Abstract:
A method for operating a network having a ring topology, in which a faulty connection between two stations of the network is detected by monitoring carrier signals. This method enables a faulty connection to be quickly detected in a network. Data may be advantageously rerouted in response to the detection.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method for operating a network having a ring topology, a device for carrying out this method, and a corresponding network. 
     BACKGROUND INFORMATION 
     The importance of networks or network services has risen steadily in recent years. In addition to their generally known use as a communication platform, e.g., the Internet, the use of networks in industrial environments is also growing in importance, e.g., in networked control and automation systems. 
     In industrial applications, in particular, an error-free, continuously available network connection between the individual stations is important to avoid production problems or even outages. 
     However, a connection between the stations which remains uninterrupted over time can never be guaranteed, since problems may always occur within the connection, e.g., cable ruptures and the like. 
     In network technology, therefore, a number of methods exists for detecting connection problems and eliminating them, if necessary. 
     FDDI is a network standard which is often used for backbones. Optical waveguides (OWG), i.e., glass fibers, which provide optimum protection against electromagnetic interference, are generally used for transmission. More economical copper lines are also used for short transmission paths which provide the same transmission rate. 
     FDDI is an ANSI (American National Standards Institute) network standard whose network topology is designed in the shape of a ring. Most of the parameters are defined in ANSI X3T9.5, and parts have been adopted by ISO (International Organization for Standardization). The current version of the standard is defined in ANSI X3T12. 
     The FDDI standard supports multiple designs of the network topology, and the dual-ring structure is described below by way of example. 
     An FDDI network having a dual-ring structure includes a primary (p) ring and a secondary (s) ring. Each station has one input interface (E), i.e., an input pE, sE, and one output interface (A), i.e., an output pA, sA, for each ring. The primary and secondary rings have opposite directions of transmission. 
     During normal data transmission, each station forwards the data it has received at one input to the corresponding output. This takes place regardless of whether the data is intended for that station and is therefore also processed by this station. When the data is returned to the original sender, the data transmission has been completed correctly, and the original sender takes the data from the ring. 
     The secondary ring remains unused in normal, error-free operation. Nevertheless, null data is transmitted to continuously check whether this ring is free of errors. 
     If the dual ring is interrupted, e.g., due to a cable defect, the data transmitted by a station on the primary ring is not returned to that station. 
     If an error occurs, a ring interruption is detected or a time limit is exceeded, a claim process is initiated. If this process is unsuccessful, a beacon process is triggered. 
     Stations which do not receive any corresponding frames during the course of the beacon process either identify the preceding station or the fiber-optic cable as defective and initiate a ring reconfiguration. 
     To do this, stations which are located upstream from the cable rupture in the direction of the ring stop forwarding the data received via the pE port and instead reroute it to the secondary ring via the sA port. Because this ring uses the opposite direction of data flow, the rupture point is thereby bypassed. 
     A method for detecting a line interruption is discussed in International patent application WO 02/065219 A2, in which a master evaluates its own transmitted telegrams which are returned to the master via the dual ring. If the master&#39;s telegrams fail to be returned, this is evaluated as a line interruption. 
     The above-discussed method may require a great deal of time to detect and subsequently eliminate a fault, which may hinder its use in systems which require a nearly uninterrupted connection, such as industrial manufacturing systems. 
     There is also a method for quickly detecting a line break in dual-ring OWG structures via missing input signal edges and subsequently reconfiguring the two rings (“Fehlertolerantes Kommunikationssystem für hochdynamische Antriebsregelungen” [Error-Tolerant Communication Systems for Highly Dynamic Drive Regulation Systems], S. Schulze, Dissertation, Darmstadt, 1995). Glass fibers are highly sensitive in their handling and require complex connecting techniques. In addition, the use of OWG technology increases the cost of providing and maintaining the transmission medium itself as well as plugs, network cards, etc. 
     SUMMARY OF THE INVENTION 
     Against this background, a method is provided, according to the present invention, for operating a network having a ring topology, as well as a device which uses this method, and finally a corresponding network, according to the independent patent claims. Advantageous embodiments are described herein. 
