Abstract:
This backup SIP server (BSS) comprises:
       means (LMM) for detecting whether an Internet protocol link is not working, and enabling the use of a backup SIP signaling link to the main site via a SIP gateway and a public telephone network when the Internet protocol link is not working;   means for transferring SIP signaling information on this backup link;   means for, when receiving a registration request from a terminal of the remote site while the Internet protocol link is not working, registering this terminal locally and forwarding the registration request to the main site via the backup link;   means (PQM) for storing policies defining what services, supplied by the main SIP server, are compatible with said backup SIP signaling link, and for altering the content of at least one field in each SIP signaling message addressed to the main SIP server before transferring this SIP signaling message on the backup link, this content being altered according to said policies.

Description:
This application is the national phase under 35 U.S.C. §371 of PCT International Application No. PCT/EP2012/051060 which has an International filing date of Jan. 24, 2012, which claims priority to European patent application number EP 11305279.9 filed Mar. 15, 2011; the entire contents of each of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to the enterprise telecommunication networks. An enterprise telecommunication network often links terminals that are spread on a plurality of sites, and today it uses the Internet Protocol (IP) and the Session Initiation Protocol (SIP). Voice samples, data packets, and signaling messages are carried by an IP network that is independent of the public telephone switched network (PSTN), though the enterprise network is also linked to the PSTN by a gateway in order to communicate with the world outside the enterprise. 
     Each SIP terminal runs a SIP User Agent. All the sip user agents are interconnected through an IP network comprising at least one SIP server that acts as a registrar server, a redirect server, a proxy server, and possibly as a presence server, in order to set up telephone communications, and also provide numerous telephony services. For instance: message waiting indications, presence state indications, conference membership notifications, call statistics, messaging . . . . Generally an enterprise network comprises a single SIP server, located in a main site of the enterprise, and the terminals of the remote sites communicate with the SIP server of the main site via an IP network. 
     In case of failure of this IP network, part or totality of the services previously provided by a SIP server in the main site is no longer available in the remote sites. 
     2. Description of the Prior Art 
     Some solutions are known:
         Calls may be rerouted to the PSTN but none out of band service information is conveyed through this way.   Some local proxy servers may offer specific services in case of loss of the link with the SIP server of the main site, but they do not exchange any information with it as long as the IP network is down; so, all the services hosted in the SIP server of the main site are no longer available in the remote site.   Global IP routing through the PSTN, by means of a fully backed up infrastructure. The cost of deployment and the cost of the required bandwidth are very high.       

     The aim of the present invention is to provide at least some of these telephony services to the SIP terminals located on remote sites, when the IP link to the SIP server of the main site is down, at a reasonable cost. 
     SUMMARY OF THE INVENTION 
     A first object of the invention is a backup SIP server for the survivability of an enterprise network using session initiation protocol, this network comprising a main site and at least one remote site, the main site comprising a main SIP server and the remote site comprising said backup SIP server, these two sites exchanging SIP signaling messages via an Internet protocol link through an Internet protocol network; 
     characterized in that it comprises:
         means for detecting whether the Internet protocol link is not working, and enabling the use of a backup SIP signaling link to the main site via a SIP gateway and a public telephone network when the Internet protocol link is not working;   means for transferring SIP signaling information on this backup link;   means for, when receiving a registration request from a terminal of the remote site while the Internet protocol link is not working, registering this terminal locally and forwarding the registration request to the main site via the backup link;   means for storing policies defining what services, supplied by the main SIP server, are compatible with said backup SIP signaling link, and for altering the content of at least one field in each SIP signaling message addressed to the main SIP server before transferring this SIP signaling message on the backup link, this content being altered according to said policies.       

