GB2368404A - Hybrid cable with twisted pairs of metallic electrical cables alternating with optical cables - Google Patents

Abstract

A communications cable 10, which is especially suited to overground or aerial use, has twisted metallic conductor pairs 14 and dielectric optical fibre elements with a sheath 22 which has at least one optical fibre 18. The twisted pairs and the optical fibre elements are disposed in an annular arrangement around a longitudinal axis of the cable, neighbouring conductor pairs being separated by one of the optical fibre elements and a further dielectric member 12 is disposed in the region enclosed by the annular arrangement. The arrangement of the twisted pairs and the dielectric elements is such that the twisted pairs are screened from each other and hence crosstalk that may occur between metallic wires is reduced.

Description

HYBRID CABLE

The present invention relates to a communications cable. In particular, but not exclusively, the invention relates to a hybrid cable suitable for overhead use.

Cables for communications routes are well known. Optical fibre cables are known which include one or a few metallic conductor pairs to provide supervisory circuits or for use (for example for linesmen's telephones) during installation and commissioning of the cable. It has also been proposed to deploy what are known as"fibre-ready" cables, that is cables which have electrical conductors for the provision of initial communications circuits and, in addition, ductlets to receive optical fibres (which would be installed using an air blown fibre (ABF) technique, refer to our granted patent EP-B-108590. Typically, however, a choice needs to be made between installing a wired circuit or a fibre circuit. While fibre circuits are attractive in terms of available bandwidth, and reliability, the terminal equipments tends to be significantly more expensive than digital subscriber line (DSL) terminals and, in general, very little of the available bandwidth is likely to be used by any individual customer or customer group. Consequently, the choice of cable from the telephone exchange to the customer premises tends to go towards metal wired circuits. With the advent of the various DSL technologies, wired circuits can carry very significant bit rates, and hence private fibre circuits are even less likely to be used in the short term. However, the continued surge in bandwidth usage and the limited range of DSL technologies, suggest that"fibre to the home"may well become worthwhile within the next five years. In view of the high cost of installing cable, there is some attraction therefore in using a composite (fibre and wire) cable for where new cables need to be installed, at least for those installations where high bandwidth usage looks likely to grow soonest.

It has been found that when conventional metal wire cables are operated at the data rates of either asymmetric digital subscriber line (ADSL) or very high rate digital subscriber line (VDSL), cross talk between the signals carried on different metal wire circuits may occur. Cross talk will reduce the quality of service. For example, data rates may decrease as some time slots will not be available. In some circumstances data may be lost.

WO 99/07002 discloses a cable including four annular electrical conductors suitable for carrying high data rates, eg. Upto 622Mbit/s. In order to reduce cross talk between the annular conductors a positioner (also referred to as a symmetriser) of a polymeric material is provided. The positioner extends through the length of the cable and is in the form of a cross with four uniformly spaced arms. Bores are provided through each arm, each bore housing one annular conductor. The bores are arranged so that the conductors occupy radial positions equidistant from the central axis of the cable. The positioner is made of a material having a low dielectric constant. The use of the positioner is said to reduce cross talk between the conductors. WO 99/07002 also discloses that unsheathed optical fibres may be disposed within the annular conductors. This document also discloses that unsheathed optical fibres may be disposed in the cavity defined by the external surface of the positioner and the inner surface of the cable's outer sheath. The optical fibres in either of these arrangements do not, however, provide any shielding effect between the conductors. Further these arrangements suffer from the disadvantage that should the optical fibres be used for data transmission, breaking the fibres out for connection to any terminal equipment would be extremely difficult as the optical fibres disclosed are not"self surviving".

We have found that by disposing optical fibre elements between metallic wire circuits and any other metallic member, say for example, any steel strengthening members, the optical fibre elements can shield the metallic wire circuits from one another and can reduce the level of crosstalk between the wire circuits to insignificant levels. In addition, this solution not only reduces crosstalk, but the optical fibre elements provide a useful upgrade route.

We have also found that by careful selection of the dimensions of the cable components, such a cable may be particularly suitable for aerial use.

