Abstract:
A multifiber connector comprising a rectangular-shaped ferrule having a plurality of fibers, alignment structure positioned on a front face of the ferrule, the alignment structure being at least one of alignment pins and pin-receiving holes, a housing defining a ferrule opening, the ferrule opening sized and shaped to accommodate disposition of the ferrule therein, wherein when disposed within the ferrule opening the ferrule extends forward from a front face of the housing; the housing having an outer periphery, wherein the outer periphery is defined by at least a top wall, a bottom wall, and first and second side walls, each of the side walls defining a recess having a curved surface, each recess positioned at an intermediate point along the height of the first and second side walls, and a key on the outer periphery of the housing on at least one of the top wall and the bottom wall, the key being offset to one side of a centerline bisecting the connector into first and second side portions, wherein the key prevents the multifiber connector from mating with a standard adapter which would otherwise mate with the multifiber connector but for the location of the key.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This present application is a continuation application of application Ser. No. 14/182,900, filed Feb. 18, 2014, which application is a continuation of application Ser. No. 13/839,732, filed Mar. 15, 2013, now U.S. Pat. No. 8,794,849, which is a continuation of application Ser. No. 13/447,613, filed Apr. 16, 2012, now U.S. Pat. No. 8,708,573, which is a continuation of application Ser. No. 12/876,943, filed Sep. 7, 2010; a continuation-in-part of application Ser. No. 12/061,064, filed Apr. 2, 2008, now U.S. Pat. No. 7,789,572; a continuation-in-part of application Ser. No. 11/930,751, filed Oct. 31, 2007, now U.S. Pat. No. 7,651,277; a continuation-in-part of application Ser. No. 11/254,356, filed Oct. 20, 2005, now U.S. Pat. No. 7,325,976; a continuation-in-part of application Ser. No. 10/982,374, filed Nov. 5, 2004, now U.S. Pat. No. 7,207,724; a continuation-in-part of application Ser. No. 11/025,090, filed Dec. 29, 2004, now U.S. Pat. No. 7,182,523; a continuation-in-part of application Ser. No. 11/108,489, filed Apr. 18, 2005, now U.S. Pat. No. 7,118,286; a continuation-in-part of application Ser. No. 09/908,140, filed Jul. 17, 2001, now U.S. Pat. No. 6,960,025, which is a non-provisional application of Provisional Application No. 60/218,705, filed Jul. 17, 2000. The application hereby claims priority to all of the aforementioned applications, which are also all hereby incorporated by reference in their entirety. 
    
