Abstract:
To allocate telecommunication channels of different channel capacity, e.g. the ISDN D channel and DECT channels to one another (e.g. simulation of the ISDN channel structure by the DECT channel structure) with good (economic) utilization of the bandwidth and minimum technical complexity in a hybrid telecommunication system, particularly an “ISDN  DECT-specific RLL/WLL” system, a DECT-specific C s  channel and a DECT-specific C f  channel are used in dependence on the amount of information transmitted on the ISDN D channel for transmitting information in the hybrid telecommunication system.

Description:
BACKGROUND OF THE INVENTION 
     In communication systems comprising an information transmission link between an information source and an information sink, transmitting and receiving devices are used for information processing and transmission, in which devices 
     1) the information processing and information transmission can take place in a preferred direction of transmission (simplex mode) or in both directions of transmission (duplex mode), 
     2) the information processing is analog or digital, 
     3) the information transmission over the long-distance transmission link is wire-connected or takes place wirelessly on the basis of various FDMA (Frequency Division Multiple Access), TDMA (Time Division Multiple Access) and/or CDMA (Code Division Multiple Access) information transmission methods—e.g. according to radio standards such as DECT, GSM, WACS or PACS, IS-54, PHS, PDC etc. [Compare IEEE Communications Magazine, January 1995, pages 50 to 57; D. D. Falconer et al.: “Time Division Multiple Access Methods for Wireless Personal Communications”]. 
     “Information” is a generic term which stands both for the intelligence (information) and for the physical representation (signal). Even if an information contains the same intelligence—i.e. has the same information content—different signal forms can occur. Thus, for example, an information relating to an object, can be transmitted 
     (1) in the form of an image, 
     (2) as a spoken word, 
     (3) as a written word, 
     (4) as a coded word or image. 
     In this connection, the type of transmission according to (1) . . . (3) is normally characterized by continuous (analog) signals whilst discontinuous signals (e.g. pulses, digital signals) are usually produced in the case of the type of transmission according to (4). 
     Based on this general definition of a communication system, the invention relates to methods for allocating telecommunication channels of different channel capacity in a hybrid telecommunication system, particularly an “ISDN  DECT-specific RLL (Radio Local Loop)/WLL (Wireless Local Loop)” system according to the precharacterizing clause of claim  1 . 
     Hybrid telecommunication systems are, for example, communication systems containing different—wireless and/or wire-connected—telecommunication subsystems. 
     FIG. 1 shows—representative for the multiplicity of hybrid telecommunication systems—on the basis of the printed documents “Nachrichtentechnik Elektronik, Berlin 45 (1995) Vol. 1, pages 21 to 23 and Vol. 3 pages 29 and 30” and IEE Colloquium 1993, 173; (1993), pages 29/1-29/7; W. Hing, F. Halsall: “Cordless access to the ISDN basic rate service” on the basis of a DECT/ISDN Intermediate System DIIS according to ETSI Publication prETS 300xxx, Version 1.10, September 1996, an “ISDN  DECT-specific RLL/WLL” telecommunication system IDRW-TS (Integrated Services Digital Network  Radio in the Local Loop/Wireless in the Local Loop) with an ISDN telecommunication subsystem I-TTS [compare printed document “Nachrichtentechnik Elektronik, Berlin 41-43, Part: 1 to 10, P1: (1991) Vol. 3, pages 99 to 102; P2: (1991) Vol. 4, pages 138 to 143; P3: (1991) Vol. 5, pages 179 to 182 and Vol. 6, pages 219 to 220; P4: (1991) Vol. 6, pages 220 to 222 and (1992) Vol. 1, pages 19 to 20; P5: (1992) Vol. 2, pages 59 to 62 and (1992) Vol. 3, pages 99 to 102; P6: (1992) Vol. 4, pages 150 to 153; P7: (1992) Vol. 6, pages 238 to 241; P8: (1993) Vol. 1, pages 29 to 33; P9: (1993) Vol. 2, pages 95 to 97 and (1993) Vol. 3, pages 129 to 135; P10: (1993) Vol. 4, pages 187 to 190”] and a DECT-specific RLL/WLL telecommunication subsystem RW-TTS. 
     In this arrangement, the DECT/ISDN intermediate system DIIS and, respectively, the RLL/WLL telecommunication subsystem RW-TTS are preferably based on a DECT (Digital Enhanced (previously European) cordless telecommunication)/GAP system DGS; compare (1): Nachrichtentechnik Elektronik 42 (1992) January/February No. 1, Berlin, DE; U. Pilger “Struktur des DECT-Standards” (Structure of the DECT standard), pages 23 to 29 in conjunction with ETSI Publication ETS 300175-1 . . . 9, October 1992; (2): Telcom Report 16 (1993), No. 1, J. H. Koch: “Digitaler Komfort f{umlaut over (u)}r schnurlose Telekommunikation  DECT-Standard eröffnet neue Nutzungsgebiete” (Digital comfort for cordless telecommunicationDECT standard opens up new fields of application), pages 26 and 27; (3): tec 2/93—The technical magazine by Ascom “Wege zur universellen mobilen Telekommunikation” (Approaches to universal mobile telecommunication), pages 35 to 42; (4): Philips Telecommunication Review Vol. 49, No. 3, September 1991, R. J. Mulder: “DECT, a universal cordless access system”; (5): WO 93/21719 (FIGS. 1 to  3  and associated description)]. The GAP (Generic Access Profile) standard is a subset of the DECT standard which has the task of ensuring the interoperability of the DECT air interface for telephone applications (compare ETSI Publication-prETS 300444, April 1995). 
