Abstract:
In one aspect of the invention, emergency modes of operation for an adapter unit comprising a channel transmitter/receiver unit, a data packet transmitter/receiver unit, and a data insertion/data retrieval unit which inserts signaling data transmitted by the channel receiver unit into data packets and forwards the same to the data packet transmitter unit while retrieving signaling data from the data packets received by the data packet receiver unit and forwarding the same to the channel transmitter unit is provided. The adapter unit especially allows terminals of a branch to be connected in a fail-safe and simple manner to a central telecommunication system.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is the US National Stage of International Application No. PCT/EP2005/050097, filed Jan. 11, 2005 and claims the benefit thereof. The International Application claims the benefits of German application No. 102004002680.7 DE filed Jan. 19, 2004, both of the applications are incorporated by reference herein in their entirety. 
   FIELD OF INVENTION 
   The invention relates to a unit for the operation of an exchange line of a public telecommunications network at a telecommunication system. The unit contains a channel send/receive unit which sends signaling data to an exchange of a circuit-switched telecommunications network and which receives signaling data from the exchange. The signaling data complies with e.g. the protocol DSS1 (Digital Signaling Systems No. 1) of the ITU-T (International Telecommunication Union-Telecommunication Standardization Sector). 
   BACKGROUND OF INVENTION 
   Various private subscribers are attached to a public telecommunications network via individual lines or telecommunication systems. Exchanges of the public telecommunications network switch connections depending on the call numbers and monitor the charges and the utilization of user facilities. 
   SUMMARY OF INVENTION 
   By contrast, the telecommunication system is used for operating a private telecommunications network, e.g. a company telephone network. The telecommunication system is a system of the type HiPath 3000 or HiPath 4000 from the company Siemens AG. Attached to a telecommunication system are a multiplicity of subscribers whose call numbers and user facilities are predetermined by the telecommunication system. In particular, the telecommunication system is used for switching connections between the terminals of the private telecommunications network or for switching connections to or from a public telecommunications network or telephone network. 
   It is possible to conceive of a solution in which the unit is an adapter unit containing the following modules for a normal operating mode:
     the channel send/receive unit,   a data packet send/receive unit which sends data packets into a data packet transfer network and which receives data packets from the data packet transfer network, and   a data insertion/extraction unit which inserts signaling data coming from the channel receive unit into data packets and forwards them to the data packet send unit and which extracts signaling data from data packets that are received from the data packet receive unit and forwards it to the channel send unit.   

   Using such an adapter unit, an exchange line leading to a branch office can be connected to a remote central telecommunication system with little switching effort, in particular without the need for a local telecommunication system in the branch office. It is nonetheless possible to utilize advantages which are otherwise only available using a local telecommunication system in the branch office. 
   However, it must also be ensured that terminals in the branch office remain operational even if the central telecommunication system fails or if the data packet transfer network fails. 
   The invention addresses the problem of specifying an adapter unit which offers simple construction and functional reliability. A method for operating a telecommunications network will also be specified. 
   The problem in relation to the unit is solved by an adapter unit having the features specified in the independent claim. Developments are specified in the dependent claims. 
   The adapter unit contains:
     a channel send/receive unit which sends signaling data to an exchange of a circuit-switched telecommunications network and which receives signaling data from the exchange,   a data packet send/receive unit which at least in a normal operating mode sends data packets into a data packet transfer network and which receives data packets from the data packet transfer network,   a data insertion/extraction unit which at least in a normal operating mode inserts signaling data coming from the channel receive unit into data packets and forwards them to the data packet send unit and which extracts signaling data from data packets that are received from the data packet receive unit and forwards it to the channel send unit, and   an operating mode switchover unit which, in the event of a fault on the side of the data packet transfer network or in the event of faults in a telecommunication system operating on the data packet transfer network, switches over into an emergency operating mode in which telecommunication via the circuit-switched telecommunications network is ensured.   

   It is consequently a straightforward matter to allow telephone operation in the branch office even in the event of faults in a data packet transfer network to a central telecommunication system or in the event of a fault in the central telecommunication system. 
