Abstract:
A method for transferring data from a first switch to a second switch selectively by line-switching or by packet-switching as wellas to a switch for carrying out the method. Data packets are thereby first transferred packet-switched through a packet-switching network to the second switch. With the presence of a corresponding control signal a line-switching connection is established from the first switch to the second switch and the data are then transferred through this connection. Where applicable a renewed changeover to a packet-switching transfer is carried out. A flexible packet-switching or line-switching data transfer linked with dynamic costs between the junctions of a telecommunications network is enabled.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS AND CLAIM FOR PRIORITY 
     This application is a continuation of application Ser. No. 11/165,280 filed Jun. 22, 2005, now U.S. Pat. No. 7,145,902, which is a division of application Ser. No. 09/147,970 filed Mar. 23, 1999, now U.S. Pat. No. 6,954,453, which is a National Stage Entry of International Application No. PCT/DE97/02363, filed Oct. 7, 1997. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention relates to a method for transferring data from a first switch to a second switch selectively by line switching or by packet switching, and to a switch for carrying out the method. 
     The present-day situation in telecommunications is marked by a division between two different connecting and switching technologies. These are the synchronous line-switching technology (line-switching or circuit switching) and asynchronous packet-switching technology (packet-switching). 
     Line-switching connections use line switches, alias line switching equipment, between the individual line sections, each of which copies over 1 byte packets and has a corresponding buffer size. Packet-switching connections use packet switches, alias packet switching equipment, between the individual line sections of a network, each of which copies over multi-byte packets. The buffer size of a packet switch correspondingly amounts to n bytes where n stands for the number of bytes in the copied data packets. The term “switch” is used below so that it includes both a line switch of a line switching network and a packet switch of a packet switching network. 
     A line switch, alias line switching equipment, is called telecommunications apparatus (TK apparatus) in the private sector, and exchanges of the network supplies in the public sector. A packet switch, alias packet switching apparatus, is also called a router, an IP switch or a host computer. 
     Line switching connections are synchronous, i.e., data transfer is carried out substantially without any time delay from one line section to an adjoining line section through a switch (here, a line switching apparatus). 
     When a line-switching call is put through, a connection is continually provided in real time with the complete bandwidth of a channel between two points. Even if no useful information is being sent e.g., during a pause in telephone conversation, the transmission channel is occupied or engaged. Line-switching connections are expensive, particularly during telephone conversations since the costs accumulate irrespectively of the information actually transferred. The advantage of a line switching connection is that it is free of any time delay and has a fixed bandwidth. 
     The other important type of data exchange nowadays is the packet exchange. With packet exchange, data, e.g., audio data, video data or computer files, are packeted and transferred as data packets. Packet switching works in the asynchronous transfer mode, i.e., data is transferred time-delayed between two adjoining line sections by a switch (here, a packet switching apparatus). In the case of packet-switching exchanges, and quite differently from line-switching exchanges, a fixed connection does not have to be maintained. It is connection-less, i.e., each packet is treated individually and not in conjunction with others. 
     Packet switching is used in particular on the Internet. The data packets are termed there as IP packets (IP=Internet Protocol). Each IP packet contains a header which contains, inter alia, sender and receiver addresses. The IP packets form a data flow which is transferred through packet switching apparatus (alias IP switches or Routers or Host computers) in the Internet from the sender to the relevant receiver. 
     As a result of the length of the IP packets (16 bytes or more), a time delay occurs in the packet switching apparatus when copying. This time delay can be so great, when there is a heavy load on the packet switching apparatus which passes a data packet over the route to the destination address, that certain applications are no longer possible. 
     These delays are of considerable significance particularly in the case of the Internet. With Internet telephony, a cost-conscious caller uses the normal Internet with approximately 8 kbit/s bandwidth and a time delay of 0.5 seconds. When the Internet is overloaded, the time delay of the individual packets becomes so great that an acceptable conversation connection between telephone partners is no longer possible. 
     Internet telephony is marked by a great advantage that only the relevant local telephone charges to the next POP (Point of Presence), the access point to the Internet offered by an Internet Service Provider ISP, as well as time charges calculated by the ISPs for the length of the Internet access as well as volume charges, but not expensive long distance telephone charges are incurred. 
