Abstract:
The invention relates to a method for transmitting data having a reduced bandwidth wherein the amount of data intended for transfer over a data transfer network is reduced by a reduction method, wherein the type of reduction performed is signaled by a source switching device to a terminal switching device in the data transfer network, such that in the terminal switching device, transferred data is processed using a partner method to the reduction method in order to recover the original data.

Description:
CLAIM FOR PRIORITY 
     This application claims priority to International Application No. PCT/EP01/12367 which was published in the English language on Jun. 14, 2002. 
     TECHNICAL FIELD OF THE INVENTION 
     The invention relates to a method for transmitting data with a reduced bandwidth in which the quantity of the data to be transmitted over a data transmission network is reduced by a reduction method. 
     BACKGROUND OF THE INVENTION 
     The data is transmitted in data packets or in transmission channels with a fixed bandwidth in the data transmission network. The data packets are usually of a permanently predefined size. Information relating to the destination is specified in a packet header. The user information data to be transmitted is located in a packet body. 
     During the transmission in transmission channels, in the simplest case a line is used for each user information connection. However, a plurality of user information connections are usually transmitted over one transmission link, fixed time windows being predefined in one time frame for the transmission of the user information data of a user information channel. 
     As a result of opening the telecommunications market, there are network operators which do not have their own transmission links or switching devices.  FIG. 1  shows a telecommunications network  10  whose operator has leased the greater part of the switching and transmission equipment from a conventional network operator and which is therefore referred to below as a new operator. The telecommunications network  10  contains three local switching offices  12  to  16  which are associated with the conventional network operator and are used by the new operator. The network operator of the telecommunications network  10  also has leased transmission links  20  to  26  from the conventional network operator. The transmission link  20  connects the local switching office  12  to a subscriber line concentrator  30  to which a multiplicity of subscribers are connected, two subscribers Tln 1  and Tln 2  of which are shown in  FIG. 1 . A telecommunications system  32 , which is used by a private company for toll-free switching of connections within the company&#39;s premises being also connected to the concentrator  30 . The concentrator  30  has also been leased by the conventional network operator. A PCM30 (pulse code modulation) system, for example, is used for transmission on the transmission link  20 . 
     The transmission link  22  connects the local switching office  14  to a subscriber line concentrator  34  to which a multiplicity of subscribers are also connected, subscribers Tln 3  and Tln 4  of which are illustrated in  FIG. 1 . A telecommunications system  36  is connected directly to the local switching office  14  via a transmission link  38 . 
     The transmission link  24  lies between the local switching offices  12  and  14 . The local switching offices  14  and  16  are connected using the transmission link  26 . A telecommunications system  40  is connected to the local switching office  16  via the public telecommunications network  42 . Voice data according to the ISDN (Integrated Services Digital Network) standard are transmitted in the public telecommunications network  42 . 
     The network operator of the telecommunications network  10  attempts to utilize the leased transmission links  20  to  26  as well as possible. Compression algorithms for voice are defined, for example, in the standards of the ITU-T (International Telecommunication Union—Telecommunication), for example in the Standards G.723 to G.729. According to the standards, the voice information to be switched within the scope of a normal voice connection can be reduced to bit rates of up to 8 kbit/s on certain links or sections without a disruptive loss of voice quality. This permits a basic channel with a bit rate of 64 kbit/s to be used for transmitting up to eight voice connections. If facsimile data, program data or file data are transmitted over a connection, suitable measures can also be taken to reduce the bandwidth. 
     The voice quality of a connection depends in particular on the following factors:
         falsification due to voice processing, for example due to voice compression/decompression,   delays due to transmission, packetizing and/or voice processing,   echo effects, and   information losses, for example due to transmission errors or due to losses of packets or cells.       

     Multiple transcoding leads to audible degradations in the voice quality, for example to:
         voice noise and relatively large voice falsifications,   reduction in the clarity and comprehensibility,   the increased difficulty in recognizing the speaker by his voice, and   listener and speaker echo owing to the relatively long delay times in the transmission of transcoded voice.       