   In a first alternative development, in the emergency operating mode the operating mode switchover unit entrusts the forwarding of the signaling data to a different telecommunication system than in the normal operating mode. Conversely, in the emergency operating mode signaling data coming from this different telecommunication system is sent via the channel send unit. The functions of this different telecommunication system are preferably significantly limited in comparison with the central telecommunication system which is used in the normal operating mode, such that the switching-related and administrative burden is acceptable for the telecommunication system which is used in the emergency operating mode. For example, the telecommunication system which is used in the emergency operating mode supports only a few terminals, in particular fewer than 16 terminals. In addition, this telecommunication system does not contain any redundant modules. 
   In a development, the functions of the telecommunication system which is used in the emergency operating mode are provided in a telephone, in particular in an IP telephone. Modules and software in the IP telephone can be used both for providing the functions of the emergency telecommunication system and for providing the functions of the IP telephone when telephoning in both operating modes. 
   In a very simple second alternative development, in the emergency operating mode the operating mode switchover unit entrusts the forwarding of the signaling data to a subscriber terminal. Conversely, in the emergency operating mode signaling data coming from the subscriber terminal is sent via the channel send unit. This development is used in particular if the exchange line is a multiple device interface. 
   In a third alternative development, the adapter unit contains a protocol conversion unit which in the emergency operating mode is entrusted by the operating mode switchover unit to perform a protocol conversion of the signaling data into a signaling protocol for a data packet transfer network and preferably also vice versa. In the third alternative development, the adapter unit also contains a network access unit which in the emergency operating mode is entrusted by the operating mode switchover unit to perform network access functions for terminals of a data packet transfer network, in particular a gatekeeper function in accordance with a protocol of the H.323 protocol family of the ITU-T (International Telecommunication Union-Telecommunication Standardization Sector) or a SIP registrar function in accordance with the SIP protocol (Session Initiation Protocol) of the IETF (Internet Engineering Task Force) or a protocol which is based thereupon. The telephones of the branch office register with the adapter unit in the emergency operating mode. 
   In a fourth alternative development, the adapter unit again contains a protocol conversion unit, namely a so-called proxy unit, which in the emergency operating mode is entrusted by the operating mode switchover unit to perform a protocol conversion of the signaling data in accordance with e.g. DSS1 into a signaling protocol for a data packet transfer network and preferably also vice versa. In the fourth development, the adapter unit also contains a terminal unit which in the emergency operating mode is entrusted by the operating mode switchover unit to perform the function of a terminal in terminal-to-terminal connections of a data packet transfer network, in particular in peer-to-peer connections. Therefore the adapter unit does not have to provide a central registering function simultaneously for all telephones of the branch office. 
   In other alternative developments, other measures are adopted in order to maintain the telephone operation in a branch office in which the adapter unit is installed even in the event of a fault. 
   The invention relates in particular to a method for operating a telecommunications network, wherein said method includes the following steps being executed in a normal operating mode:
     receiving signaling data of a circuit-switched telecommunications network ( 20 ,  22 ),   tunneling ( 160 ,  190 ) the received signaling data via a data packet transfer network ( 12 ),   processing the tunneled signaling data in a telecommunication system ( 14 ) in accordance with a signaling protocol.   

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is explained below with reference to the appended drawings, in which: 
       FIG. 1  shows the normal operating mode of a private telecommunications network in which voice data is also transferred via the Internet, and 
       FIG. 2  shows a protocol stack for the normal operating mode in an adapter unit and in a central telecommunication system of the private telecommunications network, 
       FIG. 3  shows an exemplary embodiment of an operating mode of the private telecommunications network, 
       FIG. 4  shows another exemplary embodiment of an operating mode of the private telecommunications network, 
       FIG. 5  shows another exemplary embodiment of an operating mode of the private telecommunications network, and 
       FIG. 6  shows another exemplary embodiment of an operating mode of the private telecommunications network. 
   

   DETAILED DESCRIPTION OF INVENTION 
     FIG. 1  shows the normal operating mode of a private telecommunications network  10  in which voice data is also transferred via the Internet  12 . The telecommunications network  10  contains a central telecommunication system  14  and e.g. two branch office data packet transfer networks  16  and  18  in branch offices F 1  and F 2  respectively. The telecommunication system  14  switches telephone connections or video phone connections between the terminals of the private telecommunications network  10  and/or between these terminals and terminals on a public circuit-switched telecommunications network (PSTN—Public Switched Telecommunication Network), of which three local networks  20 ,  22  and  24  are illustrated in  FIG. 1 . In the exemplary embodiment, the local networks  20  to  24  belong to the same network operator. In the case of other exemplary embodiments, the local networks  20  to  24  belong to various network operators. In another exemplary embodiment, the telecommunication system  14  and the adapter unit  90  are connected to different exchanges or to different modules of an exchange in the same local network. 