     From U.S. Pat. No. 4,996,685 a method and device are known which allow in an ISDN communications network, during an existing connection between a user and a host computer, a dynamic change between a line switching connection through an ISDN B channel and a packet-switching connection through an ISDN D channel. A command to change between a line-switching and a packet-switching connection thereby always emanates from the Host computer. 
     The method disclosed in U.S. Pat. No. 4,996,685 is restricted to undertaking on an ISDN connection a change between a line-switching and a packet-switching data transfer whereby a line-switching transfer is carried out on a B channel and a packet-switching transfer is carried out on the D channel. A method of this kind is indeed expedient to produce effective access from an end subscriber to a host computer, possibly an exchange point of the telephone network or an access point to the internet, but does not relate to the transfer of data between switches or routers of a network. 
     WO 95/31060 A1 describes a method for transferring data between an information source and a destination device wherein the data to be transferred are transferred as data packets. Depending on type of information of the data packets, the data are transferred automatically either solely by line-switching or solely by packet-switching. More particularly in the case of small amounts of data to be transferred, a packet-switching transfer is chosen and in the case of large amounts of data, a line-switching data transfer is chosen. 
     WO 95/25407 A1 describes a method for transferring data between a data source and transceiver either through a packet-switching network or a line-switching network. A control device is thereby provided which uses certain criteria to decide which network and which method of transfer is best suited for the transfer and then selects same. 
     U.S. Pat. No. 4,903,260 describes a digital coupling network and a coupling field chip which are designed so that paths leading from any input to any output can, depending on requirements, either be switched through for line-switching connections or can be preset for packet-switching information. Preset paths for the packet-switched information thereby form a network whose junctions lie in the coupling field chip of the coupling network. Those function devices which are required to send each data packet on the path preset for same are integrated in the coupling field chips. It is thus possible to divide up a single coupling network depending, on requirements, dynamically into a line-switching network and a packet-switching network. 
     SUMMARY OF THE INVENTION  
     Based on the prior art, the present invention is concerned with the problem of providing a method for transferring data from a first switch to a second switch and providing a switching for carrying out the method which, depending on the data origin and headers of a user or network management system, allows flexible data transfer between the switches and more particularly cost-effective data transfer with real time properties. 
     The solution according to the present invention makes it possible during pocket-switching connection between two switches to achieve a dynamic change-over to line-switching connection without interrupting the connection. This is always advisable if a data build up of data packets exists before the switches of the packet-switching network. Through the establishment of a line-switching connection between the switches, a bypass is produced according to the invention on which data can be transferred with fixed bandwidth and slight time delays substantially in real time so that the data blockage is bypassed. Since a line switching connection is established only when required, i.e., when a packet-switching data transfer no longer has the desired bandwidth, the invention allows a flexible, most cost-effective data transfer. 
     The term “switch” is used in the sense of the present invention as already explained so that it includes both a line-switch of a line-switching network which copies over 1-byte packets, and a packet-switch (router) of a packet-switching network which copies over multi-byte packets. Data to be transferred can be any type of data, such as audio data, video data or computer files. 
     The present invention provides for carrying out the method according to the present invention, on switches which allow both line-switching and packet-switching, and combine the functions of a line-switch and a packet-switch. A switch according to the present invention has a packeting device for packeting and unpacketing data, an IP switching device for routing data packets, a line-switching device for establishing connections for switching through data channels and a control device which directs incoming data either to the IP switching device or to the line switching device depending on the control signals. 
     The corresponding control signals are triggered by a user, or at the command of a network management system and are transferred together with other signaling data to the switch. Alternatively, the switch itself automatically produces a corresponding control command upon exceeding a certain bandwidth of the packet-switching transfer. 
     The network which consists of interconnected switches according to the present invention forms an Intranet wherein data transfer can be interchanged dynamically between line switching and packet switching and ensures, under normal conditions, data transfer substantially in real time through the possibility of establishing when required, a line-switching connection of a fixed bandwidth. This is particularly important for internet telephony. 
     There are numerous useful areas for the switches according to the present invention. The switches according to the present invention can even replace conventional line-switches such as TK equipment and exchanges as well as packet switches. More particularly they can be used to build up new networks with real time capacity (intranets) which can operate both by line-switching and by packet-switching. 
