Abstract:
A network device (NWE) for a digital transmission network with synchronous digital hierarchy receives data steams containing frames with data packets mapped therein and addressed by a phase reference identifier. Internally, the network device has redundant transfer paths which potentially cause different delay. The network element compensates for that delay by adjusting the phase reference identifier allocated to a respective data packet by a predetermined phase correcting value, leading in the phase, which corresponds to a maximum expected delay for transfer of the data packets on internal transfer paths, and by buffering the data packet by a buffering time such that its buffering time and its delay actually needed for passing through the network device in total correspond to the maximum expected delay taken into account by the phase adjustment.

Description:
[0001]    The invention is based on a priority application DE 10064988.2 which is incorporated by reference herein. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates to the field of telecommunications and more particularly to a network device for delay compensation of data packets, in particular a network device for a telecommunications network with synchronous digital hierarchy, as well as to a method for delay compensation of data packets.  
         BACKGROUND OF THE INVENTION  
         [0003]    In a transfer of data packets in a time multiplex process data packets are transferred within defined time slots or time channels, which can be repeatedly re-used for data transfer after a predetermined time. A group of time channels or else a single time channel provide so-called frames for transferring data packets. The frames in the synchronous digital hierarchy are, for example, referred to as synchronous transport modules (STM) and the data packets as so-called virtual containers. The so-called overhead or frame header of each frame contains phase reference identifiers, i.e., so-called pointers, for the data packets transported in the respective frame, which serve for determining the position of the corresponding packet within the respective frame.  
           [0004]    When the frames and the data packets contained therein pass through a network device, they are subject to a delay (i.e. a propagation time). This delay can take on various values, if, for example, in a first scenario a first data packet passes through the network device on a first transfer path between input and output stage of the network device, causing a first delay, and a second data packet associated with the first data packet passes through on a second transfer path causing a second delay. The two transfer paths can, for example, lead over various modules of a switching matrix, input/output modules and cable strings, which with a more complex structure of the network device can easily be arranged spatially far apart, so, for example, the first data packet has to cover a 200 meters longer transfer path in the network device than the second data packet. However, two data packets are, due to different transfer paths, no longer in the same phase relation to one another at the output stage of the network device as at the input stage.  
           [0005]    A second scenario relates to a network device with redundant devices, for example, with double switching matrices and double cable strings between input and output stage of the network device. The data packets pass through a first transfer path between input and output stage and in parallel through a second transfer path as a data packet copy. Ideally, at the output stage it should be possible at any time to switch over without loss of data from the first to the second transfer path and vice versa. This requires, however, that a data packet and its copy must be available exactly synchronously on the output side of the output stage. As a result of the first and second transfer paths possibly causing different delays, this is, however, not possible. It would admittedly be possible to construct the transfer paths of the network device in such a way that they cause perfectly identical delays by using identical switching matrices and cables of identical length and kind. However, this causes a considerable expense, if, for example, modules arranged directly side by side have to be connected via a cable of, e.g. 200 meters in length.  
         SUMMARY OF THE INVENTION  
         [0006]    It is therefore an object of the present invention to provide a network device and a method for delay compensation of data packets, which can pass through the network device on several transfer paths, so a defined delay behaviour of the respective data packets during passing through the network device is guaranteed.  
           [0007]    This object is achieved by a network device, in particular for a telecommunications network with synchronous digital hierarchy, for delay compensation of data packets, which delay occurs during passage of the data packets through an input stage and an output stage of the network device. The output stage of the network device is connected to the input stage via a first transfer path and via a second transfer path. A first delay is caused by the first transfer path and a second delay is caused by the second transfer path. The data packets are transferred in multiplex frames, each containing at least one data packet to be transferred, as well as at least one phase reference identifier for determining the respective position of the data packet within the corresponding frame. According to the invention, the network device has a phase correcting means for adjusting the phase reference identifier allocated to a respective data packet by a predetermined phase correcting value, leading in the phase, which corresponds to a maximum expected delay for a transfer of the data packets on the first transfer path or the second transfer path, and a buffer means for buffering the data packets by buffering times such that for each respective data packet its buffering time and its delay actually needed for passing through the network device in total correspond to the maximum expected delay taken into account in its allocated, adjusted phase reference identifier.  