In comparison with conventional wire-only cables, we have developed a lightweight cable in which not only is crosstalk reduced but which is capable of supporting far greater bandwidth than wire-only cables.

In comparison to the cable disclosed in WO 99/07002, our cable does not require an additional positioner.

We have developed a cable which is not only relatively cheap to construct, but also extremely versatile. With respect to cost, experience has shown that the relative cost

of installing optical fibre to the home is frequently greater than the cost of the fibre itself, so although our cable requires both wires and more expensive optical fibres, the cost of the fibres may not be significant in comparison to the costs involved with installing the final cable.

Although hybrid cables, including both wire pairs and fibres are known, such cables were not designed to operate high bit rate data transfer over the wire circuits. Typically, in known hybrid cables, the wire pairs were provided only for simple telephony or supervisory use. Crosstalk was not a consideration in this application as the data rates carried over the fibre were so low as not to generate crosstalk, and wire circuits were not shielded from one another, as this was neither necessary nor a consideration. Our cable provides the advantage over known hybrid cables that the wire circuits are shielded from one another.

The cables we have developed are, in particular, suitable as drop cables. The drop cables of the present invention may be suspended from poles in a catenary. Between the poles and between the final pole and the customer premises, the drop cable sags due to its own weight : the extent of the sag on installation is determined by the tension in the drop cable and is designed to be within a range of values determined by the acceptable drop cable tension and the acceptable extent of eventual sag to avoid hazard. In addition to the suspension load of the weight of the drop cable itself, externally mounted drop cables are subject to additional variable loading due to wind force and settling of moisture or ice formation. In the UK, for example, drop cables in coastal regions experience high wind loading. This additional loading results in strain in the drop cable and will affect all the components of the cable including the data carrying components. In order for a cable to be suitable for aerial routes, its weight to strength ratio is important, as is its cross section.

In contrast to heavier prior art hybrid cables suitable for underground installation, the cables of the present invention have been developed to have small diameters. Their diameter and circular or tear drop transverse cross sections have particular application for aerial routes. They are in particular suitable for use for the drop from the telegraph pole to the customer premises.

Preferably, the fibre optic elements include a plurality of optical fibres disposed in a common sheath. It has been found that with suitable design this sheathed structure including a plurality of optical fibres provides a cable with sufficient strength and robustness that steel strengthening members and anti-buckling members may not be necessary. Instead, it is sufficient to provide dispersed strength member around the outer circumference of the cable. Thus, steel strengthening members or anti-buckling members necessary in prior art cables are not necessary in cables according to the preferred embodiments of the present invention.

Preferably, the fibre optic elements include a plurality of sheathed quad optical fibre elements jacketed with a tape. It has been found that such a structure may provide a cable having still further robustness and a useful degree of rigidity. Again, in contrast to prior art cables, neither steel strengthening members nor anti buckling members are necessary according to this preferred embodiment of the present invention.

We have further found that if the sheathed quad optical fibre elements are encapsulated by the tape, the tension in the tape ensures that the sheathed quad optical fibre elements are held securely in a bundle. The bundle having a transverse cross section that is cigar shaped. The tension in the tape is such that the sides of the cigar shaped cross-section are substantially parallel, and are maintained a minimum distance from one another, where that distance depends on the diameter of the sheathed quad members. The encapsulated sheathed elements thus, provide the further advantage that the metallic wire conductor pairs and any steel strengthening members, which may be present in the cable, are maintained a minimum distance from one another, that distance being approximately the diameter of the sheathed quad optical fibre elements. By maintaining a minimum distance a certain shielding effect is guaranteed. Also, since the cable designer knows with certainty the spacing of the metallic wire pairs in the final cable, his control over the composition within the cable is increased.

