Abstract:
Cables including a plurality of twisted pairs of insulated conductors, a separator disposed among the plurality of twisted pairs so as to physically separate a first twisted pair from a second twisted pair, and a jacket surrounding the plurality of twisted pairs and the separator. The jacket may include a plurality of protrusions extending away from an inner circumferential surface of the jacket toward a center of the cable. The plurality of protrusions are configured so as to hold the plurality of twisted pairs away from the inner circumferential surface of the jacket, and may provide an air gap between the plurality of twisted pairs of insulated conductors and the inner circumferential surface of the jacket. In one example, a boundary of the air gap adjacent the inner circumferential surface of the jacket is arc shaped.

Description:
RELATED APPLICATIONS 
       [0001]    This application is a continuation of, and claims priority under 35 U.S.C. § 120 to, co-pending U.S. application Ser. No. 11/584,825 entitled “Data Cable with Cross-Twist Cabled Core Profile,” filed Oct. 23, 2006, which is a continuation of, and claims priority under 35 U.S.C. § 120 to, U.S. application Ser. No. 11/445,448 entitled “Data Cable with Cross-Twist Cabled Core Profile,” filed on Jun. 1, 2006 and now abandoned, which is a continuation of, and claims priority under 35 U.S.C. § 120 to, U.S. application Ser. No. 11/197,718, entitled “Data Cable with Cross-Twist Cabled Core Profile,” filed on Aug. 4, 2005 and now U.S. Pat. No. 7,135,641, which is a continuation of, and claims priority under 35 U.S.C. § 120 to, U.S. application Ser. No. 10/705,672, entitled “Data Cable with Cross-Twist Cabled Core Profile,” filed on Nov. 10, 2003 and now U.S. Pat. No. 7,154,043, which is a continuation-in-part of, and claims priority under 35 U.S.C. § 120 to, U.S. application Ser. No. 10/430,365, entitled “Enhanced Data Cable With Cross-Twist Cabled Core Profile,” filed on May 5, 2003 and now abandoned, which is a continuation of, and claims priority under 35 U.S.C. § 120 to, U.S. application Ser. No. 09/532,837 entitled “Enhanced Data Cable With Cross-Twist Cabled Core Profile,” filed on Mar. 21, 2000 and now U.S. Pat. No. 6,596,944, which is a continuation of, and claims priority under 35 U.S.C. § 120 to, U.S. Application Ser. No. 08/841,440, filed Apr. 22, 1997 entitled “Making Enhanced Data Cable with Cross-Twist Cabled Core Profile” (as amended) now U.S. Pat. No. 6,074,503. Each of the above-identified patents and patent applications is herein incorporated by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of Invention 
         [0003]    The present invention relates to high-speed data communications cables using at least two twisted pairs of wires. More particularly, it relates to cables having a central core defining plural individual pair channels. 
         [0004]    2. Discussion of Related Art 
         [0005]    High-speed data communications media include pairs of wire twisted together to form a balanced transmission line. Such pairs of wire are referred to as twisted pairs. One common type of conventional cable for high-speed data communications includes multiple twisted pairs that may be bundled and twisted (cabled) together to form the cable. 
         [0006]    Modern communication cables must meet electrical performance characteristics required for transmission at high frequencies. The Telecommunications Industry Association and the Electronics Industry Association (TIA/EIA) have developed standards which specify specific categories of performance for cable impedance, attenuation, skew and crosstalk isolation. When twisted pairs are closely placed, such as in a cable, electrical energy may be transferred from one pair of a cable to another. Such energy transferred between pairs is referred to as crosstalk and is generally undesirable. The TIA/EIA have defined standards for crosstalk, including TIA/EIA-568A. The International Electrotechnical Commission (IEC) has also defined standards for data communication cable crosstalk, including ISO/IEC 11801. One high-performance standard for 100Ω cable is ISO/IEC 11801, Category 5, another is ISO/IEC 11801 Category 6. 
         [0007]    In conventional cable, each twisted pair of a cable has a specified distance between twists along the longitudinal direction, that distance being referred to as the pair lay. When adjacent twisted pairs have the same pair lay and/or twist direction, they tend to lie within a cable more closely spaced than when they have different pair lays and/or twist direction. Such close spacing may increase the amount of undesirable crosstalk which occurs between adjacent pairs. Therefore, in some conventional cables, each twisted pair within the cable may have a unique pair lay in order to increase the spacing between pairs and thereby to reduce the crosstalk between twisted pairs of a cable. Twist direction may also be varied. 
         [0008]    Along with varying pair lays and twist directions, individual solid metal or woven metal pair shields are sometimes used to electromagnetically isolate pairs. Shielded cable, although exhibiting better crosstalk isolation, is more difficult and time consuming to install and terminate. Shielded conductors are generally terminated using special tools, devices and techniques adapted for the job. 