           [0008]    The object is therefore based on the idea that the network device adjusts the phase reference identifier allocated to a respective data packet by a predetermined phase correcting value, leading in the phase, which corresponds to a maximum expected delay for transfer of the data packets on the first or the second transfer path. The maximum possible delay for passing through the network device is therein, so to speak, “programmed into” the phase reference identifier, for example a delay occurring on a transfer path of approximately 200 meters between input and output stage, if the modules of the network device are arranged spatially far apart. The phase reference identifier is changed in such a way that the data packets are further leading in phase, in order to balance the maximum expected delay. The actual position of the respective data packet to the frame containing the phase reference identifier is also modified.  
           [0009]    The delay between input and output stage can actually, however, be far smaller than provided in the modified phase reference identifier, so the data packets transferred on the first transfer path cover, for example, only 3 meters, while the data packets on the second transfer path pass through a distance of 180 meters through cables and modules. To match the actual delay to the maximum expected delay already taken into account in the phase reference identifier and thus finally to balance delay differences occurring on the different transfer paths of the network device, the network device buffers the data packets, for example in buffer memories serving as buffer means, so that the total delay actually needed for passing through the network device corresponds to the maximum expected delay taken into account in the phase reference identifier allocated to a data packet.  
           [0010]    The invention can advantageously be used in any system in which data packets are transferred in a time division multiplexed fashion and can be displaced within frames or containers in relation to their respective phase. In a preferred embodiment the invention is used in a network device of a transmission network with synchronous hierarchy, for example in a cross-connect of an SDH transmission network (SDH=synchronous digital hierarchy) or in a SONET device (SONET=Synchronous Optical Network). Such networks are defined in ITU-T G.707 (2000), which is incorporated by reference herein. The multiplex frames are then SDH frames and the phase reference identifiers are contained in the control information of the SDH frames. The data packets are transmitted in virtual containers or form virtual containers, which are contained in corresponding SDH frames and are displaceable in relation to the phase.  
           [0011]    It is in principle also conceivable that one or more data packets are transported in frames designated as containers, wherein the data packets are displaceable in the containers in relation to the phase and in which a correspondingly adjustable phase reference identifier is provided.  
           [0012]    Further advantageous configurations of the invention are found in the dependent claims and in the description.  
           [0013]    In principle the buffer means can be adjusted to the, in particular spatial, circumstances given by a suitable structure, for example by a depth of memory, predetermined or able to be set by configuration data. It is, however, advantageous, in relation to the expense of configuration, if the buffer means ascertain the delay of the data packets actually needed for passing through via the at least one first or the second transfer path and to adjust the respective buffering time to the delay actually needed. This can be done, for example, with the aid of a reference clock, provided to the buffer means by the network device.  
           [0014]    The invention can advantageously be used in any network device, which, for example, owing to a modular structure, with several possible internal transfer paths has different delay behaviour. This is the case in particular if the network device is constructed as a redundant network device, wherein the at least one first transfer path leads over at least one first device, which, e.g. is a switching matrix with one or more matrix modules connected to one another via connecting leads, and the second transfer path is guided over at least one second device, redundant to the at least one first device, which in the example is also a switching matrix.  
           [0015]    The maximum expected delay is advantageously ascertained substantially by means of maximum lengths of connecting leads arranged on the transfer paths. Advantageously, however, the delays caused by the other devices arranged on the respective transfer paths, for example the previously mentioned switching matrix, are also taken into account.  
           [0016]    The input stage and the output stage allocated to it can be arranged on separate modules. Advantageously they are combined into one joint module or are formed by modules which preferably can be configured as both, input stage or output stage.  
           [0017]    The input stage and the output stage can in principle only serve as input or output interface of the network device. Advantageously the input stage and the output stage already form stages of a switching matrix or are allocated to a switching matrix, so by connecting together the two stages a two-stage switching matrix can already be constructed. Advantageously in addition a further matrix module is connected between the input and output stage constructed as matrix stages, so a three-stage matrix, referred to as a three-stage Clos matrix is formed.  
           [0018]    Needless to say, in the network device also more than two or three matrix stages or other devices connected between the input and output stage can be provided.  