The present invention seeks to provide a cable which permits metal pair circuits to operate at high data rates without experiencing significant cross talk. DSL

technologies and SDH technologies, are such technologies which would be used to establish such circuits to operate at high data rates. In accordance with a first aspect of the invention, there is provided a communications cable including twisted metallic conductor pairs for carrying data and a plurality of dielectric elements, said plurality of dielectric elements including a plurality of optical fibre elements for carrying data, each of said optical fibre elements include a sheath in which at least one optical fibre is disposed, the twisted pairs and the optical fibre elements being disposed in an annular arrangement around a longitudinal axis of the cable, neighbouring conductor pairs being separated by one of said optical fibre elements, wherein a further dielectric member is disposed in the region enclosed by the annular arrangement, the arrangement of the twisted pairs and the dielectric elements being such that the twisted pairs are screened from each other.

Thus, in accordance with the invention, the metal data-carrying wires are shielded from cross talk by the optical fibre elements.

In accordance with a second aspect of the present invention, there is provided a communications cable for aerial use including a non-metallic member, a plurality of metallic elements, the plurality of metallic elements including at least two twisted metallic wire pairs for carrying data, and optical fibre elements each of which includes a sheath in which one or more optical fibres are disposed, wherein said twisted pairs and said optical fibre elements are disposed about said non-metallic member, the arrangement being such that each metallic element is separated from its neighbouring metallic element by one of said optical fibre elements.

In accordance with a third aspect of the present invention, there is provided a method of operating a communications route over twisted metal pair carriers in which said twisted metal pair carriers are disposed in a cable, the method including the steps of: providing at least two circuits over respective twisted metallic pair carriers of a cable, the cable including optical fibre elements including a sheath in which at

least one optical fibre is disposed which shield the twisted metallic pair carriers of the circuits so as to reduce the crosstalk between the circuits. In order that the invention may be more fully understood embodiments thereof will now be described by way of example only, and by way of contrast with a prior art cable, reference being made to the accompanying drawings in which: Figure 1: shows a cross section of a cable according to a first embodiment of the present invention; Figure 2: shows a cross section of a cable according to a second embodiment of the present invention; Figure 3: shows a cross section of a fibre unit; Figure 4: shows a cross section of a cable according to a third embodiment of the present invention; Figure 5: shows a cross section of a cable according to a fourth embodiment of the present invention; Figure 6: shows a cross section of a cable according to a fifth embodiment of the present invention; Figure 7: shows a cross section of a cable according to a sixth embodiment of the present invention; Figure 8: shows a cross section of a cable according to an seventh embodiment of the present invention; Figure 9: shows one application of cable according to the present invention and its installation; Figure 10: shows the termination of cable according to the present invention at a customer's premises.

Detailed description Referring to the drawings, Figure 1 shows the transverse cross section of a cable 10.

The cable 10 includes cable components 12,14, 16,18, 22 disposed in a filler material 11 and enclosed in a sheath 19. The filler 11 is high density polyethylene, which serves to contain and protect the cable components. The sheath may be made from the same material as the filler 11, typically with added carbon-black of other

fillers to improve resistance to UV light and other envirnmental factors, or may be of a polymeric material having enhanced abrasion resistance, such as polyimide. The optical fibres 18 are housed in tubular members 22. The tubular members 22 may be made from any of the following: poly (butylen teraphthalate), polypropylene or poly (ethylene teraphthalate) (PET). The tubular members have an internal diameter of approximately 0. 5-1.5mm, and an outside diameter of approximately 1.5-2mm.

The mean dielectric constant of the tubular member and optical fibre combination will depend on the relative proportions of optical fibre, tubular member and air, which together with the optical fibre is in the tubular member. Typically, for outside use, the tubes are filled with a conventional thixotropic water barrier gel.

The cable 10 includes a central fibre-reinforced plastics member 12. The fibrereinforced member, which is conventional, serves to prevent buckling of the cable and to strengthen the cable, and is made of glass filaments in a plastics matrix.

Disposed around the fibre reinforced member 12 are two unscreened twisted pairs 14 and four optical fibres 18. The two unscreened twisted pairs 14 are disposed opposite one another on opposite sides of the fibre-reinforced plastics member 12.

The four optical fibres 18 disposed in tubular members 22, are spaced around the central fibre-reinforced plastics member. Each optical fibre 18 containing tubular member 22 is disposed closest to one of the unscreened twisted pairs 14 and furthest from a second unscreened twisted pair 14. The distances between each optical fibre and its closest and furthest unscreened twisted pairs 14 are substantially the same for all of the optical fibres 18.