   The data is transferred in the Internet  12  in data packets in accordance with the Internet protocol. Each data packet contains a data header including a destination address and a sender address. Payload data e.g. voice data or signaling data is contained in a data body part. The data packets are transferred in a connectionless manner, i.e. without connection setup phase and connection cleardown phase, on at least one lower protocol level. The Internet  12  contains a multiplicity of local data packet transfer networks LAN (Local Area Network) and long-range transfer networks WAN (Wide Area Network). 
   The telecommunication system  14  is e.g. of the type HiPath 4000 from the company Siemens AG and contains:
     a control unit  30 ,   a connection matrix  32 ,   at least one exchange module  34 ,   at least one subscriber connection module  36 ,   and at least one IP module  38 .   

   The control unit  30  controls inter alia switching processes in the telecommunication system  14 , checks the utilization of user facilities by the subscribers of the private telecommunications network  10 , and records charging data for the internal charging within the private telecommunications network  10 . The control unit contains one or more processors, for example. 
   The connection matrix  32  is controlled by the control unit  30  and is used for switching call data from or to the exchange module  34 , the subscriber connection module  36  or the IP module  38 . The switching in the connection matrix  32  takes place e.g. by means of the interleaving of time slots. 
   Connected to the exchange module  34  is an exchange line  40  or a plurality of exchange lines  40  to  46 , whose other end is connected to an exchange (not shown) of the local network  20 . The call data is transferred on up to 30 speech/data channels and one signaling channel in accordance with the ISDN protocol (Integrated Service Digital Network) on each exchange line using time-division multiplexing, for example. The exchange module  34  processes signaling messages coming from the local network  20  in accordance with the ISDN protocol and sends signaling messages into the local network  20  in accordance with the ISDN protocol. 
   Connected to the subscriber connection module  36  via connection lines  50  to  56  is a multiplicity of terminals  60  to  64  e.g. terminals of the type Optiset from the company Siemens AG. The voice data between the terminals  60  to  64  and the subscriber module  36  is only transferred in voice channels using time-division multiplexing and not in data packets. The subscriber module  36  generates signaling messages in accordance with a proprietary stimulus protocol e.g. the protocol CorNet TS (Corporate Network Telephony Support). The terminals  60  to  64  are used by employees of the company which operates the private telecommunications network  10 . 
   The IP module  38  is connected to the Internet  12  via a line  70 . Data packets are sent into the Internet  12  and received from the Internet  12  with the aid of the IP module  38 . The IP module  38  processes signaling data in accordance with the IP protocol and the TCP protocol (Transmission Control Protocol) and in the case of one alternative, and in particular for preparing a voice transfer, performs IP signaling on higher protocol levels e.g. in accordance with SIP (Session Initiation Protocol) as specified by the IETF (Internet Engineering Task Force) or in accordance with an ITU-T protocol of the H.323 protocol family. 
   Also in the IP module  38 , voice data from circuit-switched transfer channels coming from the connection matrix  32  is extracted and inserted into data packets which are then transferred via the Internet  12 . Conversely in the IP module  38 , voice data coming from the Internet  12  is extracted from data packets and switched onwards in transfer channels of the connection matrix  32 . 
   Connected to the IP data packet transfer network  16  in the branch office F 1  are:
     a line  72  to the Internet  12  and   IP telephones  80  to  84 , e.g. fewer than ten IP telephones, e.g. of the type Optipoint from the company Siemens AG.   

   The IP telephone  80  contains e.g. as a plug-in card an S 0  adapter  90  which is connected both to the data packet transfer network  16  (see connection  92 ) and to an exchange connection line or so-called exchange line  100  which leads to an exchange (not shown) of the local network  22 . Signaling for the interface S 0  takes place in accordance with the ISDN protocol on the exchange line  100 . 
   Signaling data which arrives on the exchange line  100  is packed into data packets in the S 0  adapter  90  without protocol-based processing and tunneled via the Internet  12 ; see the signaling path  102  of the signaling data as indicated by the broken line from the S 0  adapter  90  via the Internet  12  and the IP module  38  to the control unit  14 . The control unit processes the signaling data in the same way as signaling data which comes from the exchange module  34 . In this case, control operations are carried out by the control unit  30  and the signaling data is answered in accordance with the ISDN protocol. The reply signaling data is routed to the IP module  38  where it is packed into data packets and tunneled back to the S 0  adapter  90  via the Internet  12 ; see broken line signaling path  102 . 