     Multiple transcodings are therefore to be avoided as much as possible. 
     However, multiple transcodings occur to an increasing degree due to the following causes:
         connections are switched between different networks,   the use of reduction methods in private networks, and   the inclusion of new techniques in voice transmission, for example voice transmission over the Internet.       

     SUMMARY OF THE INVENTION 
     It is an aspect of the invention to transmit data with a reduced bandwidth according to a simple method whose use avoids multiple transcodings. In addition, an associated switching device and an associated remote connection unit are provided. 
     According to an aspect of the invention, a method for transmitting data with a reduced bandwidth, in which the quantity of data to be transmitted over a data transmission network is reduced is provided. The type of reduction is signaled by a source switching device for switching data in the data transmission network to a terminal switching device of the data transmission network. The data processed by the reduction method is transmitted to the terminal switching device. The transmitted data is processed as a function of the signaling using a partner method which is associated with the reduction method and which restores the original data in unchanged form or in an essentially equivalent form. 
     According to an aspect of the invention, the type of reduction is signaled from a source switching device for switching data in a data transmission network to a terminal switching device of the data transmission network. The data which is processed by the reduction method is transmitted to the terminal switching device. The transmitted data is processed as a function of the signaling using a partner method which is associated with the reduction method and which restores the original data in identical form or an essentially qualitatively equivalent form. 
     The signaling of the type of reduction to the following switching device in the transmission link ensures that one of a plurality of reduction methods can optionally be used and nevertheless the associated partner method can be determined. In the next switching office, in particular a further reduction with the same method as would be the case for the section-by-section reduction and immediate execution of the partner method at the end of a section, can also be avoided. 
     According to an aspect of the invention, the partner method is executed only in the terminal switching device. The terminal switching device determines, by reference to the destination of the data to be transmitted, that the partner method is to be executed by the terminal switching device itself. In switching devices upstream of the terminal switching device along the transmission link, it is detected, by reference to the destination call number, that the partner method is not yet to be executed. 
     According to another aspect of the invention, multiple transcoding with the same reduction method along the transmission link can be avoided. The consequence is an improved transmission quality, in particular an improved voice quality. 
     The bandwidth of the data to be transmitted is reduced by the use of the reduction method. The transmission capacity of transmission links in the data transmission network and thus the transmission capacity of the data transmission networks can consequently be used very well. In particular, on leased transmission links, the use of the method according to the invention leads to better use of the transmission capacity with a simultaneously high transmission quality. According to another aspect of the invention, the reduction method is carried out in the source switching device. The partner method is carried out in the terminal switching device. The terminals can remain unchanged as a result of this measure. 
     According to another aspect of the invention, the reduction method used is either a method for demodulating fax data, a method for compressing voice data or a method for handling the TFO (Tandem Free Operation) mode. The selection is carried out automatically. A selection unit monitors the data transmitted on the transmission link and selects a suitable reduction method as a function of the data type. The selection method senses the signal tones of various fax devices and modems. If a device is not detected, the fax data is transmitted without processing by the reduction method. The selection unit also contains devices with which a transmission mode for mobile radio data in a fixed network can be detected, said mode also being known under the name “TandemFree Operation”, abbreviated as TFO mode. In this mode, stop data are added to user information data with a bandwidth of 32 kbit/s so that the bandwidth of 64 kbit/s required for transmission in the fixed network is produced. In order to be able to take the TFO mode into account, what is referred to as a TRAU unit Transcoder/Rate Adapter Unit), such as is usually used only at a network gateway between the fixed network and the mobile radio network, is arranged in the selection unit. 
     According to a further aspect of the invention, the data processed by the reduction method is first transmitted from the source switching device to a transit switching device of the data transmission network. The signaling also initially takes place from the source switching device to the transit switching device, and only then from the transit switching device to the terminal switching device. Stop data is added to the transmitted data in the transit switching device. The data processed using the reduction method is transmitted together with the stop data from the transit switching office to the terminal switching device. While the first transmission section between the source switching device and the transit switching device is effectively utilized in terms of its bandwidth, a higher transmission capacity is required on the second transmission section from the transit switching device to the terminal switching device and is actually necessary. This results in a situation in which it is possible to dispense with transmitting the data with specific modules in multiply used basic channels in the transit switching device. Methods in which there are a plurality of transit switching devices between the source switching device and terminal switching device can also be applied. 