         [0009]    One popular cable type meeting the above specifications is Unshielded Twisted Pair (UTP) cable. Because it does not include shielded conductors, UTP is preferred by installers and plant managers, as it may be easily installed and terminated. However, conventional UTP may fail to achieve superior crosstalk isolation, as required by state of the art transmission systems, even when varying pair lays are used. 
         [0010]    Another solution to the problem of twisted pairs lying too closely together within a cable is embodied in a shielded cable manufactured by Belden Wire &amp; Cable Company as product number 1711A. This cable includes four twisted pair media radially disposed about a “star”-shaped core. Each twisted pair nests between two fins of the “star”-shaped core, being separated from adjacent twisted pairs by the core. This helps reduce and stabilize crosstalk between the twisted pair media. However, the core adds substantial cost to the cable, as well as material which forms a potential fire hazard, as explained below, while achieving a crosstalk reduction of only about 5 dB. Additionally, the close proximity of the shield to the pairs within the cable requires substantially greater insulation thickness to maintain desired electrical characteristics. This adds more insulation material to the construction and increases cost. 
         [0011]    In building design, many precautions are taken to resist the spread of flame and the generation of and spread of smoke throughout a building in case of an outbreak of fire. Clearly, it is desired to protect against loss of life and also to minimize the costs of a fire due to the destruction of electrical and other equipment. Therefore, wires and cables for in building installations are required to comply with the various flammability requirements of the National Electrical Code (NEC) and/or the Canadian Electrical Code (CEC). 
         [0012]    Cables intended for installation in the air handling spaces (i.e. plenums, ducts, etc.) of buildings are specifically required by NEC or CEC to pass the flame test specified by Underwriters Laboratories Inc. (UL), UL-910, or it&#39;s Canadian Standards Association (CSA) equivalent, the FT6. The UL-910 and the FT6 represent the top of the fire rating hierarchy established by the NEC and CEC respectively. Cables possessing this rating, generically known as “plenum” or “plenum rated”, may be substituted for cables having a lower rating (i.e. CMR, CM, CMX, FT4, FT1 or their equivalents), while lower rated cables may not be used where plenum rated cable is required. 
         [0013]    Cables conforming to NEC or CEC requirements are characterized as possessing superior resistance to ignitability, greater resistant to contribute to flame spread and generate lower levels of smoke during fires than cables having a lower fire rating. Conventional designs of data grade telecommunications cables for installation in plenum chambers have a low smoke generating jacket material, e.g. of a PVC formulation or a fluoropolymer material, surrounding a core of twisted conductor pairs, each conductor individually insulated with a fluorinated ethylene propylene (FEP) insulation layer. Cable produced as described above satisfies recognized plenum test requirements such as the “peak smoke” and “average smoke” requirements of the Underwriters Laboratories, Inc., UL910 Steiner test and/or Canadian Standards Association CSA-FT6 (Plenum Flame Test) while also achieving desired electrical performance in accordance with EIA/TIA-568A for high frequency signal transmission. 
         [0014]    While the above-described conventional cable, including the Belden 1711A cable due in part to their use of FEP, meets all of the above design criteria, the use of fluorinated ethylene propylene is extremely expensive and may account for up to 60% of the cost of a cable designed for plenum usage. 
         [0015]    The solid, relatively large core of the Belden 1711A cable may also contribute a large volume of fuel to a cable fire. Forming the core of a fire resistant material, such as FEP, is very costly due to the volume of material used in the core. Solid flame retardant/smoke suppressed polyolefin may also be used in combination with FEP. However, solid flame retardant/smoke suppressed polyolefin compounds commercially available all possess dielectric properties inferior to that of FEP. In addition, they also exhibit inferior resistance to burning and generally produce more smoke than FEP under burning conditions than FEP. 
       SUMMARY OF INVENTION 