   The voice data is extracted from the B channel (B 1 , B 2 ) of the exchange line  100  and e.g. transferred via the Internet  12  to the IP module  38 ; see voice data path  104 . A real-time protocol, e.g. the RTP (Real Time Protocol) and the UDP (User Datagram Protocol) as specified by the IETF, is used for transferring the voice data, for example. Voice compression as per ITU-T standard G.723 or G.729 in the direction of the Internet  12  and voice decompression in the direction of the exchange line  100 , for example, are performed in the S 0  adapter  90 . Therefore tunneling does not take place in relation to the voice data. 
   In the exemplary embodiment, a telephone connection is to be set up from the local network  22  to the IP telephone  80  in accordance with the signaling data which arrives via the exchange line  100 . The control unit  30  therefore notifies the IP telephone  80  of the incoming call via the Internet  12  with the aid of the IP module  38 ; see signaling path  106 . A proprietary stimulus protocol, for example, is used for signaling to the IP telephone  80 , e.g. the Siemens-internal protocol CorNet TS (Corporate Network-Telephony Support) tunneled via the proprietary protocol CorNet TC (Corporate Network-Tele Commuting) which is described in greater detail below and TCP/IP. The protocol messages of this protocol are tunneled via the Internet  12  and relate to simple signaling operations such as “Key x depressed” or “LED x on or off”. 
   In order to prepare for the transfer of the voice data, e.g. the protocol SIP (Session Initiation Protocol) or a protocol of the H.323 protocol family can be used. For signaling on the signaling path  106 , the protocol SIP or a signaling protocol as per the H.323 protocol family is used as an alternative instead of the protocol CorNet TS. 
   If the subscriber using the IP telephone  80  has accepted the incoming call, the voice data which arrived in the IP module  38  via the voice data path  104  is also transferred from the IP module  38  to the IP telephone via the Internet  12 ; see voice data path  108 , on which e.g. a real-time transfer protocol without tunneling is again used. If the subscriber using the IP telephone  80  speaks, the voice data which is thus generated is routed in a reverse direction via the voice data paths  108  and  104  to the adapter  90 , unpacked and forwarded via the exchange line  100  in a time slot. 
   Also if the IP telephone  80  wanted to set up a connection into the local network  22 , the signaling paths  106  and  102  and the voice transfer paths  108  and  104  are used. In this case, ISDN signaling data is generated by the control unit  30  and tunneled via the signaling path  102  to the S 0  adapter  90  and sent to the exchange of the local network  22  after conversion. The reply signaling data arriving from the exchange of the local network  22  is then tunneled from the S 0  adapter  90  via the Internet  12  to the control unit  14 . 
   The procedure is the same if the IP telephone  82  of the branch office F 1  or an IP telephone  84  is used instead of the IP telephone  80 . 
   Connected to the IP data packet transfer network  18  in the branch office F 2  are:
     a line  110  to the Internet  12  and   an IP telephone  112  and an IP telephone  114 , e.g. IP telephones of the type Optipoint from the company Siemens AG.   

   The IP telephone  112  contains e.g. as a plug-in card an S 0  adapter  116  which is connected both to the data packet transfer network  18  (see connection  118 ) and to an exchange line  120  which leads to an exchange (not shown) of the local network  24 . Signaling for the interface S 0  takes place in accordance with e.g. the ISDN protocol on the exchange line  120 . The above-described operations for the branch office F 1  also apply for the branch office F 2  when the local network  24  is involved in a telephone connection instead of the local network  22 . 
   In relation to the normal operating mode,  FIG. 2  shows two protocol stacks  130  and  132  in the adapter unit  90  and two protocol stacks  140  and  142  in the central telecommunication system  14 . The functions of the protocols belonging to the protocol stacks  130  to  142  are performed e.g. by a circuit arrangement without a processor or by a circuit arrangement with a processor which executes instructions which are stored in a storage unit. 