     The method according to the present invention is used in a first embodiment of the invention between two switches which are part of a line-switching network, but not directly part of a packet-switching network. Therefore, for a packet-switching transfer, first a connection is established through the line-switching network from a first switch to an access point to the packet-switching network (such as Internet access point). The data are transferred line-switched to the access point to the packet-switched network, where they are packeted if they do not already exist as packets, and are transferred from the access point packet-switched through the packet-switching network to the second switch. The data are thereby preferably already packeted in the first switch and transferred as data packets line-switched to the access point. 
     If both switches are part of both a line-switching network and a packet-switching network, then in a second embodiment of the method according to the present invention, a packet-switched data transfer can take place directly between the switches. With both embodiments, with a presence of a corresponding control signal, a line-switching connection is built up through the line-switching network directly to the second switch. If there is no longer any need for a line-switching transfer, then a change back to a packet-switching transfer takes place. 
     In one embodiment of the method according to the present invention, the data packets remain, after the change-over to a line-switching data transfer, as data packets and are then transferred as such by line-switching. In an alternative embodiment, the data packets are unpacketed, more particularly the headers of the data packets are removed, and only the data are then transferred by line-switching. The advantage of the first embodiment lies in the fact that if the data is once again to be transferred over the packet-switching network, they already exist as data packets and therefore time is saved when switching. The advantage of the second embodiment is that by removing the headers from the individual data packets, the effective bandwidth of the data transfer is increased. 
     In a preferred embodiment of the method according to the present invention, the same data channel is used for transferring the data packets from the first switch to the access point to the packet-switching network, and for transferring data from a first switch to the second switch through the line-switching network. This embodiment has the advantage that only one data channel is constantly engaged which, depending on the type of transfer, transfers data either to the access point to the packet-switching network, or to the other switch. More particularly in the case of an ISDN network, the same B data channel is used for both sending data to the access point to the packet-switching network, and for sending data through a bypass to another switch. 
     A data transfer from the first switch to the access point to the packet-switching network always takes place by line-switching. Thus compared with a packet-switching transfer to the access point (e.g., through an ISDN D channel), which is also possible, a larger and fixed bandwidth is ensured up to the access point. If an ISDN network exists, then an ISDN B channel is used as the data channel. Data packets are thereby sent through the B channel by applying them to the ISDN framework. This is known per se and fixed in the PPP protocol. 
     In a further embodiment, two data channels are provided for data transfer from a first switch, whereby through the first data channel the data packets are transferred to the access point to the packet-switching network, and through the second data channel the data are transferred to the second switch through line-switching. Depending on the type of transfer, either the one data channel or the other data channel is used. This has the advantage that data can be transferred simultaneously by packet-switching and line-switching. By way of example, less important data such as correspondence is transferred by packet-switching and audio data is transferred by line-switching. 
     In a further preferred embodiment of the present invention, with a line-switching data transfer between the first switch and the second switch or between the first switch and the access point to the packet-switching network, the data of several users are multiplexed on a data channel by forming sub-channels of fixed bandwidth. It is thereby provided that the data of one user are transferred after its selection by line-switching with a transfer rate which corresponds to only a fraction of the transfer rate of the bandwidth which is available as standard to the user. 
     More particularly with an ISDN network on the B channels sub channels of a bandwidth of 32, 16, 8, 4, 2 or 1 kbit/s are available. To implement the sub-channels, only each n-th byte or each n-th bit of an ISDN frame is copied over immediately and forwarded on the data channel to the next switch or to the computer network access point. 
     The formation of sub-channels on a data channel, possibly an ISDN B channel or a data channel of the GSM mobile phone system, allows additional flexibility for the data transfer. In many cases, it is entirely adequate that the bandwidth only uses up a part of the bandwidth which is available on a data channel. The use of the sub-channel thereby has an advantage for the user that according to the bandwidth of the sub-channel, lower costs are incurred but a fixed bandwidth is still available. Sub-channels of different bandwidth thereby define different service quality. 
     Thus a packet-switching transfer, a line-switching transfer with a part of the available bandwidth of a data channel, and a line-switching transfer with the complete bandwidth of the data channel are available as alternatives. 
     In a further development of the method according to the present invention, with a change from a packet-switching transfer to a line-switching transfer, the address information of the data packets are evaluated and sorted according to network topology. Thus for each data packet whose destination addresses relate to the same topological area of the network, a switch located in this area is selected, a line-switching connection (bypass) is established to the selected switch and the corresponding data or data packets are transferred to the switch by line-switching. 