    
     FIELD OF INVENTION 
     The present invention relates generally to connectors for use in telecommunication networks such as voice, data or video networks. More specifically, to a connector system in which only certain plugs can mate with certain receptacles to provide discriminating access to particular information networks. 
     BACKGROUND 
     A need has developed to limit user access in data networks for security or other purposes. In recent years, buildings/offices are being equipped with different information networks, each having access to different data. It is important to restrict access to these networks to only authorized users. While some restrictions may be achieved using software approaches, such as passwords, the applicants have identified the need to restrict access further using some type of “physical barrier” to the networks. The present invention fulfills this need among others. 
     SUMMARY OF INVENTION 
     The following presents a simplified summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not an extensive overview of the invention. It is not intended to identify key/critical elements of the invention or to delineate the scope of the invention. Its sole purpose is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented later. 
     The present invention provides a connector system which uses physical barriers to prevent unauthorized users from connecting to data networks. More specifically, the applicants recognize that the best protection against unauthorized users “hacking” into data networks containing confidential information is to prevent them from even connecting to the network. This can be accomplished using physical barriers which prevent plugs from mating with receptacles. To this end, the present invention facilitates discriminating mating among similar, but different, plugs and receptacles by using a system of geometrically matched connector components which allows certain combinations of plugs and receptacles—i.e., mating pairs—to mate, while preventing other combinations from mating. Thus, the connector system of the present invention imparts physical security to a particular data network by ensuring that only authorized users who possess a particular connector component can physically connect to the particular data network. 
     In a preferred embodiment, the network comprises: (a) a set of optical plugs, each plug having a housing and a ferrule, the housing having a front and back orientation and having a front face defining an opening, the ferrule being disposed within the opening, the housing defining a first keying element on the front face around the opening, the keying element for each optical plug of the set of optical plugs being different; and (b) a set of optical receptacles, each receptacle having an opening to receive the plug and a ferrule-receiving portion to receive the ferrule, the ferrule-receiving portion defining a second keying element to cooperate with the first keying element, the second keying element for each receptacle of the set of the optical receptacles being different and being adapted to cooperate with one and only one of the first keying elements, wherein plugs and receptacles having keying elements that cooperate are mating pairs. 
     Having the keying element located on the face of the plug provides for a number of benefits. First, these features can be molded with a relatively small change to the mold dies. Specifically, the opening around the ferrule is typically defined in the molding process by a core pin which is inserted into the outer mold. Changing core pin configurations is a relatively inexpensive and easy step compared to altering the configuration of the outer molds. Therefore, as mentioned above, the connector system of the present invention provides for a variety of different plug configurations with only slight modifications to the molding process. 
     Having the security features on the front face of the plug also provides for an early indication of non-mateability. Specifically, since the features are located on essentially the leading edge of the plug, they are positioned optimally to “stub” as soon as possible when a plug is inserted into a non-mating receptacle. The applicants recognize that interference between connector components which are non-mating should be made as soon as possible to minimize the possibility of coupling light between connectors. That is, if close enough, optical connectors are able to couple, albeit with high loss, even if the connectors are not mechanically engaged. This condition can be meliorated by preventing the light carrying elements from getting too close—hence the desire to stub early. Stubbing early also provides an early signal to the user that the plug is non-mating and avoids the tendency of trying to force a plug into a non-mating receptacle. 
     Additionally, by locating the keying feature on the leading surface of the plug, the corresponding keying feature on the receptacle may be located internally and still provide an early indication of non-mateability. This is beneficial since it is desirable to locate the keying feature of the receptacle internally to minimize the ability of the keying feature to be tampered with or otherwise overridden. As discussed below, this is of particular importance in the configuration of the MT-RJ and LC connectors in which the plug defines the slot and the receptacle defines the key. If the key is removed, the security feature is breached. Having the key located within the receptacle reduces this risk. 
     Yet another benefit of having the keying features located on the front face of the plug is the visual indication the plug provides with respect to its keying features. That is, one can readily determine the keying configuration of the plug by visual inspection of its front face. There is no need to look into an opening to inspect the internal geometry of the plug to determine its keying configuration. 
     Another aspect of the present invention is an economical process for producing the plugs by altering their geometry at their front end though a simple mold modification. In a preferred embodiment, the process comprises: (a) molding a first housing for a first plug of a set of plugs using a core pin to define an opening having a first keying element in a first position; and (b) molding a second housing for a second plug of the set of plugs by adjusting only the core pin to define the opening having a first keying element in a second position different than the first position. 
     Yet another embodiment of the present invention is a multi-connector assembly in which two or more of the connectors have different secure features. In one embodiment, the connector system comprising a multi-plug connector, each plug having a housing and a ferrule, the housing having a front and back orientation and having a front face defining an opening, the ferrule being disposed within the opening, the housing defining a first keying element on the front face around the opening, at least two housings of the multi-plug connector having different the first keying elements. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  shows a mating pair of the present invention in which a plug is being inserted into a receptacle. 
         FIG. 2  shows a non-mating pair in which a plug has a slot which is not in the proper position to accept a key of a receptacle. 
         FIG. 3  shows an end view of a first plug showing a first keying element (a slot) in a first position to accept a second keying element (a key) of a first mating receptacle as shown in  FIG. 4 .  FIG. 3A  shows an end view of second plug with a first keying element (a slot) in a second position which accepts a second keying element (a key) of a second mating receptacle as shown in  FIG. 4   a.    
         FIG. 4  shows an end view of the first receptacle having a second keying element (a key) in a first position to accept a first keying element of the first plug shown in  FIG. 3  to form a first mating pair.  FIG. 4 a    shows an end view of the second receptacle having a second keying element (a key) in a second position to accept a first keying element of the second plug in  FIG. 3 a    to form a second mating pair. 
         FIG. 5  shows a plug having a slot configuration capable of mating with jacks having keys in different positions. 
         FIGS. 6( a )-6( c )  show top perspective, front and rear views, respectively, of an MT-RJ connector plug having security features of the present invention. 
         FIGS. 7( a ) and 7( b )  show top perspective and front views, respectively, of an MT-RJ connector receptacle. 
         FIG. 8  shows a front and side perspective view of an LC connector plug having security features of the present invention. 
         FIG. 9  shows a front perspective view of an LC connector receptacle having security features of the present invention. 
         FIG. 10  shows schematically the discrete positions available for the first keying element. 
         FIG. 11  shows a series of LC connector plugs in which the first geometries are different. 
         FIG. 12  shows a hybrid adapter. 
         FIGS. 13( a ) and ( b )  show duplex and quad connectors having receptacles with the same security features. 
         FIGS. 14( a ) and ( b )  show duplex and quad connectors having receptacles with different security features. 
         FIG. 15  is an exploded perspective view of a prior art MT-RJ connector and illustrating alignment pins/receiving holes. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The present invention relates to a connector system comprising a series of connector components which interconnect with each other in a discretionary way. Referring to  FIG. 1 , a preferred embodiment of a mating plug  101  and receptacle  100  of the connector system is illustrated. As shown, the plug  101  is partially inserted into the receptacle  100 , which, in this embodiment, is a jack having a tub portion  102 . Although a jack is discussed herein in detail, it should be understood that the receptacle of the present invention is not restricted to a jack and may be any structure configured to receive a plug, including, for example, an adapter for connecting two plugs together or an integral connector on an active device (e.g., transceiver) or passive device (e.g., splitter). 
     The plug typically contains a conductive element, such a fiber or wire, which mates with a similar element in the receptacle. In fiber optic applications, it is common for the conductive element to be contained in a ferrule  150 , which in turn is housed by the plug  101  as shown in  FIG. 1 . In a preferred embodiment, the ferrule is an MT-type ferrule, which, as known, is rectangular having side surfaces  151 ,  152 , a bottom surface  153  and a top surface (not shown). 
     The outer surface of the plug  101  and the inner surface of the tub  102  have first and second geometries, respectively, which cooperate to allow only certain pairs of plugs and receptacles to mate (herein “mating pairs,” “mating plug and jack,” or “keyed pair”), and which physically interfere for all other combinations of plugs and jacks (herein “non-mating pairs,” “non-mating plugs and jacks” or “non-keyed pairs”), thereby preventing non-mating plugs and jacks from effecting an optical or electrical coupling. 
     The first and second geometries may embody any known keying mechanism which discriminates between connector components. Such keying mechanisms include, for example, a key and slot relationship between the plug and jack, a receptacle dimensioned to receive only certain sized or shaped plugs, and even a magnetic signature for either attracting (for mating pairs) and repulsing (non-mating pairs). Preferably, the keying mechanism involves just a slight modification to the plug and jack such that essentially the same molds can be used to manufacture connectors of different keyed pairs. Although molding is preferred, it is should be understood that other techniques for producing the first and second geometries can be used including, for example, over molding and machining. 
     In a preferred embodiment, the invention uses a key and slot mechanism. For simplicity, the term “keying elements” refers collectively to the key and the slot. Specifically, the slot can be embodied in the first or second geometry and the key can be embodied in the other geometry. In the particularly preferred embodiment shown in  FIGS. 1-4 , the key is part of the second geometry, while the skit is part of the first geometry; that is, the plug  101  has a slot  103  and the tub portion  102  of the jack has a key  104 . 
     This configuration is preferred since the key may cooperate with other “ribs” on the connector for pre-alignment purposes. More specifically, with particular reference to  FIG. 3 , an end view of housing  301  of the plug  101  is shown. The housing comprises four walls each wall having a slot  103 ,  302   a ,  302   b , and  302   c , respectively.  FIG. 4  depicts an end view of housing  401  of the tub  400  in which the key  104  having a bottom surface portion  470  and ribs  402   a ,  402   b , and  402   c  having left, right and top surface portions  471 ,  472 ,  473 , respectively, are disposed on the walls of the housing. The key  104  and the ribs  402   a ,  402   b , and  402   c  cooperate with the slots  103 ,  302 ,  302   a ,  302   b , and  302   c , respectively, to effect pre-alignment of the ferrule located within the plug with the jack before final mating of the connector plug with the connector jack. The final mating may be between the conductive elements of the connector system, such as, for example, between a couple of MT-type ferrules, which employ precise alignment pins/receiving holes  28  ( FIG. 15 ) on the ferrule face. Such ferrules are well known in the art. By pre-aligning the MT ferrules through the synergistic use of the key and slot, the inter-engagement of the closely-toleranced alignment pins/receiving holes is facilitated. The above-described synergistic keying and aligning feature of the present invention is realized with the MT-RJ connector (Tyco Electronics, Harrisburg, Pa.). 
     In a preferred embodiment, the mating end of the key  104  contains a flat portion shown as  105  and the mating end of the plug  101  has a chamfers  106  on the corners of the edges of the slot  103 , while the remainder of the mating end of the plug comprises a flat portion  107 . The radius corners on the key  106  and the chamfers on the plug  101  work as a guiding device and provide for the necessary alignment between the key and the slot when the plug is inserted into the tub of the jack. On the other hand, as shown in  FIG. 2 , when a user attempts to mate two non-mating plug and jack components, the flat portion of the key  105  contacts the flat portion of the plug  101  and provides for definite physical interference between the plug and jack when the slot and key do not correspond. Accordingly, the use of this geometry prevents a user from forcing two non-mating plugs and jacks together. Therefore, the physical interference provided between the flat portion  105  of the tub and the flat portion  107  of the key assures that only desired combinations of plugs and jacks will mate. 
     The position of the key  104  on the tub  102  and the slot  103  on the plug  101  can be varied in such a manner so that a plurality of mutually-exclusive slot and key positions are formed. In one embodiment, the series of key and slot locations are mutually exclusive so that there is a one-to-one correspondence between jacks and plugs. In another embodiment, certain plugs may be configured to mate with a variety of different jacks. For example, it may be worthwhile to give network administers or people with high security clearance certain “master” plugs which are capable of mating with a number of jacks having different slot positions. Referring to the figures,  FIG. 5  shows an embodiment of a master plug  501  which has a slot  502  that is configured (which, in this embodiment, means it is wide enough) to mate with jacks  503  and  504  which have different key positions  505  and  506 , respectively. Although a wide slot is used in this embodiment to effect mating with two or more jacks having different key configurations, it should be understood that other embodiments are possible, such as, for example a plug with two or more slots. 
     The number of possible mutually exclusive mating pairs for a given plug and receptacle is a function of the physical parameters of the plug and the receptacle. More specifically, with reference to  FIGS. 1-4 , mutual exclusivity is ensured by adhering to the following relationships:
 
