Cable is essentially types of wires or groups of wires capable of carrying voice and data transmissions.
Metallic wires were used almost exclusively in telecommunications networks for the first 80 years, until the development of microwave and satellite radio communications systems. Copper wire was found to be the most suitable medium for transmission but since the availability and cost reduction in Fibre, this is becoming more and more common, and is now the preferred medium for transmission.
Wire cable includes twisted pair, coaxial, and twin axial. Optical fibre cable includes single, dual, quad, stranded, and ribbon. Copper based cable is still the most common form of transmission medium, and is used for virtually all links except long distance. A cable medium often introduces unwanted changes to the signal, which limits the speed (how fast we can send information) and frequency range of the signals that can be transmitted. Failure to terminate each end of the cable, or a break in the cable, causes the network to fail.
- Twisted pair cable consists of two insulated copper wires twisted around each other to reduce induction, or interference from one wire to the other. Twisted pair cable is the most commonly used transmission media.
- Category 3 (CAT 3) cable is used mainly as an interior wire for single line instruments e.g. analogue telephones, fax machines, answering machines, etc. This cable type consists of 2-pair or quad wiring – 4 separate copper wires, each covered in plastic insulation, and all four wrapped in an outer plastic covering. Category 3 cable does not have the capacity to carry data at high speeds. CAT 3 is suitable for applications requiring a carrier frequency of up to 16 MHz.
- Category 5 (CAT 5) cable is highly suitable for both voice and data transmission. It is the most popular cable for LAN connectivity and can support applications requiring a carrier frequency of up to 100 MHz. There are essentially two types: Unshielded Twisted Pair (UTP) and Shielded Twisted Pair (STP).
- Unshielded Twisted Pair (UTP) cable is the predominant cable in use today. It is used for telephone wiring inside buildings, as telephone cables to link customer houses and buildings to telephone switching exchanges, and for implementing local area networks.
- Shielded Twisted Pair (STP) cables are manufactured with a metallic foil or shield that is designed to insulate the pair against electromagnetic interference. Shielded twisted pair is often used in high-noise environments, where the cable must be run in proximity to electric motors or other sources of ambient electromagnetic interference which can distort the signal.
- Optical fibre cable is a technology in which light is used to transport information from one point to another. More specifically, fibre optics are thin filaments of glass through which light beams are transmitted over long distances carrying enormous amounts of data. Benefits include: high bandwidth capabilities, relatively low cost, low power consumption, small space requirements, and insensitivity to electromagnetic interference.
Open Wire is a transmission facility typically consisting of pairs of bare (un-insulated) conductors supported on insulators which are mounted on poles to form an aerial (above ground) pole line. This was the most basic of all practical types of transmission media and is seldom used nowadays. It was used extensively in rural areas to provide both communication (12 speech channels) and power over a single wire atop telephone poles.
Open wire cable has been replaced by microwave systems or newer technologies. Being exposed to weather, dust particles and salt spray were deposited on the insulators that secured the cable to the telegraph pole. When storms and wet weather arrived, the dust turned to mud, and provided a path of least resistance for the signal, thus the signal was diverted down the pole to ground and significantly reduced in strength. Lightning strikes were also a hazard and special ceramic arrestors were used to protect equipment attached to open wire lines.
Twisted Pair Cable
The open overhead wires servicing users from the telephone centress were replaced by multi-strand cable, which was buried underground and protected by many sheaths of polyurethane plastic. This prevented water seeping into the cable and affecting the signal. It also helped overcome some of the limitations that open wire systems had. The major problem now is people digging them up!
These multi-strand cables used a pair of wires for each user, and there were about 500 pairs or more per cable. Each wire was twisted around each other wire in order to try and reduce unwanted noise (hence the term twisted pair). Cable was laid by the telephone company, along each street, from the telephone centres to the customer’s house. Thus each customer had a physical set of wires, which ran from their telephone set all the way into the telephone exchange. The use of multi-strand underground cable, still widely used today, enabled the delivery of low-cost telephone services to the general public.
