Patent Publication Number: US-9900179-B2

Title: Communication network and method for operating a communication network

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to PCT Application No. PCT/EP2013/068679, having a filing date of Sep. 10, 2013, based on DE 102012219176.3 having a filing date of Oct. 22, 2012, the entire contents of which are hereby incorporated by reference. 
     FIELD OF TECHNOLOGY 
     The following relates to a communication network and a method for operating a communication network. The communication network may particularly be an Ethernet network. 
     BACKGROUND 
     Communication networks are being used to an ever greater extent for measuring, controlling and regulating complex technical systems. By way of example, networks are increasingly used in motor vehicles in order to form vehicle control systems. Corresponding complex and safety-relevant technical systems make great demands on the availability of the control elements provided as network devices. When single components fail, such as sensors or control devices, this must not result in the failure of the overall system. Particular relevance to safety applies to drive by wire systems, e.g. steer by wire systems, in which the steering wheel position is converted into wheel positions by electric motor using a network coupling comprising sensor, control and actuator devices. 
     In the past, redundant designs of particularly critical components have been used, so that in the event of an error the respective backup or redundant component can undertake the respective task. When there are a plurality of redundant components, it is necessary to ensure that only one of the two or more control devices has the respective control sovereignty. Furthermore, contradictory control demands must not arise for the same control functionalities. It is therefore desirable for all the control components to have the same information or data in the network. 
     In this respect, errors in the form of inconsistent data, which may be corrupt in the event of data transmission via the network that is used, for example, need to be recognized A standard network environment that is in widespread use is based on the Ethernet protocol. The use of Ethernet infrastructures has the advantage that standardized network devices and methods can be used. In the past, however, proprietary data buses were also used in order to link control components having internal redundancy, that is to say duplicate functionality, to one another. 
     Furthermore, it is possible for nodes used in the network to be erroneous. By way of example, error types that involve a network device using a high frequency to send data to the network that contains no data that the other control devices can use are known. The term “babbling idiot” is also used. The network infrastructure can then be burdened by high data rates such that it is no longer possible for genuine control or sensor data to be interchanged between the network devices that are still functioning. It is desirable to deal with particularly such erroneous behavior in safety-relevant networks and to process the available data in a suitable manner in order to ensure reliable operation of the unaffected devices in the network. 
     In the past, methods have been proposed in which the data interchange between prescribed communication partners has been subject to bandwidth limiting. However, faulty network nodes can also produce data packets with incorrect address data, this being unable to be handled satisfactorily for dedicated bandwidth limiting in every network topology, particularly in a ring-shaped network topology. 
     Furthermore, methods are known that are based on synchronized communication among the network nodes. In this case, particular time slots are defined for the data interchange between prescribed communication partners. Such time slot methods require sophisticated synchronization and special hardware devices. 
     SUMMARY 
     An aspect relates to an improved communication network. 
     Accordingly, a communication network having a multiplicity of network devices is proposed. The communication network has a plurality N of virtual networks that are implemented in the communication network such that each of the safety-critical network devices is coupled to the communication network via at least two virtual networks. In addition, the communication network has a plurality of network segments. In this case, each of the network segments has an associated subset of the network devices, wherein each network device arranged at a marginal region of a network segment has N limiting units, wherein the respective instance of the N limiting units is associated with a respective instance of the N virtual networks and is set up to limit data reception of data associated with the respective virtual network to a threshold value for the data transmission rate, which threshold value is predetermined for the respective virtual network. 
     In this case, the threshold values are predetermined for the respective virtual network, that is to say in particular are set specifically for the virtual network. 
     The monitoring of the data transmission rate and the possible limiting of data reception particularly allows what are known as “babbling idiots” to be handled, that is to say network devices in the network that, out of a fault, send meaningless data or data that cannot be interpreted by other devices in the network. Frequently, said “meaningless” data are sent by the erroneous devices at a high frequency, so that a burden on the network infrastructure and communication paths can arise. Limiting the affected reception ports or blocking data reception for excessive data rates nevertheless allows reliable data communication, even if there are babbling idiots present. 