           [0019]    The phase correcting means acting on the phase reference identifier can in principle be arranged in any way, e.g. allocated completely to the input stage or the output stage or else to a device arranged on the respective transfer paths between the input and output stage, for example a center stage module of a three stage switching matrix. Or part of the phase correction can be performed by the input stage and another part by the output stage or by some other means arranged on a transfer path of the network device.  
           [0020]    The buffer means can also in principle be arranged in any way, similarly to the phase correcting means. For example, the buffer means could be allocated completely to the output stage, which then adjusts different delays of the data packets occurring on the respective transfer paths of the network device, for example in buffers on the input side, to the values taken into account in their corresponding phase reference identifiers.  
           [0021]    It is also possible for the buffers to be arranged completely on devices arranged on the at least one first or second transfer path, for example on center stage modules of a switching matrix.  
           [0022]    Advantageously, however, the buffers of the buffer means are arranged at different points of the network device, preferably on the input side of the output stage as well as on devices arranged on the transfer paths. The delay of a data packet is then increased by the buffers in such a way that the delay on a portion of the transfer path connected in series to the buffer and the buffering time in total correspond to an expected maximum delay. If, for example, a cable of 90 meters leads to a buffer of a first module and a second cable of 10 meters leads to a buffer of a second module and the expected delay is structured for a cable of 100 meters, the buffer allocated to the first cable has to buffer the received data packets for a delay occurring with a cable of 10 meters in length and the buffer allocated to the second cable for a delay occurring with a cable of 90 meters in length.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]    The invention and its advantages are illustrated below using an embodiment example with the aid of the drawings.  
         [0024]    [0024]FIG. 1 shows schematically a network device NWE according to the invention, with an input stage INST and an output stage OUTST as well as matrix stages ST 1 , ST 2 , ST 3 .  
         [0025]    [0025]FIG. 2 shows a schematic illustration of the embodiment of the method according to the invention.  
         [0026]    [0026]FIG. 3 shows a data stream FRSa with frames FR 1 , FR 2  and data packets DP 1 , DP 2 , DP 3  transferred therein.  
         [0027]    [0027]FIGS. 4 a,    4   b,    4   c  show a modification of a phase reference identifier and a buffering of the data packet DP 1  contained in the frame FR 1  from FIG. 3, using the method from FIG. 2.  
         [0028]    [0028]FIG. 5 shows an illustration of buffering times and delays in the network device NWE. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0029]    The network device NWE shown schematically in FIG. 1 has an input stage INST and an output stage OUTST as well as matrix stages ST 1 , ST 2 , ST 3  connected between the input stage INST and the output stage OUTST, which form a three-stage Clos matrix. The network device NWE is, for example, a cross-connect for an SDH transmission network, on which data packets are transferred in so-called SDH frames, referred to as synchronous transport modules (STM). From the SDH transmission network transmission lines VIN 1 , VIN 2  lead to input ports IO 11 , IO 12  of the input stage INST. On the output side the network device NWE is connected to transmission lines VOUT 1 , VOUT 2  of the SDH transmission network via output ports IO 21 , IO 22  of the output stage OUTST. By means of the matrix stages ST 1 , ST 2 , ST 3  the network device NWE can switch any number of internal transfer paths for data packets between the input ports IO 11 , IO 12  and the output ports IO 21 , IO 22 . Transfer paths TRP 1   a,  TRP 1   b  are shown as examples. The transmission lines VIN 1 , VIN 2 , VOUT 1 , VOUT 2  serve as examples for not shown further transmission lines and the input/output ports IO 11 , IO 12 , IO 21 , IO 22  serve as examples for, not shown, further input/output ports of the network device NWE.  
         [0030]    Matrix stage ST 1  contains the matrix modules S 11 , S 12 , each of which can receive data streams with data packets from each of the input ports IO 11 , IO 12  via connecting leads V 1 X. The input ports IO 11 , IO 12  can demultiplex the respective data streams and thus extract the data packets contained therein from the data streams. The input ports IO 11 , IO 12  can, if necessary, further multiplex these data packets again on to internal data streams, e.g. passing through the network device NWE. The internal data streams preferably have a higher clock frequency than the external data streams. In the embodiment example, however, for reasons of simplicity external and internal data streams flowing in the network device NWE are constructed and clocked in the same way.  
