Networks use copper media because it is inexpensive, easy to install, and has low resistance to electrical current. However, copper media is limited by distance and signal interference.
Data is transmitted on copper cables as electrical pulses. A detector in the network interface of a destination device must receive a signal that can be successfully decoded to match the signal sent. However, the longer the signal travels, the more it deteriorates in a phenomenon referred to as signal attenuation. For this reason, all copper media must follow strict distance limitations as specified by the guiding standards.
The timing and voltage values of the electrical pulses are also susceptible to interference from two sources:
To counter the negative effects of EMI and RFI, some types of copper cables are wrapped in metallic shielding and require proper grounding connections.
To counter the negative effects of crosstalk, some types of copper cables have opposing circuit wire pairs twisted together which effectively cancels the crosstalk.
The susceptibility of copper cables to electronic noise can also be limited by:
There are three main types of copper media used in networking:
|Coaxial cable||Unshielded twisted-pair||Shielded Twisted-pair|
These cables are used to interconnect nodes on a LAN and infrastructure devices such as switches, routers, and wireless access points. Each type of connection and the accompanying devices have cabling requirements stipulated by physical layer standards.
Different physical layer standards specify the use of different connectors. These standards specify the mechanical dimensions of the connectors and the acceptable electrical properties of each type. Networking media use modular jacks and plugs to provide easy connection and disconnection. Also, a single type of physical connector may be used for multiple types of connections. For example, the RJ-45 connector is widely used in LANs with one type of media and in some WANs with another media type.
Unshielded twisted-pair (UTP) cabling is the most common networking media. UTP cabling, terminated with RJ-45 connectors, is used for interconnecting network hosts with intermediate networking devices, such as switches and routers.
In LANs, UTP cable consists of four pairs of color-coded wires that have been twisted together and then encased in a flexible plastic sheath which protects from minor physical damage. The twisting of wires helps protect against signal interference from other wires.
As seen in the figure, the color codes identify the individual pairs and wires in the pairs and aid in cable termination.
Shielded twisted-pair (STP) provides better noise protection than UTP cabling. However, compared to UTP cable, STP cable is significantly more expensive and difficult to install. Like UTP cable, STP uses an RJ-45 connector.
STP cable combines the techniques of shielding to counter EMI and RFI and wire twisting to counter crosstalk. To gain the full benefit of the shielding, STP cables are terminated with special shielded STP data connectors. If the cable is improperly grounded, the shield may act like an antenna and pick up unwanted signals.
Different types of STP cables with different characteristics are available. However, there are two common variations of STP:
The STP cable shown uses four pairs of wires, each wrapped in a foil shield, which are then wrapped in an overall metallic braid or foil.
For many years, STP was the cabling structure specified for use in Token Ring network installations. With the decline of Token Ring the demand for shielded twisted-pair cabling also waned. However, the new 10 GB standard for Ethernet has a provision for the use of STP cabling which is providing a renewed interest in shielded twisted-pair cabling.
Coaxial cable, or coax for short, gets its name from the fact that there are two conductors that share the same axis. As shown in the figure, coaxial cable consists of:
Note: There are different types of connectors used with coax cable.
Coaxial cable was traditionally used in cable television capable of transmitting in one direction. It was also used extensively in early Ethernet installations.
Although UTP cable has essentially replaced coaxial cable in modern Ethernet installations, the coaxial cable design has been adapted for use in:
All three types of copper media are susceptible to fire and electrical hazards.
Fire hazards exist since cable insulation and sheaths may be flammable or produce toxic fumes when heated or burned. Building authorities or organizations may stipulate related safety standards for cabling and hardware installations.
Electrical hazards are a potential problem since the copper wires could conduct electricity in undesirable ways. This could subject personnel and equipment to a range of electrical hazards. For example, a defective network device could conduct currents to the chassis of other network devices. Additionally, network cabling could present undesirable voltage levels when used to connect devices that have power sources with different ground potentials. Such situations are possible when copper cabling is used to connect networks in different buildings or on different floors of buildings that use different power facilities. Finally, copper cabling may conduct voltages caused by lightning strikes to network devices.
The result of undesirable voltages and currents can include damage to network devices and connected computers, or injury to personnel. It is important that copper cabling be installed appropriately, and according to the relevant specifications and building codes, in order to avoid potentially dangerous and damaging situations.
|Separate of data and power cabling||Connect correctly||Inspect damage||Ground equipment|