     If, by way of example, packets need to be routed through a network segment because otherwise the network devices next to this network segment would be unreachable or unreachable via disjunct paths, the threshold values (bandwidth limit) that are routed through the relevant network segment can be limited in a specific manner, particularly in a VLAN-specific manner. This makes it possible to prevent a data packet that has been produced by a babbling idiot outside a network segment from being the cause of this actually intact network segment being isolated. 
     The virtual networks are implemented in the communication network via disjunct paths, in particular physical disjunct paths. 
     A data transmission rate is understood to mean the digital volume of data that is transmitted via a transmission channel or communication path within a unit of time. The terms data transfer rate, data rate, transmission speed, connection speed, bandwidth or capacity are also used. An established indication of a data transmission rate is bits per second. 
     The maximum data transmission rate is preferably determined on the basis of the provided bandwidth of the communication network. If the topology of the network and the functions to be performed reveal all data transfer rates provided for error-free operation between the network nodes and network devices or transmission and reception ports, the limiting units can be set up with corresponding sensitivity. 
     In addition, the error situation of a babbling idiot is handled by checking the data transfer rate and if need be limiting reception such that at least one communication path transmitting without error is available for data that are not disturbed or sent by a faulty device. 
     The use of virtual networks in the communication network also allows minimization of the threshold values of individual limiting units, particularly in the marginal region of the individual network segments. This minimizes the overall probability of failure of the communication network overall system. 
     Implementation of the concept of the limiting units particularly requires no kind of change to the hardware of the standard switch components. Either the features of high-quality switch hardware can be used or the switch hardware can be augmented by a relatively simple ballast. The available bandwidth is not or only slightly adversely affected, depending on the embodiment. 
     By way of example, the respective network segment can be outwardly partitioned by means of a relatively lower threshold value for the data transmission rate than in the central region or interior of the network segment. Consequently, limiting units in the marginal region have a lower threshold value for the data transmission rate than the limiting units in the central region of the network segment. The central region of the network segment can also be provided with no limiting units. 
     Overall, a particularly reliable network arrangement is obtained that functions reliably even in the event of disturbances in network nodes. The redundant communication as a result of the use of the at least two virtual networks allows consistent controller communication and low-sophistication error analysis and correction and low-sophistication handling of errors by babbling idiots. 
     The limiting units may be embodied particularly as part of the switch devices. Implementation as a program or program code for operating one of the switch devices is additionally conceivable. 
     In one embodiment, each of the N threshold values of the N limiting units of one of the network devices is lower than a maximum data transmission rate of the communication network. This ensures that the maximum data transmission rate of the communication network cannot be exceeded. 
     In a further embodiment, each network device comprises a control device and a switch device, coupled to the control device, that has a reception port and a transmission port for sending and receiving data via the communication network at no more than a maximum data transmission rate, wherein the N limiting units are associated with the reception port and are each set up to limit the data reception of data associated with the respective virtual network at the respective associated reception port to the threshold value for the data transmission rate that is specific to the respective virtual network. 
     The combination of a transmission and reception port can also be understood to mean a communication port of the respective device. The virtual networks are particularly in the form of virtual local area networks (VLANs). 
     In a further embodiment, exclusively the network devices arranged at a marginal region of a network segment are each equipped with the N limiting units. This minimizes the number of limiting units that are necessary. Hence, this embodiment is very inexpensive. 
     In a further embodiment, the network devices comprise at least two network devices that are redundant with regard to their functionality, wherein subsets or network segments are in a form such that each of the subsets has at most one of the redundant network devices associated with it. Network devices that are redundant with regard to their functionality are arranged in different network segments, so that in the event of failure of one network segment there is at least still one redundant network device in place in another network segment and hence the overall functionality of the overall system is not endangered. 
     In a further embodiment, the subsets are disjunct subsets. 
     In a further embodiment, the virtual networks are implemented in the communication network so as to prevent overshooting of the threshold value for the data transmission rate at a limiting unit of a network segment on account of reception of data from another network segment. 