         [0031]    The data packets received from the input ports IO 1 , IO 12  are buffered on the input side of matrix stage ST 1  in buffers B 11 , B 12  acting as buffer means, which are allocated to and connected in series to the matrix modules S 11 , S 112 . Matrix module S 11  and buffer B 11 , owing to an advantageous configuration of the invention, are redundant to matrix module S 12  and buffer B 12  and spatially separated from them.  
         [0032]    Needless to say, for simplification of the illustration matrix modules S 11 , S 12  and buffers B 11 , B 12  serve only as examples of an arrangement of matrix modules and can be formed, for example, by a single or by several electrical and/or optical matrix boards. The same applies by analogy to the input ports IO 11 , IO 12 , which can be arranged, e.g. on separate electrical and/or optical boards comprising further input ports.  
         [0033]    Matrix stage ST 2  contains matrix modules S 21 , S 22 , which comprises buffers B 21 , B 22 , acting as buffer means, as well as pointer processors PA 21 , PA 22 , acting as phase correcting means, for adjusting phase reference identifiers, allocated to the data packets passing through matrix stage ST 2 . The pointer processors PA 21 , PA 22  are arranged on the input side of matrix stage ST 2 . Pointer processor PA 21  and thus matrix module S 21  are connected to matrix modules S 11  or S 12  via connecting leads VSI 11 , VSI 21  and pointer processor PA 22  and thus matrix module S 21  are connected to matrix modules S 11  or S 12  via connecting leads VSI 12 , VSI 22 . Buffers B 21 , B 22  serve to adjust matrix modules S 31 , S 32  to possibly different delays, caused by the existing different lengths of connecting leads VSI 11 , VSI 12 , VSI 21 , VSI 22 .  
         [0034]    However, one matrix module S 21 , S 22 , one buffer B 21 , B 22  and one pointer processor PA 21 , PA 22  form in the present case one matrix unit SM 1 , SM 2  and are, e.g. arranged on an electrical and/or optical board or are formed from one constructional unit with several boards of this kind. Matrix units SM 1 , SM 2  are matrix units redundant to one another, which are constructed identically and can perform identical functions. Matrix units SM 1 , SM 2  can, however, comprise one or more electrical or optical boards.  
         [0035]    On the output side connecting leads VSO 11 , VSO 12  lead from matrix module S 21  to matrix modules S 31 , S 32  of matrix stage ST 3 . Connected in series to this on the input side is a buffer B 31 , B 32 , acting as buffer means. From matrix module S 22  connecting leads VS 021 , VS 022  lead to buffers B 31 , B 32  connected in series to matrix modules S 31 , S 32 . Buffers B 31 , B 32  serve for adjusting matrix modules S 31 , S 32  on the input side to possibly different delays, caused by the existing different lengths of connecting leads VSO 11 , VSO 12 , VSO 21 , VSO 22 . Matrix module S 31  and the buffer B 31  allocated to it on the one hand and matrix module S 32  and its buffer B 32  on the other hand are structured as separate constructional units and redundant to one another. For reasons of redundancy these constructional units are advantageously arranged spatially apart. A constructional unit of this kind can be formed by one or more electrical and/or optical devices, e.g. electro-optical boards.  
         [0036]    On the output side matrix stage ST 3  is connected to the output stage OUTST via connecting leads VS 2 X, so from each of the matrix modules S 31 , S 32  a connection can be constructed to each of the output ports IO 21 , IO 22  and thus data packets can be sent to each output port IO 21 , IO 22 .  
