Patent Abstract:
A network switch and method of switching are provided, in which first and second signal converters convert electrical signals to optical signals and vice versa. The network switch monitors alarm contacts and power drawn by the first and second signal converters, and switches from one to the other in the case of an alarm condition or power loss. In order to force a corresponding switch at the remote end, power is removed from one of the signal converters in order to force an alarm in the corresponding signal converter at the remote end.

Full Description:
FIELD OF THE INVENTION 
     The present invention is related to protection circuits for communication networks. More specifically, the present invention is related to a protection switch to support the redundant application of electrical and optical signal converters. 
     BACKGROUND OF THE INVENTION 
     Fiberoptic communication networks are capable of carrying tremendous volumes of voice and data traffic. Businesses and individuals rely more and more on voice and data communications and are therefore becoming more heavily impacted by failure in communications equipment. In a fiberoptic network, optical fibers can become damaged or severed, and lasers can fail. Further, repeater equipment, and electrical and optical converter equipment, among others, are also sources of failure. Therefore, in order to increase the reliability of communication networks, backup systems have been developed. Protection channels are known in the art and are described, for example, in the U.S. Pat. No. 4,451,916 to Casper et al. Disadvantageously, Casper requires a twisted pair copper wire link coupled along the end terminal stations and each repeater station along the network for the purpose of monitoring and fault isolation in the event of a failure. Other protection schemes have been devised for high-speed data networks which require sophisticated add-drop multiplexing equipment to monitor the content of the signals being transmitted. 
     Typically, at either end of the fiberoptic network, signal converting equipment converts optical signals into electrical signals which can then be further processed to pull individual multiplexed data channels from a high-speed signal. It would be advantageous to provide multiple optical-to-electrical converter units at either end of multiple optical fibers to provide a redundant fiberoptic network. Thus, if one set of fibers failed, or if one signal converter unit at either end of an optical fiber failed, a backup path could be placed into service. Therefore, it would be desirable to provide a simple mechanism for automatically switching between redundant fiberoptic paths through multiple signal converting units. 
     SUMMARY OF THE INVENTION 
     The above described disadvantages are overcome and other advantages are realized by providing a network switch for connecting redundant signal converting devices to a communications port. The signal converting devices preferably convert optical signals to electrical signals for downstream transmission to subscriber equipment, and electrical signals from subscriber equipment to optical signals for upstream transmission. When the switch is in a first state, electrical signals from the first signal converting device are connected to the communication port. When the switch is in a second state, electrical signals from the second signal converting device are connected to the communication port. The network switch monitors alarm contacts of both signal converting devices, as well as the nominal current drawn by each signal converting device. If the alarm contacts are activated for the active signal converting device, or if the nominal current drawn by the active signal converting device stops, the network switch switches to the second state. The network switch also terminates power to the active signal converting device temporarily, to force an alarm condition at the far end, causing a corresponding remote network switch to switch to the corresponding backup signal converting device. 
     According to a further aspect of the invention, the network switch provides output alarm contacts, and closes a relay between the output alarm contacts if the alarm contacts associated with the first signal converting device are closed. 
     According to another aspect of the invention, a method of providing backup communications is described. Two signal converting devices are provided, which preferably each convert optical signals to electrical signals. The electrical signals from each signal converting device are connected to a network switch, which in turn can switch between a first state, in which the electrical signals from the first signal converting device are connected to a communication port, and a second state in which the electrical signals from the second signal converting device are connected to the communication port. The method further includes monitoring the alarm contacts associated with the first signal converting device, as well as measuring the nominal current drawn by the first signal converting device. If an alarm is detected, or if the nominal current decreases substantially, the method includes removing power from the first signal converting device temporarily, and switching to the second state, so that the electrical signals from the second signal converting device are connected to the communication port. 
     According to yet another aspect of the invention, the above method further includes closing output alarm contacts if an alarm is detected in the alarm contacts associated with the first signal converting device. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other aspects of the invention will be more readily apparent upon consideration of the following description and the attached drawings in which: 
     FIG. 1 is a block diagram of a redundant network in accordance with an embodiment of the present invention; 
     FIG. 2 is a perspective view of two signal converting units connected to a network switch, the units and switch being mounted in a housing, in accordance with an embodiment of the present invention; 
     FIG. 3 is a block diagram of a network switch according to an embodiment of the present invention; and 
     FIG. 4 is a state diagram illustrating the functionality of a network switch constructed in accordance with an embodiment of the present invention. 
    
    
     In the accompanying drawings, like numerals will be understood to refer to like features. 