     In this case, the effect achieved by taking account of the different virtual networks, for example different VLANs, is that a segment cannot be isolated on account of packets that have been fed in outside the segment. This is achieved by virtue of the VLANs being purposely configured such that packets are not routed through a segment, particularly an outer ring. 
     As an alternative to VLANs, other suitable virtualization technologies can be used. 
     In a further embodiment, the virtual networks are implemented in the communication network such that each of the network devices is coupled to the communication network via at least two virtual networks and none of the virtual networks has a ring topology. The avoidance of ring topologies ensures that no circulating packets can arise in the network. 
     In a further embodiment, the virtual networks are implemented in the communication network such that a maximum link transmission rate is set for each communication path between a transmission port and a reception port, wherein the sum of the N threshold values of the N limiting units is less than or equal to the maximum link transmission rate of the associated reception port. By way of example, it is possible to gauge during design of the communication network what link transmission rates may be present. The limiting units can then be sensitized accordingly, so that the data reception is limited when the maximum link transmission rate is exceeded. 
     In a further embodiment, the data are associated with at least two priority classes, wherein the N limiting units are set up to limit exclusively the data reception of a higher priority class. By way of example, a first priority class can relate to particularly safety-relevant data and a second priority class can relate to less critical data. During the design of the communication network, the maximum data transfer rates are then stipulated and the limiting units are set accordingly. The various priority classes are recorded at the limiting units and the data are filtered or limited on the basis thereof. 
     The communication network may comprise an Ethernet infrastructure. The switch device can also be referred to as a bridge or router device. For network devices, the terms network nodes, nodes, network components or network elements are also used. 
     Suitable control devices that are provided in the network devices are a CPU, a microprocessor or else other programmable circuits, for example. A control device can additionally be understood to mean a sensor or actuator device. 
     The communication network or network protocol preferably provides for point-to-point connections from one node or one network device to another. In this case, bidirectional or duplex communication may be possible. 
     In a further embodiment, the network devices are each embodied as an individual FPGA, ASIC, IC chip or hardwired microcircuit. 
     In a further embodiment, the communication network comprises the multiplicity of the network devices and a number of coupling switch devices, wherein the coupling switch devices are each coupled to the communication network exclusively via a single virtual network. 
     Preferably, at least two limiting units of different network devices are allocated various or different threshold values for the data transmission rate. The sum of these threshold values is less than or equal to the prescribed maximum data transmission rate in the communication network. The threshold values for the data transmission rate can also be referred to as a bandwidth limit. 
     The use of different threshold values particularly makes it possible for different segments or network segments to be formed in the communication network. In this case, the network segments are formed such that they can fail as a whole segment without putting the overall system into a dangerous state, e.g. network segments with a shared power supply. An overall system that is supplied with power by a plurality of power supplies must anyway be designed such that it can cope with the failure of one power supply. 
     In this case, in particular, possible delaying of a block, when a network device is babbling just below the bandwidth limit, by segments that are delimited by means of limiting units with lower bandwidth limit values is purposefully limited to one segment. 
     By way of example, the respective threshold value can be indicated by a percentage based on the maximum physical data transmission rate of the network arrangement. It is thus possible to set various threshold values at 20%, 15%, 10%, 5% and 0% of the maximum physical data transmission rate, for example. In this case, a threshold value of 0% corresponds to a block. By way of example, the threshold values can be determined on the basis of the scheduled data transmission rates provided via the respective link, possibly with the addition of a safety supplement. 
     The communication network can also be referred to as a network arrangement and is particularly part of a vehicle. 
     The network devices may be sensor devices or actuator devices. Conceivable sensor devices are rotation speed sensors, braking control devices or switching control devices. It is also possible to use control devices that allow drive by wire, for example. This involves steering or acceleration pulses, for example, being transferred to appropriate actuators electronically via the network, so that the desired reaction from the vehicle begins. 