     In the method according to the present invention for operating a network having a ring topology, a faulty connection between two network stations is detected by monitoring the carrier signal, also referred to as the carrier. In wireline communications technology, a carrier frequency, in particular, which is present at the connecting line in the form of an a.c. voltage, is referred to as the carrier signal or carrier. Depending on the transmission standard, the carrier satisfies other requirements. In particular, it receives modulated data, for example payload data or IDLE data, which is transmitted on the carrier during transmission pauses. 
     In the method according to the present invention, the physical connection between two stations advantageously corresponds to an Ethernet standard using electrical data routing. In principle, the Ethernet standard supports not only electrical data routing, but also optical data routing via optical waveguides. The advantageous use of an Ethernet standard using electrical data routing, for example according to the IEEE 802.3 standard, in a network having a ring topology, in particular in a time-critical environment, makes it possible to use tried and tested components that are economical to provide and easy to maintain. Of course, an Ethernet standard using optical data routing may also be used. 
     In the method according to the present invention, it is advantageous to use the false carrier indication signal for monitoring the carrier signal. This signal is provided by Ethernet interface circuits, which makes it easy to use. The use of this signal enables a faulty connection to be detected within just a few hundred nanoseconds. 
     The method according to the present invention is advantageously used in a network having a contradirectional dual-ring structure in which each station has one input interface and one output interface for each ring. By using a dual-ring structure, it is possible to maintain network operation by appropriately rerouting the data even if one connecting path fails. A network whose stations are able to reroute data in this manner is known as a self-healing network. 
     In the exemplary embodiment of the method, at least one station monitors the carrier signal at one input interface. 
     In another exemplary embodiment, with the method according to the present invention, if, after a faulty connection is detected, the station may route the data from its own output interface, which is connected to the input interface of a second station, to its own input interface, which is connected in a faulty manner to the output interface of the second station. This advantageous routing of data after detecting a faulty connection enables network operation, e.g., in a dual-ring network, to be easily maintained. 
     In another exemplary embodiment, the method according to the present invention is used in a network according to a SERCOS standard. For example, a network according to the SERCOS III standard may be used, which supports the use of Ethernet components in a SERCOS connection. 
     The device according to the present invention for operating a network having a ring topology includes an arrangement for detecting a faulty connection between two network stations by monitoring the carrier signal. 
     The exemplary device according to the present invention may include an arrangement for implementing one or all advantageous exemplary embodiments of the method according to the present invention. In particular, the exemplary device according to the present invention may be used in an Ethernet network, the false carrier indication signal, for example, being used to monitor the carrier signal. 
     In another exemplary embodiment, the exemplary device according to the present invention includes two input interfaces, two output interfaces and two multiplexer circuits which connect the input interfaces to the output interfaces in such a way that, during error-free operation, each input interface is connected to a corresponding output interface and, upon detection of a faulty connection at one input interface, the corresponding output interface is connected to the other input interface. This exemplary embodiment is easy and economical to produce. 
     A network according to the present invention includes at least one exemplary device according to the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a network having a dual-ring structure, a connection between two stations being interrupted. 
         FIG. 2  shows an exemplary embodiment of the exemplary device according to the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     In  FIG. 1 , a network having a ring topology is identified as a whole by reference numeral  100 . Network  100  includes four stations  110 ,  120 ,  130 ,  140 , a primary ring  150 ,  151 ,  152 ,  153  and a secondary ring  160 ,  161 ,  162 ,  163 . Each station  110 ,  120 ,  130 ,  140  has one output interface  111 ,  121 ,  131 ,  141  and one input interface  112 ,  122 ,  132 ,  142  for primary ring  150 ,  151 ,  152 ,  153 . Each station  110 ,  120 ,  130 ,  140  also has one output interface  113 ,  123 ,  133 ,  143  and one input interface  114 ,  124 ,  134 ,  144  for secondary ring  160 ,  161 ,  162 ,  163 . 
     In this manner, each station is connected to an adjacent station via two ring connecting sections, via one input interface and one output interface. For example, station  110  is connected to station  120  via primary ring section  150  from output interface  111  of station  110  to input interface  122  of station  120 . 
     The two stations  110 ,  120  are also connected via ring section  160  of the secondary ring via interfaces  123  and  114 . 