     As an alternative, the DECT/ISDN intermediate system DIIS and, respectively the RLL/WLL telecommunication subsystem RW-TTS, can also be based on a GSM system (Groupe Sp{acute over (e)}ciale Mobile or Global System for Mobile Communication; compare Informatik Spektrum 14 (1991) June, No. 3, Berlin, DE; A. Mann: “Der GSM-Standard—Grundlage f{umlaut over (u)}r digitale europ{umlaut over (a)}ische Mobilfunknetze” (The GSM standard—basis for digital European mobile radio networks), pages 137 to 152). In the context of a hybrid telecommunication system it is also possible, instead, for the ISDN telecommunication subsystem I-TTS to be constructed as a GSM system. 
     In addition, further possibilities to be considered for implementing the DECT/ISDN intermediate system DIIS and, respectively, the RLL/WLL telecommunication subsystem RW-TTS or the ISDN telecommunication subsystem I-TTS are the systems mentioned initially, and future systems which are based on the known multiple-access methods FDMA, TDMA, CDMA (Frequency Division Multiple Access, Time Division Multiple Access, Code Division Multiple Access) and hybrid multiple-access methods formed from these. 
     The use of radio channels (e.g. DECT channels) in traditional line-connected telecommunication systems such as the ISDN is gaining increasing significance, particularly against the background of future alternative network operators without their own complete wire-line network. 
     Thus, it is intended to provide the ISDN subscriber with ISDN services at standard ISDN interfaces by means of the wireless RLL/WLL (Radio in the Local Loop/Wireless in the Local Loop) line interfacing technique, e.g. including the DECT system DS, for example in the RLL/WLL telecommunication subsystem RW-TTS (compare FIG.  1 ). 
     In the “ISDN  DECT-specific RLL/WLL” telecommunication system IDRW-TS according to FIG. 1, a telecommunication subscriber (user) TCU (Telecommunication User) with his terminal equipment TE (also Terminal Endpoint) is tied in, e.g. via a standardized S interface (S-BUS), the preferably DECT-specific DECT/ISDN intermediate system DIIS (first telecommunication subsystem) contained in the RLL/WLL telecommunication subsystem RW-TTS and constructed as local information transmission loop, a further standardized S interface (S-BUS), a network termination NT and a standardized U interface of the ISDN telecommunication subsystem I-TTS (second telecommunication subsystem) into the ISDN world and all the services available therein. 
     The first telecommunication subsystem DIIS essentially consists of two telecommunication interfaces, a first telecommunication interface DIFS (DECT Intermediate Fixed System) and a second telecommunication interface DIPS (DECT Intermediate Portable System) which are connected to one another wirelessly, e.g. via a DECT air interface. Because of the quasi-stationary first telecommunication interface DIFS, the first telecommunication subsystem DIIS forms the local information transmission loop defined above in this connection. The first telecommunication interface DIFS contains a radio fixed part RFP, an interworking unit IWUI and an interface circuit INC 1  to the S interface. The second telecommunication interface DIPS contains a radio portable part RPP and an interworking unit IWU 2  and-an interface circuit INC 2  to the S interface. In this arrangement, the radio fixed part RFP and the radio portable part RPP form the known DECTIGAP system DGS. 
     For a DECT-specific RLL system as bearer for all ISDN services, if possible, in the subscriber loop, the following general problems are given: 
     a) Simulation of the ISDN channel structure (D channel and 2 B-channels), especially of the D channel in the text which follows, 
     b) good economy of bandwidths; particularly significant for ISDN since some services already need two. DECT channels for the B-channel data rate of 64 kb/s, 
     c) minimum technical complexity. 
     Simulation of the D channel 
     Properties of the D channel: 
     Common signalling channel on the C plane for all terminal endpoints TE connected to the ISDN line. 
     The TE-specific signalling channels to the network are separated there by TE-individual addresses TEI (Terminal Endpoint Identifiers). 
     The order of information items is ensured TE-individually by the access mechanism to -the. D channel. 
     Throughput: 16 kb/s 
     Usage: depends on many criteria, as a rule lower than maximum capacity; congestion situations are possible but can be rapidly cleared because of the high capacity. 
     DECT channels: 
     FIG. 2 shows the TDMA structure of the DECT/GAP system TKS in accordance with the printed document “Nachrichtentechnik Elektronik 42 (1992) January/February, No. 1, Berlin, DE; U. Pilger: “Struktur des DECT-Standards” (Structure of the DECT standard), pages 23 to 29 in conjunction with ETS 300 175-1 . . . 9, October 1992”. With respect to the multiple-access methods, the DECT/GAP system is a hybrid system in which radio messages can be sent on ten frequencies in the frequency band between 1.88 and 1.90 GHz according to the FDMA principle within a predetermined time sequence according to the TDMA principle according to FIG. 2 from the base station RFP to the mobile part RPP and from the mobile part RPP to the base station RFP (duplex mode). The time sequence is determined by a multi-timeframe MZR which occurs every 160 ms and which has 16 timeframes ZR having in each case a duration of 10 ms. Within this timeframe ZR, information relating to a C, M, N, P, Q channel defined in the DECT standard is transmitted separately from the base station RFP and the mobile part RPP. If information for several of these channels is transmitted within one timeframe ZR, the transmission takes place in accordance with a list of priorities, where M&gt;C&gt;N and P&gt;N. Each of the 16 timeframes ZR of the multi-timeframe MZR is, in turn, subdivided into 24 time slots ZS having in each case a duration of 417 μs, out of which 12 time slots ZS (time slots 0 . . . 11) are intended for the “base station RFP→mobile part RPP” direction of transmission and a further 12 time slots ZS (time slots 12 . . . 23) are intended for the “mobile part RPP→base station RFP” direction of transmission. In each of these time slots ZS, information items having a bit length of 480 bits are transmitted in accordance with the DECT standard. Of these 480 bits, 32 bits are transmitted as synchronization information in a SYNC field and 388 bits are transmitted as user information in a D field. The remaining 60 bits are transmitted as additional information in a Z field and as guard information in a “guard time” field. The 388 bits of the D field, transmitted as user information, are, in turn, subdivided into a 64-bit-long A field, a 320-bit-long B field and a 4-bit-long “X-CRC” word. The 64-bit-long A field is composed of a header with a length of 8 bits, a data record with data for the C, Q, M, N, P channels with a length of 40 bits and an “A-CRC” word with a length of 16 bits. 