     Furthermore, a method for operating a communication network having a multiplicity of network devices is proposed: 
     In a first step, a plurality N of virtual networks is implemented in the communication network such that each of the network devices is coupled to the communication network via at least two virtual networks. In a second step, the network devices are arranged in a plurality of network segments, wherein each of the network segments is allocated a subset of the network devices, wherein each network device arranged at a marginal region of a network segment is equipped with N limiting units, wherein the respective instance of the N limiting units is allocated to a respective instance of the N virtual networks and is set up to limit data reception of data associated with the respective virtual network to a threshold value for the data transmission rate, which threshold value is predetermined for the respective virtual network. 
     In addition, a computer program product is proposed that prompts the performance of the method for operating a network arrangement, as explained above, on one or more program-controlled devices. 
     A computer program product such as a computer program means can be provided or delivered by a server in a network, for example as a storage medium, such as a memory card, a USB stick, a CD-ROM, a DVD, or else in the form of a downloadable file. This can be effected in a wireless communication network, for example, by transmitting an appropriate file with the computer program product or the computer program means. A suitable program-controlled device is a network device as described previously, in particular. 
     Furthermore, a data storage medium having a stored computer program with commands is proposed that prompts the performance of the method as explained above on a program-controlled device. 
     Further possible implementations of embodiments of the invention also comprise combinations—which are not explicitly cited—of method steps, features or embodiments of the method, of the network arrangement, of the network device or of a network node that are described above or below for the exemplary embodiments. In this case, a person skilled in the art will also add or modify individual aspects as improvements or additions to the respective basic form of embodiments of the invention. 
    
    
     
       BRIEF DESCRIPTION 
       Some of the embodiments will be described in detail, with reference to the following figures, wherein like designations denote like members, wherein: 
         FIG. 1  shows a schematic illustration of a first embodiment of a communication network; 
         FIG. 2  shows a schematic illustration of a second embodiment of a communication network; 
         FIG. 3  shows an embodiment of a network device; and 
         FIG. 4  shows a schematic flowchart for an exemplary embodiment of a method for operating a communication network. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a schematic illustration of a first embodiment of a communication network  100 . By way of example, the communication network  100  can be used as an Ethernet network in a vehicle. 
     The communication network  100  of  FIG. 1  comprises nine network devices  201 - 209  and also two coupling switch devices or coupling switches  211 ,  212 . The network devices  201 - 209  can subsequently also be referred to as network nodes, nodes or control components, and each have a control device (in this regard see  FIG. 3 ). The control devices are customized to accomplish particular tasks or functions. This may be sensor capture or an actuator, for example. They can also be implemented as CPUs or microprocessors. By way of example, it is conceivable for a control device to be set up to sense a pedal level or steering movement in the vehicle. It is conceivable, by way of example, for a control device to send a control signal or control data to a further control device in the network  100 . In this case, particularly in the case of safety-relevant applications in motor vehicles, for example in the case of drive by wire, it is necessary to ensure that the control data are available in consistent form at all network nodes. 
     By way of example, the network devices  201 - 206  may be in the form of sensors or actuators. The network devices  207 - 209  form three control computers. 
     In addition, two virtual networks  301 ,  302  are formed in the communication network  100  of  FIG. 1 . In this case, the reference symbol  301  characterizes such virtual lines as can be associated with a first virtual network  301 . Accordingly, the reference symbol  302  characterizes such virtual lines as can be associated with the second virtual network  302 . The coupling switches  211 ,  212  are each coupled just to a single virtual network  301  or  302 . Thus, the coupling switch  211  is coupled to the first virtual network  301 , whereas the coupling switch  212  is coupled to the second virtual network  302 . By contrast, the network devices  201 - 209  are coupled to the communication network  100  by means of both virtual networks  301 ,  302 . 
     Furthermore, the communication network  100  of  FIG. 1  contains nine network segments  401 - 409 . In this case, each of the network segments  401 - 409  has an associated subset of the network devices  201 - 210 . The subsets are disjunct subsets. Network devices  201 - 210  arranged at the marginal region of the network segment  401 - 409  have N limiting units  510 - 523 . N corresponds to the number of virtual networks  301 ,  302  in the communication network  100  of  FIG. 1  (N=2). In this case, the respective instance of the two limiting units of a network device is associated with a respective instance of the two virtual networks  301 ,  302  and set up to limit data reception of data associated with the respective network  301 ,  302  to a threshold value for the data transmission rate, which threshold value is predetermined for the respective virtual network  301 ,  302 . 