       [0016]    According to one embodiment, there is provided a cable comprising a plurality of twisted pairs of insulated conductors including a first twisted pair and a second twisted pair, each twisted pair comprising two insulated conductors twisted together in a helical manner, a separator disposed among the plurality of twisted pairs of insulated conductors so as to physically separate the first twisted pair from the second twisted pair, and a jacket surrounding the plurality of twisted pairs of insulated conductors and the separator. The jacket comprises a plurality of protrusions extending away from an inner circumferential surface of the jacket, and the plurality of protrusions cause the plurality of twisted pairs of insulated conductors to be kept away from the inner circumferential surface of the jacket. In one example, the jacket is shaped such that, between each two protrusions of the plurality of protrusions, the inner circumferential surface of the jacket has an arc shape 
         [0017]    In another embodiment, a cable comprises a plurality of twisted pairs of insulated conductors including a first twisted pair and a second twisted pair, each twisted pair comprising two insulated conductors twisted together in a helical manner, a separator disposed among the plurality of twisted pairs of insulated conductors so as to physically separate the first twisted pair from the second twisted pair, and a jacket surrounding the plurality of twisted pairs of insulated conductors and the separator. The jacket comprises a plurality of protrusions extending away from an inner circumferential surface of the jacket, and the plurality of protrusions provide an air gap between the plurality of twisted pairs of insulated conductors and the inner circumferential surface of the jacket. In one example, a boundary of the air gap adjacent the inner circumferential surface of the jacket is arc shaped. 
         [0018]    According to another embodiment, a cable comprises a plurality of twisted pairs of insulated conductors including a first twisted pair and a second twisted pair, each twisted pair comprising two insulated conductors twisted together in a helical manner, a separator disposed among the plurality of twisted pairs of insulated conductors so as to physically separate the first twisted pair from the second twisted pair, and a jacket surrounding the plurality of twisted pairs of insulated conductors and the separator, wherein the jacket comprises a plurality of protrusions extending away from an inner circumferential surface of the jacket toward a center of the cable. The plurality of protrusions are configured so as to keep the plurality of twisted pairs away from the inner circumferential surface of the jacket, thereby reducing susceptibility of the plurality of twisted pairs to alien near end crosstalk. In one example, between each two protrusions of the plurality of protrusions, the inner circumferential surface of the jacket is arc shaped. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0019]    In the drawings, which are not intended to be drawn to scale, each identical or nearly identical component that is illustrated in various figures is represented by a like numeral. For purposes of clarity, not every component may be labeled in every drawing. The drawings are provided for the purposes of illustration and explanation and are not intended as a definition of the limits of the invention. In the drawings: 
           [0020]      FIG. 1  is a cross-sectional view of a cable core according to one embodiment of the invention; 
           [0021]      FIG. 2  is perspective view of one embodiment of a perforated core according to the invention; 
           [0022]      FIG. 3  is a cross-sectional view of one embodiment of a cable including the core of  FIG. 1 ; 
           [0023]      FIG. 4  is a cross-sectional view of another embodiment of a cable core used in some embodiments of the cable of the invention; 
           [0024]      FIG. 5  is an illustration of one embodiment of a cable comprising twisted pairs having varying twist lays according to the invention; 
           [0025]      FIG. 6  is a cross-sectional view of a twisted pair of insulated conductors; 
           [0026]      FIG. 7  is a graph of impedance versus frequency for a twisted pair of conductors according to the invention; 
           [0027]      FIG. 8  is a graph of return loss versus frequency for the twisted pair of  FIG. 7 ; 
           [0028]      FIG. 9A  is a perspective view of a cable having a dual-layer jacket according to the invention; 
           [0029]      FIG. 9B  is a cross-sectional view of the cable of  FIG. 9A , taken along line B-B in  FIG. 9A ; 
           [0030]      FIG. 10  is a perspective view of one embodiment of a bundled cable according to the invention, illustrating oscillating cabling; 
           [0031]      FIG. 11  is an illustration of another embodiment of a bundled cable including a plurality of cables having interlocking striated jackets, according to the invention; 
           [0032]      FIG. 12  is a perspective view of another embodiment of a bundled cable including a plurality of cables having striated jackets, according to the invention; and 
           [0033]      FIG. 13  is an illustration of yet another embodiment of cables having jackets with inwardly extending projections, according to the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0034]    Various illustrative embodiments and aspects thereof will now be described in detail with reference to the accompanying figures. It is to be appreciated that this invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having,” “containing,” “involving,” and variations thereof herein, is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. 