Twisted pair cable consists of two insulated copper wires twisted around each other to reduce induction, or interference from one wire to the other. Twisted pair is considered to be an electrically ‘balanced’ medium. The twisting process contains the electromagnetic field within the pair thus the radiation of electromagnetic energy is reduced and the strength of the signal within the wire is improved over a distance. This is important in high-bandwidth applications, as higher frequency signals tend to attenuate (lose power) more rapidly over distance. As a rule, the more twists per metre, the better the performance of the cable. Several sets of twisted pair wires may be enclosed in a single cable.
Twisted pair cable is still the most common type of transmission media, used to connect telephone subscribers to exchanges (switching centress) and to cable buildings. In general, each twisted pair supports a single voice channel. The thicker the cable is, the better the quality of the conversation, although at a higher cost. Twisted pair is also used to interconnect PC’s on a Local Area Network (LAN).
Twisted pair used in Local Area Networks has several ratings. Category 3 has a speed rating of 10 million bits per second (the speed of Ethernet), whereas category 5 has a speed rating of 100 million bits per second. Twisted pair cable is provided in two forms, UTP (unshielded twisted pair), and STP (shielded twisted pair). Most twisted pair circuits are UTP, which involves no special shielding, just insulation.
Unshielded Twisted Pair Cable (UTP)
Unshielded Twisted Pair cable has no ground shield. UTP cable is the predominant cable in use today. Two conductors are coated with a plastic sheath then twisted around each other. These pairs are then twisted around other pairs to make a multi-pair cable. The twisting of the wires around each other helps to reduce unwanted signals being induced into the wires. It is used for telephone wiring inside buildings, as telephone cables to link customer houses and buildings to telephone switching exchanges, and for implementing local area networks.
UTP has the advantages of:
- …a high installed base
- …cheap to install
- …easy to terminate
Its disadvantages are:
- …poor voice quality
- …limited in distance
- …prone to interference from cell phones
Shielded Twisted Pair Cable (STP)
Shielded Twisted Pair cables are manufactured with a metallic foil or shield that is designed to insulate the pair against electromagnetic interference. This helps to reduce signal interference from external sources, thus making the signal travelling down the cable less prone to alteration. STP looks similar to coaxial cable as the central conductors are insulated and then surrounded by an outer conductor (shield) of steel, copper alloy, or some other metal. The STP shield absorbs the ambient noise, and conducts it to ground, thereby protecting the centre conductor. Shielded twisted pair is often used in high-noise environments, where the cable must be run in proximity to electric motors or other sources of ambient electromagnetic interference which can distort the signal.
STP has the advantages of:
- …high quality transmission
- …reduced interference
Its disadvantages are:
- …more expensive than UTP cable
Some years ago, a requirement to inter-connect telephone centress in different towns emerged. The use of open wire or multi-strand cable for this purpose was unsuitable. The capacity of each cable (number of simultaneous speech conversations per cable) was too low to make it economically viable, hence the introduction of coaxial cable. Until recently, coaxial cable was extensively used to support toll traffic and long distance links.
Coaxial cable is a two-wire conductor with a larger bandwidth than twisted pair cable. It is used mainly in television, radio, and Ethernet LAN’s. In voice communication systems, each coaxial cable supports approximately 60 speech channels. It has a single inner core conductor, which is separated from an outer conductor by an insulator medium, either plastic or mica, that acts as a shield. The signal is transmitted on the inner core. The entire cable is enclosed in polyurethane to protect it and give it some strength, although bending the cable too sharply damages the insulator which separates the inner core from the shield, thus altering the electrical characteristics of the cable.
The transmission qualities of coaxial cable are affected by the properties of the type of insulator used. Coaxial cable may carry up to 1200 speech circuits per cable and operates better than open wire, as it is buried in the ground and not exposed to the elements. The systems used to implement multi-speech channels on coaxial cable are called broadband systems.
Coaxial cable became popular in the 1980’s as a method of interconnecting computers, specifically Local Area Networks (LAN’s), because it was cheap and easy to install. It is still used extensively in networking and data communications. The networking protocol commonly used with coaxial cable is ETHERNET, which describes how data is formatted and transmitted along a shared cable system.
As open wire gave way to better systems, so coaxial cable has given way to others. Today, it is being replaced by microwave, satellite or fiber-optic links. The problem with coaxial cable is the relatively limited number of speech channels available per cable. With the high demands for data communications between computers, increased telephone circuits between cities and countries, not to mention television channels, other mediums have successfully taken over from coaxial cable. Some cable TV systems continue to use coaxial cable to supply programming content to subscribers.