 X 1− C/ 2+( D−A )+Δ&lt;= F/ 2  (1)
 
 X 2+ B/ 2&lt; A/ 2 −W   (2)
 
 X 1 a +Clear1+ Z=X 1 b   (3)
 
     wherein:
         A=the width of the plug  101 ;   B=the width of the slot  103  on the plug  101 ;   C=the width of the key  104 ;   D=the distance across the opening of the tub;   F=the width of the ferrule residing within the plug;   Δ=CLF−CLA, wherein
           CLA=centerline of the width of the plug; and   CLF=centerline of the ferrule residing within the plug.   
           X1=the distance from the center of the opening in the tub  102  to the center of the key  104  for each mutually exclusive position.   X2=the distance from the center of the plug  101  to the center of the slot  103  for each mutually exclusive position;   X1a=the X1 distance for a sequentially first key in a series of connectors;   X1b=the X1 distance for a sequentially second key in a series of connectors;   W=the wall thickness of the plug housing   Z=the minimum distance required to ensure that the flat portion of the key does not contact the flat portion of the plug  107  when a user attempts to mate a mating pair;   Clear1=the clearance distance between the center side of the key and the center side of the slot.       

     These relationships must be satisfied for the mating pairs to mate and for the non-mating pairs to definitely not mate. Specifically, for a mating pair, Relationship (1) requires that half the ferrule width must be no less than X 1  less one half of C added to the difference between the width of the tub opening D less the width of the plug added to the difference between the centerline of the ferrule within the plug and the centerline of the plug. This ensures that the key is not positioned outside of the area on which at least a portion of the ferrule will reside. By adhering to this parameter, the key will have some overlap with the ferrule, and thus will provide for pre-alignment of the ferrule in the same manner as do the ribs on the three sides of the ferrule without the key. 
     Relationship (2) requires that X 2  added to one-half of dimension B is less than one-half of dimension A less W. This assures that the slot resides on the plug within the confines of the plug walls. 
     Finally, according to Relationship (3), for each mutually exclusive position, the distance X 1  for the first connector in the system (X 1a ) added to Clear 1  added to a predefined interference interval Z would correspond to the distance X 1  for the next slot/key position (X 1b ). Z is the minimum distance required to ensure that the flat portion of the key does not contact the flat portion of the plug  107  when a user attempts to mate the two portions of a connector which is intended to mate. 
     By way of example, four mutually exclusive locations for locating the slot on the plug housing and the key on the tub are defined below for an MT-RJ connector. The MT-RJ connector has the following dimensions: 
     A=7.15±0.05 mm 
     B=1.25 mm 
     C=0.95±0.04 mm 
     D=7.24±0.04 mm 
     F=4.5±0.04 mm 
     Clear1=0.15 mm 
     W=0.8 mm 
     Based on these MT-RJ dimensions, it has been found that the following X 1  key positions satisfy the relationships above: 
     