         [0035]    Referring to  FIG. 1 , there is illustrated one embodiment of portions of a cable including an extruded core  101  having a profile described below cabled into the cable with four twisted pairs  103 . Although the following description will refer primarily to a cable that is constructed to include four twisted pairs of insulated conductors and a core having a unique profile, it is to be appreciated that the invention is not limited to the number of pairs or the profile used in this embodiment. The inventive principles can be applied to cables including greater or fewer numbers of twisted pairs and different core profiles. Also, although this embodiment of the invention is described and illustrated in connection with twisted pair data communication media, other high-speed data communication media can be used in constructions of cable according to the invention. 
         [0036]    As shown in  FIG. 1 , according to one embodiment of the invention, the extruded core profile may have an initial shape of a “+”, providing four spaces or channels  105 , one between each pair of fins  102  of the core  101 . Each channel  105  carries one twisted pair  103  placed within the channel  105  during the cabling operation. The illustrated core  101  and profile should not be considered limiting. The core  101  may be made by some other process than extrusion and may have a different initial shape or number of channels  105 . For example, as illustrated in  FIG. 1 , the core may be provided with an optional central channel  107  that may carry, for example, an optical fiber element or strength element  109 . In addition, in some examples, more than one twisted pair  103  may be placed in each channel  105 . 
         [0037]    The above-described embodiment can be constructed using a number of different materials. While the invention is not limited to the materials now given, the invention is advantageously practiced using these materials. The core material should be a conductive material or one containing a powdered ferrite, the core material being generally compatible with use in data communications cable applications, including any applicable fire safety standards. In non-plenum applications, the core can be formed of solid or foamed flame retardant polyolefin or similar materials. The core may also be formed of non-flame retardant materials. In plenum applications, the core can be any one or more of the following compounds: a solid low dielectric constant fluoropolymer, e.g., ethylene chlortrifluoroethylene (E-CTFE) or fluorinated ethylene propylene (FEP), a foamed fluoropolymer, e.g., foamed FEP, and polyvinyl chloride (PVC) in either solid, low dielectric constant form or foamed. A filler is added to the compound to render the extruded product conductive. Suitable fillers are those compatible with the compound into which they are mixed, including but not limited to powdered ferrite, semiconductive thermoplastic elastomers and carbon black. Conductivity of the core helps to further isolate the twisted pairs from each other. 
         [0038]    A conventional four-pair cable including a non-conductive core, such as the Belden 1711A cable, reduces nominal crosstalk by up to 5 dB over similar, four-pair cable without the core. By making the core conductive, crosstalk is reduced a further 5 dB. Since both loading of the core and jacket construction can affect crosstalk, these numbers compare cables with similar loading and jacket construction. 
         [0039]    As discussed above, the core  101  may have a variety of different profiles and may be conductive or non-conductive. According to one embodiment, the core  101  may further include features that may facilitate removal of the core  101  from the cable. For example, referring to  FIG. 2 , the core  101  may be provided with narrowed, or notched, sections  111 , which are referred to herein as “pinch points.” At the notched sections, or pinch points, a diameter or size of the core  101  is reduced compared with the normal size of the core  101  (at the non-pinch point sections of the core). Thus, the pinch points  111  provide points at which it may be relatively easy to break the core  101 . The pinch points  111  may act as “perforations” along the length of the core, facilitating snapping of the core at these points, which in turn may facilitate removal of sections of the core  101  from the cable. This may be advantageous for being able to easily snap the core to facilitate terminating the cable with, for example, a telephone or data jack or plug. In one example, the pinch points  111  may be placed at intervals of approximately 0.5 inches along the length of the cable. The pinch points  111  should be small enough such that the twisted pairs may ride over the pinch points  111  substantially without dipping closer together through the notched sections  111 . In one example, the pinch points may be formed during extrusion of the core by stretching the core for a relatively short period of time each time it is desired to form a pinch point  111 . Stretching the core during extrusion results in “thinned” or narrowed sections being created in the core which form the pinch points  111 . 