     A classification of the data packets is thereby preferably carried out according to geographical points of view, whereby, for data packets whose destination address relate to the same geographical area, a switch located in this geographical area is selected and a line-switching connection is established to this switch. This allows a bypass to be effectively established since for data packets with roughly the same destination, a line-switching connection is established directly to a network junction which lies, regarding network topology, in the destination area of the data packets. The establishment of an effective bypass between the individual switches has great importance in the case of packet-switching networks since a data packet can run on the way from Berlin to Munich via Paris and New York. By bringing together all data packets intended for Munich and transferring these data packets by line-switching directly from Berlin to Munich, it is possible to provide a more effective data transfer. 
     For classifying data packets according to geographical points of view, it is preferable to compare the destination address with destination addresses stored in a data bank whereby the data bank contains a link between the destination addresses and the associated geographical position. The data bank is thereby preferably integrated in the switch. If the data packets are IP data packets, then the relevant IP addresses are consulted in the data bank and assigned to a certain bypass depending on the geographical destination. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows diagrammatically a telecommunications network according to the invention; 
         FIG. 2  shows diagrammatically a telecommunications network known in the prior art; 
         FIG. 3  is a diagrammatic illustration of a telecommunications network in which switches according to the invention form an Intranet; 
         FIG. 4  shows diagrammatically a switch according to the invention; 
         FIG. 5a  shows a flow chart of the method according to the invention for transferring data between two switches; and 
         FIG. 5b  shows diagrammatically a flow chart for the method according to the invention for selecting a destination switch through topological points of view. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 2  shows a conventional telecommunications network. Data terminals such as telephone  1  or personal computer  2  are connected to an exchange  4  of the telephone network directly or by means of a telecommunications apparatus (TK-apparatus)  3  through an ISDN/POTS line. Where applicable, a local network LAN  5  is connected to the TK-apparatus  3 . The exchanges  4  pass on incoming connection requests and provide line-switching connections. Entry to a packet-switching network is possible through an access point POP (Point of Presence)  6 . Data are transferred between interlinked packet switches  10  by packet-switching through the packet-switching network. 
     The internet will now be considered as packet-switching network without restricting the present invention. Indeed any packet-switching network could be used such as mobile phone networks within the scope of the present invention. 
     The technologies used are known per se. The data transfer between terminals  1  and  2  and a line-switch (TK apparatus  3  or the exchange  4 ) takes place line-oriented, and similarly the data transfer between the individual line switches (such as between the individual exchanges  4  and the POP  6  of the Internet Service Providers IPS). The lines are switched through via coupling fields which are produced in the exchange and in the TK-apparatus. 
     The PCM  30  System is particularly wide spread in ISDN networks wherein 8 Bit codewords for each 30 useful channels within a scanning period of 125 μs are multiplexed and sent in one pulse frame. However, no multiplexing takes place on a single channel. The pulse frame is transferred in constant repetition between sender and receiver even if no useful signals are contained. In the digital coupling field, individual bytes are copied and then sent (switch of 1-byte-packet). Since during the exchange process only one byte is read into a memory each time and then read out again, only a minimal time delay occurs when exchanging the connecting path. 
     From the access point POP  6  to the Internet, the transfer of data is still only carried out packet-switched on the basis of the known network protocol UDP/IP or TCP/IP. Access to the Internet is brought about by a packet switch (hereinafter also called IP Switch) which receives data packets which are not intended for itself and passes them on to the party network whose address they support. During routing, copying of the IP packets is carried out (switch of multi-byte packets). Time delays thereby occur in the packet-switching network according to the size of the IP packet and the number of routers passing on an IP-packet. These time delays can assume such proportions in the event of overloading the IP switches  10  that in the case of the Internet telephony, delays of more than 0.5 s may occur. 
       FIG. 1  shows a telecommunications network according to the present invention with switches  7 a and  7 b according to the present invention which are shown as starred and are described in detail in  FIG. 3 . The switches  7 a and  7 b integrate the functions of a packet switch and a line switch. 
