NZ260559A

NZ260559A - Asynchronous cell mode data transmission

Info

Publication number: NZ260559A
Application number: NZ260559A
Authority: NZ
Inventors: Raymond Gass
Original assignee: Alcatel Australia
Priority date: 1993-06-11
Filing date: 1994-05-20
Publication date: 1996-06-25
Also published as: EP0629100B1; FR2706722A1; JPH0730558A; ES2162847T3; ATE205353T1; DK0629100T3; DE69428156D1; EP0629100A1; AU6338494A; DE69428156T2; JP3347877B2; FR2706722B1; AU679051B2

Abstract

Method and arrangement intended to allow transmission, via a link operated in cell mode between two telecommunication installation units, of information previously supplied to one of the two units via a link in narrow band circuit mode, when the two units are respectively managed in circuit mode. The arrangement for implementing the method according to the invention involves the implementation of at least two combined units (3) linked, on the one hand, together by a link for cell mode and, on the other hand, each linked to a unit and/or to terminals, supporting the circuit mode. Each combined unit includes a communication circuit (9 or 13) for its exchanges in each of the modes, as do the units with which it communicates in one of these modes, and a gateway circuit (14) putting the information modules transmitted by a time-division multiplex link in circuit mode into cell mode in the course of a frame and, conversely, recovery of the information modules contained in a received cell. <IMAGE>