     Properties: 
     Use of TDMA time slots. 
     In principle, one C s  channel (s=slow) is used per time slot for signalling [C plane in the DECT standard] and an associated channel is used for the user information [U plane in the DECT standard] (32 kb/s throughput). 
     Throughput of the C s  channel: 2 kb/s. 
     The DECT standard also offers other channel structures, e.g. a C f  channel (f=fast). 
     The C f  channel occupies one time slot. 
     Throughput of the C f  channel: 25.6 kb/B. 
     Based on the OSI/ISO reference model [compare (1): Unterrichtsbl{umlaut over (a)}tter (Training sheets) Deutsche Telecom Vol. 48, 2/1995, pages 102 to 111; (2): ETSI Publication ETS 300175- 1 . . . 9, October 1992; (3): ETSI Publication ETS 300102, February 1992; (4): ETSI Publication ETS 300125, September 1991; (5): ETSI Publication ETS 300012, April 1992], FIG. 3 shows a model of the C plane of the “ISDN  DECT-specific RLL/WLL” telecommunication system IDRW-TS according to FIG.  1 . 
     Based on the OSI/ISO reference model [compare (1): Unterrichtsbl{umlaut over (a)}tter (Training sheets) Deutsche Telecom Vol. 48, 2/1995, pages 102 to 111; (2): ETSI Publication ETS 300175-1 . . . 9, October 1992; (3): ETSI Publication ETS 300102, February 1992; (4): ETSI Publication ETS 300125, September 1991; (5): ETSI Publication ETS 300012, April 199], FIG. 4 shows a model of the U plane for voice data transmission of the “ISDN  DECT-specific RLL/WLL” telecommunication system IDRW-TS according to FIG.  1 . 
     Economy of bandwidths 
     The C s  channel structure offers optimum economy of bandwidths for a standard voice connection since, according to FIG. 5, on the basis of FIGS. 3 and 4 and taking into consideration the ETSI Publications (ETS 300175-1, 10/1992, Section 7; ETS 300175-3, October 1992, Section 4.1; ETS 300175-4, October 1992, Section 4), only one transmission path (bearer)—e.g. MBC with the LCNy, LCN 1  according to FIG.  5 —or, respectively, one connection or one time slot is needed. 
     According to FIG. 5, on the basis of FIGS. 3 and 4 and taking into consideration the ETSI Publications (ETS 300175-1, October 1992, Section 7; ETS 300175-3, 10/1992, Section 4.1; ETS 300175-4, October 1992, Section 4), the use of the C f  channel leads to less economy of bandwidths since the U plane itself-needs a further transmission path (bearer) or, respectively, a further connection or a further time slot; i.e. two bearers—e.g. MBC with the LCN 2 , LCNZ and MBC with the LCNy, LCN 1  according to FIG.  5 —or, respectively, two connections or two time slots are needed for a simple voice connection. 
     Moreover, three bearers—e.g. MBC with the LCNx, LCN 0 , MBC with the LCNy, LCN 1  and MBC with the LCNz, LCN 2  according to FIG.  5 —or, respectively, three connections or three time slots are required in the case where there are two ISDN B-channel connections (voice connections). 
     Although it appears to be appropriate to use the C f  channel from the point of view of channel capacity, the use of the C s  channel is appropriate from the point of view of economy of bandwidths. 
     Independently of whether it is the C f  channel or the C s  channel which is used for setting up the connection (setting up bearers), it must be ensured (compare FIG.  5 ), that it is possible to change from the C f  channel to the C s  channel and conversely at any time (changing channels between channels of unequal channel capacity). In addition, it must be ensured that it is possible to change between a first C s  channel and a second C s  channel (changing channels between two channels of equal channel capacity) because of the possibility that two connections (bearers) can be set up at the same time in the ISDN system (2 B-channels). 
     Technical complexity 
     The DECT-specific RLL system must appear to be transparent to the ISDN subscriber and the ISDN network. For its internal functions such as, for example, DECT channel selection etc., it needs control criteria which must be determined by the analysis of ISDN “Layer-2/Layer-3” messages (compare printed document “Nachrichtentechnik Elektronik, Berlin 45, P2: (1991) Vol. 4, pages 138 to 143”) of the network—ISDN subscriber (Terminal Endpoint TE) signalling if they are not explicitly available at the network interfaces. 
     To minimize the complexity, it is possible to concentrate this body of control criteria in a telecommunication interface of the telecommunication interfaces DIFS, DIPS, e.g. of the first telecommunication interface DIFS (DECT Intermediate Fixed System) and to control the other telecommunication interface in each case, in the present case the second telecommunication interface DIPS (DECT Intermediate Portable System) from there. In this constellation, the fixed system DIFS is always able to select a DECT channel structure corresponding to the ISDN service (C plane and U plane). 
     In the portable system DIPS, this is not possible without direct access to the ISDN layer  3 . It is not able to map a TE-individual connection with C and U plane unambiguously onto a corresponding DECT channel structure from the ISDN layer  2  function alone in all situations. 
     Even if this were possible, there remains the problem of the difference in throughput when only the C s  channel is used, which is economic with respect to bandwidths. 
     An approach is therefore sought which, with good economy of bandwidths and low system complexity, maps the entire D channel of an ISDN line onto a DECT channel arrangement in such a manner that the fundamental properties of the D channel are not changed and situations of congestion can be rapidly relieved. 
     A known design for the standardization of such a system has hitherto provided for the use of the C f  channel for as long as the ISDN line is active. The central control function lies in the fixed system DIFS which controls the portable system DIPS via the C f  channel. This solution is relatively simple but has the disadvantage, that the economy of bandwidths is not optimum. 