     Thanks to the means for transferring SIP signalling information on this backup link, the main SIP server can continue to provide services to the terminals of the remote site because this SIP signalling information enables the main SIP server to continue to provide some of the telephony services to the SIP terminals located on remote sites, when the IP link to the SIP server of the main site is down. 
     Thanks to the means for storing policies and for altering the content of at least one field in each SIP signalling message addressed to the main SIP server according to said policies, it is possible to continue to provide services by means of a cheap backup Link, because these means allow to discriminate the services that are compatible with the backup link and those that are not compatible. For instance, it is possible to use a backup link constituted of one or a few voice channels of the public switched telephone network by allowing only services for which the signalling can be carried by such a low bandwidth link. 
     Other features and advantages of the present invention will become more apparent from the following detailed description of embodiments of the present invention, when taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order to illustrate in detail features and advantages of embodiments of the present invention, the following description will be with reference to the accompanying drawings. If possible, like or similar reference numerals designate the same or similar components throughout the figures thereof and description, in which: 
         FIG. 1  is a block diagram showing an exemplary enterprise telecommunication network, on a main site and a remote site, comprising a backup SIP server according to the invention. 
         FIG. 2  illustrates the transmission of a REGISTER message from the SIP user agent of a terminal located in the remote site to the main SIP server located in the main site, for registering the terminal in the main SIP server. 
         FIG. 3  illustrates the transmission of a SUBSCRIBE message from the SIP user agent of a terminal located in the remote site to the main SIP server located in the main site, for subscribing to message waiting indication. 
         FIG. 4  illustrates the transmission of a NOTIFY message from the main SIP server located in the main site to the SIP user agent of a terminal Located in the remote site, for notifying the terminal that a message is waiting. 
         FIG. 5  illustrates the transmission of a SUBSCRIBE message from the main SIP server located in the main site to the SIP user agent of a terminal located in the remote site, for subscribing to call statistics publication. 
         FIG. 6  illustrates the transmission of a PUBLISH message from the SIP user agent of a terminal located in the remote site to the main SIP server located in the main site, for notifying call statistics to the latter. 
         FIG. 7  is a block diagram showing an embodiment of the backup SIP server according to the invention. 
         FIG. 8  illustrates the operations made by a link monitor manager in this embodiment. 
         FIG. 9  illustrates with more details one of the operations made by the link monitor manager in this embodiment. 
         FIG. 10  illustrates, with more details, the operations of a forward registration manager in this embodiment. 
         FIG. 11  illustrates, with more details, the operations of a policy manager in this embodiment. 
         FIG. 12  illustrates the operations of a classical SIP terminal when it collaborates with an embodiment of the backup SIP server according to the invention. 
         FIG. 13  is a block diagram showing an embodiment of the gateway according to the invention. 
         FIG. 14  illustrates operations made by this embodiment of the gateway according to the invention. 
         FIGS. 15 and 16  illustrate other operations made by this embodiment of the gateway according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The exemplary enterprise network represented on  FIG. 1  comprises, on a main site:
         SIP terminals such as T 2 ,   Time Division Multiplexing (TDM) terminals such as T 4 ,   a main SIP server MSS,   a router R 2 ,   a SIP gateway SIPGW 2  according to the invention;
 
and on a remote site:
   SIP terminals such as T 1 ,   Time Division Multiplexing (TDM) terminals such as T 3 ,   a backup SIP server BSS, according to the invention,   a router R 1 ,   a SIP gateway SIPGW 1  according to the invention.       

     All the network elements of the main site are linked to a local area network LAN 2 , except the TDM terminals that are linked to the gateway SIPGW 2 . All the network elements of the remote site are linked to a local area network LAN 1 , except the TDM terminals that are linked to the gateway SIPGW 1 . 
     The routers R 1  and R 2  are linked by an IP link IPL through an IP network WAN. The SIP gateways SIPGW 1  and SIPGW 2  are linked by a public switched telephone network PSTN that may be analog or digital. In other embodiments, it may be replaced by a public mobile network. 
     The SIP gateways SIPGW 1  and SIPGW 2  are used for calls between TDM terminals and IP or TDM terminals of the enterprise network, and for calls between terminals of the enterprise network and terminals of the network PSTN. In addition, they are used for exchanging SIP signaling messages via the public network PSTN in case the IP link IPL through an IP network WAN the IP network WAN does not work. 
     Normal Mode: 
     When the IP network WAN is working, the backup SIP server BSS is on standby. It periodically checks that the IP link IPL is working. The main SIP server MSS acts as a registrar server, a redirect server, and a proxy server, for the users of all the terminals of both sites, for setting up the calls to and from all the terminals of both sites. For instance, when the SIP terminal T 1  sets up a session with one of the terminals T 2 , T 3 , or T 4 , its sends SIP signaling message that are forwarded to the main SIP server MSS via the local area network LAN 1 , the router R 1 , the IP network WAN, the router R 2 , and the local area network LAN 2 . Then the main SIP server MSS forwards the messages to the destination terminal via the local area network LAN 2 . 
     Backup Mode: 
     When the SIP signaling link IPL, through the network WAN, does not work, the backup SIP server BSS or one of the SIP terminals of the remote site, T 1  for instance, detects the failure of the link IPL. Then the backup SIP server BSS orders the setup of a backup SIP signaling link BL over the public switched telephone network PSTN (It may be on an analog or digital trunk, according to the configuration of the public switched telephone network PSTN). In other embodiments, the backup link BL may be setup via a mobile network, for instance a third generation mobile network. It may also be a permanent link, for a quicker reactivity in case of interruption of the IP link IPL. 
     The backup SIP server BSS becomes fully activated to act as a registrar server, a redirect server, and a proxy server, for the users of alt the terminals of the remote site. So it accepts the registration of all the terminals of the remote site. This local registration enables the terminals of the remote site to set up local calls between them. 
     In addition, the backup SIP server BSS forwards their registration requests to the main SIP server MSS, via the backup SIP signaling Link BL, in order to keep the main SIP server MSS updated. This registration in the main site enables the terminals of the remote site to set up calls to terminals of the main site, and anywhere else, via the public switched telephone network PSTN. 
     In addition, the backup SIP signaling link enables the main SIP server MSS to continue to provide at least a part of the services it usually provides to the terminals of the remote site. The backup SIP server BSS can transfer, on the backup SIP signalling Link BL, various kinds of SIP signalling information. However it cannot be real time signalling or high bandwidth signalling since the backup SIP signalling link BL is carried by one or a few voice channels of the public switched telecom network PSTN. 
     According to the invention, the backup SIP server BSS allows only a subset of the services usually provided by the main SIP server MSS to the terminals of the remote site. It allows the services that require signalling information that is compatible with the backup SIP signalling link BL. For allowing or forbidding services, it parses the SIP signalling messages that are sent from the terminals of remote site to the main SIP server, on the main site; and it alters the contents of some fields of these SIP messages, by deleting some information that would be necessary for providing the forbidden services. 
     On the other hand, it propagates, on the backup SIP signalling channel BL, signalling information necessary for allowed services such as:
         SIP device registrations;   Service subscribing from local or remote users, and relevant notifications;   Service messaging;   Contextual data publications.       