     The important factor is therefore the possibility of dynamically switching between packet-switching and line-switching during one transfer, as will be described in further detail below. It is thereby possible to change over, when desired, from an asynchronous packet-switching transfer of variable bandwidth to a synchronous line-switching transfer of greater and fixed bandwidth. Internet telephony and downloading of files from a WWW server are two important uses. 
     The implementation of the switches  7 a and  7 b takes place selectively through hardware or software. The line switching, is thereby preferably implemented by hardware and the packet-switching by software. Thus with line switching, after switching through a connection the data are forwarded without further examination, whilst with packet switching the destination addresses of each data packet is evaluated and the next IP switch has to be selected from the routing tables. A switching device for the switches  7 a and  7 b which undertakes a change over between packet and line-switching is preferably likewise implemented as software. 
     The switches  7 a and  7 b can be mounted according to  FIG. 1  at different points in the telecommunications network. The switch  7 a represents a service access module for connecting the LANs or end terminals  1  and  2  to the ISDN/PSTN network and internet. On the user side, the switch  7 a has an ethernet interface for a LAN connection, a printer interface and interfaces for connecting telephones (radio telephones, ISDN telephones, analog telephones) not shown. The switch  7 a is connected to an exchange point  4  of the telephone network through a line  8 . 
     Since the switch  7 a is not a part of the internet, it is necessary for the packet-switching transfer of data through the internet to first make a connection with the access point POP  6 . This can be carried out through the exchange point  4  or even through a standing line  9  to the POP  6 . The data are transferred line-switched up to the POP  6  and are preferably already packeted. If a change to line-switching data transfer is to take place, then a line to another switch is switched through the exchange  4  and the data are transferred to this by line-switching. The switch  7 a thus routes the IP switched/line-switched channels to the line  8  controlled by same so that they are more cost-effective or more real time depending on the wishes of the user or provider of the channels. 
     The switch  7 a is integrated in the internet and connected to further IP switches  11  and/or line switches  12 . Ideally the network still only consists of switches  7 b which allow both line switching and packet switching, so that with each switch  7 b there is the possibility where necessary of providing a higher quality line-switching transfer instead of a packet-switching transfer. A line-switched transfer is thereby established as bypass, more particularly between switches where a data blockage builds up. 
       FIG. 3  shows a telecommunications network wherein switches  7  which allow a data transfer selectively by packet switching or by line switching form an Intranet within the internet. A real time communications possibility is thereby present between the switches  7 . So that this is always possible, additional real time communications channels exist between the switches  7 . These are additional ISDN/PSTN connections or additional Intranet channels. A line switching connection (bypass) between the switches  7  can thus arise not only through the telephone network, but also through separate channels. 
       FIG. 4  shows diagrammatically the establishment of a switch  7  according to the present invention. The switch  7  is part of both a packet-switching network (internet) and a line-switching network (telephone network), i.e., it is connected through lines to further network junctions to which it can transfer or receive line-switched or packet-switched data. Data coming in through a data input  74  can have any source, more particularly come from an IP switch/router, a line-switch such as an exchange point or a telecommunications unit, from a LAN or from an end terminal  1  or  2 . The data input  74  has for this purpose, in a known way, an ethernet interface, an analog interface with A/D converter and an ISDN interface. In addition where applicable, an ATM interface and an interface with a mobile phone network can also be provided. The ISDN networks are with incoming data of 8 bit long words which arrive on a multiplexed supply line of the switch  7 . 
     The switch  7  has a known IP switch  72  which copies over incoming IP packets (switch of multi-byte packets) and forwards them in the internet to suitable switches according to the address of the packets. These relate to the known internet protocol UDP/IP and TCP/IP. A data compression device  721  is integrated as an option in the IP switch  72 . For data compression, a reference is made to the international compression standards developed for individual communications, more particularly the compression process according to ITU standard G. 72 X. Furthermore, a coding device  722  for coding data packets can be provided as an option. 
     Furthermore, the switch  7  has a line switching device  73 . This has a digital coupling  731  which is known per se for switching through telephone conversation channels of the line-switching network, and a multiplex/demultiplex device  732  which produces sub-channels on existing data channels, as will be described in further detail below. 