         [0040]    The cable may be completed in any one of several ways, for example, as shown in  FIG. 3 . The combined core  101  and twisted pairs  103  may be optionally wrapped with a binder  113  and then jacketed with a jacket  115  to form cable  117 . In one example, an overall conductive shield  117  can optionally be applied over the binder  111  before jacketing to prevent the cable from causing or receiving electromagnetic interference. The jacket  115  may be PVC or another material as discussed above in relation to the core  101 . The binder  113  may be, for example, a dielectric tape which may be polyester, or another compound generally compatible with data communications cable applications, including any applicable fire safety standards. It is to be appreciated that the cable can be completed without either or both of the binder and the conductive shield, for example, by providing the jacket. 
         [0041]    As is known in this art, when plural elements are cabled together, an overall twist is imparted to the assembly to improve geometric stability and help prevent separation. In some embodiments of a process of manufacturing the cable of the invention, twisting of the profile of the core along with the individual twisted pairs is controlled. The process includes providing the extruded core to maintain a physical spacing between the twisted pairs and to maintain geometrical stability within the cable. Thus, the process assists in the achievement of and maintenance of high crosstalk isolation by placing a conductive core in the cable to maintain pair spacing. 
         [0042]    According to another embodiment, greater cross-talk isolation may achieved in the construction of  FIG. 4  by using a conductive shield  119 , for example a metal braid, a solid metal foil shield or a conductive plastic layer in contact with the ends  121  of the fins  102  of the core  101 . In such an embodiment, the core is preferably conductive. Such a construction rivals individual shielding of twisted pairs for cross-talk isolation. This construction optionally can advantageously include a drain wire  123  disposed in the central channel  107 , as illustrated in  FIG. 4 . In some examples, it may be advantageous to have the fins  102  of the core  101  extend somewhat beyond a boundary defined by the outer dimension of the twisted pairs  103 . As shown in  FIG. 4 , this helps to ensure that the twisted pairs  103  do not escape their respective channels  105  prior to the cable being jacketed, and may also facilitate good contact between the fins  102  and the shield  119 . In the illustrated example, closing and jacketing the cable  117  may bend the ends  121  of the fins  102  over slightly, as shown, if the core material is a relatively soft material, such as PVC. 
         [0043]    In some embodiments, particularly where the core  101  may be non-conductive, it may be advantageous to provide additional crosstalk isolation between the twisted pairs  103  by varying the twist lays of each twisted pair  103 . For example, referring to  FIG. 5 , the cable  117  may include a first twisted pair  103   a  and a second twisted pair  103   b . Each of the twisted pairs  103   a ,  103   b  includes two metal wires  125   a ,  125   b  each insulated by an insulating layer  127   a ,  127   b . As shown in  FIG. 5 , the first twisted pair  103   a  may have a twist lay length that is shorter than the twist lay length of the second twisted pair  103   b.    
         [0044]    As discussed above, varying the twist lay lengths between the twisted pairs in the cable may help to reduce crosstalk between the twisted pairs. However, the shorter a pair&#39;s twist lay length, the longer the “untwisted length” of that pair and thus the greater the signal phase delay added to an electrical signal that propagates through the twisted pair. It is to be understood that the term “untwisted length” herein denotes the electrical length of the twisted pair of conductors when the twisted pair of conductors has no twist lay (i.e., when the twisted pair of conductors is untwisted). Therefore, using different twist lays among the twisted pairs within a cable may cause a variation in the phase delay added to the signals propagating through different ones of the conductors pairs. It is to be appreciated that for this specification the term “skew” is a difference in a phase delay added to the electrical signal for each of the plurality of twisted pairs of the cable. Therefore, a skew may result from the twisted pairs in a cable having differing twist lays. As discussed above, the TIA/EIA has set specifications that dictate that cables, such as category 5 or category 6 cables, must meet certain skew requirements. 