   In accordance with the OSI (Open Systems Interconnection) model, the protocol stack  130  includes as its lowest protocol level  150 , i.e. as a physical layer or bit transfer layer, a TDM layer (Time Division Multiplex), such that the signaling data is transferred in a signaling channel in accordance with the time-division multiplexing method. The second protocol level  152 , i.e. the data link layer, of the protocol stack  130  is operated e.g. according to ITU-T standard Q.921. 
   While the protocol levels  150  and  152  in the adapter  90  are processed completely, signaling messages of the third protocol level  154 , i.e. the network layer, are merely recognized but not processed in the adapter  90 . The third protocol level concerns the signaling e.g. in accordance with ISDN subscriber protocol DSS1 (Digital Signaling System No. 1); see Q.931 ff. of the ITU-T. The data belonging to this protocol is copied and tunneled via the Internet  12 . 
   For the purpose of tunneling, use is made of a proprietary protocol, for example, such as the Siemens-internal protocol CorNet TC (Corporate Network-Tele Commuting), which recognizes the following basic reports:
     “Log On” or “Log Off” for signaling the setup or cleardown of a data connection,   “Keep Alive” for monitoring the data connection,   a container report in which e.g. signaling messages can be transported in accordance with other proprietary protocols or also the signaling messages to be tunneled can be transported in accordance with DSS1.   

   In terms of switching, CorNet TC belongs to the link level  2 . 
   The TCP protocol (Transmission Control protocol) is used in the fourth protocol level, i.e. in the transport protocol level of the IP protocol stack  132 . The Internet protocol IP lies underneath this in the third protocol level, i.e. in the network protocol level. The two lower protocol levels in the IP protocol stack  132  are combined into a network access protocol level  158  and represent a LAN or WAN transfer protocol e.g. an Ethernet protocol. Higher protocol levels  159  of the IP protocol stack  132  are indicated by dots in  FIG. 2 . 
   The tunneling of the signaling data in the adapter  90  is illustrated by means of a double arrow  160  in  FIG. 2  and takes place both in the direction from the exchange line  100  to the line  72  and in the opposite direction from the line  72  to the exchange line  100 . The CorNet TC container reports are sent from the adapter  90  into the Internet  12  or received from the Internet  12  as IP data packets. 
   In the exemplary embodiment, the functions of the IP protocol stack  132  are performed by the IP telephone  80 , which also uses the functions of such a IP protocol stack  132  for the IP telephony. The telephone  80  contains an internal bus system into which an S 0  card is plugged. Untunneled DSS1 messages between the S 0  card and the telephone  80  are exchanged via the internal bus system. Consequently, multiple use is made of software and hardware and the additional switching-related and software-related burden for the adapter  90  is small. 
   In another exemplary embodiment, the S 0  adapter  90  is implemented independently of an IP telephone and contains in this case a dedicated connection to the data packet transfer network  16  of the branch office. The functions of both protocol stacks  130  and  132  are also performed in the S 0  adapter  90  in the case of the alternative exemplary embodiment. 
   In its right-hand part,  FIG. 2  shows the protocol stacks  140 ,  142  which correspond to the protocol stacks  130 ,  132 . The protocol stack  140  is constructed in the same way as the protocol stack  132  and contains:
         a lower network access protocol level  170  whose functions are provided e.g. by the IP module  38 ,   a third protocol level  171  situated thereupon for implementing the IP protocol,   a fourth protocol level  172  for implementing a transport protocol, e.g. TCP, wherein the functions of the protocol levels  171  and  172  are performed e.g. likewise by the IP module  38 , and   further higher protocol layers  174  which are indicated by dots but are not explained in further detail.       

   The protocol stack  142  is constructed in a similar manner to the protocol stack  130  and contains:
         a lower protocol level  180  which works in accordance with a time-division multiplexing method,   a second protocol level which works e.g. in accordance with ITU-T standard Q.921 and creates a data transfer connection between the exchange module  34  and the exchange in the local network  20 ,   a protocol level  184  which, unlike the protocol level  154 , is fully processed and performs functions of the DSS1 protocol, and   higher protocol layers  186  which are indicated by dots but are not explained in further detail.       

   The protocol level  184  of the protocol stack  142  is used both for signaling in the direction of the local network  20  and for onward routing of signaling messages which have been tunneled through the Internet  12  to the control unit  30  or for onward routing of signaling messages which come from the control unit  30  and must be tunneled through the Internet  12 ; see double arrow  190 . 