     According to yet a further aspect of the invention, it is possible to detect, in the switching devices downstream of the source switching devices, that stop data has been added on the basis of the signaling and the transmission bandwidth of the incoming transmission channel. This stop data can be removed in the switching devices which pass through, if corresponding units are present. The following transmission sections can thus again be utilized better in terms of the bandwidth. If appropriate, stop data which is still present is to be removed in the end switching device. Only then is the partner method executed. 
     In one development, if the switching devices are switching offices in a switched telecommunications network, the signaling takes place inside or outside the transmission channel used for the user information data. The signaling of the type of reduction is carried out in particular using a signaling protocol for the signaling between switching offices. At present, signaling data is increasingly being exchanged between switching offices in accordance with the international signaling protocol No. 7 which was produced by the ITU-T (International Telecommunication Union—Telecommunication Standardization Sector) or its predecessor the CCIT (Commité Consultatif International Télégraphique et Téléphonique). The signaling of the type of reduction can be carried out, for example, by using already defined parameters, not used in the telecommunications network, of the signaling protocol number 7. In particular, the signaling of the type of reduction can advantageously be integrated into protocol-based signaling systems. Signaling which takes place in the user information channel is in principle conceivable but is associated with considerable limitations owing to the restriction on the information which can be signaled. 
     In another development, transmission links on which the data processed by means of the reduction method are transmitted with a reduced bandwidth are preferably characterized by a network administration center in such a way that these transmission links are known to the controller of a switching device or switching office. The execution of the partner method is in this case also controlled as a function of the prescriptions for the transmission links. This procedure makes it possible to inform a multiplicity of switching offices of the respective transmission links in a simple way. 
     The switching devices are generally only capable of switching channels with a specific switching bandwidth. For this reason, stop data is added to the data in transmission channels with a smaller bandwidth than the switching bandwidth before the switching operation. This stop data is either removed again immediately after the switching operation, if the user information data is to be transmitted with a reduced bandwidth, or remains with the user information data if it is not necessary to pay attention to the bandwidth during the transmission over the next transmission link. 
     The invention also relates to switching devices and a remote connection unit for carrying out the method according to the invention and to its developments. In this way, the abovementioned technical effects also apply to the switching device and to the remote connection unit. 
     The remote connection unit according to the invention is used in the method according to the invention for switching connections with a reduced bandwidth. The remote connection unit is either connected directly to a connection unit in which the method according to the invention or one of its developments is executed, or alternatively the remote connection unit is connected to a connection unit of a switching office which does not carry out the method steps according to the inventive method. However, the switching office contains connection units which execute the method according to the invention or its developments. Connections are switched with double use of a main switching matrix of the switching device both via a connection unit which carries out the method steps according to inventive methods and using a connection unit which does not carry out the inventive method. This measure makes it possible to bundle the connections of a plurality of connection units according to the invention. In addition, switching twice in the switching matrix brings about greater flexibility. 
     The invention will be explained below with reference to the appended drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a telecommunications network. 
         FIG. 2  shows a switching office having a plurality of subscriber line modules, 
         FIG. 3  shows a subscriber line module having a plurality of connection units, 
         FIG. 4  shows the signaling of the type of reduction and thus also the required transmission capacity between the switching offices, 
         FIG. 5  shows switching processes in a switching office, and 
         FIG. 6  shows a unit which is remote from a switching office. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 2  shows, in a telecommunications network  50 , a switching office  52 , also abbreviated to SWO, having a plurality of subscriber line modules  54  and  56 . The subscriber line modules are also referred to in English as “line trunk group”, abbreviated to LTG. Further subscriber line modules are indicated by points  58 . 