The unscreened twisted pairs are typically copper, but may also be silver or aluminium and form part of a communications circuit which, in use, carries data.

Also provided around the fibre reinforced member are two steel strengthening members 16. The two steel strengthening members are disposed opposite one another on opposite sides of the central fibre-reinforced plastics member 12 and on adjacent sides from the unscreened twisted pairs 14. The steel strengthening members and the unscreened twisted pairs are the metallic elements in the cable. In a clockwise direction around the central fibre-reinforced plastics member 12, starting from the"12 O'clock"position, there are disposed the first unscreened twisted pair

14, the first steel strengthening member 16, the second unscreened twisted pair 14 and the second steel strengthening member 16. The distances between the centres of the pairs of neighbouring metallic elements 14,16 are substantially equal. The optical fibres 18 are arranged so that one optical fibre is disposed between neighbouring metallic elements. Preferably, the tubular members 22 containing optical fibres 18 are disposed mid-way between the centres of neighbouring metallic elements 14,16.

The optical fibres provide a dual function. Firstly, they shield the unscreened twisted pairs from crosstalk. Secondly, they provide an upgrade option. Should further bandwidth be required, subsequent to the commissioning of the twisted pairs, the optical fibres may be commissioned.

The two types of crosstalk experienced in high speed transmission over metallic wires are far end cross talk (FEXT) which is a cumulative effect which builds with increasing transmission distance, and near end cross talk (NEXT), which is due to a launching effect of the signal onto the wire (and which does not increase with increasing transmission distance).

The cable 10 is constructed so that the cable components are as closely packed as possible, without the risk of damaging the data carrying cable components (i. e. the metallic circuits and the optical fibre carrying tubes), so that the transverse dimensions of the cable 10 are minimised. Typical diameters and dielectric constants for the components of the cable shown in Figure 1, are: each unscreened wire has a diameter of approximately 0. 5mm, thus a pair has a"diameter"of approximately 1mm. The fibre reinforced plastic member has a diameter typically of 2-3mm. The diameter of the steel strenghthening members are chosen to match those approximately of the other peripheral components, ie. the tubular members and the unscreened twisted pairs. Thus, in the embodiment shown the diameter of the steel strengthening members is between 1-2mm. By selecting the diameter of the strengthening members, the overall cross section of the cable is maintained as near to a circle as possible, it being known that cables having a circular transverse cross section perform relatively well under environmental loading.

The dielectric constant of the fibre reinforced plastic member is low. The optical fibres typically each have a diameter of 0. 25mm. The dielectric constant of optical fibre glass is the same as that of silica, and is thus low.

The overall transverse cross-sectional area of the cable 10 is preferably less than 110mm2.

The transverse cross sectional shape of the cable is preferably circular, but may also be oval depending on how much filler 11 is included. If a more robust cable is required, more filler may be added. If on the other hand, a lighter cable is required, less filler may be added.

The factors that dictate whether a fibre reinforced plastic antibuckling member and/or steel strengthening members are used are numerous and include relative cost, weight and cable dimensions. When installed a cable is subject to stress and strain. Steel is a good strengthening member because it is cheap and flexible and has a high Young's modulus. On the other hand, steel does not elastically deform under sustained stress and over time, the steel member will stretch causing the cable to sag. Steel also suffers from the disadvantage that it is heavy.

Fibre reinforced plastic on the other hand is lighter and deforms elastically. However, FRP suffers from the disadvantages that it is expensive, and it is not as strong as steel. In order to provide the same strengthening effect as a corresponding steel member, a larger transverse cross section of FRP material is required.

Figure 2 shows a second embodiment of the present invention. The cable 30 shown in Figure 2 comprises four unscreened twisted pairs 14. The four unscreened twisted pairs are disposed around a central fibre optic element 32. Also provided are four further fibre optic elements 32. Each fibre optic element 32 includes four optical fibres 34 loosely disposed in a common sheath. Preferably, the sheath is made from a UV resistant polyethylene material. Also provided within the sheath is a filler material to prevent the four optical fibres from coming into contact with each other and to prevent the passage of water through the cable. Typically, the filler is a conventional thixotropic water barrier gel. The external diameter of the sheath is approximately 23mm. The mean dielectric constant of the fibre optic element 32 is determined by the relative proportion of optical fibre, sheath material and filler material.