       
         
               
               
               
             
           
               
                   
               
               
                 Mating pair 
                 Key Position 
                 X 1   
               
               
                   
               
             
             
               
                 1 
                 1 
                   0.8 mm 
               
               
                 2 
                 2 
                   1.6 mm 
               
               
                 3 
                 3 
                 −0.8 mm 
               
               
                 4 
                 4 
                 −1.6 mm 
               
               
                   
               
             
          
         
       
     
     For example,  FIG. 3  shows an end view of a first plug housing  301  showing a first keying element  103  (a slot) in a first X 1  position to accept a second keying element  104  (a key) of a first receptacle housing  401  as shown in  FIG. 4 .  FIG. 4  shows an end view of the first receptacle housing  401  having a ferrule-receiving portion having a second keying element  104  (a key) in a first X 1  position to accept a first keying element  103  of the first plug housing  103  shown in  FIG. 3  to form a first mating pair.  FIG. 3A  shows an end view of second plug housing  301   a  with a first keying element  103   a  (a slot) in a second X 1  position which accepts a second keying element  104   a  (a key) of a second receptacle housing  401   a  as shown in  FIG. 4A .  FIG. 4A  shows an end view of the second receptacle housing  401  having ferrule-receiving portion  450  having a second keying element  104   a  (a key) in a second X 1  position to accept a first keying element  103   a  of the second plug housing  301   a  shown in  FIG. 3A  to form a second mating pair. 
     Although the data above indicates four mutually exclusive positions, it should be understood that additional positions are possible within the parameters of the MT-RJ connector. Additionally, it should be understood that the combinations of various key positions can be used to increase the number of permutations of mating pairs. For example, in addition to the four mating pairs listed above, additional mating pair configurations may obtained from the following combinations of key positions: 
     
       
         
               
               
             
               
               
             
           
               
                   
               
               
                 Mating pair 
                 Key Positions 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 5 
                 1, 2 
               
               
                 6 
                 1, 2, 3 
               
               
                 7 
                 1, 2, 3, 4 
               
               
                 8 
                 2, 3 
               
               
                 9 
                 2, 4 
               
               
                 10 
                 2, 3, 4 
               
               
                 11 
                 3, 4 
               
               
                 12 
                 1, 3 
               
               
                 13 
                 1, 4 
               
               
                 14 
                 1, 3, 4 
               
               
                 15 
                 1, 2, 4 
               
               
                   
               
             
          
         
       
     
