|  | 4.2 |
| Signals and Noise
|  | 4.2.1 |
| Signaling over copper and fiber optic cabling
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. 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. 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. 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.
Web Links
Fiber Optic Page http://www.eco.utexas.edu/faculty/Norman/ long.extra/Student.F98/ modem/FourthRight.html