The first of the optic fibre elements 32 is disposed centrally within the cable 30. The four remaining fibre optic elements 32 are disposed around the central fibre optic element 32, so that the centres of neighbouring fibre optical elements surrounding the central fibre optic elements are approximately equidistant. The outer surfaces of the sheaths of each of the surrounding fibre optic elements may either touch or be in close proximity to the outer surface of the sheath of the central optical fibre element 32.

The four unscreened twisted pairs 14 are disposed between the surrounding optical fibre elements. The unscreened twisted pairs 14 are disposed so that they lie approximately midway between the centres of neighbouring surrounding optical fibre elements.

The optical fibre elements 32 and unscreened twisted pairs 14 are embedded in high density polypropylene, the preferred filler material 11.

In the arrangement shown in Figure 2, a strength member 31 is provided. The strength member is distributed around the outside of the cable and is preferably a kevlar braid. With this arrangement neither the steel strengthening members nor the fibre reinforced plastic are necessary, as the braided cable is robust enough without requiring further strengthening.

Figure 3 shows an alternative, preferable, configuration for the optical fibre elements.

The alternative shown in Figure 3 is interchangeable with those optical fibre elements 32 shown in Figure 2.

The optical fibre elements 33 shown in Figure 3 include four optical fibres 34 disposed symmetrically about and equidistant from the longitudinal axis of the element. The fibres 34 are positioned so that their secondary coating just touch. The fibres are held in a soft buffer layer 36 which has an overall diameter of about 760 micrometres. In the example shown the buffer layer comprises a silicon-acrylate, Cablelite TM 950-701 (available rom DSM Desotech, The Netherlands). About this buffer layer 36 there is a further resin layer 37 which is a tough layer which serves to protect the buffer layer 36 and fibres 34 from mechanical and chemical attack. In this example, layer 37 comprises Cablelite 950-705, a urethane-acrylate resin, and is about 50 micrometres thick. The outer diameter of optical fibre elements 33 is

approximately 0. 9-0. 95mm. Thus, the diameter of the optical fibre elements 33 is smaller than those shown in Figure 2. The optical fibre elements 33 differ from those disclosed in our European patent, EP B1-0 521 710 only in that those shown in Figure 3 do not have glass microspheres embedded in their outer resin surface.

The fibre optic elements shown in Figures 2 and 3 are hereinafter referred to as sheathed quad optical fibre elements. The sheathed quad optical fibre elements of Figures 2 and 3 have the advantages that they provide a shielding effect to the unscreened twisted pairs, and that the cable has increased potential bandwidth available over a total of twenty fibres.

Figure 4 shows a third embodiment of the present invention. The embodiment shown in Figure 4 is similar to Figure 2, however, two of the unscreened twisted pairs have been replaced with steel strengthening members 16 to give additional strength, and the braided keviar member has been removed. These steel members 16 are the same as those described above with reference to Figure 1. The arrangement in the embodiment shown in Figure 4 differs however, as the diameter of the steel strengthening members can vary from cable to cable, and may be greater than the diameter of the unscreened twisted pairs. In order to provide the most compact cable arrangement, the sheathed quad optical fibre elements 32 are arranged as described above with reference to Figure 2. The unscreened twisted pairs 14 are disposed opposite one another on opposing sides of the central fibre optic element 32. They are further disposed between neighbouring surrounding fibre optic elements. The unscreened twisted pairs are disposed so that their outer surfaces are in close proximity to the outer sheaths of the neighbouring surrounding optic fibre elements 32. The steel strengthening members 16 are similarly disposed adjacent to one another on opposing sides of the central fibre optic element 32 between the neighbouring surrounding fibre optic elements that the unscreened twisted pairs 14 are not disposed between, so that between each neighbouring optic fibre element there is disposed either an unscreened twisted pair 14 or a steel strengthening member 16. The steel strengthening members 16 are disposed so that their outer