     It may have additional capabilities such as routing feature for local and external calls, local services, conference . . . . 
     For instance, when the SIP terminal T 1  on the remote site sends a SIP message addressed to the terminal T 2  on the main site, this message is received by the backup SIP server BSS. This tatter alters the Allow header field of this message, if necessary, in order to allow only some services; inserts the relevant route headers; and then forwards the message to the main SIP server MSS via the gateway SIPGW 1 , the backup signaling link BL through the public switched telephone network PSTN, and the gateway SIPGW 2 . Then the main SIP server MSS forwards the message to the terminal T 2 . 
     The  FIGS. 2 to 6  illustrate the transmission of different kinds of SIP signaling messages between the SIP user agent of terminal T 1  and the main SIP server MSS, and the transmission of the corresponding acknowledgements, in backup mode, for various services. 
       FIG. 2  illustrates the transmission of a REGISTER message from the SIP user agent of the terminal T 1  Located in the remote site to the main SIP server MSS located in the main site, for registering the terminal in the main SIP server MSS, in backup mode. When this terminal T 1  detects that the IP link IPL with the main SIP server no longer works, it registers to the backup SIP server BSS. The backup SIP server BSS propagates the registration request to the main SIP server MSS via the backup SIP signalling link BL setup over the public switched network PSTN. In the REGISTER message, the backup SIP server BSS may:
         rewrite the content of the field “Request-URI” to match the main SIP server registrar URI (Uniform Resource Identifier).   alter the content of the header field “Allow” (that contains a list of “SIP methods” such as SUBSCRIBE, NOTIFY, MESSAGE, PUBLISH, OPTIONS . . . ).   insert the route headers of the corresponding gateway URI SIPGW 1  and SIPGW 2  according to its configuration.       