     DETAILED DESCRIPTION OF THE INVENTION 
     A redundant network  100  in accordance with an embodiment of the present invention is shown in FIG.  1 . This network arrangement uses a pair of optical to electrical signal converters  104   a  and  104   b  at either end of redundant optical fibers  102   a  and  102   b  to provide a redundant network connection. As shown in FIG. 1, there are two fiberoptic lines  102   a  and  102   b  which carry optical signals to either end of the network. The optical fibers and signal converters shall be referred to generally as  102  and  104 , respectively. Preferably, each optical fiber  102  comprises two physical fibers to carry signals in both directions. At either end of each optical fiber  102 , there are signal converting units  104 . Each signal converting unit  104  converts optical signals from the corresponding optical fiber  102  into electrical signals which are carried toward a network switch  106  at either end of a network on electrical conductors  108 . The optical signals carried on optical fibers  102  are preferably in the form of OC3 signals, as defined in (specification document). Similarly, the electrical signals carried on conductors  108  are preferably in the form DS3 signals. It is to be understood, however, that other types of optical and digital signals can be used. 
     Each network switch  106  has two network interfaces  110   a  and  110   b  and is capable of connecting either interface  110   a  or  110   b  to communications port  112 . Electrical signals are passed from either network switch  106  through communications port  112  and on to other parts of the network through further network connections  114 . Thus, when the network switches  106  at either end of the redundant optical fiber  102  are in a first state, communications port  112  is connected to interface  110   a  and communications occur through the corresponding pair of signal converting units  104   a  and the corresponding optical fiber  102   a . If, however, the network switch  106  is in a second state, then communications port  112  is connected to interface  110   b  and communications occur through the second corresponding pair of signal converting units  104   b  and their corresponding optical fiber  102   b.    
     FIG. 2 illustrates two signal converter units  104   a ,  104   b  and a corresponding network switch  106  connected together and mounted in a housing  116 . The signal converter units  104   a ,  104   b  each have a pair of optical fibers  102  connected to the front panel of the signal converter  104 . 
     Each of these converter units  104   a ,  104   b  also has a pair of electrical conductors  108   a ,  108   b  for carrying electrical signals connected to the front panel of the signal converter unit  104 . The electrical conductors  108   a ,  108   b  carry electrical signals from the signal converter units  104   a ,  104   b  to the network switch  106 . The network switch  106 , in turn, selects one of the parallel electrical conductors  108   a ,  108   b  and connects them to the rest of the network via electrical conductors  114 . The pair of signal converters  104   a ,  104   b  and the network switch  106  are shown mounted in a housing  116 . The pair of signal converters  104   a ,  104   b  and the network switch  106  mounted in the housing  116  corresponds to one half of the network depicted in FIG.  1 . Each of the devices  104   a ,  104   b ,  106  are preferably designed to comply with a standard 200 mechanic form factor. Each of the units shown in FIG. 2 have additional electrical connections along a backplane (not shown). The backplane electrical connections include additional features such as alarm contacts and power supply terminals. 
     A preferred embodiment of the network switch  106  is illustrated in FIG.  3 . Transmit and receive lines  108   a  are connected to the transmit and receive lines  114  through selection relay  118 . Transmit and receive lines  108   a  are connected to one of a pair of signal converting devices  104   a  and  104   b  (FIG.  1 ). Transmit and receive lines  108   b  are connected to the other one of the signal converting device pair. Selection relay  118  connects the transmit and receive lines from either the first or the second signal converting device  104  to the output lines  114 . Selection relay  118  is controlled by switch logic device  120 . Switch logic device  120  also controls timers  122 ,  124  and LEDs  126 ,  128 , the functions of which are described below. 
     A series of electrical connections indicated generally at  129  in FIG. 3 are preferably provided along a backplane. The backplane connections  129  allow the network switch  106  to sense the status of each signal converting device  104 , and further to manipulate the devices in order to cause a remote network switch  106  to switch to the appropriate signal converting device  104  and optical fiber  102 . 