         [0045]    In addition, in order to impedance match a cable to a load (e.g., a network component), the impedance of a cable may be rated with a particular characteristic impedance. For example, many radio frequency (RF) components may have characteristic impedances of 50 or 100 Ohms. Therefore, many high frequency cables may similarly be rated with a characteristic impedance of 50 or 100 Ohms so as to facilitate connecting of different RF loads. The characteristic impedance of the cable may generally be determined based on a composite of the individual nominal impedances of each of the twisted pairs making up the cable. Referring to  FIG. 6 , the nominal impedance of a twisted pair  103   a  may be related to several parameters including the diameter of the wires  125   a ,  125   b  of the twisted pairs making up the cable, the center-to-center distance d between the conductors of the twisted pairs, which may in turn depend on the thickness of the insulating layers  127   a ,  127   b , and the dielectric constant of the material used to insulate the conductors. 
         [0046]    The nominal characteristic impedance of each pair may be determined by measuring the input impedance of the twisted pair over a range of frequencies, for example, the range of desired operating frequencies for the cable. A curve fit of each of the measured input impedances, for example, up to 801 measured points, across the operating frequency range of the cable may then be used to determine a “fitted” characteristic impedance of each twisted pair making up the cable, and thus of the cable as a whole. The TIA/EIA specification for characteristic impedance is given in terms of this fitted characteristic impedance. For example, the specification for a category 5 or 6 100 Ohm cable is 100 Ohms, +−15 Ohms for frequencies between 100 and 350 MHz and 100 Ohms+−12 Ohms for frequencies below 100 MHz. 
         [0047]    In conventional manufacturing, it is generally considered more beneficial to design and manufacture twisted pairs to achieve as close to the specified characteristic impedance of the cable as possible, generally within plus or minus 2 Ohms. The primary reason for this is to take into account impedance variations that may occur during manufacture of the twisted pairs and the cable. The further away from the specified characteristic impedance a particular twisted pair is, the more likely a momentary deviation from the specified characteristic impedance at any particular frequency due to impedance roughness will exceed limits for both input impedance and return loss of the cable. 
         [0048]    As the dielectric constant of an insulation material covering the conductors of a twisted pair decreases, the velocity of propagation of a signal traveling through the twisted pair of conductors increases and the phase delay added to the signal as it travels through the twisted pair decreases. In other words, the velocity of propagation of the signal through the twisted pair of conductors is inversely proportional to the dielectric constant of the insulation material and the added phase delay is proportional to the dielectric constant of the insulation material. For example, referring again to  FIG. 6 , for a so-called “faster” insulation, such as fluoroethylenepropylene (FEP), the propagation velocity of a signal through the twisted pair  103   a  may be approximately 0.69c (where c is the speed of light in a vacuum). For a “slower” insulation, such as polyethylene, the propagation velocity of a signal through the twisted pair  103   a  may be approximately 0.66c. 
         [0049]    The effective dielectric constant of the insulation material may also depend, at least in part, on the thickness of the insulating layer. This is because the effective dielectric constant may be a composite of the dielectric constant of the insulating material itself in combination with the surrounding air. Therefore, the propagation velocity of a signal through a twisted pair may also depend on the thickness of the insulation of that twisted pair. However, as discussed above, the characteristic impedance of a twisted pair also depends on the insulation thickness. 
         [0050]    Applicant has recognized that by optimizing the insulation diameters relative to the twist lays of each twisted pair in the cable, the skew can be substantially reduced. Although varying the insulation diameters may cause variation in the characteristic impedance values of the twisted pairs, under improved manufacturing processes, impedance roughness over frequency (i.e., variation of the impedance of any one twisted pair over the operating frequency range) can be controlled to be reduced, thus allowing for a design optimized for skew while still meeting the specification for impedance. 
         [0051]    According to one embodiment of the invention, a cable may comprise a plurality of twisted pairs of insulated conductors, wherein twisted pairs with longer pair lays have a relatively higher characteristic impedance and larger insulation diameter, while twisted pairs with shorter pair lays have a relatively lower characteristic impedance and smaller insulation diameter. In this manner, pair lays and insulation thickness may be controlled so as to reduce the overall skew of the cable. One example of such a cable, using polyethylene insulation is given in Table 1 below. 
         [0000]    
       