     In a preferred embodiment, the key and slot components are combined with the industry standard MT-RJ connector.  FIG. 6  and  FIG. 7  show the key-slot combination added to the MT-RJ connector as produced by Tyco Electronics of Harrisburg, Pa. 
       FIGS. 6 ( a )-( c )  show the plug  602  of the MT-RJ connector combined with the slot  601  of the present invention.  FIGS. 7( a ) and 7( b )  show the center tub portion  703  of an MT-RJ connector jack. The key is shown as  701  located in one of the plurality of possible positions. The three pre-alignment ribs are shown as  702   a ,  702   b , and  702   c . The key  701  functions as the discriminating member for allowing or preventing mating with a plurality of plugs, while at the same time functioning as the pre-alignment member for the remaining side of the ferrule not aligned with ribs  702   a ,  702   b , and  702   c.    
     To provide a simple and readily apparent indication to the user of which plugs mate with which receptacles, it is preferable to mark mating pairs with indicia or color to indicate their compatibility. In a preferred embodiment, the components of a mating pair are a similar color different from all others used in the connector system. 
     Referring to  FIGS. 8 and 9 , another embodiment of the connector system of the present invention is shown.  FIG. 8  shows a plug  800 , which is one of a set of different plugs in the system. Each plug has a housing  801  which defines a first geometry. The first geometry comprises a front face  804  with an opening  802  (demarcated with dotted line), and a ferrule (not shown) within said housing and disposed in said opening. Around said opening  802  is a first keying element  803 . The keying element for each different optical plug of said set of optical plugs is different. 
       FIG. 9  shows a receptacle  900  for receiving a particular plug (not shown) and is one of a set of different receptacles. The receptacle  900  has a second geometry configured to receive the first geometry of a plug. The second geometry comprises a cavity  901  to receive a plug and a ferrule-receiving portion  904  having a borehole  902  to receive the ferrule of the plug. The ferrule-receiving portion  904  defines a second keying element  903  to cooperate with a first keying element of a particular plug. The second keying element for each receptacle of said set of said optical receptacles is different and is adapted to cooperate with one and only one first keying element. Plugs and receptacles having keying elements that cooperate are referred to herein as “mating pairs.” 
     Although the LC connector system described above is a single-fiber ferrule rather than a multifiber ferrule, the general keying features are essentially the same as those described above with respect to the MT-RJ connector. Further, the keying features of the plug  800  and receptacle  900  of the present invention may be implemented in any well known optical connector including, for example, other single-fiber ferrule connectors such as MU, SC, ST, or FC connectors. For illustrative purposes, the security features are described with respect to the LC connector system, which includes the LC plug (plug  800 ) and LC adapter (receptacle  900 ). Aside from the security features described herein, these connector components are the same as those specified in the LC Standard available on-line or from OFS (Japan), and the common features between them will not be addressed herein. 
     Like the MT-RJ embodiment described above, the keying features of the LC connector are contained on the front face of the plug. This is important for a number of reasons. First, these features can be molded with a relatively small change to the mold dies. Specifically, the opening around the ferrule is typically defined in the molding process by a core pin which is inserted into the outer mold. Changing core pin configurations is a relatively inexpensive and easy step compared to altering the configuration of the outer molds. Therefore, as mentioned above, the connector system of the present invention provides for a variety of different plug configurations with only slight modifications to the molding process. 
     Having the security features on the front face of the plug also provides for an early indication of non-matability. Specifically, since the features are located on essentially the leading edge of the plug, they are positioned optimally to “stub” as soon as possible when a plug is inserted into a non-mating receptacle. The applicants recognize that interference between connector components which are non-mating should be made as soon as possible to minimize the possibility of coupling light between connectors. That is, if close enough, optical connectors are able to couple, albeit with high loss, even if the connectors are not mechanically engaged. This condition can be meliorated by preventing the light carrying elements from getting too close—hence the desire to stub early. Stubbing early also provides an early signal to the user that the plug is non-mating and avoids the tendency of trying to force a plug into a non-mating receptacle. 
     Additionally, by locating the keying feature on the leading surface of the plug, the corresponding keying feature on the receptacle may be located internally and still provide an early indication of non-matability. This is beneficial since it is desirable to locate the keying feature of the receptacle internally to minimize the ability of the keying feature to be tampered with or otherwise overridden. As discussed below, this is of particular importance in the configuration of the MT-RJ and LC connectors in which the plug defines the slot and the receptacle defines the key. If the key is removed, the security feature is breached. Having the key located within the receptacle reduces this risk. 
     Yet another benefit of having the keying features located on the front face of the plug is the visual indication the plug provides with respect to its keying features. That is, one can readily determine the keying configuration of the plug by visual inspection of its front face. There is no need to look into an opening to inspect the internal geometry of the plug to determine its keying configuration. 
     The keying elements that may be used in the LC connector are the same as those described above with respect to the MT-RJ embodiment. In a preferred embodiment, the keying elements comprise a slot and a key. The slot can be embodied in the first or second geometry and the key can be embodied in the other geometry. In a first configuration, the slot is embodied in the first geometry and the key is embodied in the second geometry, while in a second configuration, the key is embodied in the first geometry and the slot is embodied in the second geometry. 
     The LC connector shown in  FIGS. 8-9  has a first configuration. This configuration is advantageous for a number of reasons. First, the first keying features do not prevent a plug from mating with an ordinary receptacle. This is particularly beneficial since a plug with keying elements can be nevertheless “mated” with standard equipment used for the polishing, testing and inspection of the ferrule. Specifically, the polishing, testing and inspection equipment for single fiber ferrules typically comprises a ferrule receiving interface, similar to that of a receptacle, which receives just the ferrule disposed in the opening of the housing. The housing is not engaged. If a key protrudes into the space between the opening and the ferrule, it would preclude coupling with this existing equipment. Conversely, by having slots extend radially outward from the opening, and thereby maintain the space between the opening and the ferrule, a standard ferrule receiving interface, which does not have keying features, can be used. For example, a plug having a first keying element can be coupled to a standard LC ferrule receiving interface connected to a polishing device for polishing the ferrule, or to a microscope for inspecting the endface geometry of the ferrule, or to a photodetector for testing optical attenuation of the ferrule assembly. 
     Furthermore, since the physical “barrier”—i.e., the key—is located on the receptacle in the first configuration, it will serve to facilitate discriminatory mating among, not only plugs employing security features, but also existing plugs which have no security features of the present invention. Specifically, if a slot in the plug is necessary to accommodate the key of the receptacle, then plugs without slots will not mate with receptacles having the key. Therefore, ordinary, non-secure type plugs which do not have the slot in the proper position will not mate with the receptacle. In contrast, a non-secure receptacle will mate with a secure plug of the first configuration. Specifically, since the physical barrier is absent from the receptacle, any ordinary or secure plug can mate with it. As discussed below, the situation with the second configuration is opposite from that of the first, meaning that a secure plug cannot mate with a non-secure receptacle but a secure receptacle can mate with a non-secure plug. To provide for discrimination between secure and non-secure connectors components, a secondary key is added to the system as discussed below. 
     A connector system having the second configuration offers certain benefits, but also presents certain challenges. One benefit is that the space consuming security feature—i.e., the slot—resides in the receptacle which is typically larger than the plug and better suited for accommodating this feature. That is, since a slot is defined by the material around it, a slot requires more room than a key. The receptacle does not have the same space constraints as a plug (which is designed to be inserted in the receptacle) and may be more capable of accommodating the slot than the plug. Additionally, it may be preferable to have one “master” plug which plugs into all receptacles having security features. This is easily accomplished with a connector system of the second configuration. Specifically, the master plug would simply be one having no key to interfere with the first geometry of the receptacle. The simplicity in offering a master plug in the connector system of the second configuration also gives rise to a challenge facing the system—the ability of non-secure plugs to mate with secure receptacles (discussed below). 
     Referring to  FIG. 10 , a preferred embodiment of the first keying element  803  is shown schematically. The figure shows the opening  802  in which the ferrule is disposed and which is configured to receive the ferrule-receiving portion  904 . Positioned around the opening  802  are spatially discrete positions  101 ( a )-( h ) for the first keying element. Similar discrete positions exist around the ferrule-receiving portion  904  (see  FIG. 9 ) to define the location of the second keying element. In a preferred embodiment, the first keying element comprises one or more slots in a combination of positions  101 ( a )-( h ) and the second keying element comprises keys in corresponding positions. It should be understood that to facilitate cooperation between the first and second keying elements, the combination of slot positions in the plug must be the same as the combination of key positions in the ferrule receiving portion  904 . In other words, each slot must correspond to a key in the same relative position to facilitate a mating pair. For example, a plug having a first keying element which comprises slots in positions  1001   a ,  1001   d ,  1001   e , and  1001   d , will mate with a receptacle having a second keying element comprising keys  905 ,  906 ,  907 , and  908  is the same relative positions (see  FIG. 9 ). 
     The number of slots in the combination of first keying elements depends upon the number of possible positions of the slots. Specifically, the number of possible permeations of different mating pairs is given by the following equation: 
     
       
         
           
             nCr 
             = 
             
               
                 n 
                 ! 
               