     A connecting line  60  connects the switching office  52  to a concentrator unit  62 . Digitized voice data, for example, according to the PCM30 (pulse code modulation) standard are transmitted on the connecting line  60  in 30 basic channels which each have a transmission capacity of 64 kbit/s. The connecting line  60  ends at the subscriber line module  54 , which is used to connect lines which come from subscribers, for example from a subscriber Tln 5 . A further connecting line  64  which is connected to the subscriber line module  54  leads, for example, to a telecommunications system (not illustrated). The subscriber line module  54  is of similar design to the subscriber line module  56 , whose design is explained below with reference to  FIG. 3 . 
     A connecting line  66  connects the subscriber line module  56  to a switching office  68 . Voice data, for example according to the PCM30 standard but in a bandwidth-reduced form, is transmitted on the connecting line  66 . The reduction in the bandwidth is represented below by double lines. 
     The switching office  52  also contains a main switching matrix  70  and a central processor  72 . All the subscriber line modules  54  to  56  are connected to the main switching matrix  70  via connecting lines, connecting lines  76  and  78  of which are illustrated in  FIG. 2 . 
     Each connecting line  76 ,  78  has a transmission capacity of, for example, eight Mbit/s so that in each case voice data or control data can be transmitted over 128 transmission channels with a bandwidth of 64 kbit/s. The central processor  72  controls the switching of connections in the switching matrix  70 . For example, the central processor  72  switches a voice channel  80  via which a subscriber Tln 5  which is connected to the switching office  52  can speak with a subscriber Tln 6  connected to the switching office  68 . A signaling channel  82  is used by the subscriber line module  54  to transmit control data to the central processor  72  before the connection set-up. After the switching of the connection using the voice channel  80 , further control data is exchanged between the subscriber line module  54  and the central processor  72 . Control data is exchanged via a signaling channel  84  between the subscriber line module  56  and the central processor  72 . 
     The voice channel  80  and the signaling channels  82 ,  84  have a transmission capacity of, for example, at least 64 kbit/s if the main switching matrix  70  is only capable of switching connections with such a bandwidth (switching bandwidth). 
       FIG. 3  shows the design of the subscriber line module  56  which contains four connection units, two connection units  100  and  102  of which are illustrated in  FIG. 3 . Further connection units are indicated by points  104 . The connecting line  66  is connected to the connection unit  100 . A connecting line  105  on which compressed voice data is transmitted is connected to the connection unit  102 , a plurality of user information channels being transmitted in a basic channel with a bandwidth of 64 kbit/s. 
     The subscriber line module  56  also contains a group switching matrix  106  and a group processor  108 . The connection units  100 ,  102  are connected to the group switching matrix  106  via connecting lines  110 ,  112  so that connections between the connection units  100 ,  102  and an interface  114  can be switched in the group switching matrix  106 . The interface  114  is connected to the group switching matrix  106  via a connecting line  116  and to the group processor  108  via a connecting line  118 . The connecting line  76  is connected to the other side of the interface  114 . 
     The connection illustrated in  FIG. 2  for the call between the subscribers Tln 5  and Tln 6  is switched using a connection  119  in the group switching matrix  106 . The connection units  100 ,  102  are connected to the group processor  108  via a BUS system (not illustrated). The group processor  108  also controls the switching of connections in the group switching matrix  106  via the BUS system. 
     The subscriber line module  56  contains a signal unit  120  which contains, inter alia, a sound generator, a time pulse generator and a dual tone multi frequency (DTMF) receiver. The signal unit  120  is connected via connections (not illustrated) to the connection units  100 ,  102 , the group switching matrix  106  and to the group processor  58 . 
     In addition, the subscriber line module  54  contains a storage unit  122  which is connected via a BUS  124  to the group processor  108 . Programs during whose processing by the group processor  108  the signaling and the connection control are implemented are stored in the storage unit  122 . 
     A compression unit  132  monitors the data switched in the group switching matrix  106  and selects a suitable reduction method as a function of the switched data type. Voice data is compressed. Fax data is demodulated and transmitted with a reduced bandwidth. Data in the TFO mode is also suitably processed. 