     For instance, if: 
     BackupSIPServer.com is the URI of the backup SIP server BSS, 
     MainSIPServer.com is the URI of the main SIP server MSS, 
     SIPUA@BackupSIPServer.com is the URI of the terminal T 1 , 
     SIPUA@IPSIPUA is the contact URI of terminal T 1  formed by its IP address, then the terminal T 1  sends to the backup SIP server BSS a message: 
     Register 1 : registrar.BackupSIPServer.com 
     From: SIPUA@BackupSIPServer.com 
     Contact: SIPUA@IPSIPUA 
     Allow: INVITE, ACK, BYE, REGISTER, SUBSCRIBE, NOTIFY, MESSAGE, PUBLISH, OPTIONS, . . . 
     So the terminal T 1  has been registered in the backup SIP server BSS, and this latter has indicated that it allows the SIP methods INVITE, ACK, BYE, REGISTER, SUBSCRIBE, NOTIFY, MESSAGE, PUBLISH, OPTIONS . . . 
     Then the backup SIP server BSS sends, to the gateway SIPGW 1  a message: 
     Register 2 : registrar.MainSIPServer.com 
     From: SIPUA@MainSIPServer.com 
     Contact: SIPUA@IPSIPUA 
     Allow: REGISTER, SUBSCRIBE, NOTIFY, MESSAGE, PUBLISH, OPTIONS . . . 
     In the message Register 2 , the field “Allow” has been modified by the backup SIP server BSS in order to suppress some SIP methods (INVITE, ACK, etc) among those that were allowed by the terminal T 1 . The suppressed methods correspond to services (such as establishment of real time media sessions and attached services like transfer, . . . , for instance) that need a real time signalling or a wide band signalling that cannot be supported by the backup SIP signalling link BL. Then the backup SIP server BSS sends an acknowledgement 200ok to the terminal T 1 . 
     Then the gateway SIPGW 1  sends, to the gateway SIPGW 2 , via the backup SIP signaling link BL, a message: 
     Register 3 : registrar.MainSIPAppliServer.com 
     From: SIPUA@MainSIPServer.com 
     Contact: SIPUA@IPSIPUA 
     Allow: REGISTER, SUBSCRIBE, NOTIFY, MESSAGE, PUBLISH, OPTIONS, . . . 
     Path: SIPGW 1 .MainSIPServer.com 
     In the message Register 3 , a field Path has been added by the gateway SIPGW 1  in order to indicate a path towards the backup SIP server BSS, conforming to standard RFC3327. 
     The gateway SIPGW 2  sends, to the main SIP server MSS, a message: 
     Register 4 : registrar.MainSIPServer.com 
     From: SIPUA@MainSIPServer.com 
     Contact: SIPUA@IPSIPUA 
     Allow: REGISTER, SUBSCRIBE, NOTIFY, MESSAGE, PUBLISH, OPTIONS, . . . 
     Path: SIPGW 2 .MainSIPServer.com, SIPGW 1 .MainSIPServer.com 
     The field Path has been completed by the gateway SIPGW 2  in order to indicate a full path towards the backup SIP server BSS, conforming to standard RFC3327. 
     Then the main SIP server MSS sends an acknowledgement message “200ok” to backup SIP server BSS via the gateway SIPGW 2  and the gateway SIPGW 1 . 
     The backup SIP server according to the invention also enables service subscribing from the remote site. In the following examples, the backup SIP server BSS shall alter the relevant headers, including the Route header when needed. 
     Examples of Services: 
     a) Message waiting indication (see  FIGS. 3 and 4 ): Each user Located on the remote site can use a Voice Mail service located in the main SIP server MSS. This service notifies the user with the state of his/her mailbox, in whatever mode (normal/backup) the system is. For such purpose, after registration of a terminal, the backup SIP server BSS propagates the relevant subscribe request (Message Waiting Indication EventPackage) to the main SIP server MSS. Upon any significant change of the user&#39;s voice mailbox state, relevant notification shall be sent from the main SIP server MSS and propagated to the corresponding user. An application or equipment can use this notification to inform the user through an appropriate way (Popup window, icon, blinking LED, tone . . . .) 
     b) Resource presence state: SIP devices of the remote site may display global resource state information (remote user presence state, services availability as Voicemail services . . . ). For such a purpose, a SIP user agent sends a subscribe request (Presence Event Package) to the backup SIP server BSS. This latter forwards such requests to the main SIP server MSS and processes subsequent notifications to the corresponding user agents. 
     c) Conference membership notification: A SIP user agent may participate to a conference hosted by a conference server in the main SIP server MSS, and require the respective presence states of all participants, through a subscribe request (Conference Event Package) to the backup SIP server BSS which shall forward such requests and shall process subsequent notifications. 
       FIG. 3  illustrates the transmission of a SUBSCRIBE message from the SIP user agent of the terminal T 1  located in the remote site to the main SIP server MSS located in the main site, for subscribing to the service called “message waiting indication”, in backup mode. 
     The terminal T 1  sends, to the backup SIP server BSS, a message: 
     Subscribe 1 : SIPUA@vmail.BackupSIPServer.com 
     From: SIPUA@BackupSIPServer.com 
     Event: message-summary 
     where SIPUA@vmail.BackupSIPServer.com is the URI of the Voice Mail service attached to Terminal  1  in Backup mode, 
     and where SIPUA@BackupSIPServer.com is the URI of Terminal  1  in Backup mode. 
     Then the backup SIP server BSS sends, to the gateway SIPGW 1 , a message: 
     Subscribe 2 : SIPUA@vmail.MainSIPServer.com 
     From: SIPUA@MainSIPServer.com 
     Route: SIPGW 1 .MainSIPServer.com, SIPGW 2 .MainSIPServer.com 
     Event: message-summary 
     In the message Subscribe 2 , a field Route has been added by the gateway SIPGW 1  for routing the message to the main SIP server MSS. 
     Then the gateway SIPGW 1  sends, to the gateway SIPGW  2 , via the public switched telephone network PSTN, a message: 
     Subscribe 3 : SIPUA@vmail.MainSIPServer.com 
     From: SIPUA@MainSIPServer.com 
     Route: SIPGW 2 .MainSIPServer.com 
     Event: message-summary 
     In the message Subscribe 3 , the header field Route has been altered by the gateway SIPGW 1  by suppressing the URI of the SIP Gateway SIPGW 1  which has just been crossed. 
     Then the gateway SIPGW  2  sends, to the main SIP server MSS a message: 
     Subscribe 4 : SIPUA@vmail.MainSIPServer.com 
     From: SIPUA@MainSIPServer.com 
     Event: message-summary 
     In the message Subscribe 4 , the field Route has been altered by the gateway SIPGW 2  by suppressing the URI of the SIP Gateway SIPGW 2  which has just been crossed. 
     Then the main SIP server MSS sends an acknowledgement message “200ok” to the terminal T 1  via the gateway SIPGW 2 , the public switched telephone network PSTN, the gateway SIPGW  1 , and the backup SIP server BSS. 
     The backup SIP server according to the invention enables publications and notification from the remote side.
         a) Publications (An example will be described below with reference to  FIG. 6 )
           Presence state publication (RFC 3856 and PUBLISH method):   Devices of the main site (Conference servers, messaging applications, routing . . . ) may need Presence state information relative to the users located in the remote site. These presence data are conveyed from user agents in the remote site to a Presence agent associated to main SIP server MSS, via the backup SIP server BBS and the backup SIP signalling link.   Call statistics publication (PUBLISH method):   Some devices of the main site (Billing application, conference servers, routing, voice quality system monitoring . . . ) need permanent call statistics information. These call statistics are conveyed from user agents in the remote site to a Call Statistics agent associated to main SIP server MSS, via the backup SIP server BBS and the backup SIP signalling link.   
           b) Notifications (An example will be described below with reference to  FIG. 4 )
           Call statistic notifications: according to RFC6035, the subscribe/notify mechanism may also be used in such a case.   Call Terminated notification: An automatic redial feature using Dialog Event Package (RFC4235) may be invoked by a user A located on the main site, as this user A is trying to contact a user B located on the remote site and that is busy. A notification of call termination is forwarded to the main SIP server MSS when the user B becomes available, in order to allow an automatic redial for the user A.   
               