         
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Twist Lay Length 
                 Diameter of Insulation 
               
               
                 Twisted Pair 
                 (inches) 
                 (inches) 
               
               
                   
               
             
             
               
                 1 
                 0.504 
                 0.042 
               
               
                 2 
                 0.744 
                 0.040 
               
               
                 3 
                 0.543 
                 0.041 
               
               
                 4 
                 0.898 
                 0.040 
               
               
                   
               
             
          
         
       
     
         [0052]    This concept may be better understood with reference to  FIGS. 7 and 8  which respectively illustrate graphs of measured input impedance versus frequency and return loss versus frequency for twisted pair  1 , for example, twisted pair  103   a , in the cable  117 . Referring to  FIG. 7 , a “fitted” characteristic impedance  131  for the twisted pair (over the operating frequency range) may be determined from the measured input impedance  133  over the operating frequency range. Lines  135  indicate the category 5/6 specification range for the input impedance of the twisted pair. As shown in  FIG. 7 , the measured input impedance  133  falls within the specified range over the operating frequency range of the cable  117 . Referring to  FIG. 8 , there is illustrated a corresponding return loss versus frequency plot for the twisted pair  103   a . The line  137  indicates the category 5/6 specification for return loss over the operating frequency range. As shown in  FIG. 8 , the measured return loss  139  is above the specified limit (and thus within specification) over the operating frequency range of the cable. Thus, the characteristic impedance could be allowed to deviate further from the desired 100 Ohms, if necessary, to reduce skew. Similarly, the twist lays and insulation thicknesses of the other twisted pairs may be further varied to reduce the skew of the cable while still meeting the impedance specification. 
         [0053]    According to another embodiment, a four-pair cable was designed, using slower insulation material (e.g., polyethylene) and using the same pair lays as shown in Table 1, where all insulation diameters were set to 0.041 inches. This cable exhibited a skew reduction of about 8 ns/100 meters (relative to the conventional cable described above—this cable was measured to have a worst case skew of approximately 21 ns whereas the conventional, impedance-optimized cable exhibits a skew of approximately 30 ns or higher), yet the individual pair impedances were within 0 to 2.5 ohms of deviation from nominal, leaving plenty of room for further impedance deviation, and therefore skew reduction. 
         [0054]    Allowing some deviation in the twisted pair characteristic impedances relative to the nominal impedance value allows for a greater range of insulation diameters. Smaller diameters for a given pair lay results in a lower pair angle and shorter non-twisted pair length. Conversely, larger pair diameters result in a higher pair angles and longer non-twisted pair length. Where a tighter pair lay would normally require an insulation diameter of 0.043″ for 100 ohms, a diameter of 0.041″ would yield a reduced impedance of about 98 ohms. Longer pair lays using the same insulation material would require a lower insulation diameter of about 0.039″ for 100 ohms, and a diameter of 0.041″ would yield about 103 ohms. As shown in  FIGS. 7 and 8 , allowing this “target” impedance variation from 100 Ohms may not prevent the twisted pairs, and the cable, from meeting the input impedance specification, but may allow improved skew in the cable. 
         [0055]    According to another embodiment, illustrated in  FIGS. 9A and 9B , the cable  117  may be provided with a dual-layer jacket  141  comprising a first, inner layer  143  and a second, outer layer  145 . An optional conductive shield  147  may be placed between the first and second jacket layers  143 ,  145 , as illustrated. The shield  147  may act to prevent crosstalk between adjacent or nearby cables, commonly called alien crosstalk. The shield  147  may be, for example, a metal braid or foil that extends partially or substantially around the first jacket layer  143  along the length of the cable. The shield  147  may be isolated from the twisted pairs  103  by the first jacket layer  143  and may thus have little impact on the twisted pairs. This may be advantageous in that small or no adjustment may need to be made to, for example conductor or insulation thicknesses of the twisted pairs  103 . The first and second jacket layers may be any suitable jacket material, such as, PVC, fluoropolymers, fire and/or smoke resistant materials, and the like. In this embodiment, because the shield is isolated from the twisted pairs  103  and the separator  101  by the first jacket layer  143 , the separator  101  may be conductive or non-conductive. 
         [0056]    According to another embodiment, several cables such as those described above may be bundled together to provide a bundled cable. Within the bundled cable may be provided numerous embodiments of the cables described above. For example, the bundled cable may include some shielded and some unshielded cables, some four-pair cables and some having a different number of pairs. In addition, the cables making up the bundled cable may include conductive or non-conductive cores having various profiles. In one example, the multiple cables making up the bundled cable may be helically twisted together and wrapped in a binder. The bundled cable may include a rip-cord to break the binder and release the individual cables from the bundle. 