       FIG. 4  shows the signaling of the compression method and, thus, also of the required transmission capacity between the switching offices  52 ,  68  and switching offices  200 ,  202 . The switching office  68  contains not only a main switching matrix  204  but also a subscriber line module  206  connected to the connecting line  66 , as well as a subscriber line module  208 . The subscriber line module  208  is connected to a connecting line  210  which leads from the switching office  68  to the switching office  200 . In the switching office  200 , the connecting line  210  is connected to a subscriber line module  212  which transmits user information data which is incoming on the connecting line  210  to a main switching matrix  214  of the switching office  200 . There is also a subscriber line module  216  in the switching office  200 . 
     The switching office  202  contains a main switching matrix  218  and two subscriber line modules  220  and  222 . The switching offices  200  and  202  are connected to one another by means of a connecting line  224 , one of whose ends is connected to the subscriber line module  216  and the other end of which is connected to the subscriber line module  220 . 
     It will be assumed that the subscriber Tln 5  which is connected to the switching office  52  has set up a connection to a subscriber Tln 7 , which is connected to the switching office  202 . The switching office  52  is therefore referred to below as a source switching office. The switching office  202  is referred to as a terminal switching office. The switching offices  68  and  200  are transit switching offices which lie between the source switching office  52  and the terminal switching office  202 . The connection between the subscriber Tln 5  and Tln 7  must be set up via the connecting lines  66  and  224  which have been leased by another network operator. For this reason, the data must be transmitted on the connecting lines  66  and  224  in a compressed form. The user information data is transmitted on the connecting line  210  in compressed form together with stop data in user information channels with a bandwidth of 64 kbit/s. This is represented by two continuous lines which are parallel to one another, and one dashed line parallel thereto. 
     As explained above with reference to  FIG. 3 , the subscriber line module  56  contains a compression unit  132 . In the transmission direction from the subscriber Tln 5  to the subscriber Tln 7 , the compression unit  132  operates as a voice compression unit, demodulation unit or bandwidth reduction unit, depending on the data to be transmitted. As a result of the compression carried out in the compression unit  132 , it is possible to transmit a plurality of user information channels with a transmission bandwidth of, for example, 16 kbit/s each in a basic channel with a transmission capacity of 64 kbit/s on the connecting line  66  to the switching office  68 . The connecting line  66  was characterized, from a network maintenance center (not illustrated) in the switching center  52  and in the switching office  68 , as a connecting line on which data with a reduced bandwidth is transmitted. 
     If what is referred to as a transparent connection is requested by the subscriber Tln 5 , that is to say a connection which is to expressly have a bandwidth of 64 kbit/s, such a connection is also set up in the switching office  52 . The compression unit  132  is not used for such connections. If, on the other hand, a voice connection or a 3.1 kHz audio connection is requested, the voice compression and the bandwidth reduction take place in the compression unit  132 . The bandwidth reduction takes place either at fixed subrates of 64 kbit/s or in packet-oriented fashion. In the latter case, the channel structure or subchannel structure of the connecting line  66  is generally eliminated. In particular, when silence suppression is applied, the packet-oriented bandwidth reduction is preferred. If the connection temporarily becomes a fax/modem connection, the voice processing in the compression unit  132  is switched off. The signaling data of the fax device or of the modem is switched on. This is possible because the compression unit has control over the fax or modem protocols which are usually used. By demodulating the sound signals coming from the fax device or from the modem, the transmission bandwidth can be reduced if the connected device transmits with a transmission rate which is less than 64 kbit/s. 
     If the subscriber Tln 5  is in a mobile radio network and if the compression unit  132  detects the TFO (Tandem Free Operation) mode of place, stop data which is already added at another place is removed again by the compression unit  132 . 
     If the switching office  52  is a network gateway between the mobile radio network and fixed network, the compression unit  132  prevents stop data from being inserted despite the TFO mode during the transmission in the fixed network. To transmit on the connecting line  66 , only one channel with a bandwidth of less than 64 kbit/s is required, for example of 32 kbit/s. 
