By Steven Totolo, President, Total Voice Control

Properties of new wires

AC current flowing through household wiring generates a magnetic field. When this field comes near another conductor, current flow is induced in the wire. With twisted cables, the magnetic field will cause the current to flow in one direction before the twist and the reverse direction after the twist. Therefore, the current induced into the cable will cancel out.(Figure 3)

Standard telephone cabling is constructed with either 2,3, or 4 insulated conductors gathered together and jacketed. The wires are randomly placed in the jacket causing conductors to parallel each other or sometimes, become twisted. This is known as Category 1 (CAT1) wire or Plain Old Telephone Service (POTS) wire.

 Typical CAT3 wire has 8 conductors in 4 pairs, with each pair twisted together at 0.8 to 2.4 twists per cm of wire. It has a usable data bandwidth of 10 MHz and is most commonly used for low-speed data applications, such as computer modems and telephony. Despite the low cost for CAT3, its demand is dwindling because price differentials between it and the superior CAT5 are diminishing.

CAT5 wire has 4 pairs with each pair twisted together at least 1 twist per cm, resulting in a bandwidth of 100 MHz. The amount of twists per cm is proportional to the amount of noise rejection. In simple terms, more twists per cm result in better noise rejection and a larger bandwidth. 

There are CAT2 and CAT4 cables, but they are not widely used. CAT5E (enhanced CAT 5 100 MHz bandwidth), CAT6 (250 MHz bandwidth), and CAT7 (600 MHz bandwidth -proposed) are currently beyond home requirements and their higher cost are unwarranted.

RG-6 is an improved version of the standard television cable RG-59. It has a slightly larger diameter, but the reduction in the signal level at high frequencies is less than the RG-59. This means that cable channels at the higher frequencies (i.e. channels 70-125) will not attenuate as much. Consequently, satellite television dish signals will travel further using RG-6.

Fibre optic cable has glass or plastic strands that are used to carry the signal. Fibre optics use light pulses instead of electrical signals for transmitting information, eliminating the concern about electromagnetic interference. More information can be transmitted using fibre optic cable since the light pulse levels degrade less over distances as compared to copper cables. This results in a bandwidth greater than 500 MHz per kilometer.

Fibre optic cables consist of a glass or plastic core and cladding surrounded by a protective coating. The core and cladding are part of the same glass or plastic rod, but have different optical properties. Light pulses are injected into the core. As light pulses travel down the cable, the cladding, acting as a mirror, reflects the pulse back to the center of the core. A plastic protective coating, called a buffer, surrounds the core and cladding. Figure 5 shows the construction of a fibre optic cable.

There are two basic types of fibre optic cables: multimode and single-mode. Multimode fibres provide a number of paths for light pulses to follow as they traverse a cable. Single-mode has only one path. The wavelength of the light source and size of the core determine the number of modes. Multimode fibre has a core width of 62.5 microns (um). Single-mode fibre has an 8.3 um core. By comparison, the average human hair is 80 um thick.

Structured wiring is a new technology created for the home automation and home networking markets. The term "structured" is derived from the construction of a cable assembly of several cable types. The bundle is wrapped with a spiral nylon rope or encased in a material such as polypropylene. The assembly is comprised of two RG-6 and two CAT5 cables. Some companies also incorporate two 62-um fibre cables. A uniquely different colour sheaving identifies each cable. One company colours the RG6 black and pink, the CAT5 lines blue and yellow and the fibre cables orange and gray. This package enables the installation of up to 6 cables with a single cable run, reducing installation time. Since the bundle is approximately 1.6 cm in diameter, installing the cable in tight locations can be difficult. This is especially true for structured wires that have overall sheaving since the diameter and stiffness of the bundle increase.

Part 1 - Traditional household cabling
Index
Part 3 - Installation
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Steven Totolo is president of tvcAutomation, a home automation specialist and a member of the CABA Standards Committee. He can be reached at (613) 795-7117; fax (613) 737-5323; email: sales@tvcAutomation.com