     For the limiting units used in  FIG. 1 , the following syntax applies for the reference symbols: the hundreds position  5  characterizes the unit as a limiting unit. The tens position shows the association with the virtual network  301  or  302 . The ones position indicates the threshold value of the limiting unit. Therefore, the following limiting units are used in  FIG. 1 :
       510 : Limiting unit (hundreds position: 5) of the first virtual network (tens position: 1) with a threshold value of 0% (threshold value: 0),     511 : Limiting unit of the first virtual network with a threshold value of 5%,     512 : Limiting unit of the first virtual network with a threshold value of 10%,     513 : Limiting unit of the first virtual network with a threshold value of 15%, and     514 : Limiting unit of the first virtual network with a threshold value of 20%.     515 : Limiting unit of the first virtual network with a threshold value of 30%.     520 : Limiting unit of the second virtual network with a threshold value of 0%,     521 : Limiting unit of the second virtual network with a threshold value of 5%,     522 : Limiting unit of the second virtual network with a threshold value of 10%,     523 : Limiting unit of the second virtual network with a threshold value of 15%, and     524 : Limiting unit of the second virtual network with a threshold value of 20%.     525 : Limiting unit of the second virtual network with a threshold value of 30%.   

     Overall,  FIG. 1  shows a parallel redundant communication network  100  with three control computers  207 - 209 , to which are connected two outer rings with nodes  201 ,  202  and  203 - 206 . 
     The scheduled traffic between the individual network segments  401 - 409  may be organized as follows, for example: 5% between the network devices  201 ,  202  and the coupling switches  211 ,  212 , 10% between the network segments  403 ,  404  and the coupling switches  211 ,  212  and 15% in each case between the network segments  405 - 407  and the coupling switches  211 ,  212 . 
     The two virtual network  301 ,  302  shown are subject to bandwidth limiting, for example 30%, at all nodes  201 - 212 . Such nodes as are not connected to a virtual line  301 ,  302  of a particular virtual network also do not forward packets from this virtual network. By way of example, all nodes in  FIG. 1  are existent in redundant form, the nodes  203  and  206  thus being able to be redundant in respect of one another, for example. The node  204  is redundant in respect of the node  201 , and the node  205  is redundant in respect of the node  202 . In addition, the coupling switches  211  and  212  may be redundant in respect of one another. The control computers  207 - 209  are also redundant in respect of one another. If one of the nodes  201 - 212  fails or is isolated from the communication, for example, the overall system remains operational. By way of example, redundant can denote the supply of power by different circuits in this case. If a power supply were to fail, one of each pair of redundant nodes, that is to say of the same type, would continue to be supplied with power and connected to the communication network  100 . The network segments  401 - 409  are accordingly chosen such that at least one of each pair of redundant nodes  201 - 212  continues to be connected to the communication network  100  when a network segment  401 - 409  is isolated. That is to say that if two redundant nodes, for example the nodes  203  and  206 , are installed in an outer ring, the bandwidth between them is limited with the computed threshold values or bandwidth limit values in both directions. Otherwise, it is also possible for band limiting units  510 - 523  to be provided only in selected positions. 
     On the basis of the ring-shaped topology of the communication network  100 , the virtual connections  301 ,  302  form two disjunct, loop-free paths between all control computers  207 - 209  themselves and between the control computers  207 - 209  and the nodes  201 - 206 . 