     SUMMARY OF THE INVENTION 
     The object forming the basis of the invention consists in allocating telecommunication channels of different channel capacity, e.g. the ISDN D channel and DECT channels to one another (e.g. simulation of the ISDN channel structure by the DECT channel structure) with good (economic) utilization of the bandwidth and minimum technical complexity in a hybrid telecommunication system, particularly an ISDN  DECT-specific RLL/WLL system. 
     The basic concept of the invention consists in using the DECT-specific C s  channel and the DECT-specific C f  channel for the information transmission in a hybrid telecommunication system in dependence on the amount of information transmitted in the ISDN D channel. In this arrangement, it is especially the technical facts with respect to the object forming the basis of the invention, which have been discussed in the introduction to the description for hybrid telecommunication systems, particularly a ISDN  DECT-specific RLL/WLL system, which are being taken into consideration. 
     The aim of the solution is a dynamic adaptability of the capacity of a DECT channel arrangement carrying the D channel to the current throughput -demand of the D channel whilst largely retaining the properties of the known draft standard explained in the introduction to the description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features of the present invention which are believed to be novel, are set forth with particularity in the appended claims. The invention, together with further objects and advantages, may best be understood by reference to the following description taken in conjunction with the accompanying drawings, in the several Figures of which like reference numerals identify like elements, and in which: 
     FIG. 1 depicts a prior art telecommunication system; 
     FIG. 2 depicts a TDMA structure of the FIG. 1 system; 
     FIG. 3 depicts a model of the C plane of the FIG. 1 system; 
     FIG. 4 depicts a model of the U plane for voice data transmission of the FIG. 1 system; 
     FIG. 5 depicts channel structure of the FIG. 1 system; 
     FIG. 6 depicts a configuration of transmitting and receiving sections according to the present invention; 
     FIGS.  7 - 10  depict channel changes according to the present invention; and 
     FIGS.  11 - 15  depict signal flow in the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 6 shows, on the basis of FIGS. 1 to  5 , the basic configuration of a transmitting section and receiving section in each case for the first telecommunication interface DIFS and the second telecommunication interface DIPS, which is of significance for the analysis of the ISDN “layer- 2 /layer- 3 ” messages and, respectively, the amount of information transmitted via these (compare printed document “Nachrichtentechnik Elektronik, Berlin 41, P2: (1991) Vol. 4, pages 138 to 143”) on the “ISDN network  ISDN subscriber (Terminal Endpoint TE) transmission link. 
     In the transmitting section of the first telecommunication interface DIFS and, respectively, the second telecommunication interface DIPS, the NWK layer (Network Layer) transfers ISDN “layer- 2 /layer- 3 ” information and DECT control information in familiar manner via a first queue WSD, constructed as a buffer, to the DLC (Data Link Control) layer. A MAC/DLC controller STE of the transmitting section measures the loading ratio in the queue WSD and stimulates from this the MAC (Medium Access Control) layer and DLC layer. As long as the loading ratio remains below a threshold SD, the DLC layer deposits the information (message) to be transmitted in a second queue WSS, also constructed as a buffer, from which it transmits the MAC layer on the C s  channel to the receiving section. 
     When the threshold SD is exceeded, the DLC layer deposits the information in a third queue WSF, again constructed as a buffer, from which it transmits the MAC layer to the receiving section on a C f  channel which is set up for this purpose. The C s  channel is used again when the first queue WSD and the third queue WSF are empty. 
     From the transmitting section and/or the receiving section, e.g. the first telecommunication interface DIFS and/or the second telecommunication interface DIPS detect(s) the necessity of producing a change of channel (change from one subsystem channel to another subsystem channel). The stimulus for the change of channel is formed by the result of the analysis. The configuration of the transmitting section and receiving section shown in FIG. 6 can thus be used for controlling the change of channel. 
     For the resultant change of channel between the C s  channel and the C f  channel, it is supposed that the C s  channel  C f  channel association in the first telecommunication interface DIFS and the second telecommunication interface DIPS is known, using the DECT standard. Like the C s  channel, naturally, the C f  channel can also be used for transmission in the opposite direction if it already exists. 
     FIGS. 7 to  10  show a first illustrative embodiment for the change of channel. 
     The operation in detail 
     As long as the loading ratio of the first queue WSD is below the threshold SD, the DLC layer uses the DECT A-field format (DECT standard) for feeding the second queue WSS. After the threshold SD has been exceeded, the third queue WSF is fed in the DECT B-field format. Switching over to transmitting from the third queue WSF is done after the C f  channel has been set up, when the second queue WSS is empty or the C f  channel is ready. 
     There are two possibilities for changing from the A format to the B format: 
     a) The queue WSS only contains complete A-field DLC frames: 
     Then, switching over is always done at DLC frame boundaries. There are three criteria for dimensioning the DLC frames: 
     shortest possible frames so that the delay of switching over to transmitting from the third queue WSF remains as short as possible, 
     on the other hand, the DLC PDU (Protocol Data Unit) data overhead rises when the maximum DLC frame length is not utilized, 
     bridging the set-up time for the C f  channel. 
     To control the C s  channel  C f  channel switch-over, DLC (Data Link Control) procedures are used. 
     Thus, e.g., the standard DECT procedures “Class B acknowledged suspension/Class B resumption” are used in modified form specifically for this application (compare DECT standard ETS 300175-4, October 1992, Section 9.2.7). 
     C s  channel—C f  channel according to FIG. 7 
     When the first queue WSS is empty, i.e. the last I frame has been acknowledged in accordance with the HDLC protocol, the initiating telecommunication interface DIFS, DIPS (e.g. the second telecommunication interface DIPS) sends a “SUSPEND” command on the C s  channel. If the other station (the first telecommunication interface DIFS) itself still has I frames to send from the first queue WSS, it will terminate this as early as possible at the next frame boundary (remaining frames will be transferred to the third queue WSF), wait for the last acknowledgement on the C s  channel and then accept the “SUSPEND” command on the C s  channel. 