     The internal control commands, as to whether a packet switching is to take place through the IP switch or a line switching is to take place through the line switching device  73 , are produced in a control device  71 . The device  71  is substantially a switch which forwards the incoming data either as data packets to the IP switch  72  or as bit flow to the line switching device  73 . To this end, the control information of the incoming data are evaluated. The change-over control unit  711  monitors and controls which open connections are present (i.e., which and how many data channels are connected) and which bandwidth the individual data channels require. 
     In detail the control device  71  has a change-over control unit  711 , two packeting/unpacketing devices  713  and  714 , and an intermediate register  712 . The change-over control unit is connected to a topography data bank  75  which contains geographical data for a number of IP addresses. 
     If the incoming data are IP packets, then the headers of the IP packets are evaluated by the change-over control unit  711 . If the incoming data are a continuous data stream, then the signaling information of the signaling channel (in band signaling or outband signaling) are evaluated by the change-over control unit  711 . The basic state thereby provides that the incoming data are sent into the internet through the IP switch  72 . If the incoming data do not yet exist as IP packets then they are packeted into corresponding IF packets in the packeting/unpacketing device  714  and sent to the IF switch. 
     If the data exist as IP packets but are to be transferred line-switched through the line-switching device  73  then the data are, where applicable, unpacketed in the packeting/unpacketing device  713 . More particularly the headers, of the data packets are removed. Unpacketing is optional however and not absolutely necessary since data packets can be transferred line-switched where applicable according to the PPP protocol. The (packeted or non-packeted) data are transferred as bit stream to the line switching device  73  by the change-over control unit  711 . 
     Through a control command which is sent by an end terminal or another switch and for example triggered by a user by pressing a certain button on the terminal or by the network management system, the type of communication is switched over to line-oriented or packet-oriented communication. 
     A corresponding signaling command for changing between packet and line switching is, for example, represented by a certain bit sequence wherein the switching unit  71  stores the detailed incoming data in an intermediate register  712  and compares it with stored bit sequences. If a certain bit sequence exists, then a change over to a different type of switching is carried out. Alteratively, it can also be possible for the change-over control device  711  to monitor the bandwidth of a transfer and on understepping or exceeding a certain bandwidth and/or in the event of a time delay when forwarding IP data packets to automatically release a control command to change over to the relevant other type of transfer. 
     To change from packet switching to line switching, first at the command of the control unit  71 , a connection is made via the line-switching unit  73  (bypass) with another switch (destination switch). To this end, the ISDN signaling command SETUP is sent to the next exchange point. After the connection is established, all the incoming data of the communications connection considered are no longer directed through the IP-switch  72  but through the line-switching unit  73 . The data are now transferred by line-switching with fixed bandwidth through the established bypass to the other switch. 
     The change-over control unit  711  thereby checks, within the scope of the change-over process and prior to sending the data to the device  73 , whether they are IP packets and whether unpacketing is to take place in the packeting/unpacketing device  713 . The decision on this is made dependent on control signals of the network management system or the end terminal or alternatively by the change-over control unit  711  itself dependent on the data arrival. The control signals here contain corresponding transfer parameters. In each case, the data after being sent to the device  73  are then exposed in the coupling field  731  to an ISDN data frame. 
     To establish the most effective line-switching connection possible, it is important to select a suitable destination switch where the bypass is established. To this end, a switch is selected as destination switch which lies in a geographical area which coincides with the destination address of numerous IP packets. Then in particular, these IP packets are transferred through the bypass to the corresponding destination switch so that the data packets still only have a short transfer path from the destination switch to the final destination. 
     The classification of the IP packets and selection of a corresponding destination switch takes place by means of the topology data bank  75  which contains a geographical link between a number of IP addresses and their geographical position. In the line switching device  73 , the IP destination address of each packet is compared with the addresses stored in the data bank  75  and in the event of a successful association of the IP address, this is given a code. This can be a number which characterizes a certain geographical region. This code is recognized by the coupling field  731  and the data packet is then switched through to the corresponding destination switch. 
     Since it would result in too much time delay to interrogate the data bank  75  for each data packet, the change-over control unit  711  contains a cache which can be quickly accessed and in which the result of the last data bank inquiry is stored. If the IP address of a data packet arriving through the data input  74  is stored in the cache, then the corresponding code can be quickly given. 