         [0037]    For reasons of operational safety matrix units SM 1 , SM 2  are arranged spatially apart from one another, wherein matrix unit SM 1  is arranged, e.g. in a room together with the input stage INST and matrix unit SM 2  in a separate room. An arrangement of this kind, spatially distanced, is designated as “room protection”. Between the rooms and thus between the components of the network device NWE arranged in each of the rooms considerable distances have to be bridged, which can, for example, have a length of up to 200 meters. Corresponding to the spatial arrangement of matrix stages ST 1 , ST 2 , ST 3 , the connecting leads VSI 11 , VSI 12 , VSI 21 , VSI 22  and VSO 11 , VSO 12 , VSO 21 , VSO 22 , constructed as electrical or optical connections, are of different lengths and cause delays of data packets, transferred on them differing from one another. The connecting leads VSI 12 , VSI 22 , VSO 21 , VSO 22 , leading to matrix unit SM 2 , are in the present case in comparison with connecting leads VSI 11 , VSI 21 , VSO 11 , VSO 21 , leading to matrix unit SM 1 , approximately 200 meters longer, as indicated in FIG. 1 by interrupted lines.  
         [0038]    The network device NWE also comprises further devices, not illustrated, for example a central control module or board, an interface to a network management system and one or more clock generator modules, which supply the modules shown, for example matrix modules S 11 , S 12 , S 21 , S 22 , S 31 , S 32 , with a master clock signal and a slave clock signal redundant to this. The respective internal master/slave clock signals are formed by the, not shown, clock generator modules using external clock signals, derived from a received data signal at one of the input ports IO 11 , IO 12 . The external clock signals are, for example, contained in STM frames. The internal master/slave clock signals contain on the one hand so-called frame clock signals, which are transferred, for example, at a bit rate of 2 megabits per second and comprise several frame clocks, e.g. one at one Hz and one at 8 kHz. The internal master/slave clock signals additionally advantageously contain simple clock signal pulses, e.g. with a frequency of 2.43 MHz, typical for SDH, for fine synchronisation of the components of the matrix stages ST 1 , ST 2 , ST 3  and of the input and output stages INST, OUTST.  
         [0039]    The network device NWE receives, e.g. on the transmission line VIN 1  a time division multiplexed data stream FRSa, which is shown in FIG. 3 over a time axis t. In data stream FRSa data packets DP 1 , DP 2 , DP 3 , designated as virtual containers, are transferred in SDH frames FR 1 , FR 2 , so-called synchronous transport modules. Data packet DP 3 , forming a so-called payload of the frame FR 2 , can be, for example, a so-called VC- 4  container. In the SDH frames FR 1 , FR 2  frame headers FA 1 , FA 2 , designated as overheads, are provided, which form a pattern in the data stream FRSa, recurring cyclically with the frame clock cycle, and in which are contained phase reference identifiers P 1 , P 2 , P 3  for determining the respective position of the data packets DP 1 , DP 2 ; DP 3  within the frames FR 1 , FR 2 .  
         [0040]    The network device NWE receives the data stream FRSa at the input port IO 11 . For example by means of a destination identifier contained in the frame FR 1  or the data packets DP 1 , DP 2  or by means of pre-setting by a network management system, not shown, the network device NWE ascertains that the data stream FRSa and thus the data packets DP 1 , DP 2  are to be transferred to the output port IO 22 . The present network device NWE, operating as an SDH cross-connect, leads the entire data stream FRSa from the input port IO 11  to the output port IO 22 . Transfer path TRP 1   a,  which leads via the matrix modules S 11 , S 21 , S 31 , is, for example, suitable for this.  
         [0041]    To simplify the following embodiments the data stream FRSa, received from outside on the connection VIN 1 , is not modified below by the input stage INST, for example multiplexed into a faster clocked internal data stream, and forwarded to matrix stage ST 1  as an internal data stream.  
         [0042]    For reasons of redundancy a second transfer path TRP 1   b  is additionally provided, leading over matrix modules S 11 , S 22 , S 32 , which is an independent transfer path, redundant to transfer path TRP 1   a  and leading over redundant devices. By contrast to transfer path TRP 1   a,  transfer path TRP 1   b  is longer, however, so data packets DP 1 , DP 2  in a transfer on transfer path TRP 1   b  arrive later at the output port IO 22  than in a transfer on transfer path TRP 1   a,  if the measures according to the invention, explained below, are not applied.  
         [0043]    A distributing module C 11 , allocated to the input port IO 11 , transmits on the one hand data stream FRSa on transfer path TRP 1   a  to matrix module S 11  and on the other hand a data stream copy FRSb of data stream FRSa to matrix module S 12 . A distributing module C 12 , corresponding in its function to the distributing module C 11 , is allocated to the input port IO 12 .  