Description

<div class="application article clearfix" id="description"> 26 0 5 5 9 PATENTS ACT 1953 COMPLETE SPECIFICATION " ASYNCHRONOUS CELL MODE TRANSMISSION " WE, ALCATEL AUSTRALIA LIMITED, C_Ao/ ©oo O05 3<»J) A Company of the State of New South Wales, of 280 Botany Road, Alexandria, New South Wales, 2015, Australia, hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: 260 559 This invention relates to a method and arrangement whereby data can be transmitted in cell mode between two units of a telecommunications facility, the data being first supplied to one of these units, and then restored by the other unit in narrow band circuit mode. This method is more specifically designed to be implemented in telecommunications facilities made up of units, some of which service apparatus and/or equipment operating in narrow band circuit mode, while others are servicing devices and/or equipment operating in cell mode, when these facilities contain links between units specifically organised as a function of the units they interconnect. This method may be implemented in telecommunications facilities such as the above-mentioned facilities, which may consist of geographically scattered modular sub-assemblies, and which are designed for communications between various devices and equipment, hereinafter indiscriminately referred to as terminals. These terminals being capable of operating according to different specific modes, some of them operating in narrow band circuit mode, of the digital telephone type, while others operate in packet mode, broad band circuit mode, or cell mode. For the transmission of data issued from one of these terminals, it may be interesting to use a mode different from the mode this terminal is designed for. This is in particular the case for terminals operating in narrow band circuit mode, since the basic lominal bit rate of 2048 Mbit/s designed for PCM time 26 0 5 division multiplex links is far lower than the 155.52 Mbit/s designed for links operating in cell mode. With regard to the facilities envisaged, it is therefore conceivable to use links operating in cell mode, namely between remote sub-assemblies within a facility, for communications between terminals which are only designed to operate in narrow band circuit mode, especially if these terminals are linked to different sub-assemblies solely linked via links operated in cell mode. According to the invention, there is provided a method whereby data can be transmitted, via a link operated in cell mode between two units of a telecommunications facility, the data being first supplied to one of these units in a time division multiplexed form, hence in a modular form, via one or several basic time channels within a link operated in narrow band circuit mode, when both units are operated respectively in circuit mode, synchronously or by two plesiochronous clocks wherein time allocated to each cell is shorter than the time provided for a frame time slot dedicated to a basic channel during each frame in narrow band circuit mode, is used to force, over the cell mode operated link, the transmission priority of each data module previously in narrow band circuit mode which is to be sent from one unit to the other, such that, once received by the unit in charge of the transmission, each of said data modules can be inserted in the first cell transmitted by the said unit, and such that, the said data modules transmitted in cell mode to this destination unit can be cyclically reinserted in one or 3 260 559 several preset time channels, under the control of the clock driving the narrow band circuit mode at destination unit level, thus making it possible to fully restore the data to its original form. According to a further aspect of the invention, there is provided an arrangement for implementation of the method as claimed in claim 1, within a telecommunications facility including narrow band circuit mode support units supporting terminals operating in this mode, said units being point-to-point interconnected via links specifically fitted and time controlled for such a mode of operation, and cell mode support units also point-to-point connected via links specifically fitted and controlled for cell mode operation, via one or several switching components of a cell switching arrangement within the facility, wherein said arrangement includes at least two composite units interconnected via a cell mode link, each composite unit being connected to a unit and/or terminals supporting the narrow band circuit mode, each composite unit being provided with a communication circuit to provide for data transfer in each of these modes, this applies as well to the units with which they communicate in one of these modes, and further including a gateway circuit to transform into cells the data modules transmitted during one frame via a narrow band circuit mode time division multiplex link, and conversely, to restore the data modules contained within the cell received to their original time position, before changing to cell mode, during the time frame within which they are restored in narrow band circuit mode. 260559 In order that the invention may be readily carried into effect, embodiments thereof will now be described in relation to the accompanying drawings, in which: Figure 1 represents a principle diagram showing the units making up a telecommunications facility, and involved in the implementation of the transmission method in accordance with the invention. Figures 2 and 3 represent principal diagrams showing the main components involved in the implementation of the method in accordance with the invention, in relation with figure 1 units with which they are associated or to which they are integrated. The telecommunications facility components shown in figure 1 include narrow band circuit support units (1), cell mode support units (2) and units referred to as composite units (3) in so far as they support the narrow band circuit mode for communication with units supporting this mode and the circuit mode with units supporting this circuit mode, hence in particular between them. These units (1,2 and 3) may for instance be scattered in subassemblies (4) issued from the core of the multiservice facility, these subassemblies being likely to be geographically remote. Each sub-assembly (4) groups various units fitted according to common knowledge to enable communication between various terminals which are usually designed to operate in a preset mode which is not necessarily common to all terminals. 