               
                 
                   r 
                   ! 
                 
                 · 
                 
                   
                     ( 
                     
                       n 
                       - 
                       r 
                     
                     ) 
                   
                   ! 
                 
               
             
           
         
       
     
     wherein: 
     n equals the number of spatially discrete positions for the keying elements, and 
     r is the number of positions occupied. 
       n C r  therefore provides for the number of mutually exclusive combinations or permeations of mating pairs. 
     Below is a table providing data on the theoretical number of mating pairs,  n C r , for different n and r values. 
     
       
         
               
               
               
             
               
               
               
             
           
               
                   
               
               
                   
                   
                 Number of Mutually 
               
               
                 Number of Spatially 
                 Number of 
                 Exclusive Combinations 
               
               
                 Discrete Positions n 
                 Positions Occupied r 
                   n C r   
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 4 
                 1 
                 4 
               
               
                   
                 2 
                 6 
               
               
                   
                 3 
                 4 
               
               
                   
                 4 
                 1 
               
               
                 5 
                 1 
                 5 
               
               
                   
                 2 
                 10 
               
               
                   
                 3 
                 10 
               
               
                   
                 4 
                 5 
               
               
                   
                 5 
                 1 
               
               
                 6 
                 1 
                 6 
               
               
                   
                 2 
                 15 
               
               
                   
                 3 
                 20 
               
               
                   
                 4 
                 15 
               
               
                   
                 5 
                 6 
               
               
                   
                 6 
                 1 
               
               
                   
               
             
          
         
       
     
