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Open wire cable has been replaced by microwave systems or newer technologies.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.
The open overhead wires servicing users from the telephone centres 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.
UTP has the advantages of:
STP has the advantages of:
Some years ago, a requirement to interconnect telephone centres 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.
Coaxial cable became popular in the 1980s 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 with a shared cable system.
As open-wire gave way to better systems, so the coaxial cable has given way to others. Today, it is being replaced by microwave, satellite or fibre-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:
The 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 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:
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 for 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 transcontinental and trans-oceanic telecommunications cables.
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.
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.
The 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 the 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.