       FIG. 4  illustrates the transmission of a NOTIFY message from the main SIP server MSS located in the main site to the SIP user agent of the terminal T 1  located in the remote site, for notifying the terminal T 1  that a message is waiting, in backup mode. 
     The main SIP server MSS sends, to the gateway SIPGW 2 , a message: 
     Notify 1 : SIPUA@MainSIPServer.com 
     From: SIPUA@MainSIPServer.com 
     Route: SIPGW 2 .MainSIPServer.com, SIPGW 1 .MainSIPServer.com 
     Event: message-summary 
     Messages-Waiting: yes 
     The main SIP server MSS adds a field Route for routing the message to the backup SIP server BSS. 
     Then the gateway SIPGW 2  sends, to the gateway SIPGW 1 , via the public switched telephone network PSTN, a message: 
     Notify 2 : SIPUA@MainSIPServer.com 
     From: SIPUA@MainSIPServer.com 
     Route: SIPGW 1 .MainSIPServer.com 
     Event: message-summary 
     Messages-Waiting: yes 
     The gateway SIPGW 2  modifies the field Route in the message Notify 2 , by suppressing the URI of the SIP gateway SIPGW 2  which has just been crossed. 
     Then the gateway SIPGW 1  sends, to the backup SIP server BSS, a message 
     Notify 3 : SIPUA@MainSIPServer.com 
     From: SIPUA@MainSIPServer.com 
     Event: message-summary 
     Messages-Waiting: yes 
     The gateway SIPGW 1  modifies the field Route in the message Notify 3 , by suppressing the URI of the SIP gateway SIPGW 1  which has just been crossed. 
     Then the backup SIP server BSS sends, to the terminal T 1 , a message 
     Notify 4 : SIPUA@BackupSIPServer.com 
     From: SIPUA@BackupSIPServer.com 
     Event: message-summary 
     Messages-Waiting: yes 
     Then the terminal T 1  sends an acknowledgement message “200ok” to the main SIP server MSS via the backup SIP server BSS, the gateway SIPGW 1 , the public switched telephone network PSTN, and the gateway SIPGW 2 . 
     The backup SIP server according to the invention enables subscription from the main site. 
       FIG. 5  illustrates the transmission of a SUBSCRIBE messages from the main SIP server MSS located in the main site to the SIP user agent of a terminal T 1  located in the remote site, for subscribing to call statistics publication, in backup mode. 
     The main SIP server MSS sends to the gateway SIPGW 2  a message: 
     Subscribe 1 : SIPUA@MainSIPServer.com 
     From: callStat@MainSIPServer.com 
     Route: SIPGW 2 .MainSIPServer.com, 
     SIPGW 1 .MainSIPServer.com 
     Event: vq-rtcpxr 
     In the message Subscribe 1 , the main SIP sever MSS adds a field Route for routing the message to the backup SIP server BSS. 
     Then the gateway SIPGW 2  sends, to the gateway SIPGW 1 , via the public switched telephone network PSTN, a message: 
     Subscribe 2 : SIPUA@MainServer.com 
     From: callStat@MainSIPServer.com 
     Route: SIPGW 1 .MainSIPServer.com 
     Event: vq-rtcpxr 
     Where callStat@MainSIPServer.com is the URI of the call statistics publication service. 
     Then the gateway SIPGW 1  sends, to the backup SIP server BSS, a message: 
     Subscribe 3 : SIPUA@MainSIPServer.com 
     From: callStat@MainSIPServer.com 
     Event: vq-rtcpxr 
     Then the backup SIP server BSS sends, to the terminal T 1 , a message: 
     Subscribe 4 : SIPUA@BackupSIPServer.com 
     From: callStat@BackupSIPServer.com 
     Event: vq-rtcpxr 
     Then the terminal T 1  sends an acknowledgement message “200ok” to the main SIP server MSS via the backup SIP server BSS, the gateway SIPGW 1 , the public switched telephone network PSTN, and the gateway SIPGW 2 . 
     The backup SIP server according to the invention enables publications from the remote side. 
       FIG. 6  illustrates the transmission of a PUBLISH message from the SIP user agent of the terminal T 1  located in the remote site to the main SIP server MSS located in the main site, for notifying call statistics to the latter, in backup mode. 
     Terminal T 1  sends, to the backup SIP server BSS, a message: 
     Publish 1 : SIPUA@BackupSIPServer.com 
     From: SIPUA@BackupSIPServer.com 
     Event: vq-rtcpxr 
     Then the backup SIP server BSS sends, to the gateway SIPGW 1  a message 
     Publish 2 : SIPUA@MainSIPServer.com 
     From: SIPUA@MainSIPServer.com 
     Route: IPGW 1 .MainSIPServer.com, 
     SIPGW 2 .MainSIPServer.com 
     Event: vq-rtcpxr 
     The backup SIP server BSS adds a field Route for routing the message to the main SIP server MSS. 
     Then the gateway SIPGW 1  sends, to the gateway SIPGW 2 , via the public switched telephone network PSTN, a message: 
     Publish 3 : SIPUA@MainSIPServer.com 
     From: SIPUA@MainSIPServer.com 
     Route: SIPGW 2 .MainSIPServer.com 
     Event: vq-rtcpxr 
     Then the SIP gateway SIPGW 2  sends to the main SIP server MSS, a message: 
     Publish 4 : SIPUA@MainSIPServer.com 
     From: SIPUA@MainSIPServer.com 
     Event: vq-rtcpxr 
     Then the main SIP server MSS sends an acknowledgement message “200ok” to the terminal T 1  via the gateway SIPGW 2 , via the public switched telephone network PSTN, the gateway SIPGW 1 , and the backup SIP server BSS. 
       FIG. 7  is a block diagram showing an embodiment BSS of the backup SIP server according to the invention. It comprises:
         a management and configuration module MMC;   a classical SIP server CSS;   and a backup application server BAS for implementing the backup operations according to the invention.       