         [0057]    According to one embodiment, illustrated in  FIG. 10 , the bundled cable  151  may be cabled in an oscillating manner along its length rather than cabled in one single direction along the length of the cable. In other words, the direction in which the cable is twisted (cabled) along its length may be changed periodically from, for example, a clockwise twist to an anti-clockwise twist, and vice versa. This is known in the art as SZ type cabling and may require the use of a special twisting machine known as an oscillator cabler. In some examples of bundled cables  151 , each individual cable  117  making up the bundled cable  151  may itself be helically twisted (cabled) with a particular cable lay length, for example, about 5 inches. The cable lay of each cable may tend to either loosen (if in the opposite direction) or tighten (if in the same direction) the twist lays of each of the twisted pairs making up the cable. If the bundled cable  151  is cabled in the same direction along its whole length, this overall cable lay may further tend to loosen or tighten the twist lays of each of the twisted pairs. Such altering of the twist lays of the twisted pairs may adversely affect the performance of at least some of the twisted pairs and/or the cables  117  making up the bundled cable  151 . However, helically twisting the bundled cable may be advantageous in that it may allow the bundled cable to be more easily bent, for example, in storage or when being installed around corners. By periodically reversing the twist lay of the bundled cable, any effect of the bundled twist on the individual cables may be substantially canceled out. In one example, the twist lay of the bundled cable may be approximately 20 inches in either direction. As shown in  FIG. 10 , the bundled cable may be twisted for a certain number of twist lays in a first direction (region  153 ), then not twisted for a certain length (region  155 ), and then twisted in the opposite direction for a number of twist lays (region  157 ). 
         [0058]    Referring to  FIG. 11 , there is illustrated another embodiment of a bundled cable  161  according to the invention. In this embodiment, one or more of the individual cables  117  making up the bundled cable  161  may have a striated jacket  163 , as shown. The striated jacket  163  may have a plurality of protrusions  165  spaced about a circumference of the jacket  163 . In one example, the cables  117  may not be twisted with a cable lay. In this example, the protrusions  165  may be constructed such that the protrusions  165   a  of one jacket  163   a  may mate with the protrusions  165   b  of another jacket  163   b  so as to interlock two corresponding cables  117   a ,  117   b  together. Thus, the individual cables  117  making up the bundled cable  161  may “snap” together, possibly obviating the need for a binder to keep the bundled cable  161  together. This embodiment may be advantageous in that the cables  117  may be easily separated from one another when necessary. 
         [0059]    In another example, the individual cables  117  may be helically twisted with a cable lay. In this example, the protrusions  165  may form helical ridges along the length of the cables  117 , as shown in  FIG. 12 . The protrusions  165  may thus serve to further separate one cable  117   a  from another  117   b , and may thereby act to reduce alien crosstalk between cables  117   a ,  117   b . The plurality of cables  117  may be wrapped in, for example, a binder  167  to bundle the cables  117  together and form the bundled cable  161 . 
         [0060]    According to another embodiment, the cable  117  may be provided with a striated jacket  171  having a plurality of inwardly extending projections  173 , as shown in  FIG. 13 . Such a jacket construction may be advantageous in that the projections may result in relatively more air separating the jacket  171  from the twisted pairs  103  compared with a conventional jacket. Thus, the jacket material may have relatively less effect on the performance characteristics of the twisted pairs  103 . For example, the twisted pairs may exhibit less attenuation due to increased air surrounding the twisted pairs  103 . In addition, because the jacket  171  may be held further away from the twisted pairs  103  by the protrusions  173 , the protrusions  173  may help to reduce alien crosstalk between adjacent cables  117  in a bundled cable  175 . The cables  117  may again be wrapped in. for example, a polymer binder  177  to form the bundled cable  175 . 
         [0061]    Having thus described several aspects of at least one embodiment of this invention, it is to be appreciated various alterations, modifications, and improvements will readily occur to those skilled in the art. For example, any of the cables described herein may include any number of twisted pairs and any of the jackets, insulations and separators shown herein may comprise any suitable materials. In addition, the separators may be any shape, such as, but not limited to, a cross- or star-shape, or a flat tape etc., and may be positioned within the cable so as to separate one or more of the twisted pairs from one another. Such and other alterations, modifications, and improvements are intended to be part of this disclosure and are intended to be within the scope of the invention. Accordingly, the foregoing description and drawings are by way of example only and the scope of the invention should be determined from proper construction of the appended claims, and their equivalents.