     From this data, it is clear that the maximum number of permutations (i.e.,  n C r ) is reached when the number of positions occupied equals n divided by 2. Therefore, in the preferred embodiment, either n/2 slots (if n is an even integer) or (n±1)/2 slots (if n is an odd integer) of spatially discrete positions are occupied by either a slot with respect to the plug or a key with respect to the receptacle. (For purposes of simplicity, hereinafter, n will be presumed to be an even number.) Therefore, using the equation above, the embodiment shown in  FIGS. 8, 9 and 10 , in which n equals 8 and r equals 4, the maximum number of permutations of mating pairs is 70. 
     Referring to  FIG. 11 , different of plugs  1101 - 1110  of a set are shown in which the first keying elements comprise slots in different combinations of positions as defined in  FIG. 10  and accompanying text. In these drawings, the opening  802  which is constant in all the plugs and the slot positions are shown with a phantom line. Specifically, plug  1101  shows slots in a combination of positions  1001   c ,  1001   d ,  1001   e , and  1001   f ; plug  1102  shows slots in a combination of positions  1001   e ,  1001   f ,  1001   g , and  1001   h ; plug  1103  shows slots in a combination of positions  1001   a ,  1001   b ,  1001   g , and  1001   h ; plug  1104  shows slots in a combination of positions  1001   a ,  1001   b ,  1001   c , and  1001   d ; plug  1105  shows slots in a combination of positions  1001   b ,  1001   d ,  1001   e , and  1001   g ; plug  1106  shows slots in a combination of positions  1001   b ,  1001   c ,  1001   e , and  1001   h ; plug  1107  shows slots in a combination of positions  1001   a ,  1001   c ,  1001   f , and  1001   h ; plug  1108  shows slots in a combination of positions  1001   a ,  1001   d ,  1001   f , and  1001   g ; plug  1109  shows slots in a combination of positions  1001   a ,  1001   d ,  1001   e , and  1001   h ; and plug  1110  shows slots in a combination of positions  1001   b ,  1001   c ,  1001   f , and  1001   g . It should be understood that each of the plugs described above will mate with a receptacle having a key in the same position. For example, plug  1109  will mate with receptacle  900  which has keys  904 ,  905 ,  906  and  907  in the same positions as the slots (i.e.,  1001   a ,  1001   d ,  1001   e , and  1001   h ). 
     In a preferred embodiment, the connector system of the present invention may contain one or more master plugs of varying levels. That is, there may be lower-level master plugs, which can mate with receptacles of two different networks, or higher-level master plugs, which can mate with receptacles of three or more networks. The difference in the level of the mater plug is a function of the r number of slots occupying n possible positions—the more slots there are, the higher the plug&#39;s level. Specifically, the master plug comprises a first keying element having a third combination of greater than n/2 slots, in which the slots occupy the positions of at least two different first combinations as described above. Higher level master plugs have slots which occupy the positions of three or more different first combinations. 
     Aside from showing the different combinations of keying elements,  FIG. 11  illustrates the ease with which the various plugs can be made. Specifically, in a preferred embodiment, the process of manufacturing an optical connector comprises molding different plugs by adjusting the core pin which defines the opening  802  while leaving the outer molds essentially the same. In other words, rather than using different molds to modify the outside of the housing—which can be expensive, the present invention involves simply adjusting the core pin—which is relatively inexpensive. Referring to  FIG. 11 , the process is described in greater detail. The process comprises first molding a first housing  1101   a  for a first plug  1101  of a set of plugs  1101 - 1110  using a core pin (not shown) to define an opening  802  and a first keying element in a first combination of positions  1001   c ,  1001   d ,  1001   e , and  1001   f . Next, a second housing  1102   a  for a second plug  1102  is molded by adjusting only said core pin to define first keying element in a second combination of positions  1001   e ,  1101   f ,  1001   g , and  1101   h , which is different from first combination of positions. 
     To effect the different combinations of positions, the core pin is preferably adjusted by rotating it in θ increments, in which θ is equal to 360°/m, wherein m is an integer. Preferably m is an integer from 2-18, more preferably from 2-5, and even more preferably from 3-4. In the embodiment shown in  FIG. 10 , m is 4, thus the core pin is adjusted by rotating it in 90° increments. It should be clear that rotating this core pin in 90° increments in subsequent molding operations will produce plugs  1103  and  1104 , respectively. Plugs  1105 - 1108  were prepared using a different core pin which was also rotated in 90° increments. Plugs  1109  and  1110  were prepared using yet a different core pin which was rotated in a 90° increment. It is worthwhile to mention that since the combination of positions  1001   b ,  1001   c ,  1001   f , and  1001   g  is symmetrical with respect to two axes, the core pin can only be rotated by one 90° increment before repeating the same combination of positions. 
     Once the housings are prepared, a ferrule is disposed in the opening of each housing to form a subassembly. A fiber may be terminated in the ferrule either before or after the preparation of the subassembly. For field-terminatable connectors, it may be preferable to dispose just a fiber stub in the ferrule. This configuration facilitates field installation of a fiber as discussed, for example, in WO2005004285. Regardless of whether a stub or a fiber is terminated in the ferrule, the preferred keying arrangement of the present invention in which slots radiate outwardly from the opening  802  allows the subassembly of the LC connector to be polished, inspected, and tested using standard polishing equipment as mentioned above. 
     An advantage of the connector system of the present invention is that different receptacles may be combined to form “hybrid” adapters. More specifically, aside from the second keying element extending outward from the ferrule receiving portion, the receptacles are the same as those used for standard connectors. This allows different receptacles to be combined back to back to form hybrid adapters. In a particularly preferred embodiment, a secure receptacle is combined with a nonsecure receptacle by ultrasonically welding, or other known technique. Such a configuration is particularly useful in situations in which the nonsecure end of the adapter is located in an inherently secure area, for example, behind a wall or panel, where access is already limited. In other words, since connectors within cabinets and walls cannot be accessed readily after construction, the advantages derived from a secure connector at those ends would be minimal. Thus, it is preferable to use a nonsecure connector in these situations so the installer need not concern himself with the “proper” secure connector configuration during the installation of the infrastructure wiring. 
     To discriminate between secure and non-secure connector systems, the present invention provides for a secondary key &amp; slot configuration, which is either non-existent or in a different position for all plugs and receptacles which are outside of the given connector system  800 . For example, referring to  FIG. 8 , the first geometry comprises a secondary plug  810 , which is shown in the same relative position for all plugs of a given set, but which may be in different positions as discussed below. Referring to  FIG. 9 , the second geometry of the geometry of the receptacle comprises a secondary slot  910  are preferably, but not necessarily, in the same position for all the receptacles of a given set of receptacles. The secondary slots  910  are adapted to receive secondary keys  810 . This way, only plugs and receptacles of a given set of having accommodating secondary keys/slots will mate. In a preferred embodiment, at least a portion of the secondary key  810  is disposed in the plug and is an extension of the side loading structure which is an LC connector standard. Therefore, in the preferred embodiment, the secondary key not only provides for discriminating mating between secure and non-secure connectors, but also enhances side load strength. 
     It is worthwhile to note that the use of the secondary key/slot adds another security feature to the connector system—essentially another keying mechanism. This additional keying feature increases the number of permutations within a given connector system. That is, rather than maintaining the same secondary key and slot location for all connectors within a system, it can be moved to form different classes within the same family 
     Preferably, the keying elements (primary and secondary) are positioned such that not mating pairs “stub” at about the same axial position relative to one another regardless of whether the connectors are interfering because they are different types of secure connectors or whether they are interfering because they are secure/non-secure connectors. This way, the user becomes accustomed to the point at which non-mating connector components interfere, thereby reducing the risk of the user forcing non-mating components together. 
     To provide a simple and readily apparent indication to the user of which plugs mate with which receptacles, it is preferable to mark mating pairs with indicia or color to indicate their compatibility. In a preferred embodiment, the components of a mating pair are a similar color different from all others used in the connector system. 