surfaces come into close proximity with the outer surfaces of the fibre optic elements 32 between which each steel strengthening member is disposed. Figure 5 shows a fifth embodiment of the present invention. The embodiment shown in Figure 5 is similar to that shown in Figure 2. However, the optical fibre elements 33 are the alternative shown in Figure 3. Three of the sheathed quad optical fibre elements 33 are held in a 3 x 1 matrix by two lengths of tape 52. The central fibre optic element 33 together with two adjacent surrounding fibre optic elements 33 are held together by the tape to form a ribbon structure 52 having dimensions of one optic fibre element 33 by three optic fibre elements 33. The two remaining fibre optic elements 33 are opposite one another and disposed one on either side of the central fibre optic element held in the ribbon structure 52. The four unscreened twisted pairs 14 are disposed approximately equidistant from one another, each one lying between one sheathed quad fibre optic element 33 and one ribboned fibre optic element 52.

The sheathed quad elements 34 may either be encapsulated within a single length of tape, edge bonded or held between two separate lengths of tape. Melanex is one example, of such a commercially available tape. The fibre optic element configured as a ribbon in Figure 5 provides additional robustness and rigidity to the cable structure.

One consequence of this is that the steel members and fibre reinforced plastic members are unnecessary in the arrangement shown in Figure 5. Additional strength is provided by a distributed strength member 31, preferably in the form of a kevlar braid 31. The tape held element 33 is approximately 6-9mm wide and approximately 2-3mm thick. The diameter of the cable 50 slightly exceeds the width of the tape held element 52. If the sheath quad elements are encapsulated within a single length of tape as shown in Figure 5, the taped fibre optic element has a cigar shaped transverse cross section. The opposed straight edges are substantially parallel to one another. There should be sufficient tension in the tape enclosing the sheathed quad fibre optic elements 33 so that when the cable is assembled, the sheathed quad fibre optic elements 33 and metallic wires 14 disposed around the taped fibre optic element do not fall into the interstices between the sheathed quad fibre optic elements contained within the ribbon. Thus, the encapsulated taped fibre optic element provides the further advantage that the metallic elements in the cable

including the metallic wires 14 are maintained at a minimum distance from one another. Thus, because the metallic wires are maintained a minimum distance from one another, a certain level of shielding is maintained. Also, with the taped arrangement, the cable designer will know with certainty the spacing of the metallic wires in the finished cable. This increases the amount of control the designer has over the composition within the cable.

Figure 6 shows a fifth embodiment of the present invention. The embodiment shown in Figure 6 is similar to that shown Figure 5, however, in order to provide alternative strength to the cable 60, the distributed strength member 31 has been removed. Instead, two of the unscreened twisted pairs have been replaced with steel strengthening members 16. The steel members 16 are the same as those described above with reference to Figure 1. The steel members 16 and unscreened twisted pairs 14 are arranged in the manner described in connection with Figure 4. In particular, in order to provide the most compact cable arrangement, the unscreened twisted pairs 14 are disposed opposite one another on opposing sides of the central fibre optic element 33. They are further disposed between neighbouring surrounding fibre optic elements. The unscreened twisted pairs 14 are disposed so that their outer surfaces either come into contact with or are in close proximity to the outer sheaths of the neighbouring surrounding optic fibre elements 52. The steel strengthening members 16 are similarly disposed adjacent to one another on opposing sides of the central fibre optic element 33 between the neighbouring surrounding fibre optic elements that the unscreened twisted pairs 14 are not disposed between, so that between each neighbouring optic fibre element there is disposed either an unscreened twisted pair 14 or a steel strengthening member 16. The steel strengthening members 16 are disposed so that their outer surfaces either touch or come into close proximity with the outer surfaces of the fibre optic elements 33,52 between which each steel strengthening member is disposed.