26 0 5 For some, this mode may for instance be the narrow band circuit mode, other terminals may be designed to operate in other modes, in particular at high bit rate, for instance in cell mode, packet mode, or broad band circuit mode. In figure 1, two sub-assemblies within the same facility have been considered, each is assumed to include several units, the only units represented being two narrow band circuit mode support units (1), one cell mode support unit (2) and one circuit/cell composite unit (3) per subassembly. In the preferred implementation layout, all circuit mode support units within the facility are assumed to be point-to-point interconnected via bidirectional time division multiplex links itemised (NL), within the subassembly containing them, such as units (1) and (2) in the two subassemblies (4). In this instance, the links (NL) are assumed to have an identical physical structure and are designed to support transmissions based on the same time frame structure. For instance and according to common knowledge, this common frame has a period equal to 125 microseconds and is assumed to be divided into "p" equal time slots enabling the transfer of data converted to digital form by "p" channels having a unit bit rate of 64 kbit/s, there being 32 channels for the standard basic time division multiplex link, controlled by a pilot clock at 2048 MHz. The same data flow can thus be transmitted in the form of a series of data modules cyclically transmitted during sequential time 260 559 frames, i.e. one at a time via an elementary channel, or in groups via several elementary time channels preset within the same time division multiplex link (IML). The various links (NL) within a facility are not necessarily synchronous since several plesiochronous clock generators, individually producing the same clock frequency, referred to as first pilot clock, may act simultaneously within the facility, each over different links (NL). This is the case in particular if, as in the example studied, two facility sub-assemblies are not connected by any link (NL) and if they rely on non-synchronised clock generators to use the links (NL) they respectively contain. The two facility sub-assemblies shown in figure 1 are also assumed to include at least one, anrt generally several cell mode support units, of which the composite units (3) form part. In the implementation layout envisaged, all cell mode support units (2 and 3) in the facility are assumed to be interconnected by high rate transmission links (BL), i , each relevant sub-assembly. This point-to-point link between units of the same sub-assembly may be carried out via a switching unit (not shown here and which may be duplicated) of the cell switching arrangement (5) within the facility, when the sub-assembly in question contains several cell mode support units. According to the configuration of the facility envisaged, the cell switching arrangement (5) may contain switching units integrated within each 26 0 5 of the sub-assemblies containing cell mode support units and eventually at secondary cell mode switching stage level, then interleaved between the subassemblies, this stage not being specifically shown here. The various cell switching units in the sub-assemblies and eventually in the stage making up the switching arrangement (5) are for instance made up of broad band switching components, such as mentioned in the paper entitled "Techniques large bande" published in Alcatel Telecommunications Review No. 1 volume 65. Irrespective of the configuration selected, the links (BL) between the sub-assembly circuit mode support units and those uniting these subassemblies have an identical physical structure and are inter-synchronised. In the proposed example, a second pilot clock frequency of 155.52 MHz for instance, or a multiple of this frequency, namely 622.08 MHz, is selected for all links (BL) within the same facility, thus making up a synchronous network within this facility. The moment the bit rate over a link (BL) operating in cell mode is greater - and in this instance far greater - than the rate admissible for a link (NL) operating in narrow band circuit mode, it is possible to transmit, without loss nor distortion, a data supplied in the form of sequential modules by one or several time channels in the link (NL), during sequential time frame. Indeed, it is common knowledge that, in cell mode all cells contain the same number of bits and that the time required for sequential cell bit transmission 260559 by a unit at the rate of a pilot clock common to the units supporting this mode has a fixed value. With the pilot clocks provided above and in particular when their respective frequencies are 2048 and 155.52 MHz, the transmission time allocated per cell is shorter - and in this case much shorter - than the duration of the frame time slot dedicated to an elementary time channel in narrow band circuit mode. It is therefore possible to guarantee the transmission - in cell mode - of data modules cyclically supplied at the rate of sequential frames in narrow band circuit mode, within a time which, for each of these modules, is not likely to exceed a preset threshold lower than the normal delay between modules sequentially transmitted during sequential frames in narrow band circuit mode. To this end, priority is allocated to the data modules supplied to a composite circuit/cell destination unit in order to be transmitted in cell mode by this unit, in general with some other data supplied or eventually reorganised for their transmission in the form of cells. It then being possible to guarantee reception, without loss or distortion, of the data initially supplied in narrow band circuit mode s;nd transmitted in this manner, despite the variations in the transmission time inherent to cell mode, in so far as these variations can thus be maintained at a value lower than the above-mentioned threshold. In the preferred proposed implementation layout under the above-mentioned conditions, in particular regarding the clock, it is planned to force 9 260 5 the priority of the data modules sequentially supplied in narrow band circuit mode to a composite unit in view of their transmission in cell mode to another unit, via a link (BL), such that