   In the telecommunication system  14 , the DSS1 signaling messages which have been tunneled through the Internet  12  are unpacked from the CorNet TC container reports and forwarded to the control unit  30 . Conversely, the DSS1 signaling messages which must be tunneled through the Internet  12  are inserted into CorNet TC containers and then into IP data packets. 
   If the Internet  12  or the central telecommunication system  14  fails, it is not possible to control the telephones  80  to  84 ,  112 ,  114  in the branch offices F 1 , F 2  or the exchange lines  100 ,  120  leading to the branch offices F 1 , F 2  without the additional measures explained below. 
     FIG. 3  shows a first variant of an emergency operating mode, in which the functions of the adapter unit  90  are performed by an adapter unit  90   a . The IP telephone  80  detects the failure of the Internet  12  or the failure of the telecommunication system  14 . 
   The IP telephone  80  which includes the S 0  adapter unit  90   a  thereupon assumes the role of a small telecommunication system (PBX—Private Branch Exchange) including the IP telephones  80  to  84  as extension terminals. The IP telephone  80  operates the S 0  interface of the S 0  adapter unit  90   a  in the same way as a small telecommunication system which has an S 0  ISDN switching center interface. 
   For their part, the IP telephones  82 ,  84  detect the failure and switch to a substitute IP address of the emergency telecommunication system. The IP telephones  82 ,  84  automatically register themselves in the emergency operating mode with the small telecommunication system which is situated in the IP telephone  80  or in the S 0  adapter unit  90   a . In the emergency operating mode, therefore, all IP telephones  80 ,  82  and  84  can handle incoming and outgoing switching center calls or exchange calls via the unit comprising IP telephone  80  and S 0  adapter unit  90   a  as extension subscribers of the emergency telecommunication system. 
   In the emergency operating mode, the IP telephone  80  and its S 0  adapter unit  90   a  work as the central telecommunication system  14  in relation to the IP telephones  82 ,  84 . In the exemplary embodiment, the telecommunication system  14  is a telecommunication system of the type HiPath 4000 from the company Siemens AG. The IP telephones  82 ,  84  as so-called HFA-IP telephones (HiPath Feature Access) therefore switch over from the central telecommunication system  14  to IP telephone  80  as an emergency telecommunication system. The interface protocols are retained when switching over, particularly proprietary interface protocols such as the CorNet TS protocol or the CorNet IP protocol. 
   In the exemplary embodiment, the IP telephone  80  itself is likewise operated at the emergency telecommunication system in the emergency operating mode. In an alternative exemplary embodiment, the telephone  80  is operated as a main connection line telephone in the emergency operating mode, wherein the emergency telecommunication system and the telephone share the S 0  bus. 
   In the exemplary embodiment which is illustrated in  FIG. 3 , no DSS1 signaling messages are tunneled via an IP network in the emergency operating mode. The DSS1 signaling messages arrive at the IP telephone  80  via an internal bus system of the IP telephone  80 . 
     FIG. 4  shows a very simple second variant of an emergency operating mode, in which the functions of the adapter unit  90  are performed by an adapter unit  90   b . According to the second variant, in the emergency operating mode the IP telephone  80  and the S 0  adapter unit  90   b  work as an ISDN switching center telephone or an ISDN telephone which is directly connected to an exchange. 
   The IP telephone  80  detects the failure of the Internet  12  or the failure of the telecommunication system  14 . As a result of this, the IP telephone  80  uses the S 0  interface in the same way as an ISDN telephone directly at an exchange. Therefore the IP telephone  80  can handle incoming calls from the local network  22  and outgoing calls into the local network  22  as an ISDN telephone in the emergency operating mode. The other IP telephones  80 ,  82  cannot handle calls in the emergency operating mode; see cancellations  200  and  202 . 
   In the emergency operating mode according to  FIG. 4 , it is likewise the case that no DSS1 signaling messages are tunneled via a data packet transfer network. The DSS1 signaling messages are transferred between S 0  card and telephone  80  via an internal bus system of the IP telephone. Therefore no functions of the IP protocol stack  132  are performed in the emergency operating mode. 
     FIG. 5  shows a third variant of an emergency operating mode, in which the functions of the adapter unit  90  are performed by an adapter unit  90   c . According to the third variant, in the emergency operating mode the S 0  adapter unit works as an H.323/ISDN gateway or as a SIP/ISDN gateway in an alternative exemplary embodiment. A gateway is also referred to as a network interworking unit between networks having different signaling protocols. 