         [0044]    In as far as delay differences occur on the connecting leads V 1 X during transfer of data streams FRSa, FRSa 2 , these are balanced by the buffers B 11 , B 12 . Buffers B 11 , B 12  therein delay the data streams FRS, FRSc by buffering times TB 11 , TB 12  (FIG. 5). Buffers B 11 , B 12  contain, for example, shift registers, the memory depth of which is dimensioned accordingly for delays occurring on the connecting leads V 1 X. The respective memory depth can also be configurable, wherein, for example, on constructing the network device NWE the lead lengths of the connecting leads V 1 X are ascertained and configuration data corresponding to these lengths are loaded into the network device NWE for configuration of the buffers B 11 , B 12 . Advantageously, however, buffers B 11 , B 12  are adaptive buffers, which ascertain the necessary buffering times according to the lead lengths of the connecting leads V 1 X, e.g. by means of the above-mentioned frame clock signal or some other reference signal and are set to the respective lead lengths. For this purpose buffers B 11 , B 12  are, for example, equipped as memories, the memory cells of which are scanned by means of a multiplexer. The memory cells to be scanned can be pre-provided to the multiplexer, for example by circulating counters, the respective starting values of which are set according to the lead lengths of the connecting leads V 1 X.  
         [0045]    Needless to say, with equal lead lengths of the connecting leads V 1 X the buffers B 11 , B 12  are not absolutely necessary and the buffers B 11 , B 12  could also be allocated to the input stage INST.  
         [0046]    [0046]FIG. 2 illustrates the synchronous reception S 11 N of the data streams FRSa, FRSb at the matrix modules S 11 , S 12  of matrix stage ST 1  thanks to buffers B 11 , B 12 . FIG. 2 is a three-dimensional diagram with a time axis designated as “t” and with time axes S 11   t,  S 12   t;  S 21   t,  S 22   t;  S 31   t,  S 32   t  respectively allocated to the matrix modules S 11 , S 12 ; S 21 , S 22 ; S 31 , S 32 , pointing in the X-direction. Allocated to each of the matrix stages ST 1 , ST 2 , ST 3  is a horizontal plane ST 1   e,  ST 2   e,  ST 3   e,  located on top of one another along a Y-axis designated as STn. In the Z-direction extend axes ST 1   n,  ST 2   n,  ST 3   n,  which together with time axes S 11   t,  S 21   t,  S 31   t  span planes ST 1   e,  ST 2   e,  ST 3   e,  in which time axes S 12   t;  S 22   t,  S 32   t,  parallel to time axes S 11   t,  S 21   t,  S 31   t,  are located.  
         [0047]    The data streams FRSa, FRSb are transmitted from the matrix modules S 21 , S 22  to matrix stage ST 2  in transmission processes designated as TR 1 . The inputs of the data streams FRSa, FRSb in the pointer processors PA 21 , PA 22  are designated as S 211 , S 221 . Due to the greater lead length of connecting lead VSI 22  in comparison with connecting lead VSI 11 , data stream FRSb compared with data stream FRSa needs a greater delay for transfer from matrix stage ST 1  to matrix stage ST 2  and is thus displaced with respect to this by a phase difference.  
         [0048]    The pointer processors PA 21 , PA 22  form phase correcting means for adjusting phase reference identifiers, allocated to data packets contained in the data streams FRSa, FRSb.  
         [0049]    The pointer processors PA 21 , PA 22  modify the phase reference identifiers by pre-determined phase correcting values, leading in the phase, which correspond to a maximum expected delay TPA during transfer of the data packets on transfer paths TRP 1   a,  TRP 1   b.  The adjustment of the phase reference identifiers is designated as TR 2  in FIG. 2. By means of FIGS. 4 a,    4   b  an adjustment of this kind of the phase reference identifiers is explained using the example of frame FR 1  explained in connection with FIG. 3.  