     The classical SIP server CSS comprises a registrar module RGM and a proxy server PRX. The backup application server BAS comprises:
         a forward registration manager FRM that duplicates and sends to the main SIP server MSS, via the proxy PRX, the registration data that are being written into the registrar module RGM;   a policy manager POM that checks the “SIP methods” (INVITE, ACK, BYE, REGISTER, SUBSCRIBE, NOTIFY, MESSAGE, PUBLISH, OPTIONS, . . . ) indicated in a message being sent by a terminal, for keeping or deleting each of these methods in the message before forwarding the message to the main SIP server MSS;   and a link monitor manager LMM that monitors the IP link IPL carried by the IP network WAN. If it detects any failure of the IP link IPL, it enables the registrar module RGM of the backup SIP server BSS for registering the local terminals, and it sets up a backup SIP signaling channel BL via the public switched telephone network PSTN. The signaling information concerning services is forwarded to the main SIP server MSS, whereas the signaling concerning voice is dealt with by the backup SIP server BSS.   Examples of methods selected by the policy manager POM:
           INVITE:
               Non media session (Case Content-Type header &lt; &gt; sdp: depends on Content-Length header): cf media type description (http://www.iana.org/assignments/media-types)   Media session (Content-Type header: application/sdp): NO   Content-Type absent: NO (supposed to be SDP in Ack, . . . )   
               CANCEL:   BYE:   ACK:
               If from a non media session: YES   Otherwise: no   
               REFER: NO   REGISTER: YES   SUBSCRIBE/NOTIFY: Depends on Event header contents; may also depend on the Content-Length header value:
               Message-summary: YES (rfc3842)   vq-rtcpxr: YES (rfc6035)   presence: YES (rfc3856)   winfo: YES (rfc3857)   dialog: YES (rfc4235)   conference: YES (rfc4575)   . . . (list is not exhaustive)   
               MESSAGE: depends on the Content-Length value (and the rate: all data may not be transmitted if traffic is too important)   OPTIONS: YES   INFO: depends on the Content-Length value and the rate   PUBLISH: if Event header is accepted (rfc3903)   
               