     The system described allows for a series of mutually-exclusive connectors to be used in a manner which provides physical security to a network system. In light of the often highly sensitive data stored on many of the networks in use today, this is a highly desirable feature. The present invention is an effective way to segregate separate networks and assure that the proper users are connecting to the desired network. Additionally, the present invention may be employed in the manufacture of devices in which fibers or wires need to be connected in particular arrangements. More specifically, the discriminating connectors of the present invention can be engineered into a system such that, during manufacturing, the correct connection of the fibers/wires is ensured by the mating pairs and their ability to prevent all other “incorrect” connections. Applications requiring particular routing of fibers or wires include, for example, routers, backplane assemblies, and even component devices such as multiplexers/demultiplexers. 
     Considering, for example, routers/backplane assemblies, the connector system of the present invention can be used to organize switch racks and, more specifically, to manage patch cables to reduce clutter and improve the ease, reliability, and security of the patch cable installation by providing customer specific patch cables/backplane connections. 
     By way of background, although network architectures may vary, common to most networks, and of particular interest herein, are switch rack systems. Such systems involve multiple-port switches mounted in a rack. Each activated port of a switch is connected to an aggregation box in the panel with a patch cable. The aggregation box, in turn, is connected to a deaggregation or breakout box with a trunk cable. The breakout box breaks out the trunk into individual channels again. The interconnections between the ports and the aggregation box and between the aggregation and breakout boxes may be accomplished using optical fiber or electrical conductor. 
     One of the objectives in designing switch racks is to minimize floor space. To this end, efforts are generally concentrated on increasing port density. This means increasing the number of ports on a particular switch and increasing the number of switches that fit into a particular rack or panel. A challenge in designing and installing such high port density switch racks is organizing the patch cables interconnecting the ports to the aggregator. Each activated port requires a discrete connection to the aggregator. This can lead to a great quantity of patch cables and general clutter, which creates a strong likelihood that a patch cable will be connected to the wrong port in error. Even trained technicians find it difficult to work around such clutter effectively and without making errors. If a patch cable is in fact connected to the wrong port, it may take hours to troubleshoot and resolve the problem in the mass of interconnections. Therefore, a need exists for a switch rack system that reduces the likelihood of improper interconnections. 
     The connector system of the present system fulfills this need among others. Specifically, the patch cables may be configured with one or more particular secure connectors to ensure that the correct plug is plugged into the correct receptacle. In one embodiment, the interconnections are not only secure, but customized for a particular user. Specifically, Applicants recognize that many of the interconnections involve multi-connector assemblies such as duplex and quad connectors. Applicants also realized that the multi-connector assemblies provide an opportunity to increase the number of permutations of the secure connectors to the extent that particular duplex and quad connector assemblies can be provided on a per customer basis. 
     By way of background, referring to  FIGS. 13( a ) and 13( b ) , one embodiment of the invention is shown as a duplex and quad connectors having the same secure configuration for each connector. Specifically, the duplex connector  1301  has to two identical receptacles  1302 ,  1303 , each having the same D-type configuration. (It should be apparent that this receptacle configuration corresponds to the plug  1101  shown in  FIG. 11 .) Likewise, the quad connector  1304  shown in  FIG. 13( b ) , comprises four identical receptacles  1305 . (Again each receptacle corresponds to the plug  1101  shown in  FIG. 11 .) 
     Although the connector embodiments of  FIGS. 13( a )  &amp; ( b ) work well to ensure a secure connection, they are limited in the number of unique configurations they can make. Specifically, referring to  FIG. 13( a ) , the duplex connector  1301  with identical receptacles  1302  and  1303  only has four unique combinations using the primary tooling. Likewise, the quad receptacle  1304  shown in  FIG. 13( b )  is also limited to just four unique combinations using primary tooling. As used herein, the primary tooling refers to the tooling required to make embodiments  1101 - 1104  as shown in  FIG. 11( b ) . As mentioned above, one of the advantages of the connector system shown in  FIG. 11 , is the ability to provide different receptacle and plug configurations simply by rotating the core pin. In other words, different configurations can be manufactured using the same tooling. This is desirable from a production cost standpoint. 
     Although the number of permutations of the quad shown in  FIG. 13( b )  could be increased to forty (40) by using the ten (10) alternative configurations keying configuration illustrated in plugs  1101 - 1110  in  FIG. 11 , this requires additional tooling because plugs  1105 - 1110  cannot be produced by rotating the core pin used in the manufacture of plugs  1101 - 1104 . 
     In addition to a desire to avoid additional tooling requirements, Applicants have identified a need to further increase the number of unique connector combinations so that certain multi-connector configurations can be customized for a particular user. To this end, Applicants disclose herein multi-connector assembly in which two or more of the geometries of the connectors of a given assembly are different. For example, one embodiment of the duplex connector  1401  of the present invention is shown in  FIG. 14( a ) . In this embodiment, the duplex connector  1401  comprises one receptacle  1402  having a geometry similar to the receptacles shown in  FIG. 13A , but its second receptacle  1403  has a different geometry, corresponding to the plug  1103  shown in  FIG. 11 . By configuring the duplex connector  1401  such that at least two of the receptacles have different first geometries, the number of unique combinations increases from four to sixteen. 
     Even more permutations can be derived from the quad connector  1404  as shown in  FIG. 14( b ) . Specifically, quad connector  1404  can be arranged such that at least two of the receptacles have different geometries. As shown in  FIG. 14B , quad  1404  has four receptacles  1405 ,  1406 ,  1407 ,  1408 , all of which have a different geometry. Specifically,  1405  corresponds to plug  1101 , receptacle  1406  corresponds to plug  1102 , receptacle  1407  corresponds to plug  1104  and receptacle corresponds to plug  1103 . It should be understood that while each of these is different, the number of different plug/receptacle geometries can carry from two to four in this assembly. By altering the geometries within the quad connector, the number of unique permutations increases dramatically. For example, just using the primary tooling described above, 256 combinations are possible. When all ten keying combinations are used as shown in  FIG. 11, 1600  unique combinations are possible. 
     In addition to increasing the number of unique connector assemblies, varying the geometry among the connectors of a given assembly also improves the resilience of the connector to a “forced” connection. Specifically, when the geometries are aligned as they are in the connectors of  FIGS. 13( a )  &amp; ( b ), a user can force the connection by biasing the mating connector to one side or the other. For example, if a user wanted to force a connection to the duplex connector  1301  of  FIG. 13( a ) , the user could urge the plug assembly to the left of the duplex connector  1301  during mating. However, by varying the geometries as in the embodiment of  FIGS. 14( a ) and ( b )  such that there is no polarization or alignment of the geometries, a user can no longer urge the mating connector to one side or the other to force the connection. Thus, the embodiments of  FIGS. 14( a ) and ( b )  are also less prone to forced connections. 
     It should be understood that specific plug assemblies such as a duplex and quad are not illustrated particularly herein, but that such connectors assemblies are produced by clipping together simplex connectors, such as those shown in  FIG. 11 , with a known clip (see, for example, U.S. Pat. No. 7,500,790, incorporated herein by reference.) 
     Because the present invention provides for so many unique combinations, it is possible to designate certain unique configurations for certain customers. In this regard, the color of the connector itself may be used to render it unique for a certain customer. For example, the quad connector embodiment shown in  FIG. 14( b )  may be provided in green for one customer and be provided in blue for a different customer. Thus, the physical connectors themselves may be the same but the colors are different based on the user. 
     In one embodiment, the housings are color coded as described above. In another embodiment, the connectors comprise strain relief boots extending rearwardly (not shown) that are color coded to increase the number of color permutations possible. Such strain relief boots are well known and are disclosed, for example, in U.S. Pat. No. 7,695,197, incorporated herein by reference. 
     It should be understood that while the multiple connector configurations were discussed in terms of a patch cords, the application is by no means limited to patch cord applications. Indeed, the unique multi-connector arrangements can be applied in any situation requiring multi-connector connections. In this regard, it should be understood that the foregoing is illustrative and not limiting and that obvious modifications may be made by those skilled in the art without departing from the spirit of the invention. Accordingly, the specification is intended to cover such alternatives, modifications, and equivalence as may be included within the spirit and scope of the invention as defined in the following claims.