each of these modules be integrated into the first cell transmitted by the composite unit in charge of transmission, after it has been received by this unit. Figures 2 and 3 diagrams define the main components either associated with or integrated into the units within the telecommunications facility which are used to implement the invention. To this end, figure 2 shows two composite units (3) assumed to belong to two remote sub-assemblies (4) within the same facility, these composite units are interconnected via a high rate transmission link (BL), such as defined above, which may be established as a point-to-point connection via one or several units of the cell switching arrangement (5) within the facility, not shown here. These composite units (3) are assumed to be linked to several narrow band circuit mode support units (1), to the sub-assemblies respectively containing them, and to cell mode support units, not shown in figure 2. Each narrow band circuit mode support unit (1) is assumed to be point-to-point connected to each of the other units supporting this mode within the sub-assembly containing it via a bidirectional time division multiplex link (NL), the only link shown in each sub-assembly being the (NL) link connecting the sub-assembly unit (No.1 - the only one shown) to the composite unit (3) 10 260 5 corresponding to this sub-assembly. As is common knowledge, one of the possible alternatives for the narrow band circuit mode support unit (1) is for instance designed to be used for connecting terminals (6) operating in this mode, and in particular, telecommunications devices such as digital telephone terminals, each of which communicate via one or several time channels within an NL link. In this instance, it is assumed as is common knowledge that the terminals (6) are connected to the unit (1) servicing them, via one or often several connection interfaces (7), individually connected to this unit (1) via one or several individual time division multiplex links grouping the communications of several terminals. The units (1) may be constituted differently depending on the type of terminals, i.e. on the type of devices or equipment within the facility they service. Generally, they contain a command structure (8) organised around a microprocessor to which are associated some memories and a time base, not shown, as well as a communication circuit (9) providing for the transfer of data transmitted in narrow band circuit mode between terminals, such as (6), via the interface(s) (7), the control logic (8) of the relevant unit (1) and the other narrow band circuit mode support units (1 or 3) to which the relevant unit is point-to-point connected via NL links. Such a communication circuit (9) for narrow band circuit mode is known therefore it shall only be briefly mentioned here. 11 260 5 The composite units (3) supporting the narrow band circuit mode for communications with units (1) and supporting the cell mode for communications between themselves and units (2), may also be constituted differently. In general, they include a command structure (10), most of the time supervised by a microprocessor to which is associated a set of memories, not shown in figure 2. A set of time bases is also associated with the command structure of a composite unit (3); in the proposed implementation, this set includes two modules itemised (11) and (12) in one of the units (3) and itemised (1V) and (12) in the other. Within each unit (3), a first communication circuit for narrow band circuit mode ensures communication of this unit (3) with each of the units (1) to which it is individually and directly point-to-point connected via a link (NL), within the sub-assembly to which it belongs. This first circuit may for instance be of the same type as the circuit (9) proposed in the units (1) as assumed in figure 2. It is connected to the command structure (10) such that the latter may communicate with the units (1) connected to this circuit via NL links and with this circuit itself. To this end, it is controlled by the pilot clock pulse selected for narrow band circuit mode communications, this pilot clock being distributed by the time base module (11 or 11'), located in the same composite unit as the circuit. Depending on the operating situation, a time base module (11 or 11') may be arranged such that it supplies the base clock used for synchronisation of the 260559 other narrow band circuit mode support units within the sub-assembly containing the circuit, and eventually within other sub-assemblies within the facility. Alternately, it may be arranged such that it is controlled in relation with another internal clock, or eventually another clock external to the facility considered. A communication circuit (13) between cell mode support units is proposed within each unit supporting this mode and in particular within each composite unit (3). The cell mode communication circuit (13) is connected to the command structure (10) which monitors it in the composite unit where it is housed and to at least one other cell mode support unit, in general via at least one cell switching unit within the cell switching arrangement (not shown in figure 2) of the facility considered. In the example shown in figure 2, the respective communication circuits (13) of the two composite units represented are interconnected via a high rate transmission link (BL) established via the cell switching arrangement in the facility, this arrangement being assumed to be located as shown in dotted lines at the centre of the BL link shown. Each communication circuit (13) is time controlled by a second pilot clock, selected for cell mode communications via the synchronous network made up of the BL links within the facility considered in this instance, this pilot clock being distributed by a time base module (12) located in the same unit (composite in this instance) as this circuit. 