     Some priorities may also be associated to methods, events or users (SDP invite might be authorized for a determined user or with a determined low priority), depending on the value and the capacity of the backup signalling link, the transfer may whether or not be guaranteed. 
       FIG. 8  illustrates the operations made by the link monitor manager LMM in the backup application server BAS: 
     Step  81 : At initialization, the backup SIP server MS is on standby, i. e. its port is dosed. 
     Step  82 : The link monitor manager LMM periodically sends a keep-alive message to the main SIP server MSS via the IP network WAN, in order to detect a possible default of the IP link through the network WAN. 
     Step  83 : Then the link monitor manager LMM checks whether it receives a response from the main SIP server MSS via the IP network WAN. If it receives a response, it continues to keep the backup SIP server BSS on standby (Return to step  81 ). 
     Step  84 : If it does not receive a response, the link monitor manager LMM activates the backup SIP server BSS in order to forward the registration requests to the main SIP server MSS. 
     Step  85 : Then the link monitor manager LMM awaits a recovery of the IP network WAN: It periodically sends a keep-alive message to the main SIP server MSS via the IP network WAN. 
     Step  86 : Then the link monitor manager LMM checks whether it receives a response from the main SIP server MSS via the IP network WAN. If it does not receive a response, the backup SIP server remains active (Return to step  84 ). 
     Step  87 : If it receives a response from the main SIP server MSS via the IP network WAN, i. e. if the IP link IPL carried by IP network WAN is working again, it unregisters the SIP user agents that it had registered locally, and then puts the backup SIP server BSS back to standby state (Return to step  81 ). 
       FIG. 9  illustrates, with more details, the step  84  made by the link monitor manager LMM in the backup application server BAS. If it does not receive a response, the link monitor manager LMM activates the backup SIP server BSS: 
     Step  91 : It activates the register module RGM so that it accepts to register the users of the terminals, of the remote site, which request registration. 
     Step  92 : Then it sends a SIP message INVITE to the gateway SIPGW 1 , for requesting it to setup a call to the gateway SIPGW 2  via the public network PSTN, to set up the backup SIP signaling link BL. 
     Step  93 : If the gateway SIPGW 1  responds negatively or does not respond, the link monitor manager LMM tries again (Return to step  92 ). 
     Step  94 : If the gateway SIPGW 1  responds positively, a backup SIP signaling link BL is now available via the public switched telephone network PSTN. Then the link monitor manager LMM proceeds to the step  85 . 
       FIG. 10  illustrates, with more details, the operations of the forward registration manager FRM: 
     When the SIP user agent of a SIP terminal, located in the remote site, sends a SIP register request RRQ 1 , via the proxy PRX, this request is locally dealt with by the forward registration manager FRM. If the backup SIP server BSS is activated:
         The forward registration manager FRM forwards the registration request by sending a SIP register request message RRQ 2  to the main SIP server MSS, via the proxy PRX. Then the main SIP server MSS acknowledges that it has registered the user. Then the forward registration manager FRM stores, in a data base D that is part of the registrar module RGM, an indication that the main SIP server MSS has registered the user.   The forward registration manager FRM allows a local registration, in a registrar memory RM that is part of the registrar module RGM, and allows the sending of an acknowledgement message ROK, 200ok, to the terminal that sent the register request RRQ 1 .       

       FIG. 11  illustrates, with more details, the operations of the policy manager POM: All the requests that transit through the proxy PRX are dealt with by the policy manager POM. When the SIP user agent of a SIP terminal, located in the remote site, sends a SIP request RRQ 1 , via the proxy PRX, the policy manager POM supplies a response RS, to the proxy PRX, that may be one of the following ones according to predetermined rules:
         Continue treatment without modification (for instance for a Local call, or a mere phone call via the public network PSTN).   Negative response (For instance refusing a video call via the public switched telephone network PSTN because the available bandwidth is insufficient, or refusing a service that needs a real time signaling that the public switched telephone network PSTN cannot support).   Forward the request to its destination (with header modifications).   No action (For instance, on  FIG. 2 , there is no action done when the backup SIP server BSS receives the message 200ok, because the message Register 1  has already been acknowledged previously by sending a message 200ok to the terminal T 1 ).       

     It is possible to use classical SIP terminals, without any modification in these SIP terminals, when a backup SIP server according to the invention is installed in an enterprise network. 
       FIG. 12  illustrates the operations of a classical SIP terminal T 1  located in the remote site, when it collaborates with an embodiment of the backup SIP server according to the invention. This terminal stores the address of a main SIP server and the address of a backup SIP server. 
     Step  121 : The terminal is in its initial state. Generally, in this initial state, the terminal has been previously registered on the main SIP server. 
     Step  122 : The terminal periodically sends a SIP message, REGISTER, requesting a registration on any SIP server. 
     Step  123 : Then it checks that the main SIP server responds. If the main SIP server responds, then the terminal stays in its initial state (Return to step  121 ), or continues the normal steps of a call if a call is ongoing. 
     Step  122 ′: Between two periodic automatic checking, the user of the terminal may attempt to make a call while the IP link with the main SIP server does not work anymore. The attempt triggers the sending of a SIP message, INVITE, addressed to the main SIP server. 
     Step  123 ′: Then the terminal checks that the main SIP server responds. If the main SIP server responds, then the terminal continues the normal steps of a call. 
     Step  124 : If the main SIP server does not respond, the terminal sends a SIP message, REGISTER, in order to be registered on the backup SIP server (For instance the backup SIP server BSS described above). 
     Step  125 : Then it checks whether a SIP server responds. If the backup SIP server has responded within a predetermined time interval, the terminal goes back to in its initial state (Return to step  121 ) in order to continue to periodically send a SIP message, REGISTER, requesting a registration on the main SIP server. 
     Step  126 : If the backup SIP server has responded that it has registered the terminal, then the terminal enters into a new state “Registered on backup SIP server”. It can benefit from the backup SIP server for using services hosted by the main SIP server. 
       FIG. 13  is a block diagram showing an embodiment SIPGW 1  of the gateway according to the invention. The peer gateway SIPGW 2  located in the main site is similar. This embodiment SIPGW 1  comprises a classical gateway CGW, a proxy PRO and a data compression module DC. The classical gateway CGW comprises:
         A management and configuration module MC that stores parameters such as the parameters of IP interfaces, SIP and trunk layers, and route configuration.   An analog or digital trunk interface module TI for coupling the gateway to a trunk of the public network PSTN. Its type depends on the type of trunk.   A media Server module MS coupled to the enterprise network: It is located on the IP network side, and manages the RTP/RTCP/T38 . . . flows.   A transcoder module TC: It comprises several types of transcoding resources, such as audio coders, a HDLC (High-Level Data Link Control) transcoder, and a modem, in order to convert the protocol used in the enterprise IP network protocol, for the data payload, to the protocol used in a trunk of the public network PSTN, and reciprocally.   A call control module CC: It manages a call according to the type of the signalization (SIP, Q931, analog . . . ) used by the terminal of a calling party, and it drives the Media Server module MS, the transcoder module TM, and the trunk module TM.       

