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4.2 | |
Signals and Noise | |
| 4.2.1 | |
Signaling over copper and fiber optic cabling |
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On copper cable, data signals are
represented by voltage levels that represent binary ones and zeros.
The voltage levels are measured with respect to a reference level of
zero volts at both the transmitter and the receiver. This reference
level is called the signal ground. It is important that both
transmitting and receiving devices refer to the same zero volt
reference point. When they do, they are said to be properly grounded.
In order for the LAN to operate properly, the receiving device must be able to accurately interpret the binary ones and zeros transmitted as voltage levels. Since current Ethernet technology supports data rates of billions of bits per second, each bit must be recognized, even though duration of the bit is very small. The voltage level cannot be amplified at the receiver, nor can the bit duration be extended in order to recognize the data. This means that as much of the original signal strength must be retained, as the signal moves through the cable and passes through the connectors. In anticipation of ever-faster Ethernet protocols, new cable installations should be made with the best available cable, connectors, and interconnect devices such as punch-down blocks and patch panels. There are two basic types of copper cable: shielded and unshielded. In shielded cable, shielding material protects the data signal from external sources of noise and from noise generated by electrical signals within the cable. Coaxial cable is a type of shielded
cable. There are two types of twisted-pair
cable: shielded twisted-pair (STP) and unshielded twisted pair (UTP).
STP cable contains an outer conductive shield that is electrically grounded to insulate the signals from external electrical noise. STP also uses inner foil shields to protect each wire pair from noise generated by the other pairs. STP cable is sometimes called screened twisted pair (ScTP). STP cable is more expensive, more difficult to install, and less frequently used than UTP. UTP contains no shielding and is more susceptible to external noise but is the most frequently used because it is inexpensive and easier to install. Fiber optic cable is used to transmit
data signals by increasing and decreasing the intensity of light to
represent binary ones and zeros.
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It
consists of a solid copper conductor surrounded by insulating
material, and then braided conductive shielding. In LAN applications,
the braided shielding is electrically grounded to protect the inner
conductor from external electrical noise. The shielding also helps
eliminate signal loss by keeping the transmitted signal confined to
the cable. This helps make coaxial cable less noisy than other types
of copper cabling, but also makes it more expensive. The need to
ground the shielding and the bulky size of coaxial cable make it more
difficult to install than other copper cabling. 
The
strength of a light signal does not diminish like the strength of an
electrical signal does over an identical run length. Optical signals
are not affected by electrical noise, and optical fiber does not need
to be grounded. Therefore, optical fiber is often used between
buildings and between floors within the building. As costs decrease
and demand for speed increases, optical fiber may become a more
commonly used LAN media. 
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