Figure 7 shows a sixth embodiment of the present invention. The cable 70 shown in Figure 7 is similar to that shown in Figure 5, however, the fibre optic taped element 72 comprises an array of 4 sheathed quad elements by 1 sheathed quad element, rather than 3 sheathed quad elements by 1 sheathed quad element. The taped

element 72 is approximately 8-12mm in width and approximately 2-3mm thick. This arrangement provides further potential available optical bandwidth. The ribbon element 72 has two possible central optic fibre elements on either opposing sides of which two further optic fibre elements are disposed. Thus, the optic fibre elements are disposed in an arrangement having a cross shaped transverse cross section. The cable 70 shown in Figure 7 further includes four unscreened wire pairs, which are disposed approximately equidistant from one another around the central fibre optic element, so that neighbouring surrounding fibre optic elements are disposed on either side of each unscreened wire pair. A distributed strength member 31 is provided around the outer circumference of the cable 70.

The cable 70 has a teardrop transverse cross section. It has been found that such a cross section is particularly suitable for aerial use. However, the the installed cable is twisted around its central axis. This avoids aeolian vibrations that may occur, which cause the cable 70 to flap (or to"gallop") when subject to environmental loading.

Figure 8 shows an seventh embodiment of the present invention. The cable 80 shown in Figure 8 is similar to that shown in Figure 7. However, strength is provided to the cable 80 by replacing two of the unscreened twisted pairs with steel strength members 16. The steel members 16 and unscreened twisted pairs 14 are arranged in the manner described in connection with Figures 4 and 6. In particular, in order to provide the most compact cable arrangement, the unscreened twisted pairs 14 are disposed adjacent to one another on opposing sides of the central fibre optic element 33. They are further disposed between neighbouring surrounding fibre optic elements. The unscreened twisted pairs 14 are disposed so that their outer surfaces either come into contact with or are in close proximity to the outer sheaths of the neighbouring surrounding optic fibre elements 72. The steel strengthening members 16 are similarly disposed adjacent to one another on opposing sides of the central fibre optic element 72,33 between the neighbouring surrounding fibre optic elements that the unscreened twisted pairs 14 are not disposed between, so that between each neighbouring optic fibre element there is disposed either an unscreened twisted pair 14 or a steel strengthening member 16. The steel strengthening members 16 are disposed so that their outer surfaces either touch or come into close proximity with

the outer surfaces of the fibre optic elements 33, 52 between which each steel strengthening member is disposed.

Further the distributed strength member 31 is omitted.

In the embodiments described above, the metallic wires for carrying data are unscreened twisted pairs. The invention is not restricted to unscreened twisted pairs, and encompasses screened twisted pairs. Unscreened twisted pairs are preferred, however, because they are cheaper and easier to couple to than screened twisted pairs. Screened twisted pairs have reduced electromagnetic emissions due to a shielding effect provided by the braiding that surrounds the data carrying wires.

Thus, braiding reduces the chance of cross talk occurring. However, screened twisted pairs have the disadvantage that they are more expensive and more difficult to couple to than unscreened twisted pairs. Further, it will be appreciated that the twisted pair count is not restricted to either two or four pairs, but cables according to the present invention may include more than four pairs.

The cables previously described are assembled using conventional techniques. The cable components are assembled in a single process. Drums carrying each of the cable components feed each of the components into a common extruder at an appropriate tension for each component. For example, optic fibre elements are fed at a lower tension than metallic wire components. The filler 11 is added at the same time as the other components are fed. The cable is provided with a temperature and abrasion resistant sheath of, for example, polyimide. The sheath may also be made from polyetherimid, which in addition to temperature and abrasion resistance, provides a sheath having added flexibility. Other considerations, such as flammability, toxicity of smoke produced, UV insensitivity etc may also need to be taken into account for some applications and an appropriate jacket selected.

Figure 9 shows one possible application and installation of cable 10 according to the present invention. In the example shown the cable 10 is for a telecommunications application. The cable is installed as an aerial drop cable running overground to a telegraph pole to the ground. The metallic wires are fed from a first underground tube assembly 91 through a duct 94 within (or secured to) the telegraph pole 90 to a box

92, which houses insulation displacement connectors to a first manifold 95. The optical fibre elements, similarly, are fed from a second underground tube assembly 93 through a duct 96 within (or secured to) the telegraph pole to a second manifold 97. The manifolds 95,97 include chambers having a number of inputs and outputs which distribute the metallic and optical fibre elements appropriately. In the case of metallic wires, these are distributed to the appropriate insulation displacement connectors. In the case of optic fibre elements, these are distributed to the duct 96.