     The use of the backup SIP server BSS implies modification of the management and configuration module MC and of the call control module CC of the classical gateway CGW. The management and configuration module MC is modified in order to receive and store the following parameters:
         A rescue call number that will be used for establishing the backup link BL.   A data compression flag (to enable/disable compression).   Value of the bandwidth of the backup Link BL (optional).       

     The call control module CC is modified so as to start (respectively stop) the relevant HDLC resources of the transcoder module TC in case the called number matches the rescue call number (instead of starting (respectively stopping) resources of media server MS and audio coder resources of the transcoder module TC, as it is normally done for any other call). 
     The proxy module PRO forwards each SIP message according to the IP address contained in a SIP Request-URI. The SIP message is either sent to the Call Control module CC if its IP address is the address of the gateway SIPGW 1  (normal ISDN call), or to the Data compression module DC if the IP address is the address of the main SIP Server MSS. In the latter case the proxy adds its own IP address in Path header on each register method. For each other SIP method sent to the Main SIP Server MSS, the proxy PRO suppresses its own IP address in the Route header. 
     The data compression module DCM applies a compression according to the configuration parameters, in order the decrease the size of SIP packets, thus increasing the maximal signalling data rate on the backup link BL. 
       FIG. 14  illustrates operations made by this embodiment of the gateway according to the invention for call establishment on the SIP Gateway SIPGW 1  (and SIPGW 2 ). There are two types of calls managed by the gateway SIPGW 1 :
         Basic outgoing/incoming calls used from/to a local phone in order to make a voice call.   An outgoing rescue call, based on the rescue call number, when the call control module CCM must establish a back up link BL for the SIP signalling data.       

     There are two types of calls managed by the gateway SIPGW 2 :
         Basic outgoing/incoming calls used from/to a local phone in order to make a voice call.   An incoming rescue call, based on the rescue call number, when the call control module CCM of SIPGW 1  must establish a back up link BL for the SIP signalling data.   Step  140 : The call control module CC receives a request to set up an outgoing call (SIPGW 1 )/incoming call (SIPGW 2 ) to/from the public network PSTN.   Step  141 : The call control module CC checks the called number, on the outgoing/incoming call establishment request.   Step  142 : If the number matches the rescue call number, then the call control module CC sets up a backup link BL supporting the HDLC protocol.   Step  143 : If the number does not match the rescue call number, then the call control module CC sets up a classical voice link supporting a RTP (Real Time Transport) protocol on the side of the enterprise network, and starts the data transcoding in the transcoder module TC.       

       FIG. 15  illustrates operations made by this embodiment SIPGW 1  of the gateway according to the invention, for sending SIP requests and responses to the main site:
         Step  150 : A SIP message originating from the remote site is received by the proxy PRO of the gateway SIPGW 1 .   Step  151 : The proxy PRO checks the content of its destination URI (request-URI or identified destination for a SIP response message):   Step  152 : If URI=XXX@SIPGWAddress, the proxy PRO forwards the message directly to the call control module CC of the gateway SIPGW 1 .   Step  153 : If URI=XXX®MainSIPGServerAddress, the proxy PRO forwards the message to data compression module DC, in order to save bandwidth on the backup link BL.   Step  154 : Then the compressed message is sent to the transcoder module TC (HDLC coder) of the gateway SIPGW 1 , in order to transmit the compressed message via the PSTN network, on the backup link BL.       

       FIG. 16  illustrates operations made by both the SIPGW 1  in remote site and the peer gateway SIPGW 2  in the main site for processing SIP egress traffic. The proxy may receive SIP requests or responses from the local network, or from the remote entity (either in the main site or in the Local site) via the public network PSTN:
         Step  160 : The gateways SIPGW 1 /SIPGW 2  receive SIP a request or response from the local network.   Step  161 : The gateways SIPGW 1 /SIPGW 2  receive SIP a request or response from the remote site via the public network PSTN.   Step  162 : In this latter case, an HDLC driver receives compressed data, forwards them to a decompression module. The decompression module decompresses the data to reconstitute a SIP message and transmits the SIP message to the proxy of the gateway.   Step  163 : The proxy receives the SIP message.   Step  164 : The proxy forwards the SIP message to its destination in its local network (BSS for SIPGW 1  and main SIP server MSS for SIPGW 2 ).