         [0050]    On the input side of pointer processor PA 21 , data packet DP 1  has a phase relation P 1  with respect to the frame header FA 1 , which phase relation is recorded in the header as a so-called pointer P 1 . Pointer processor PA 21  now changes the phase relation between the frame header FA 1  and the data packet DP 1 , as a result of which the data packet DP 1  is, so to speak, moved chronologically into the past. In the specific case the frame head FA 1  is moved closer towards the data packet DP 1  and the pointer P 1  is at the same time shortened to a shorter pointer P 1   m,  which the pointer processor PA 21  records in the frame header FA 1 . In principle data packet DP 1  could therein also be shifted into a different frame, if the desired phase correction is not possible within frame FR 1 .  
         [0051]    Pointer processor PA 22  operates on the frames, phase reference identifiers and data packets contained in the data stream FRSb identically by analogy. However, pointer processors PA 21 , PA 22  correct the respective phase relations by identical, pre-determined fixed values, which correspond, as it were, to the “worst case”, namely the maximum occurring delay TPA (FIG. 5) during transfer of data streams FRSa, FRSb in the network device NWE, in the present case the delay occurring on transfer path TRP 1   b.    
         [0052]    The reception of data streams FRSa, FRSb, the data packets of which have been virtually shifted into the future, at buffers B 21 , B 22  are designated in FIG. 2 as S 212 , S 222 . Buffers B 21 , B 22  buffer the data streams FRSa, FRSb in buffering processes TR 3  by buffering times TB 21 , TB 22 . This process is pictorialised for buffer B 21  in FIG. 4 c,  in which frame FR 1  is delayed in total by a buffering time TB 21 .  
         [0053]    At this point it should be noted that the relations shown in FIG. 4 and FIG. 5 are neither true to scale with one another nor in total reproduce the actual chronological circumstances of the network device NWE. The arrow lengths shown are for pictorialisation in particular in relation to the buffering times longer than in reality, in particular in relation to the delays shorter.  
         [0054]    The buffering times TB 21 , TB 22  of buffers B 21 , B 22  are dimensioned differently and provide that the different delays TVSI 11 , TVSI 12  needed on the connecting leads VSI 11 , VST 22  are compensated on the input side of matrix modules S 21 , S 22  of matrix stage ST 2  and the data streams FRSa, FRSb arrive synchronously at matrix modules S 21 , S 22 . The respective reception of the data streams FRSa, FRSb at matrix modules S 21 , S 22  is designated as S 213  or as S 223 .  
         [0055]    Matrix modules S 21 , S 22  transmit the data streams FRSa, FRSb in transmission processes TR 4  to matrix stage ST 3 , where they enter at buffers B 31 , B 32  allocated to matrix modules S 31 , S 32 . The respective receptions are designated as S 311  or S 321 . Buffers B 31 , B 32  have substantially the same function as buffers B 21 , B 22 , namely to compensate delays of connecting leads connected in series, in the present case delays TVSO 11 , TVSO 22 , of connecting leads VSO 11 , VSO 22 , of different lengths, so that the data streams FRSa, FRSb arrive synchronously at the matrix modules S 31 , S 32  of matrix stage ST 3 . The buffering processes with accordingly shorter buffering time TB 31  of buffer B 31  and longer buffer time TB 32  of buffer B 32  are designated as TR 5  in FIG. 2.  
         [0056]    In the embodiment example the data streams FRSa, FRSb leave matrix stage ST 3  in a transmission process S 30 UT synchronously and with phase relations between the frames and data packets contained therein correlating to each other. Matrix module S 31  transmits data stream FRSa and matrix module S 32  transmits data stream FRSb to the output port IO 22  of the output stage OUTST. Selection means SW 2  are connected ahead of the output port, which select at any time one data stream from the received synchronous data streams FRSa, FRSb, which is free of errors. If, for example, a problem occurs on transfer path TRP 1   a,  e.g if matrix module S 21  fails, the selection means SW 2  can switch over from data stream FRSa to data stream FRSb without a phase jump and forward data stream FRSb to output port IO 22 .  