     After that, the second telecommunication interface DIPS initiates the resumption of the data link by a “RESUME” command on the C f  channel. This will be acknowledged on the C f  channel by the first telecommunication interface DIFS. The two telecommunication interfaces DIFS, DIPS then continue the transmission on the C f  channel. 
     C f  channel—C s  channel according to FIG. 8 
     The channels are switched back when the first queue WSD and the third queue WSF on both sides are empty and the last I frame has been acknowledged. 
     A distinction is made between two cases: 
     The condition is met first in the case of the telecommunication interface which has initiated the switch-over (the second telecommunication interface DIPS). 
     the second telecommunication interface DIPS sends the “SUSPEND” command on the C f  channel. 
     the first telecommunication interface DIFS rejects the “SUSPEND” command on the C f  channel and continues to transmit information on the C f  channel. 
     thereafter, the first telecommunication interface DIFS will thus also take over the initiative for switch-over to the C s  channel and, in turn, initiates the “suspension/resumption” when the C f  channel is no longer needed. In the meantime, the second telecommunication interface DIPS could also spontaneously use the C s  channel again if it needs to do so. 
     The condition in the case of the telecommunication interface DIFS, DIPS, which has previously caused or retained the switch-over to the C f  channel is met later. 
     This case ends the use of the C f  channel and switches back to the C f  channel. 
     In this case, the answering telecommunication interface DIFS, DIPS accepts the “suspension” on the C f  channel. The suspending telecommunication interface DIFS, DIPS then initiates the “resumption” on the C s  channel. 
     b) Switch-over within I frames 
     This approach avoids the additional overheads for optimum DLC frames but presupposes that the C s  channel—C f  channel switch-over is gapless for the DLC layer and the precise point of switch-over is also detectable by the receiver. 
     The DLC layer in the transmitting section presets a frame length L after the start of a frame in the second queue WSS but must expect that it will necessary to switch over to the third queue WSF within the frame and that the frame will there be terminated in the B-field format. For this case, it stores L and all data already transferred to the second queue WSS and can use this to form the frame termination (fill octets, check sum) in accordance with B-field rules. 
     An expansion of the previously standardized functions of the MAC layer can be used for controlling the switch-over which is gapless for the DLC layer. This expansion affects the A-field as follows (see ETS 300 175-3, 7.2.5, especially 7.2.5.3 et seq.). 
     In the MAC message header, one of the code points which is still free is occupied by the MAC command type “C s  channel/C f  channel switch-over”. 
     The remainder of the A-field essentially contains the following information under this command: 
     Reference of the MAC connections between which the C s  channel/C f  channel switch-over is to take place (the already defined ECN (Exchanged Connection Number) will be used). 
     Specific C s  channel—C f  channel/C f  channel—C s  channel switch-over command. 
     Acknowledgement: Switch-over accepted/not accepted, confirmation of the correct reception of the “acknowledgement” command. 
     Blank field (use wait function if acknowledgement is not immediately possible). 
     The B-field of the time slots containing these MAC control information items either carries user information (U plane) if the C s  channel is used, or the signalling information itself or, respectively, no information, if the C s  channel is used. 
     In the I frame, the switch-over proceeds in accordance with a similar arrangement to that outlined above at point a): 
     C s  channel—C f  channel according to FIG. 9 
     After the MAC connection has been set up for the C f  channel, the initiating telecommunication interface DIFS, DIPS sends, instead of an I frame segment, the C s  channel—C f  channel switch-over command on the C s  channel. The other end acknowledges acceptance on the C s  channel (there is no reason for rejection in this case). Both telecommunication interfaces DIFS, DIPS then continue the transmission on the C f  channel. 
     C f  channel—C s  channel according to FIG. 10 
     When the telecommunication interface DIFS, DIPS initiating the C f  channel no longer needs this channel, it sends the C f  channel—C s  channel switch-over command on the C f  channel. If the other end also no longer needs this channel at this time (WSD, WSF empty), it acknowledges the acceptance of the switch-back. Otherwise, it rejects the switch-back and thus, in turn, accepts the initiative for a new activation of the switch-back if it no longer needs the C f  channel. As long as the C f  channel is active, it can also be used again by the other end. 
     Note 
     Naturally, the method can also be used at I frame boundaries. 
     There are then two possibilities: 
     MAC commands and acknowledgements are used, i.e. sent, at DLC frame boundaries. 
     MAC commands and acknowledgements are preventatively inserted already in current transmissions of DLC frames but the time of effectiveness is defined for DLC frame ends. 
     This results in the advantage of a gain in time because negotiations and possible subsequent operations can already take place in parallel with an ongoing transmission. 
     Miscellaneous 
     According to DECT rules, the C f  channel can be set up by both telecommunication interfaces DIFS, DIPS, if needed. Collisions in this case should lead to a common channel. 
     In the case of collision between set-up and clear-down, clear-down has priority. 
     The use of the C f  channel can also be additionally stimulated by other criteria. 
     FIGS. 11 to  15  will be used to explain a second illustrative embodiment of the change of channel on the basis of FIG.  6 . 
     FIGS. 11 to  15  show various event/state diagrams which represent possible sequences during the change of channel. 
     Using FIGS. 1 to  6  as a basis, FIG. 11 shows a first event/state diagram which represents the basic control sequence for a change of channel. 
     The first telecommunication interface DIFS is connected on a first bearer having a first logical connection number LCNx by a first subsystem channel C x  to the second telecommunication interface DIPS. In addition, there is a further telecommunication connection between the first telecommunication interface DIFS and the second telecommunication interface DIPS on a second bearer having a second logical connection number LCNy by a second subsystem channel Cy or, as an alternative, a further telecommunication connection can be set up between the first telecommunication interface DIFS and the second telecommunication interface DIPS on a second bearer having a second logical connection number LCNy by a second subsystem channel C y . 
