX)). This travels across the PSTN 22  to the VPN server  29  in a “transparent” way that is to say no number translation or protocol conversion is carried out but rather the original message is carried within the DASS2 compatible UUD. 
     The processor  40  of VPN server  29  then removes from the UUD, the DPNSS message P 1_CALL (SERVICE+ 332XXX). It then transmits the message to PBX 23 . Thus it will now be seen that to PBX 23  it appears that it has received the message directly from PBX 25  that is to say it is a DPNSS call. 
     PBX 23  then returns an acknowledgement message P 1_ACK followed by P   1_ANSWER. These are loaded by the VPN server 29 into UUDs to form P   2_UUD (P   1_ACK) and P   2_UUD (P   1_ANSWER) and they are sent over the PSTN22 to VPN server 27. VPN server 27 then removes the DPNSS messages and sends them to PBX25. Speech can then be initiated.