     From the source switching office  52 , the method which has been carried out for bandwidth reduction in a respective channel by the compression unit  132 , cf. arrow  225 , is signaled using the signaling protocol to the switching office  68 . Signaling is carried out for each logic channel number used on the connecting line  66 . 
     The compressed data transmitted on the connecting line  66  is received in the subscriber line module  206 . 
     Owing to the administrative characterization of the connecting line  66 , the data received on the connecting line  66  is already processed. Initially, the data is divided up according to user information channels. Subsequently, stop data is added to the user information data in each user information channel using adaptation units  226  so that a bandwidth of 64 kbit/s is obtained. Then, the user information data and the stop data are switched via the main switching matrix  204  and the subscriber line module  208 , as in a known switching office. With packet-oriented bandwidth reduction, a 64 kbit/s channel is used exclusively for switching onward the arriving packets of a specific connection in the switching office  204 . 
     Within the scope of inter-office signaling between the switching offices  52  and  68 , the source switching office  52  signals in particular the type of voice compression and thus, inter alia, also the required net bandwidth during the setting up of a connection to the following switching office  68 . The transit switching office  68  detects through use of the destination call number that it is a transit switching office and thus that no voice decompression, fax/modem remodulation or processing in terms of the TFO method is necessary. The outgoing connecting line  210  was not administratively characterized as a connecting line on which data with a reduced bandwidth have to be transmitted. For this reason, the stop data is not removed on the output-end subscriber line module  208 . The compressed user information data is transmitted to the switching office  200  together with the stop data in 64 kbit/s channels via the connecting line  210 . 
     Within the scope of the inter-office signaling, in addition to the previously used signaling data, the switching office  68  signals onward to the switching office  200  the type of compression and the net bandwidth of the data received by the source switching office  52 , cf. arrow  228 . 
     Owing to the forward signaling of the compression type of the connection and the fact that the user information data arrives via an input-end connecting line  210  which is not optimized in terms of bandwidth, the switching office  200  is capable of detecting the content of the 64 kbit/s channel coming from the transit switching office  68  as compressed data with stop data. The output-end connecting line  224  is determined by the destination call number. As already mentioned, the connecting line  224  is characterized as a connecting line on which the data can be transmitted with a reduced bandwidth. As the data is already present in a compressed form, only the stop data is removed from the respective channels in the subscriber line module  216  using an adaptation unit  230 . 
     The switching office  200  signals the type of compression in a respective channel and the associated, actually required transmission bandwidth to the switching office  202 , cf. arrow  232 . The terminal switching office  202  detects by reference to the destination call number that the subscriber Tln 7  is connected within its own subscriber line region. Owing to the forward signaling of the type of compression, the terminal switching office  202  is capable of carrying out, in the input-end subscriber line module  220 , a partner method of the method carried out in the subscriber line module  56  for bandwidth reduction. The partner method is carried out in a decompression unit  234  which is located on the subscriber line module  220 . Depending on the type of bandwidth reduction, either voice decompression, voice pause generation or fax/modem remodulation is carried out in the decompression unit  234 . In this way, the transmitted data is essentially restored to its original form which was present when the data was generated by the subscriber Tln 5 . 
     The method explained by reference to  FIG. 4  ensures that excess transcodings do not occur in the transit switching offices  68  and  200 . Voice data and fax data are transmitted in a way which is optimized in terms of the bandwidths. In addition, codings can be supported in the sense of a tandem-free operation. If the terminal switching office  202  is a network gateway between the fixed network and a network with mobile subscribers, then, in the decompression unit  234 , the signal which is received via the connecting line  224  is passed on without being changed to the mobile radio network. Stop data do not have to be removed. 
     In a similar way to that explained with reference to  FIG. 4 , the data generated by the subscriber Tln 7  is directed via the terminating switching office  202 , the transit switching office  200 ,  68  and the source switching office  52  to the subscriber Tln 5 . However, a compression unit (not illustrated) which corresponds to the compression unit  132  is used in the subscriber line module  220 . Stop data is added in the adaptation unit  230 . Stop data is removed in the adaptation unit  226  and the partner method associated with the method for reducing the bandwidth is carried out in a decompression unit, corresponding to the decompression unit  234 , of the subscriber line module  56 . If the subscriber Tln 7  calls the subscriber Tln 5 , signaling of the type of compression and of the required transmission bandwidth takes place in the direction opposite to the direction illustrated by the arrows  225 ,  228  and  232 . 