     In this arrangement, the relation LCNx≢LNCy applies to the logical connection numbers LCNx, LCNy. The first subsystem channel C x  can be constructed as DECT-specific C f  channel or C s  channel. Due to the channel constellations occurring in the DECT-specific telecommunication subsystem WLL/RLL, the second subsystem channel C y  is accordingly a C s  channel or respectively a C f  channel or C s  channel. According to FIG. 11, the first subsystem channel C x  is used for transmitting information on the C plane. 
     To set up a bearer, a DECT-specific first B-field message “BEARER_REQUEST” (compare ETSI Publication ETS 300175-3, October 1992, Section 7.3.3.2) is sent as COMMAND and a DECT-specific second B-field message “BEARER_CONFIRM” (compare ETSI Publication ETS 300175-3, October 1992, Section 7.3.3.3) is sent as RESPONSE (compare ETSI Publication ETS 300175-3, October 1992, Section 10.5.1.1 to 10.5.1.3) in familiar manner. Transmission of the first B-field message “BEARER_REQUEST” is preferably initiated by the second telecommunication interface DIPS in this arrangement (compare FIGS. 9 and 10 and ETSI Publication ETS 300175-3, October 1992, Section 10.5.1.2 and 10.5.1.3). 
     As a result of the analysis of the ISDN “layer  2 /layer  3 ” messages or, respectively, the amount of information transmitted by these messages (compare printed document “Nachrichtentechnik Elektronik, Berlin 41, P2: (1991) Vol. 4, pages 138 to 143”) on the “ISDN network—ISDN subscriber (Terminal Endpoint TE) transmission link, the first telecommunication interface DIFS, for example, recognizes the necessity of initiating a change of channel (change from the first subsystem channel C x  to the second subsystem channel C y ). In this arrangement, the result of the analysis forms the stimulus for the change of channel. 
     A possible first result of this analysis can, for example, consist in no messages being transmitted between the first telecommunication interface DIFS and the second telecommunication interface DIPS on the first subsystem channel C x , preferably for a predetermined period of time. 
     A possible second result of this analysis can, for example, consist in two bearers having been set up with in each case one C plane and one U plane and the bearer, on which the C plane is being used, having to be cleared down; so that, accordingly, a change from the previously active C s  channel to be cleared down to the previously inactive C s  channel becomes necessary. 
     To minimize the complexity, it is appropriate to concentrate the analysis described above in one of the telecommunication interfaces DIFS, DIPS—e.g. advantageously the first telecommunication interface DIFS and to control the second telecommunication interface DIPS from there [MASTER-SLAVE configuration, in which the first telecommunication interface DIFS is the MASTER and the second telecommunication interface DIPS is the SLAVE]. In this constellation, the first telecommunication interface DIFS always has the possibility of selecting a DECT channel structure corresponding to the ISDN service (C plane and/or U plane). 
     Instead of the first telecommunication interface DIFS, the second telecommunication interface DIPS can also be provided for this purpose. However, this only works if the latter has direct access to the ISDN layer  3 . The second telecommunication interface DIPS is not capable of unambiguously mapping, in all situations, a TE-individual connection with C plane and U plane onto a corresponding DECT channel structure from the ISDN layer  2  function alone. 
     In the further explanation of the illustrative embodiment, the MASTER-SLAVE configuration described above is used as a basis. 
     After the first telecommunication interface DIFS has recognized a necessity of a change of channel, it will preferably confirm (answer) all unconfirmed (unanswered) information transmitted and completely received on the first subsystem channel C x  in accordance with the HDLC (High level Data Link Control) protocol, the so-called I frames (information packet) with a DECT-specific first DLC message “RECEIVE_READY” sent as RESPONSE (compare ETSI Publication ETS 300175-4, October 1992, Section 7.11.2), if no further I frame is sent. 
     In accordance with the HDLC protocol, it is possible, for example, to transmit the information (I frames) in transmission sequences (windows) and to acknowledge each transmission sequence (each window) separately. In the present case, for example, the information is transmitted with a window size of k=3 before an acknowledgement is made. The window size k=3 signifies with respect to the abovementioned I frames that an acknowledgement of the three frames transmitted previously is made after each third I frame. In general, the following relation applies to the window size k: 
     
       
         1≦k≦5 n where nεN  
       
     
     Due to the transmission of a first message “SWITCHING_REQUEST”, which, for example, can either be defined in the DECT standard (compare MAC message “ATTRIBUTES_T._REQUEST” in FIGS. 12 to  15  according to ETSI Publication ETS 300175-3, October 1992, Section 7.2.5.3.8) or is still to be defined in the latter, the wish by the first telecommunication interface DIFS to transfer the transmission of the system information from the first subsystem channel C x  to the second subsystem channel C y , is conveyed to the second telecommunication interface DIPS. As already mentioned, the wish can have arisen due to stimulation or without any trigger. 
     As a result of the transmission of this message, the first telecommunication interface DIFS can either—preferably—interrupt its own information transmission on the C plane or continue the transmission of the information on the C plane. Interruption means that the first telecommunication interface DIFS will send no further information for a predetermined period of time. The interruption can occur, for example, before, on or after the transmission of the message. 
     In addition, the message can be sent at the I frame boundaries and within one I frame. 
     On or after receipt of the message “SWITCHING_REQUEST”, the second telecommunication interface DIPS will preferably delete all I frames incompletely received and it can either interrupt or continue its own information transmission on the C plane like the first telecommunication interface DIFS, on or after receipt of the message “SWITCHING_REQUEST”. 
     In addition, the second telecommunication interface DIPS can confirm (answer) all unconfirmed (unanswered) information items transmitted on the first subsystem channel C x  in accordance with the HDLC (High level Data Link Control) protocol and completely received, the so-called I frames, with the DECT-specific first DLC message “RECEIVE_READY” sent as response (compare ETSI Publication ETS 300175-4, October 1992, Section 7.11.2) if its own transmitter is idle. 
     As an alternative to the direct interruption, it is also possible for the second telecommunication interface DIPS to conclude the transmission of an I frame before the interruption. 