Coaxial cable has the following advantages:
- widely used and cheap to install
- conforms to popular standards
- greater capacity than UTP for voice and data traffic
Its disadvantages are:
- limited transmission distance
- limitations in number of connections
- terminations and connections must be done precisely
Optical fibre cable is a transmission medium consisting of a core of pure silica (glass) or plastic, encased in a protective cladding, strengthening material and outer jacket. Strands of the fibre - thinner than a human hair - are coated with a reflective surface. When fibre is used for transmission purposes, digital signals are converted to modulated light using a light transmitter – either a laser or light emitting diode. Light is shown into the strand, travels along the fibre, and is prevented from escaping by the reflective layer. Light travels in a fibre at approximately 2/3 the speed of light in a vacuum.
There are two types of optical fibre:
Single mode fibre is more efficient. It offers low dispersion, travels great distances without repeaters and has enormous information-carrying capacity. Multimode fibre is no longer the preferred method of optical telecommunications. The relatively large core of multimode fibre allows light pulses to zig-zag along many different paths. It is more suited to light sources larger than lasers, such as LED’s.
Optical fibre is an ideal transmission medium for the following reasons:
- Transmission is high-speed, and data losses are very small.
- Bandwidth is greater than any other transmission medium in existence today.
- Optical fibre is immune to electromagnetic interference. It is safer than copper because it provides electrical isolation, and as it carries no current it can be used in hazardous or flammable environments such as factory floors, elevator shafts and on battleships, etc.
- Fibre does not radiate, unlike copper cable which is subject to distortion and interference. Many strands of fibre can carry large amounts of information in the same bundle and they won’t interfere with each other.
- Each fiber optic strand can support numerous speech channels and multiple TV channels simultaneously.
- The basic raw material for optical fibre – silica (sand) - is the second most abundant element on earth.
- Scientists continue to discover more ways of sending greater amounts of information down a single strand of fibre.
- Fibre optic cable is useless to cable thieves!
The only major disadvantage of optical fibre is its insulating properties. Metallic conductors need to be included to power repeater amplifiers required over long-distance fibre networks. However, these form the strengthening components that are necessary in the laying of fibre cables. Optical fibre is also costly and more difficult to join than conventional copper or coaxial cable. Fibre applications are many, and range from use in local area network sections where there is a high degree of electrical interference, to trans-continental and trans-oceanic telecommunications cables.
“THE GLASS STORY”
Multimode and Single-Mode Optical Fibres
Two optical fibres categories with distinctive operational attributes are multimode and single-mode fibres. Within the multi-mode category, another important characteristic is between step index and graded index. Further definition of fibres relates to physical size, optical performance, coatings and strength.
All fibres consist of a number of substructures including:
- A core, which carries most of the light, surrounded by
- A cladding, which bends the light and confines it to the core, surrounded by
- A substrate layer (in some fibres) of glass which does not carry light, but adds to the diameter and strength of the fiber, covered by
- A primary buffer coating, which provides the first layer of mechanical protection, covered by
- A secondary buffer coating, this protects the relatively fragile primary coating and the underlying fiber.
Single-mode fibres have the very broadest bandwidth, lowest cost and lowest attenuation of any available optical fibre. Therefore, they are universally used in long-distance telephony and cable television applications.
Optical performance of fibres is relatively standardized, in that the same optical characteristics may be found in fibres of the same type produced by several fibre manufacturers.
The physical characteristics of the fibre, however, are not necessarily uniform across the industry.
For instance, the level of proof test determines the inherent strength of the glass, and the ultimate lifetime of the fibre.
The preservation of this strength is the function of the buffer materials and the cable structure. The multiple-layer tight-buffer coating, if fabricated with the proper materials and technology, provides excellent physical, mechanical and environmental protection for each fibre in the cable. It prevents the accumulation of moisture near the glass surface which could cause stress corrosion and ultimate fibre breakage. It also buffers or reduces the sensitivity of the fibre to repetitive small bends, referred to as “micro bends”, which cause an increase in fibre attenuation. The cable structure isolates and protects the fibres from the installation and the installed environment.