260 55 Where a composite unit (3) is designed firstly, to help transmit, in cell mode to another composite intermediate destination unit and via a BL link, the data supplied in narrow band circuit mode by at least one other unit (1) which supports this mode and is connected to this composite unit via an NL link, and secondly, to allow the composite intermediate destination unit (3) to restore this data in narrow band circuit mode, to at least one other unit (1) connected to this composite unit, a gateway circuit (14) is proposed for mode switching in order to be able to transfer data between the communication circuits (9) and (13) and the composite unit (3). Hence, each gateway circuit (14) receives data modules in narrow band circuit mode, transmitted with a fixed period corresponding to the period of the time frame implemented. These modules are conventionally made up of either one byte - this is the case in particular for elementary data modules corresponding to PCM coded speech signal samples - or of a preset and limited number of bytes. In the example provided in figure 3, the gateway circuit (14) shown is used to process the time channels corresponding to four time division multiplex links of communication circuit (9), by ensuring distribution of the data modules received in the proportion of one cell per link and per time frame in the narrow band circuit mode. To this end, this gateway circuit contains four mode changer modules (15), each used to create one cell into which are inserted the data modules supplied by a time division multiplex link 14 26 0 5 during a frame, in view of transferring these modules in cell mode. Each mode changer module ensures, for instance, in thirty-two out of the fifty-three byte positions of a standardised cell, the insertion of the bytes suitable for transmission via one of the time division multiplex links of communication circuit (9); prior to this, it uses destination data previously supplied in narrow band circuit mode via the relevant time division multiplex link to make up the standardised cell header specifying its destination. Each mode changer module (15) also ensures the reverse operation on cells it receives in order to re-insert, for later use, the data modules, contained in a cell, into a frame of the time division multiplex link it services, according to the time structure in which these modules were arranged before they were transmitted in cell mode. In the proposed implementation, a switching component (16) is inserted between the mode changer modules (15) of the gateway circuit (14) and the communication circuit (13) to which is connected this gateway circuit within the composite unit (3) housing them. This switching component (16) ensures, as is common knowledge, multiplexing of the cells produced by the mode changer modules (15) and addressed to the associated communication circuit (13), and demultiplexing of the cells transmitted in the reverse direction by this communication circuit (13) towards one or several mode changer modules. Either way, a gateway circuit (14) may preferably not be limited to servicing 15 260 5 only the above-mentioned circuits (9) and (13), it may also service some terminals and links, in a way which will not be described further in this document since it is not directly related to the invention. It is nevertheless mentioned that cells may also be transmitted via the switching component (16) via one or several links supporting a cell mode for instance or any other compatible mode, eventually via a specialised mode changer module, not illustrated here. Cells transiting via the switching component (16) for further transmission in cell mode, via the communication circuit (13), are transmitted at the clock rate (11) set by the time base module (11), in order to be temporarily taken into account at the level of the adaptation buffer assembly ensuring the change of clock rate resulting from the change of mode before transmission. In this instance, this assembly groups buffer modules itemised 17 to 19, of the first in first out type, it therefore receives clock signals issued from each of the two time base modules (11) and (12). In the proposed implementation, each cell issued from the switching component (16) is encapsulated prior to being taken into account by the buffer assembly. This operation, performed in this instance by a specialised logic encapsulation module (20), is in particular designed to add to the cell the routing data necessary for its travel from the composite unit (3) where it is encapsulated to the composite intermediate destination unit (3) to which it will be transmitted in cell mode and where the data modules it contains will be 26 0 5 restored in narrow band circuit mode. This encapsulation may consist for instance of adding three additional bytes to each standard cell made up of fifty-three bytes, mentioned above. In this instance, the cell obtained is called encapsulated cell, a priority flag is added to it at this level. This flag is forced to high level for cells containing data modules designed to later be restored in narrow band circuit mode. Encapsulated cells issued sequentially from the encapsulation module (20) are taken over by the buffer assembly, those whose priority was forced to a high level are in this instance assumed to be sent to buffer module (17) and from there to one of the transmission buffer modules (19) which are then available for reception. A buffer module (18) takes over the encapsulated cell whose priority was not forced, in this instance these cells are assumed to be encapsulated at the level of the second encapsulation module (21) which may be connected to the switching component (16), if it is planned that the latter will transmit such cells; in the implementation shown in figure 3, the encapsulation module (21) is shown connected to the command structure (10) of the composite unit (3) housing it, for the requirements of this structure. Buffer modules (17) and (18) are connected in parallel to the inputs of the sub-assembly made up by the transmission buffer modules (19) which are also arranged in parallel, in order to be able to access any one of them indifferently. 