     The interruption of the information transmission or the continuance of the information transmission on the first subsystem channel C x  by the second telecommunication interface DIPS preferably occurs between the receipt of the first message and before the transmission of a second message “SWITCHING_CONFIRM”, which, for example, can either be defined in the DECT standard (compare MAC message “ATTRIBUTES_T._CONFIRM” in FIGS. 12 to  15  according to ETSI Publication ETS 300175-3, October 1992, Section 7.2.5.3.8) or is still to be defined in the latter. 
     The second message “SWITCHING_CONFIRM” meets, for example, the request of the first telecommunication interface DIFS for a change of subsystem channel by confirming (positively answering) it. 
     However, it is also possible that the second telecommunication interface DIPS either deliberately or unintentionally (e.g. due to the fact that it has not received the first message due to a fault in the radio transmission link) does not meet the request. 
     In the case where the request is deliberately not met, the first message “SWITCHING_REQUEST” will thus be rejected (answered negatively) either directly or indirectly, e.g. by exceeding a predetermined period of time for confirming the first message, via the second telecommunication interface DIPS. 
     In the second case, the first message “SWITCHING_REQUEST” will be rejected (answered negatively) indirectly, e.g. by exceeding a predetermined period of time for confirming the first message. 
     In both the abovementioned cases, either the first message “SWITCHING_REQUEST” is retransmitted a predetermined number of times via the first telecommunication interface DIFS or the change of channel is aborted for an undetermined time. 
     The result of the transmission of the second message “SWITCHING_CONFIRM” is that the transmission of information is continued on the second subsystem channel C y . Continuance can preferably take place on or after transmission of the message. 
     After or on receipt of the second message “SWITCHING_CONFIRM”, the first telecommunication interface DIFS will preferably also delete the unconfirmed information transmitted on the first subsystem channel C x  and incompletely received. 
     Before the information deleted by the first telecommunication interface DIFS and the second telecommunication interface DIPS is retransmitted on the second subsystem channel C y , subsystem-specific parameters such as, for example, the backward transmission counter or timer specific to the DLC layer (compare ETSI Publication ETS 300175-4, October 1992, Section 9.2.5.7) and the C T  packet number (compare ETSI Publication ETS 300175-3, October 1992, Section 7.1.2) are reset. 
     In addition, a test message which must be confirmed can be transmitted on the second subsystem channel C y  before the information deleted by the first telecommunication interface DIFS and the second telecommunication interface DIPS is retransmitted. The test message is preferably the first DLC message “RECEIVE_READY” (compare ETSI Publication ETS 300175-4, October 1992, Section 7.11.2) sent as command whereas the confirmation of the test message is preferably the first DLC message “RECEIVE_READY” (compare ETSI Publication ETS 300175-4, October 1992, Section 7.11.2) sent as response. 
     Both the test message and the deleted information are preferably transmitted with the smallest possible window size according to the HDLC protocol, i.e. k=1, at the beginning (start phase of the transmission) in order to achieve rapid synchronization on the second subsystem channel C y , and are then transmitted again with the window size k=3. 
     FIG. 12 shows a second event/state diagram based on FIG. 11, which shows the control sequence for changing from a first subsystem channel C f  to a second subsystem channel C s . 
     The first subsystem channel C f  is used for transmitting information on the C plane. The second subsystem channel C s  is not used for transmitting information on the C plane. However, the U plane is utilized. The first subsystem channel C f  has a higher transmission capacity than the second subsystem channel C s . 
     The first telecommunication interface DIFS detects that the first subsystem channel C f  is no longer necessary and sends a first MAC message “ATTRIBUTES_T._REQUEST” (compare ETSI Publication ETS 300175-3, October 1992, Section 7.2.5.3.8) to the second telecommunication interface DIPS. 
     The second telecommunication interface DIPS confirms the first MAC message “ATTRIBUTES_T._REQUEST” by sending a second MAC message “ATTRIBUTES_T._CONFIRM” to the first telecommunication interface DIFS. After that, the second subsystem channel C s  is used for transmitting information on the C plane and the first subsystem channel C f  is cleared by transmitting a third MAC message “RELEASE” (compare ETSI Publication ETS 300175-3, October 1992, Section 7.2.5.3.13). 
     FIG. 13 shows a third event/state diagram based on FIG. 11, which illustrates the control sequence for changing from the second subsystem channel C s  to a third subsystem channel C s . 
     The second subsystem C s  is used for transmitting information on the C plane. In addition, the U plane is utilized. The third subsystem channel C s  is not used for transmitting information on the C plane but the U plane is utilised. The second subsystem channel C s  has the same transmission capacity as the third subsystem channel C s . 
     The first telecommunication interface DIFS detects that the second subsystem channel C s  is no longer necessary and sends the first MAC message “ATTRIBUTES_T._REQUEST” (compare ETSI Publication ETS 300175-3, October 1992, Section 7.2.5.3.8) to the second telecommunication interface DIPS. 
     The second telecommunication interface DIPS confirms the first MAC message “ATTRIBUTES_T._REQUEST” by sending the second MAC message “ATTRIBUTES_T._CONFIRM” to the first telecommunication interface DIFS. After that, the third subsystem channel C s  is used for transmitting information on the C plane and the second subsystem channel C s  is cleared by transmitting the third MAC message “RELEASE” (compare ETSI Publication ETS 300175-3, October 1992, Section 7.2.5.3.13). 
     FIG. 14 shows a fourth event/state diagram based on FIG. 11, which illustrates the control sequence for changing from the second subsystem channel C s  to the first subsystem channel C f , the preparation for the change being initiated by the first telecommunication interface DIFS. 
     The second subsystem channel C s  is used for transmitting information on the C plane. In addition, the U plane is utilized. A bearer having a logical connection number LCN for utilizing the first subsystem channel C f  has not yet been set up. The second subsystem channel C s  has a lower transmission capacity than the first subsystem channel C f . 