     The operation of the communication network  100  in the embodiment of  FIG. 1  is illustrated with reference to the three examples below: 
     In the first example, it is assumed that the node  206  is the babbling idiot: the node  206  receives data from the nodes  203 ,  204  and  205  via the second virtual network  302 . It adds an arbitrary amount of data to this virtual network  302 , as a result of which the scheduled bandwidth of 10%, for example, is exceeded. The bandwidth limiting of the band limiting unit  522  of the coupling switch  212  rejects randomly selected packets. In the worst case, all valid packets from the nodes  203 ,  204  and  205  are rejected. However, this means that the scheduled bandwidth that is transmitted via this path  302  to the coupling switch  212  is not exceeded, as a result of which the data received from the nodes  202  and  201  are not adversely affected. Should the node  206  also send data from the first virtual network (VLAN)  301  to the bottom coupling switch  212 , said data are not forwarded by the coupling switch  212  and have no effect. 
     In the other direction, the node  206  likewise sends an arbitrary amount of data, in the worst case on the two virtual networks  301  and  302 . As a result, the bandwidth on both virtual networks  301 ,  302  is exceeded, and the limiting between the nodes  205  and  204  again rejects arbitrary packets. As a result, all packets that have been sent to the nodes  203 ,  204  and  205  via the bottom coupling switch  212  may be lost. At the same time, however, the limiting at this position ensures that the packets from the nodes  203  and  204  have sufficient bandwidth available to communicate with the control computers  207 ,  208  and  209  via the top coupling switch  211  of the first virtual network  301 , since a limiting unit  512  with a higher threshold value is provided at this point. Hence, the network segment  404  with the nodes  205  and  206  is isolated from the communication in the communication network  100 , but all other nodes can still communicate with one another. 
     In the second example below, it is assumed that the first control computer  207  is the babbling idiot: 
     In the worst case, the control computer  207  could babble in both directions on both virtual networks  301 ,  302  with the maximum bandwidth. The babbled bandwidth would then be limited to 15% at both coupling switches  211 ,  212 . This would involve the use of the band limiting units  513  and  523 . These 15% of bandwidth would advance to the two outer rings and to the other control computers  208 ,  209 . Between the nodes  201  and  202  and between the nodes  204  and  205 , they would encounter band limiting units  511 ,  521 . Hence, on account of the packets babbled into the first VLAN  301 , the nodes  202 ,  205  would no longer be able to be reached via the first VLAN  301 . At the same time, in more or less mirror-inverted fashion, packets babbled via the bottom VLAN  302  would encounter band limiting by the band limiting units  511 ,  512  between the nodes  201  and  202  and between the nodes  204  and  205 . This means that the nodes  203 ,  204  and  201  would no longer be able to reached via the bottom VLAN  302 . Hence, if babbling in all possible directions via all possible virtual networks  301 ,  302 , the babbling idiot in the control computer  207  disturbs the redundancy of all communication links, but all nodes remain connected to one another in nonredundant fashion in the communication network  100 . 
     In the third example below, it is assumed that the top coupling switch  211  is the babbling idiot. The top coupling switch  211  can only flood over the first VLAN  301  with the maximum permissible bandwidth. It has no way of sending packets to the second VLAN  302 , however, since none of its neighboring nodes accepts packets from the second VLAN  302  from it. Accordingly, a nonredundant connection continues to exist between all other nodes via the second VLAN  302 . 
       FIG. 2  shows a schematic illustration of a second embodiment of a communication network  100 . The communication network  100  of  FIG. 2  has the network devices  201 - 206 , which are in the form of sensors or actuators, for example. In addition, the communication network  100  of  FIG. 2  has four control computers  207 - 210  and two coupling switches  211 ,  212 . 
     Hence, the communication network  100  of  FIG. 2  is in the form of a triple ring-shaped network. An inner ring is formed by the four control computers  207 - 210 , whereas two outer rings are provided by means of the nodes  201 - 206 . As in the exemplary embodiment in  FIG. 1 , the nodes  201 - 206  can depict three pairs of redundant nodes. In addition, in the exemplary embodiment of  FIG. 2 , four virtual networks  301 - 304  are implemented, virtual lines of which are each denoted by means of their characteristic reference symbols  301 - 304 . Virtual lines of the first virtual network  301  are provided with the reference symbol  301 , for example. 