17 26 0 5 5 Any encapsulated cell transmitted to any buffer module (17) or (18) is sent to the first buffer module then available to receive it, in viev\- of transmitting it, in this instance via any one of transmission outputs (Bi_t or BLt'), having a standard transmission interface (22) to which are connected the BL links linking the transmitting composite unit (3) where the relevant buffer modules (17 and 18) are located, with the composite intermediate destination unit (3). For the above-mentioned reasons, the rate of the encapsulated cells established from data modules issued from a communication circuit in narrow band circuit mode is lower than the rate transmissible in cell mode between two composite units, due to clock differences. Therefore, it is always possible to have non-priority encapsulated cells between the encapsulated cells via which the above-mentioned data modules are transmitted, even if for the latter cells the traffic is maximum, this should only be envisaged as an exception in normal operation. Due to the different rate and clock existing between narrow band circuit mode and cell mode, it is always possible to have a transmission buffer module (19) available to transmit each of the high priority encapsulated cells, successively taken into account by the buffer module (17), immediately after the encapsulated cell, then being transmitted by one of the transmission buffer modules (19), hence, within a time always shorter than the set time required for each cell. The encapsulated cells thus transmitted via BL link from one composite 18 2 6 0 5 unit (3) to another composite intermediate destination unit (3), are received within the latter by a second adaptation buffer assembly acting in reverse to the buffer assembly made up of buffer modules (17 to 22) and ensuring the change of clock rate resulting from the change of mode to be performed after transmission for the data modules to be restored in narrow band circuit mode. A standard reception interface (23) is conventionally positioned between the receive buffer registers (24) connected in parallel to the second buffer assembly of each composite unit (3) and the BL links servicing it, in this instance the links are shown leading to inputs BLr and BLr' of the reception interface. In this instance, the buffer registers act in reverse to buffer registers (19). Each encapsulated cell received at composite intermediate destination unit level is initially taken over by the first receive buffer module (24) becoming available for reception, in view of being transmitted to one of the two buffers (26, 27) towards which it is directed according to the priority flag given to it during its encapsulation. This take over occurs at the cell mode fixed clock rate supplied by the time base module (12) of the relevant composite intermediate destination unit, it being reminded that the time base modules (12) are planned to be synchronous within the entire facility. The encapsulated cells sequentially taken over at buffer module level (26) are those containing a high level priority flag, while in this instance the 19 26 0 5 others are assumed to be directed towards buffer module (27). Cells of both types are then de-encapsulated via either of the two de-encapsulation modules (28, 29). Cell transmission from a buffer module (26 or 27) to the associated de-encapsulation (28 or 29) occurs at the rate of clock signals supplied by time base module (11) of the composite unit housing these modules, It is reminded that time base modules (11) of narrow band circuit mode support units within a facility may be plesiochronous only. The de-encapsulation module (28) of a composite unit is used to remove cell encapsulation data, corresponding in this instance to three additional bytes, it therefore restores a standard cell from each encapsulated cell received, each standard cell then being transmitted to the relevant composite unit communication circuit (9) in narrow band circuit mode, at the rate of clock signals supplied by clock module (11) of this relevant composite unit. The de-encapsulation module (29) performs the same function for the received encapsulated cells which do not contain the above-mentioned high priority flag and which must be transmitted under preset time conditions, less stringent and potentially graduated. De-encapsulation of these cells may lead to the restoration of data transmitted under different forms according to the requirements, in particular in the form of standard cells without high priority which may be transmitted, via switching component (16) of the relevant composite unit, or under another compatible form, \\(\ the form of file for 26055 instance, after assembly, to the command structure (10) of this composite unit, as shown in figure 3. In the p/oposed implementation, a request/acknowledgment regulation method, whereby the units wishing to transmit send a request and the 5 destination units transmit an acknowledgment, is used to prevent traffic congestion over the links, this method shall not be described further in this document since it is not directly related to the invention. This method implies the availability, in each communication circuit (13), of an automaton (30), transmit buffer modules (31) and receive buffer modules (32), functionally 10 and respectively corresponding to buffer modules (19) and (24) for transmission and reception of the proposed request and acknowledge indications added to the encapsulation data of the encapsulated cells, for instance in preset bit positions within the three encapsulation bytes provided per encapsulated cell. 15 Furthermore, the cells issued from the de-encapsulation module (28) are B routed, according to their respective headers, by the associated switching component (16), towards the mode changer modules (15) of the gateway circuit (14) in the composite unit housing them. Each changer module (15) redistributes over the time channels of the 20 time division multiplex link it services, the data modules contained within a received cell, in order to reposition them in time, as they were initially, during the frame used for their transmission. 21 26055 It is reminded that variation of the delay due to cell mode transmission is limited by the choice of cell transmission, according to the invention, to a maximum deviation corresponding to the time allocated per cell during transmission, at this level, it is aiso proposed to compensate for time variations, progressive and cyclic speed affecting the successive byte groups in the data modules addressed to the same time division multiplex link, as a function of their respective cells which transmitted them As already mentioned above, in the absence of specific synchronisation between two units between which data supplied in narrow band circuit mode is transferred in cell mode, this data is restored, under time conditions set by clock signals supplied locally at the level of the intermediate destination unit where the change of mode occurs to return to narrow band circuit mode, following transmission in cell mode. </div>