       FIG. 5  shows switching operations in the switching office  68 . The subscriber line module  206  is of similar design to the subscriber line module  56 . However, the subscriber line module  206  does not necessarily contain a unit corresponding to the compression unit  132 . The subscriber line module  206  contains four connection units  250  to  256  for connecting various connecting lines. The connecting line  66  is connected to the connection unit  250 . At the output end, the connection unit  250  is connected via a line  258  to a group switching matrix  260  of the subscriber line module  206 . The adaptation unit  226  is located between the connecting line  258  and the group switching matrix  260 . In addition, the subscriber line module  206  contains an interface  262  with the main switching matrix  204 . 
     The design of the subscriber line module  208  corresponds essentially to the design of the subscriber line module  206 . An interface  264  forms the connection point to the main switching matrix  204 . A group switching matrix  266  in the subscriber line module  208  has the same design and the same function as the group switching matrix  260 . In addition, the subscriber line module  208  contains four connection units  270  to  276 . The connecting line  210  is connected to the connection unit  274 . A connecting line  278  is connected to the connection unit  272  and leads to a further switching office (not illustrated). 
     A channel structure  280  on the connecting line  66  was defined administratively by a network maintenance center. The PCM30 system on the connecting line  66  was divided into logic transmission channels with a transmission rate which is less than 64 kbit/s. The channels with a reduced transmission rate typically have a transmission bandwidth of n×8 kbit/s, n being a natural number greater than 1. In the exemplary embodiment, a reduced bandwidth of 16 kbit/s per user information channel was selected. For this reason, from a logical point of view, there are user information channels K 1  to K 120  on the PCM30 system  120 . In each case, four user information channels are transmitted in a physical channel P 1  to P 31  with a transmission bandwidth of 64 kbit/s. For example, the four user information channels K 1  to K 4  are transmitted in the physical PCM channel P 1 . The physical channel P 16  can, in principle, be used for the transmission of signaling data and is not divided here into logic channels with a reduced transmission bandwidth. 
     During the signaling, the user information channels K 1  to K 120  are designated by their logic channel number. If a channel is seized on a connecting line which is set up in this way, the transmission rate of the designated channel and its physical position are determined from the logic channel number in the subscriber line module  206 . 
     In the adaptation unit  226 , the incoming two bit-long user information data words of the designated channel are written in a predefined fashion into a data word of a 64 kbit/s channel of a PCM frame with a word length of eight bits. Stop data is written into the remaining bit places, for example, by assigning the value zero for the bit places to be filled in. Subsequently, the eight bit-long data words in the switching office  68  are transmitted from the group switching matrix  260  to the main switching matrix  204  via the interface  262 . A data word  280  which is generated by the adaptation unit  226  contains voice data of the user information channel K 1  in the first two places. The further six places of the data word  280  are filled with stop data X. A data word  282  contains user information data of the user information channel K 6  in the first two bit places. The further bit places of the data word  282  are filled with stop data X. In this way, only data words of a bit length of eight bits in channels which are to be switched through with a transmission rate of 64 kbit/s pass from the subscriber line module  206  to the main switching matrix  204 . 
     After the data words  280  and  282  have been switched in the main switching matrix  204 , they are transmitted in unchanged form as data words  284  and  286  to the subscriber line module  208  and switched to one of the connection unit  270  to  276  in the group switching matrix  266  on the basis of the destination call number. The data word  284 , which corresponds to the data word  280 , is switched to the connection unit  274  and transmitted to the switching office  200  in a physical channel P 1  with a transmission rate of 64 kbit/s. The data word  286  passes to the connection unit  272  and is transmitted in a physical channel P 2  on the connecting line  278  with a transmission rate of 64 kbit/s. 