The cable 10 is installed at the telegraph pole 90 by breaking out the metallic wires 14 and optic fibre elements 18 separately, and connecting them to their respective manifolds 95,97.

Also provided, although not shown in the figure, are conventional securing means for attaching the cable to the top of the telegraph pole. Such attachment means are, for example, disclosed in our own International publication WO 90/07138.

Figure 10 shows the termination of the cable 10 at the customer's premises. The metallic wires 14 and optical fibres are broken out from the cable in which they are disposed at the customer's premises. The cable is fed to the customer's premises through, for example, a hole in their wall 112. The cable is fed into an OTIAN lead in box 111. The cable 10 is fed into network termination equipment 102 including, preferably, within the NTE housing an area 108 where the metallic circuits 14 are broken out of the cable and terminated at the NTE 102. The network termination equipment distributes metallic wires to those pieces of equipment requiring metallic wire connection. This may include equipment such as a telephone 105 or a modem 107.

The optical fibres are also broken out of the cable in area 108 and fed into a duct 98.

If the ducted fibres are to be fed a distance greater than approximately 2 metres, the fibres are coated in a flame retardant sheath. The duct 98 may typically be coated in polytetrafluoroethylene (PTFE). The duct 98 leads to a customer splicing point 100.

At the customer splicing point the fibre optic elements are spliced to other fibre optic elements which serve equipment. Ruggardised optical fibre tails with connectors exit the customer splicing point and connect with the equipment. The types of equipment that may be served by fibre optic elements include personal computers 101 and televisions 103.

The cable of the present invention is also suitable for any application where high bit rate transfer is likely to occur, for example, greater than 512kbit/s over metallic wires where cross talk may be a potential problem.

Claims (9)

1. A communications cable including twisted metallic conductor pairs for carrying data and a plurality of dielectric elements, said plurality of dielectric elements including a plurality of optical fibre elements for carrying data, each of said optical fibre elements include a sheath in which at least one optical fibre is disposed, the twisted pairs and the optical fibre elements being disposed in an annular arrangement around a longitudinal axis of the cable, neighbouring conductor pairs being separated by one of said optical fibre elements, wherein a further dielectric member is disposed in the region enclosed by the annular arrangement, the arrangement of the twisted pairs and the dielectric elements being such that the twisted pairs are screened from each other.

2. A communications cable for aerial use including a non-metallic member, a plurality of metallic elements, the plurality of metallic elements including at least two twisted metallic wire pairs for carrying data, and optical fibre elements each of which includes a sheath in which one or more optical fibres are disposed, wherein said twisted pairs and said optical fibre elements are disposed about said non-metallic member, the arrangement being such that each metallic element is separated from its neighbouring metallic element by one of said optical fibre elements.

3. A cable according to claim 1, wherein said further dielectric member is either an optical fibre element or a polymeric strength member.

4. A communications cable according to any of claims 1 to 3, wherein the sheath is a tube of dielectric material within which the or each of the at least one optical fibre is loosely contained.

5. A cable according to any of claims 1 to 3, wherein the sheath is a resin material in which the or each of the at least one optical fibre is embedded.

6. A cable according to any one of claims 1 to 5, wherein said optical fibre element includes an array of optical fibre elements held together by a tape.

7. A cable according to any of claims 1 to 5, wherein said optical fibre element has a centre of circular symmetry.

8. A cable according to claim 6, wherein said optical fibre element has a flat ribbon like symmetry.

9. A method of operating a communications route over twisted metal pair carriers in which said twisted metal pair carriers are disposed in a cable, the method including the steps of: providing at least two circuits over respective twisted metallic pair carriers of a cable, the cable including optical fibre elements including a sheath in which at least one optical fibre is disposed which shield the twisted metallic pair carriers of the circuits so as to reduce the crosstalk between the circuits.