Claims

<div class="application article clearfix printTableText" id="claims"> 2 C i> o 5 What we claim is:

1. A method whereby data can be transmitted, via a link operated in cell mode between two units of a telecommunications facility, the data being first supplied to one of these units in a time division multiplexed form, hence 5 in a modular form, via one or several basic time channels within a link operated in narrow band circuit mode, when both units are operated respectively in circuit mode, synchronously or by two plesiochronous clocks wherein time allocated to each cell is shorter than the time provided for a frame time slot dedicated to a basic channel during each frame in narrow 10 band circuit mode, is used to force, over the cell mode operated link, the transmission priority of each data module previously in narrow band circuit mode which is to be sent from one unit to the other, such that, once received by the unit in charge of the transmission, each of said data modules can be inserted in the first cell transmitted by the said unit, and such that, the said 15 data modules transmitted in. cell mode to this destination unit can be cyclically reinserted in one or several preset time channels, under the control of the clock driving the narrow band circuit mode at destination unit level, thus making it possible to fully restore the data to its original form.

2. An arrangement for implementation of the method as claimed in claim 20 1, within a telecommunications facility including narrow band circuit mode support units supporting terminals operating in this mode, said units being point-to-point interconnected via links specifically fitted and time controlled 23 26 0 5 5 for such a mode of operation, and cell mode support units also point-to-point connected via links specifically fitted and controlled for cell mode operation, via cne or several switching components of a cell switching arrangement within the facility, wherein said arrangement includes at least two composite units interconnected via a cell mode link, each composite unit being connected to a unit and/or terminals supporting the narrow band circuit mode, each composite unit being provided with a communication circuit to provide for data transfer in each of these modes, this applies as well to the units with which they communicate in one of these modes, and further including a gateway circuit to transform into cells the data modules transmitted during one frame via a narrow band circuit mode time division multiplex link, and conversely, to restore the data modules contained within the cell received to their original time position, before changing to cell mode, during the time frame within which they are restored in narrow band circuit mode.

3. An arrangement as claimed in claim 2, wherein each composite unit contains a gateway circuit including some mode changers, each used to create a cell into which are inserted The data modules supplied by a time division multiplex link during a frame, as well as to redistribute over the time channels of the time division multiplex link it services, the data modules contained within a received cell, in order to reposition them in time, as they were initially, during the frame used for their transmission. 260 5

4. An arrangement as claimed in claim 3, wherein each composite unit contains a cell mode communication circuit including some devices used to allocate priority to the cells to be transmitted, and in particular to allocate a high level priority to those cells transmitting data modules supplied and 5 restored in narrow band circuit mode, and some transfer buffer devices connected in parallel used for cell transmission between composite units according to allocated priorities and for high priority cell transmission within a time per cell shorter than the set cell transmission time.

5. An arrangement as claimed in claim 3, wherein each composite unit 10 contains a cell mode communication circuit further including some transfer buffer devices connected in parallel for separation of the cell received from another composite unit according to the priority flags allocated by said unit and to compensate, together with the mode changers of the gateway circuit within the same composite unit, for the variable delays resulting from cell 15 mode transmission, for each of the byte groups corresponding to the data modules, to be restored in narrow band circuit mode, contained within one of the sequentially received cells addressed to the same time division multiplex link. 25 2 6 0 5 5 i# s

6. A method and arrangement substantially as herein described will reference to Figures 1 - 3 of the accompanying drawings. 5 ALCATEL AUSTRALIA LIMITED 10 B. O'Connor Authorized Agent P5/1/1703 26 </div>