     The first telecommunication interface DIFS detects that the first subsystem channel C f  is needed. However, since there is as yet no bearer having the logical connection number LCN for the first subsystem channel C f , the first telecommunication interface DIFS sends the first MAC message “ATTRIBUTES_T._REQUEST” to the second telecommunication interface DIPS (compare ETSI Publication ETS 300175-3, October 1992, Section 7.2.5.3.8). It informs the second telecommunication interface DIPS with this message of the need for a bearer having the logical connection number LCN, e.g. the logical connection number LCN 0 , for the first subsystem channel C f . 
     The logical connection number LCN—in the present case LCN 0 —is not arbitrarily selected as identification for the bearer to be set up but deliberately in accordance with a predetermined selection criterion. Formulated generally, this criterion consists in using as logical connection number LCN the logical connection number, of the possible logical connection numbers LCN 0 , LCN 1 , LCN 2 , which is not yet being used for another bearer, that is to say is available. 
     As an alternative to the abovementioned selection criterion, it is also possible to use special features of the selection criterion for issuing the logical connection number. Thus it is possible—as in the present case—always to use, for example, the smallest available number of the logical connection numbers LCN 0 , LCN 1 , LCN 2  or the largest available number of the logical connection numbers LCN 0 , LCN 1 , LCN 2 . 
     The second telecommunication interface DIPS which, according to the explanations in the description of FIG. 11 is preferably responsible for setting up a bearer (compare ETSI Publication ETS 300175-3, October 1992, Section 10.5.1.2 and 10.5.1.3), sends the DECT-specific first B-field message “BEARER_REQUEST” (compare ETSI Publication ETS 300175-3, October 1992, Section 7.3.3.2) as command to the first telecommunication interface DIFS. After receiving the first B-field message, the first telecommunication interface DIFS thereupon sends the DECT-specific second B-field message “BEARER_CONFIRM” (compare ETSI Publication ETS 300175-3, October 1992, Section 7.3.3.3) as response to the second telecommunication interface DIPS. In this state, i.e. after receipt of the second B-field message by the second telecommunication interface DIPS, the further bearer is set up (compare ETSI Publication ETS 300175-3, October 1992, Section 10.5.1.1 to 10.5.1.3). 
     After that, the first telecommunication interface DIFS sends the first MAC message “ATTRIBUTES_T._REQUEST” (compare ETSI Publication ETS 300175-3, October 1992, Section 7.2.5.3.8) to the second telecommunication interface DIPS. 
     The second telecommunication interface DIPS confirms the first MAC message “ATTRIBUTES_T._REQUEST” by sending the second MAC message “ATTRIBUTES_T._CONFIRM” to the first telecommunication interface DIFS. After that, the first subsystem channel C f  is used for transmitting information on the C plane. 
     FIG. 15 shows a fifth event/state diagram based on FIG. 11, which illustrates the control sequence for changing from the second subsystem channel C s  to the first subsystem channel C f , the preparation for the change being initiated by the second telecommunication interface DIPS. 
     The second subsystem channel C s  is used for transmitting information on the C plane. In addition, the U plane is utilized. A bearer having a logical connection number LCN for utilizing the first subsystem channel C f  has not yet been set up. The second subsystem channel C s  has a lower transmission capacity than the first subsystem channel C f . 
     The second telecommunication interface DIFS detects that the first subsystem channel C f  is necessary. However, since there is not yet a bearer having the logical connection number LCN, e.g. the logical connection number LCN 0 , for the first subsystem channel C f , it sets up the latter. 
     The logical connection number LCN—in the present case LCN 0 -is not arbitrarily selected as identification for the bearer to be set up but again deliberately in accordance with a predetermined selection criteria. Formulated, generally, this criterion consists in using as logical connection number LCN the logical connection number, of the possible logical connection numbers LCN 0 , LCN 1 , LCN 2 , which is not yet being used for another bearer, that is to say is available. 
     As an alternative to the abovementioned selection criterion, it is also possible to use special features of the selection criterion for issuing the logical connection number. Thus it is possible—as in the present case—always to use, for example, the smallest available number of the logical connection numbers LCN 0 , LCN 1 , LCN 2  or the largest available number of the logical connection numbers LCN 0 , LCN 1 , LCN 2 . 
     To set up the bearer, the second telecommunication interface DIPS, which is preferably responsible for setting up a bearer in accordance with the explanations in the description of FIG. 11, (compare ETSI Publication ETS 300175-3, October 1992, Section 10.5.1.2 and 10.5.1.3) sends the DECT-specific first B-field message “BEARER_REQUEST” (compare ETSI Publication ETS 300175-3, October 1992, Section 7.3.3.2) as command to the first telecommunication interface DIFS. 
     After having received the first B-field message, the first telecommunication interface DIFS thereupon sends the DECT-specific second B-field message “BEARER_CONFIRM” (compare ETSI Publication ETS 300175-3, October 1992, Section 7.3.3.3) as response to the second telecommunication interface DIPS. In this state, i.e. after receipt of the second B-field message by the second telecommunication interface DIPS, the further bearer is set up (compare ETSI Publication ETS 300175-3, October 1992, Section 10.5.1.1 to 10.5.1.3). 
     This is detected by the first telecommunication interface DIFS so that the latter sends the first MAC message “ATTRIBUTES_T._REQUEST” (compare ETSI Publication ETS 300175-3, October 1992, Section 7.2.5.3.8) to the second telecommunication interface DIPS. 
     The second telecommunication interface DIPS confirms the first MAC message “ATTRIBUTES_T._REQUEST” by sending the second MAC message “ATTRIBUTES_T._CONFIRM” to the first telecommunication interface DIFS. After that, the first subsystem channel C f  is used for transmitting information on the C plane. 
     The invention is not limited to the particular details of the method depicted and other modifications and applications are contemplated. Certain other changes may be made in the above described method without departing from the true spirit and scope of the invention herein involved. It is intended, therefor, that the subject matter in the above depiction shall be interpreted as illustrative and not in a limiting sense.