Signals over copper and fiber optic cables (Core)

Signals over copper and fiber optic cables (Core)
4.2.1 This page discusses signals over copper and fiber optic cables.


On copper cable, data signals are represented by voltage levels that represent binary ones and zeros. The voltage levels are measured based on a reference level of 0 volts at both the transmitter and the receiver. This reference level is called the signal ground. It is important for devices that transmit and receive data to have the same 0-volt reference point. When they do, they are said to be properly grounded.

For a LAN to operate properly, the devices that receive data 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 bps, each bit must be recognized and the duration of each bit is very small. This means that as much of the original signal strength as possible must be retained, as the signal moves through the cable and passes through the connectors. In anticipation of faster Ethernet protocols, new cable installations should be made with the best cable, connectors, and interconnect devices such as punch-down blocks and patch panels.

The two basic types of copper cable are 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 a braided conductive shield. In LAN applications, the braided shielding is electrically grounded to protect the inner conductor from external electrical noise. The shield also keeps the transmitted signal confined to the cable, which reduces signal loss. 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.

Two types of twisted-pair cable are 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) in error. ScTP generally refers to Category 5 or Category 5e twisted pair cabling, while STP refers to an IBM specific cable containing only two pairs of conductors. ScTP 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 increases and decreases the intensity of light to represent binary ones and zeros in data transmissions. The strength of a light signal does not diminish as much as 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 unless the jacket contains a metal or a metalized strength member. Therefore, optical fiber is often used between buildings and between floors within a building. As costs decrease and speeds increase, optical fiber may become a more commonly used LAN media.

The next page explains the concept of insertion loss.