   The S 0  adapter unit  90   c  detects the failure of the Internet  12  or of the telecommunication system  14 . As a result of this, the S 0  adapter unit works as an H.323/ISDN gateway or an H.323/ISDN network access unit which assumes responsibility for the conversion between the IP signaling in accordance with H.323 and ISDN signaling (e.g. DSS1) in the case of exchange connections of the IP telephones  80  to  84  involving the local network  22 . At the same time, the S 0  adapter unit  90   c  assumes the role of the gatekeeper in relation to the telephones  80  to  84 . The IP telephones  80  to  84  register themselves with the S 0  adapter unit  90   c  as H.323 terminals in the emergency operating mode, e.g. after detecting the failure. 
   In an alternative exemplary embodiment, the S 0  adapter unit  90   c  works as a SIP/ISDN gateway in the emergency operating mode, such that a conversion between a SIP signaling and an ISDN signaling is performed in the S 0  adapter unit  90   c . The S 0  adapter unit assumes the role of the SIP registrar. The IP telephones  80  to  84  register with the S 0  adapter unit as SIP terminals. 
   In an alternative exemplary embodiment, in the emergency operating mode as per  FIG. 5  the telephone  80  is operated as a main connection line telephone, wherein the network access unit and the telephone  80  share the S 0  bus. 
   In the exemplary embodiment according to  FIG. 5 , it is again the case that no DSS1 signaling messages are tunneled via an IP network in the emergency operating mode. The DSS1 signaling messages arrive at the IP telephone  80  via an internal bus system of the IP telephone  80  and are converted there in the network access unit. The IP protocol stack  132  can nonetheless be used for the connection of the telephone  80  to the IP network  16 . 
     FIG. 6  shows a fourth variant of an emergency operating mode, in which the functions of the adapter unit  90  are performed by an adapter unit  90   d . According to the fourth variant, in the emergency operating mode the S 0  adapter unit  90   d  and the IP telephones  80 ,  82  and  84  work as peer-to-peer terminals of a peer-to-peer architecture. 
   The S 0  adapter unit  90   d  detects the failure of the Internet  12  or of the telecommunication system  14 . As a result of this, the S 0  adapter unit  90   d  works in relation to the IP telephone  80 ,  82  or  84  as a peer-to-peer terminal which implements the conversion between IP signaling in accordance with a peer-to-peer protocol and an ISDN protocol (e.g. DSS1) as a proxy or network interworking unit in the case of switching center connections of the IP telephone  80 ,  82  or  84 . The telephones  80  to  84  also detect the fault and switch over to a substitute IP address of the network interworking unit. 
   In an alternative exemplary embodiment as per  FIG. 6 , in the emergency operating mode the telephone  80  is operated as a main connection line telephone, wherein the network access unit and the telephone  80  share the S 0  bus. 
   In the exemplary embodiment according to  FIG. 6 , it is again the case that no DSS1 signaling messages are tunneled via an IP network in the emergency operating mode. The DSS1 signaling messages arrive at the IP telephone  80  via an internal bus system of the IP telephone  80 . The DSS1 signaling messages arrive at the IP telephone  80  via an internal bus system of the IP telephone  80  and are converted there in the network access unit. The IP protocol stack  132  can nonetheless be used for the connection of the telephone  80  to the IP network  16 . 
   In summary, an enterprise network includes a central facility having an access to the public telephone network and decentrally located branch offices in other local network areas or other exchanges in addition to the central facility. The enterprise network operator operates both the telephone subscribers of the branch offices and their switching center connections controlled from the central system, by tunneling the signaling of the switching center connections of the branch offices via the LAN/WAN which connects central facility and branch offices. 
   The invention provides means for implementing an inexpensive emergency operation in the event of a failure of the central facility or in the event of a failure of the LAN/WAN between the central facility and the branch offices, in particular with regard to the manufacturing and service costs. For example, if an emergency telecommunication system was installed in every branch office in order to allow local switching center connections for branch-office subscribers during emergency operation without connection to the central facility, the operating costs and service costs would be orders of magnitude higher in particular in the case of small branch offices having only few telephone subscribers and correspondingly modest telephony requirements. 
   In all of the described variants, the S 0  interface of the branch offices is either a telecommunication system connection or a multiple device connection. Alternatively, for example, an analog interface with associated analog signaling protocol is also used instead of an S 0  interface in the branch offices.