     The switching function of the switching office  68  thus remains unchanged in comparison with the function of conventional switching offices. The conversion between the logic channel number and physical position is defined administratively so that the switching functions of the switching office  52  are not affected. The conversion between logic channel number and physical position itself then takes place exclusively in the connection units, for example in the connection unit  250 . 
       FIG. 6  shows a unit  300  which is remote from the switching office  52  and which can be used to carry out the compressed transmission over a connecting line  302 , although a following switching office  304  does not have a subscriber line module which is suitable for processing channels with a transmission bandwidth of less than 64 kbit/s. The remote unit  300  is located several kilometers from the switching office  52  and is set up directly in or at the switching office  304 . The connecting line  302  corresponds essentially to the connecting line  66  so that there are transmission channels with a transmission rate of, for example, 16 kbit/s on the connecting line  302  as well. 
     The switching office  52  has already been explained above with reference to  FIG. 2  in terms of its design. The subscriber line module  56  and the compression unit  132  were explained above with reference to  FIG. 3 . In the exemplary embodiment according to  FIG. 6 , the switching office  52  contains a PCM interface  306  to which the end of the connecting line  302 , leading to the switching office  52 , is connected. The PCM interface  306  switches basic channels with a transmission bandwidth of 64 kbit/s and is connected to the main switching matrix  70  via a line  308 . Each basic channel contains four user information channels. Compressed user information data is transmitted on the line  308  from the main switching matrix  70  to the PCM interface  306 . 
     Instead of the connecting line  66 , in the exemplary embodiment according to  FIG. 6 , a connecting line  310  which leads from the connection unit  100  to a directly adjacent connection unit of the subscriber line module  56  is connected to the subscriber line module  56 . Via the connecting line  310 , the user information data coming from the connection unit  100  passes in compressed form up to the connecting line  308  via the group switching matrix  106 , a transmission link  312  and a transmission channel  314  switched in the switching matrix  70 . 
     The remote unit  300  contains a PCM interface  316  which is connected to the end, leading to the remote unit  300 , of the connecting line  302 . In addition, the remote unit  300  contains a subscriber line module  318  whose design corresponds essentially to the design of the subscriber line module  206  explained above with reference to  FIG. 5 . An adaptation unit  320  fulfills the function of the adaptation unit  226 . The subscriber line module  318  generally contains a compression unit/decompression unit. In the exemplary embodiment according to  FIG. 6 , this unit is however not used. A connecting line  322  connects the PCM interface  300  to the subscriber line module  318 . A connecting line  326  several meters long connecting line lies between the subscriber line module  318  and a subscriber line module  324 . The subscriber line module  324  is a conventional subscriber line module in the switching office  304 . In addition to a main switching matrix  328 , a further subscriber line module  330  of the switching office  304  is also illustrated in  FIG. 6 . The subscriber line module  330  is connected to the connecting line  210  which leads to the switching office  200 . 
     A control channel  332  leads from the switching office  52  via the connecting line  302  up to the subscriber line module  318 . Using the control channel  332 , the channels on which user information data is transmitted with a reduced bandwidth can be characterized in the subscriber line module  318 . Data passes in user information channels with a transmission rate of 16 kbit/s via the connecting line  302  to the PCM interface  316  and from there to the adaptation unit  320 , also with a transmission rate of 16 kbit/s. In the adaptation unit  320 , the user information data items of various user information channels are separated from one another and replaced by stop data so that the further transmission via the connecting line  326  to the subscriber line module  324  can take place with a transmission rate of 64 kbit/s per user information channel. If appropriate, the user information data is handled by the partner method which is available in the compression unit, as a result of which a bandwidth of 64 kbit/s is also produced. In the switching office  304 , decompressed user information data is switched, or user information data is switched together with the stop data, and said data passes via the connecting line  210  to the switching office  200  and from there to the switching office  202 . The method steps explained above with reference to  FIG. 4  are executed in the switching office  202 . 
     Signaling of the type of compression in the switching office  52  is maintained, as also explained above with reference to  FIG. 4 , cf. arrows  225 ,  228  and  232 .