         [0057]    A selection means SW 1 , corresponding in function to selection means SW 2 , is allocated to output port IO 21 . Additionally, buffers can be connected in series to the output ports IO 21 , IO 22 , which if necessary compensate different delays caused by connecting lead V 2 X. Buffers B 21 , B 22 , B 31 , B 32  are, like buffers B 11 , B 12 , of fixed configuration, configurable or preferably adaptive buffers, which are adjusted to the delays of the corresponding data streams transferred caused by connecting leads or other devices. In total buffers B 11 , B 12 , B 21 , B 22 , B 31 , B 32  form buffer means according to the invention, which serve to adjust in total the delay actually needed by the data packets transferred in data streams FRSa, FRSb for passing through the network device NWE to the maximum expected delay taken into account in the phase reference identifiers allocated to them, e.g. the phase reference identifier P 1 M.  
         [0058]    Further variants of the invention are easily possible. Needless to say, the buffer means could also be provided in only one of the matrix modules ST 1 , ST 2 , ST 3 . Buffer means according to the invention could also be provided in the output stage OUTST. The buffer means would, for example, be connected in series to the output ports IO 21 , IO 22  and would supplement the delays occurring on the transfer paths, for example, transfer paths TRP 1   a,  TRP 1   b,  by suitable buffering times, so that the delays and buffering times in total would correspond to the delays taken into account in the phase reference identifiers.  
         [0059]    Instead of the phase correcting means PA 21 , PA 22  arranged in matrix stage ST 2 , phase correcting means could alternatively be provided in the input stage. This is indicated in FIG. 1 by the pointer processors PA 11 , PA 12  allocated to the input ports J 11 , IO 12 .  
         [0060]    It is also possible for pointer processors PA 11 , PA 12  to cooperate with pointer processors PA 21 , PA 22  and for each pointer processor PA 11 , PA 12 , PA 21 , PA 22  to perform only a part of a phase correction.  
         [0061]    In a further variant of the embodiment example, pointer processors PA 31 , PA 32  are allocated to the output ports IO 21 , IO 22 , which is indicated by broken lines. The pointer processors act as phase correcting means for adjusting phase reference identifiers and modify the phase reference identifier of data packets received from the output stage OUTST by a phase correcting value leading in the phase. Phase correcting means PA 31 , PA 32  can cooperate with phase correcting means PA 11 , PA 12  and/or PA 21 , PA 22  and perform only a part of the necessary adjustment of the phase reference identifiers. Alternatively, pointer processors PA 31 , PA 32  could be provided alone instead of phase correcting means PA 11 , PA 12  and/or PA 21 , PA 22  and thus carry out the full adjustment of the phase reference identifiers.  
         [0062]    It is also possible that the input stage INST and the matrix stage ST 1  or corresponding parts of input and matrix stages are combined into one joint input module IOM 1  or several input modules of this kind, designed as an electric board.  
         [0063]    In another variant, matrix module S 11  is allocated to input port IO 11  and matrix module S 12  is allocated to input port IO 12 . In the same way matrix module S 31  and output port IO 21  and matrix module S 32  and output port IO 22  could also be allocated to one another and possibly also be combined into one constructional unit.  
         [0064]    In principle the input stage INST and matrix stage ST 1  can also be constructed as constructionally separate units. Further, e.g. on the one hand matrix module S 11  and input port IO 11  and on the other hand matrix module S 12  and input port IO 12  could also be combined into respective constructional units.  
         [0065]    The input ports IO 11 , IO 12  and the output ports IO 21 , IO 22  could be provided on a joint module with universal input/output ports, which can be configured according to requirement as either input ports or output ports. Matrix stages ST 1  and ST 3 , which act, so to speak, as an matrix final stage, could also be combined into one constructional unit and/or be formed by identical modules, adjustable for the respective function as matrix stage ST 1  or ST 3 . Advantageously, for reasons of redundancy, matrix modules S 11  and S 31  on the one hand and S 12  and S 32  on the other hand are then combined into one constructional unit. The components shown such as for example the buffers B 11 , B 12 , B 21 , B 22  or the pointer processors PA 21 , P 122 , of the network device NWE can be implemented as hardware, for example as integrated circuits, as so-called Field Programmable Gate Arrays (FPGA) or as Application Specific Integrated Circuits (ASIC). Some components or parts thereof can also be implemented as software in the form of one or more program modules, the program code of which can be carried out, e.g. by a control processor of an matrix module or some other processor arrangement.  
         [0066]    Needless to say any combinations of the measures and arrangements disclosed in the claims and in the description are also possible.