Characteristics of Copper Media
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:
- Electromagnetic interference (EMI) or radio frequency interference (RFI) – EMI and RFI signals can distort and corrupt the data signals being carried by copper media. Potential sources of EMI and RFI include radio waves and electromagnetic devices such as fluorescent lights or electric motors as shown in the figure.
- Crosstalk – Crosstalk is a disturbance caused by the electric or magnetic fields of a signal on one wire to the signal in an adjacent wire. In telephone circuits, crosstalk can result in hearing part of another voice conversation from an adjacent circuit. Specifically, when electrical current flows through a wire, it creates a small, circular magnetic field around the wire which can be picked up by an adjacent wire.
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:
- Selecting the cable type or category most suited to a given networking environment.
- Designing a cable infrastructure to avoid known and potential sources of interference in the building structure.
- Using cabling techniques that include the proper handling and termination of the cables.
There are three main types of copper media used in networking:
- Unshielded Twisted-Pair (UTP)
- Shielded Twisted-Pair (STP)
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:
- STP cable shields the entire bundle of wires with foil eliminating virtually all interference (more common).
- STP cable shields the entire bundle of wires as well as the individual wire pairs with foil eliminating all interference.
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:
- A copper conductor used to transmit the electronic signals.
- The copper conductor is surrounded by a layer of flexible plastic insulation.
- The insulating material is surrounded in a woven copper braid, or metallic foil, that acts as the second wire in the circuit and as a shield for the inner conductor. This second layer, or shield, also reduces the amount of outside electromagnetic interference.
- The entire cable is covered with a cable jacket to protect it from minor physical damage.
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:
- Wireless installations: Coaxial cables attach antennas to wireless devices. The coaxial cable carries radio frequency (RF) energy between the antennas and the radio equipment.
- Cable Internet installations: Cable service providers are currently converting their one-way systems to two-way systems to provide Internet connectivity to their customers. To provide these services, portions of the coaxial cable and supporting amplification elements are replaced with fiber-optic cable. However, the final connection to the customer’s location and the wiring inside the customer’s premises is still coax cable. This combined use of fiber and coax is referred to as hybrid fiber coax (HFC).
Copper Media Safety
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