     The scheduled traffic in the communication network 1 between the individual network devices is as follows, for example (in this case, the scheduled traffic for the four virtual networks  301 - 304  is concatenated with regard to a percentage of the bandwidth):
     Between nodes  201  and coupling switch  211 : 5%, 0%, 0%, 0%,   Between nodes  201  and  202 : 5%, 5%, 0%, 0%,   Between nodes  202  and coupling switch  212 : 0%, 5%, 0%, 0%,   Between nodes  206  and coupling switch  212 : 0%, 10%, 0%, 0%,   Between nodes  204  and  205 : 5%, 5%, 0%, 0%,   Between nodes  203  and coupling switch  211 : 10%, 0%, 0%, 0%,   Between control computer  207  and coupling switch  211 : 15%, 0%, 10%, 10%,   Between control computer  208  and coupling switch  211 : 15%, 0%, 10%, 10%,   Between control computer  207  and control computer  209 : 15%, 15%, 0%, 10%,   Between control computer  208  and control computer  210 : 15%, 15%, 10%, 0%,   Between control computer  209  and coupling switch  212 : 0%, 15%, 10%, 10%,   Between control computer  210  and coupling switch  212 : 0%, 15%, 10%, 10%   

     In  FIG. 2  also, the network segments  401 - 406  are chosen such that when a network segment  401 - 406  is isolated at least one of each pair of redundant nodes  203 ,  206 ;  204 ,  201 ;  205 ,  202  remains connected to the communication network  100 . The operation of the communication network  100  of  FIG. 2  is illustrated on the basis of the three examples below. 
     In the first example below, it is assumed that the node  206  is the babbling idiot: 
     The node  206  receives data from the nodes  203 ,  204  and  205  via the second VLAN  302 . The node  206  adds an arbitrary amount of data to this virtual network  302 , as a result of which the scheduled bandwidth of 10%, for example, is exceeded. The band limiting unit  522  of the bottom coupling switch  212  rejects randomly selected packets. In the worst case, all valid packets from the nodes  203 ,  204 ,  205  are rejected. As a result, the scheduled bandwidth that is transmitted to the lower coupling switch  212  via this path is not exceeded, however, as a result of which the data received from the nodes  201  and  202  are not adversely affected. 
     Should the node  206  also transmit data to the bottom coupling switch  212  on a VLAN other than the second VLAN  302 , said data are not forwarded by the coupling switch  212  and have no effect. In the other direction (upward in  FIG. 2 ), the node  206  likewise sends an arbitrary amount of data, in the worst case on the first VLAN  301  and the second VLAN  302 . In this case, the bandwidth on both VLANs  301 ,  302  is exceeded, and in this case the limiting between the nodes  205  and  204  rejects arbitrary packets. As a result, all packets that have been sent to the nodes  203 ,  204 ,  205  via the bottom coupling switch  212  may be lost. At the same time, however, the limiting at this position ensures that the packets from the node  203  and the node  204  have sufficient bandwidth available to communicate with the control computers  207 - 210  via the top coupling switch  211 , since a limiting unit  512  with a higher threshold value is provided at this point. Hence, the network segment  404  with the nodes  205  and  206  is isolated from the communication, but all other nodes can still communicate with one another. 
     In the second example below it is assumed that the control computer  207  is the babbling idiot. 
     In the worst case, the control computer  207  could babble in both directions on all VLANs  301 - 304  with the maximum bandwidth. As a result, the VLAN  303  and  304  would be completely flooded and could no longer provide secured information for transmission. Only the connection between the control computers  208  and  210  could still be used via the third VLAN  303 , since in this case a higher bandwidth of 20% is permissible. In the direction of the top coupling switch  211 , it could send packets from the first VLANs  301  with 30% bandwidth. These packets would enter the outer rings of the network segments  401 ,  402 ;  403 ,  404  and encounter bandwidth limiting between the control computers  209  and  210  between the nodes  201  and  202  and between the nodes  204  and  205 . Hence, the nodes  202 ,  205  and  206  and the control computer  210  would no longer be able to be reached via the first VLAN  301 . At the same time, packets could be sent on the second VLAN  302  in the direction of the control computer  209 , and the bandwidth is limited to 15% in this case. More or less in mirror-inverted fashion, these packets would encounter bandwidth limiting between the nodes  201  and  202  and between the nodes  204  and  205 . Hence, the nodes  203 ,  204  and  201  and the control computer  208  would no longer be able to be reached via the VLAN  301 . Hence, if babbling in all possible directions via all possible VLANs  301 - 304 , the babbling idiot  207  in the control computer ring disturbs the redundancy of all communication links, but all nodes  201 - 210  continue to be connected to one another in nonredundant fashion. 
