Selecting cable for a particular purpose has always been a head-breaking job. But, with proper information it is not so painful job as one imagines
Cables have always been the backbone for effective communication. But, this effective communication comes at one uncompromising condition-proper cable lay out. Improper establishment of cables could lead to erratic and fallacious results. Along with it, less knowledge about cables could lead to total disastrous situation. Hence, an idea about cable type and its implementation is necessary while cable is laid out.
The information given below will help to select right type of cable and its implementation in proper manner.
Basically, there are three types of cables viz., Twisted pair (UTP, STP), Coaxial cable and fibre optic cable.
Twisted pair (UTP, STP)
Twisted pair cables are available in two varieties: shielded and unshielded. Unshielded twisted pair (UTP) is the most popular type of cable. Each pair is twisted with a different number of twists per inch to help reduce (and if possible-eliminates) interference from adjacent pairs and other electrical devices.
Shielded Twisted Pair (STP) Cable
A disadvantage of UTP is that it may be vulnerable to radio and electrical frequency interference. In such cases, Shielded twisted pair (STP) should be used; however, the extra shielding can make the cables quite heavy. Shielded twisted pair is often used on networks using Token Ring topology.
Coaxial cabling has a single copper conductor at its center. A plastic layer on surface of copper wire provides insulation between the center conductor and a braided metal shield. The metal shield helps to block any outside interference from fluorescent lights, motors, and other computers. Even though coaxial cabling is hard to install, it is highly resistant to signal interference. Also, it can support greater cable lengths between network devices than twisted pair cable. The two types of coaxial cabling are thick coaxial and thin coaxial.
There are different kinds of signal loss in a cable. But, the main factors leading to signal loss are as follows:
· Near-End Crosstalk (NEXT)
Resistance (caused by head-on collision of randomly moving electrons) of a cable is one of the main reason for signal-loss. For instance, if the resistance of cable is too high, no signal will reach other end. Therefore, high resistivity of the cable can cause reduction in signal transmission leading to data loss.
Capacitance in a cable is another factor responsible for data-loss. The signal loss due to capacitance can be explained by considering a coaxial cable.
The shield of a coaxial coil consists of interweaved or foil shield or both. The current passing along the core induces signal on the shield, and current flowing through the shield induces signal on the core. In simple words, the negative charges on the shield attract positive charges on the core, giving rise to electrical interaction. This interaction causes signal loss.
Attenuation is a loss caused by the +skin effect+ and +di-electric loss+.
Skin effect is a phenomenon in which the signal flowing through a conductor will be conducted only on the outer surface (skin) of the wire as the frequency increases. At lower frequencies the current flows through the entire cross section of the conductor. As the frequency increases, the current only flows nearer the outer surface of the conductor.
Di-electric loss is another reason for attenuation, caused by the jacket. Ideally, a jacket should be completely electrical inactive, but that is not the case. PVC will have some charge present on its surface. This is electrical activeness has its effect on the signal flowing through the cable and causes some loss.
Near-End Crosstalk (NEXT)
NEXT is the energy induced by one copper wire (or pair) on to the other giving rise to +noise formation+. This usually takes place in UTP and STP cables.
Fiber Optic Cable
Fiber optic cabling comprises of a center glass core surrounded by several layers of protective materials. Fibre optics cables make use of light rather than electronic signals, thus eliminating the problem of electrical interference, moisture and lighting.
1. Fiber optic cable has the potential to transmit signals over longer distances than coaxial and twisted pair.
2. It also has the potential to carry information at a very fast speed.
3. This ability broadens communication possibilities to include services such as video conferencing and interactive services.
4. The cost of fiber optic cabling is comparable to copper cabling; however, it is more difficult to install.
|Posted : 12/5/2005|