     If the IP address is not contained in the cache, then a data bank inquiry is carried out, and the IP packets are directed onto the IP switch  72  until the result of the data bank inquiry is provided. Only then is a change-over made for this data to a line-switching transfer through a bypass. It is thereby possible that several bypasses to different destination switches exist at the same time whereby the change over control unit  711  controls the coupling field  731  so that the data packets are each time sent to the destination switch which is most favorable from the network topological point of view. The change-over control unit  711  thus informs the coupling field  731  of which data is to be sent to which destination switch. 
     If the destination address of a data packet is not contained in the data bank  75 , then those intermediate junctions of the packet-switching network are checked to be fully functioning which are normally run through when sending data packets with a certain destination address. To this end, the corresponding data are exchanged between the individual network junctions in known way by trace routing. At the appropriate intermediate junctions, i.e., the intermediate junctions with low functioning output, it is determined whether the ISDN number is known and this is requested where applicable. The change-over control unit  711  of the data bank  75  is thereby operated in the manner already described. A bypass is then established from the change-over control unit  711  to a switch which lies in the chain of switches as close as possible to the destination switch. 
     The multiplex/demultiplex device  732  of the line-switching device  73  allows, depending on the control commands of the change-over control unit  711 , a line-switching transfer to sub-channels with a bandwidth which corresponds to a fraction of the usual bandwidth of a data channel considered. Data channels are thereby bundled which are formed or determined in the coupling field  731  according to the control commands of the change-over control unit  711 . A time multiplex channel of the PCM  30  system is considered as ISDN data frame which has information of  30  data channels and two signal channels. The bandwidth of the data channels each amounts to 64 kbit/s. 
     The multiplex/demultiplex device  732  allows a multiplexing inside each of the  30  data channels of the time multiplex channel. To this end, two methods are alternatively used. In a first method only a part of the 8 bit of a PCM Word is switched through each other, thus 1, 2 or 4 bits. The bandwidth is reduced accordingly to 8, 16 or 32 kbit/s. The data of several channels are in this way multiplexed on one data channel. 
     Alternatively a PCM word (byte) of the time multiplex channel of the PCM  30  system is not switched through in each of the successive pulse frames, but only in each n-th pulse frame whereby the bandwidth is reduced to 64/n kbit/s. 
     The two multiplex methods described can also be combined. By way of example, one bandwidth of 1 kbit/s is produced for one data channel in that each eight bit in each eighth frame of the ongoing data channel stems from the data channel considered. 
     The switching through in the line switching device  731  takes place, depending on the selected data rate, and in the case of transfer rates per data channel unequal to 64 kbit/s, includes the multiplex/demultiplex device  732 . If no multiplexing takes place on a data channel, then the data are passed by the multiplex/demultiplex device  732 . 
     For the channel or sub-channel considered, a line-switching transfer takes place to the switch which represents the other side of the line-switching connection until a control command again reaches the device  71  to switch over again to packet-switching. This command is in turn coded by a certain bit sequence or is produced automatically. Then through the control device, the switched-through line is broken off and the incoming data are then again directed to the IP switch  72 . 
       FIGS. 5a and 5b  show the method sequence.  FIG. 5a  shows the course of the method when changing from a packet-switching data transfer to a line-switching data transfer between two switches. With the presence of a corresponding control signal, a line-switching connection is set up to another switch and the data sent by line-switching. 
     If a line-switching data transfer is to take place to sub-channels of fixed bandwidth then a multiplexer/demultiplexer is activated which multiplexes several data streams so that each time only every n th  bit and/or every n th  byte is reserved in the outgoing data stream for an incoming data stream. It can thereby be provided that the individual sub-channels have a different bandwidth, i.e., the different input data streams have different proportions in the outgoing data stream. With the presence of a further control signal, a change back to a packet-switching transfer is undertaken. 
       FIG. 5b  shows the selection of a suitable switch when establishing a bypass. To this end, the headers of the IP data packets are compared with the information of a data bank. If the header information is associated with a certain geographical destination, then the bypass is established to a switch mounted in this geographical area. If the header information is not associated with a certain geographical destination, then as described above, a bypass is made to an intermediate junction where the data packets pass through in the normal case. Where applicable, the switch has numerous bypasses to different switches wherein each time only data packets with the same or similar topological destination features are transferred to the individual switches within the frame of the bypass. 
     The present invention is not restricted in its design to the embodiments given above. Rather a number of variations are possible which make use of the invention even in fundamentally different types of designs.