     In the present exemplary embodiment, each station  110 ,  120 ,  130 ,  140  monitors, at each of its input interfaces  112 ,  114 ,  122 ,  124 ,  132 ,  134 ,  142 ,  144 , the carrier signal or carrier on the particular connecting section. Connecting sections  150 ,  160 ,  151 ,  161 ,  152 ,  162 ,  153 ,  163  are physically designed as Ethernet connections having electrical data transmission, for example as Fast Ethernet according to the IEEE 802.3u standard. 
     If an interruption occurs in connecting sections  151  and  161 , station  120  detects a change in the carrier signal at its input interface  124 , and station  130  does the same at its input interface  132 . 
     During error-free operation, the data undergoes multiple coding prior to transmission (e.g., PCS code, scrambler). This coding produces a high-frequency carrier signal even if only zeros or ones or even no logical data at all are transmitted. In the event of a line interruption, the carrier signal remains statically at a constant level. Stations  120  and  130  thereby determine that the connection from their input interfaces  124  and  132  to the corresponding output interfaces of the adjacent station is faulty. 
     In another exemplary embodiment of the method according to the present invention, the false carrier indication signal of the Ethernet chip may be used for detecting the interruption. In the event of a line interruption, the decoding of the carrier signal produces a random data pattern of zeros and ones. This is usually an invalid data pattern which the chip detects as being defective and to which it responds accordingly, i.e., by displaying a false carrier indication signal. 
     To obtain a valid data pattern, code groups must be transmitted in a predetermined order during the transmission of telegrams, starting with a start-of-stream sequence. During transmission pauses, i.e., when no data is being transmitted logically, the sender transmits IDLE codes. No other data patterns are allowed. The failure to transmit the IDLE codes generates invalid code groups and also causes the carrier monitoring system to respond. 
     According to this exemplary embodiment of the present invention, it is not necessary to detect a faulty connection on a higher-level network layer, for example by the fact that data transmitted to a ring via an output interface fails to return to the sender after one complete cycle. 
     According to another exemplary embodiment of the method according to the present invention, both stations  120  and  130  reroute their data accordingly. Station  120  routes its data from its output interface  121  to its input interface  124 , i.e., from the primary ring to the secondary ring. Station  130  routes its data from output interface  133  to input interface  132 , i.e., from the secondary ring to the primary ring. Of course, these stations route the data in the same manner if only one of ring sections  151 ,  161  is interrupted. 
     The ring is now closed again. Data transmitted on both rings reaches each station and ultimately returns to the transmitting station. 
       FIG. 2  shows an exemplary embodiment of a device according to the present invention. The exemplary device as a whole is identified by reference numeral  200 . Device  200  includes an input interface  210  and an output interface  211  for the primary ring, which is identified by p, and an input interface  220  and an output interface  221  for the secondary ring, which is identified by s. In particular, the device may be an integral part of a station  110 ,  120 ,  130 ,  140  from  FIG. 1 , or it may be the station itself. 
     Device  200  also includes two multiplexer circuits  230 ,  240  and two signal processing circuits  250 ,  260 . Multiplexer circuit  230  includes an input interface  231  for primary ring p, an input interface  232  for secondary ring s, and an output interface  233 . Signal processing circuit  250  is connected to input interface  210  of device  200 . The multiplexer output is connected to output interface  211  of device  200 . 
     Multiplexer circuit  240  includes an input interface  241  for secondary ring s, an input interface  242  for primary ring p, and an output interface  243 . The latter is connected to output interface  221  of device  200 . 
     Input interface  220  of device  200  is connected to the input of signal processing circuit  260 . 
     Device  200  checks the carrier signal on both rings p and s at its input interfaces  210 ,  220 . 
     If device  200  detects a faulty connection at its input interface  210  on primary ring p, multiplexer circuit  230  interrupts the connection between input interface  231  and output interface  233  and instead connects input interface  232  to output interface  233 . In this manner, a connection is established from input interface  220  of device  200  to output interface  211  of the device. Secondary ring s is thereby connected to primary ring p. 
     If device  200  detects a faulty connection at its input interface  220  on secondary ring s, multiplexer circuit  240  interrupts the connection between input interface  241  and output interface  243  and instead connects input interface  242  to output interface  243 . In this manner, a connection is established from input interface  210  of primary ring p to output interface  221  of secondary ring s.