     With continued reference to FIG. 3, a pair of electrical connectors  130  are provided to sense the status of an alarm contact output associated with the first of the pair of signal converting devices (e.g., signal converter unit  104   a ). The electrical connectors are connected to a sensing device  132  which senses whether the alarm contacts associated with the first signal converting device  104  are open or closed. A closed alarm contact preferably indicates an alarm condition, such as a failed laser or optical fiber, or other problems. The alarm contact sensing device  132  provides a control signal  134  to the switch logic device  120 , which indicates the status of the alarm contacts  130 . A duplicate alarm output relay  136  is used in combination with electrical connectors  138  to duplicate the status of contacts  130 . Similar electrical contacts  140  are provided to sense the alarm contact status of the alarm associated with the second signal converting device  104   a  and alarm contact sensing device  142  senses the status of the alarm contacts relays and provides a control signal  144  which is relayed to switch logic device  120  and a second duplicate alarm output relay  146 . The second duplicate alarm output is provided through electrical contacts  148 . 
     As shown in FIG. 3, the power supply for the first signal converting device  104  is connected to electrical connector  150 , power supply sensing device  152 , power cutoff relay  154 , and is finally provided to the first signal converting device  104   a  through electrical connector  156 . Power supply sensing device  152  senses whether a nominal current is being drawn by the first signal converting device  104   a , and relays the status of the power supply to switch logic device  120  through control line  158 . A nominal current sensed by current sensing device  152  is indicative of whether a signal converting device  104  is plugged into the backplane or not. This is important because open alarm contact relays may indicate a non-alarm condition. In this manner, a fault condition can be sensed even if the signal converting device  104  is missing, resulting in open alarm contact relays. 
     Similarly, the power supply for the second signal converting device  104   b  is routed through electrical connector  160 , current sensing device  162 , power supply cutoff relay  164 , and is finally provided to the second signal converting device  104   b  through electrical connector  166 . The status of the nominal current drawn by the second signal converting device  104   b  is provided to switch logic device  120  through control line  168 . 
     In operation, relay  118  defaults to the first signal converting device contacts. In the event that a fuse blows in the network switch  106 , or if power is lost to the network switch  106 , it is preferable that the default position of relay  118  connects one or the other of the pair of signal converting devices  104 . For illustrative purposes, the default connection is to the first of the two signal converting devices  104 . Under normal operation, switch logic device  120  monitors the status of alarm contact sensing device  132  and  142 , as well as the status of power supply sensing devices  152  and  162 . If switch logic device  120  senses that there has been a failure with the first signal converting device  104   a  (e.g., the alarm contact sensing device  132  or power supply sensing  152  indicates a failure), then switch logic device  120  controls relay  118  to switch the connection to the second signal converting device  104   b.    
     If a switch to the second signal converting device is required, the switch logic device  120  also activates timer  122  which opens power cutoff relay  154  for a predetermined amount of time. Thus, power cutoff relay  154  removes power from the first signal converting device  104   a , which results in loss of signal to the corresponding first signal converting device  104   a  at the remote end of the fiber network. This, in turn, causes an alarm condition at the remote end that is sensed by the corresponding network switch  106  at the remote end, causing the remote network switch also to switch to the corresponding second signal converting device. 
     Similarly, if relay  118  is connected to the second signal converting device  104   b  and either an alarm condition, or loss of nominal current, are sensed at alarm sensing device  142  or current sensing device  162 , then switch logic device  120  causes relay  118  to switch back to the first signal converting device  104   a . Switch logic device  120  also controls LEDs  126  and  128  to visually indicate which signal converting device is presently active. 
     FIG. 4 further illustrates the functionality of a network switch  106  through a state diagram. In the first state  200 , the output line  114  is connected to a first signal converting device. If the alarm from the first signal converting device is sensed, or if power loss to the first signal converting device is sensed and the second signal converting device is functioning properly, then state  202  is entered. In state  202 , timer  122  is activated, opening power cutoff relay  154  and thereby disconnecting power from the first signal converting device. The power cutoff is preferably 1.1 seconds, but any other length of time is contemplated to be within the scope of the invention. After the timer expires, the network switch enters state  204  in which output  114  is connected to a second signal converting device. Once in this state, if an alarm condition for the second signal converting device is sensed, or if power loss to the second signal converting device is sensed, and the first signal converting device is functioning properly, then state  206  is entered. During state  206 , timer  124  is activated, causing power cutoff relay  164  to open to remove power from the second signal converting device for a period of time which is preferably 1.1 seconds. After timer  124  expires, the system returns to state  200  and the first signal converting device is connected to output line  114 . If power to the network switch  106  is lost (e.g. due to a fuse failure), relay  118  defaults to connecting output  114  to the second signal converting device, as shown at state  208 . 
     Although only a few examples of embodiments of this invention have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiments without materially departing from the novel teachings and advantages of this invention. Accordingly, all such modifications are intended to be included in the scope of this invention as defined in the following claims.

Technology Classification (CPC): 7