     In the third example below, it is assumed that the top coupling switch  211  is the babbling idiot: 
     The top coupling switch  211  can flood the first, third and fourth VLANs  301 ,  303 ,  304  with the maximum permissible bandwidth. However, it has no way of sending packets via the second VLAN  302 , since none of its neighboring nodes accepts packets from the second VLAN  302  from it. Accordingly, at least one nonredundant connection continues to exist between all other nodes via the second VLAN  302 . 
     In a further embodiment, the embodiment of  FIG. 2  that is described above could also be used in a network of arbitrary network topology if the traffic patterns in this network are known a priori. 
     The present use of virtual networks and of the network segments and of the VLAN-based band limiting used therein allows babbling idiots to be tolerated in an Ethernet-based environment without losing connectivity, in the event of an arbitrary single error, to so many nodes, particularly to redundant nodes, that secure operation of the overall system would no longer be possible. In this context, it is possible to dispense with special hardware, and there is not even a requirement for special software in the CPU of the respective switch. Rather, a static configuration is sufficient, since the band limiting units define only thresholds for the data transmission rate, but ports and the association thereof are not changed. 
       FIG. 3  shows an embodiment of a simple network device  201  that has a control device  217  and a switch device  218 . The switch device  218  has two reception ports  215  and two transmission ports  216  coupled to the communication network  100  for the purpose of outputting and inputting data. In addition, the control device or CPU  217  is communicatively connected to the switch device  218  via transmission and reception ports  220 ,  219 . The reception ports  215  have a respective associated limiting unit  511  that limits a data transfer when a stipulated maximum reception data rate is exceeded. 
       FIG. 4  shows a schematic flowchart for an exemplary embodiment of a method for operating a communication network  100  that has a multiplicity of network devices  201 - 210 , a plurality N of virtual networks  301 - 304  and a plurality of network segments  401 - 409 . Examples of such communication networks  100  are shown in  FIGS. 1 and 2 . 
     The method of  FIG. 4  has the following steps S 1  and S 2 : 
     In step S 1 , the plurality N of virtual networks  301 - 304  is implemented in the communication network  100  such that each of network devices  201 - 210  is coupled to the communication network  100  via at least two virtual networks  301 - 304 . 
     In step S 2 , the network devices  201 - 210  are arranged in the plurality of network segments  401 - 409  such that each of the network segments  401 - 409  is allocated a subset of the network devices  201 - 210 . In this case, each network device  201 - 210  arranged at a marginal region of a network segment  401 - 409  is equipped with N limiting units, wherein the respective instance of the N limiting units is allocated to a respective instance of the N virtual networks  301 - 304  and is set up to limit data reception of data associated with the respective virtual network  301 - 304  to a threshold value for the data transmission rate, which threshold value is prescribed for the respective network  301 - 304 . 
     Although the invention has been illustrated and described in more detail by the preferred exemplary embodiment, the invention is not restricted by the disclosed examples, and other variations can be derived therefrom by a person skilled in the art without departing from the scope of protection of the invention. In particular, embodiments of the network arrangement can have further networkable elements that may also be a babbling idiot. 
     For the sake of clarity, it is to be understood that the use of “a” or “an” throughout this application does not exclude a plurality, and “comprising” does not exclude other steps or elements. The mention of a “unit” or a “module” does not preclude the use of more than one unit or module.