Transmission Mediums

A transmission medium guides signal energy in a particular direction. The key different types of transmission mediums include copper wire, coax (a form of copper wire), free space (air), glass fiber, and mechanical (acoustic wave). There are also other types of transmission mediums such as radio waveguides and stripline that use the fundamental principles of the other types of transmission mediums to help direct transmitted energy in a particular direction.

The propagation in free space (a vacuum) is the speed of light 300 million meters per second. In free space, the propagation delay (time from entry to exit) is exactly 11.7 nanoseconds per foot (a nanosecond is one Billionth of a second). Any other medium other than free space will slow the transmission speed (introduce additional delay).

When transmitting energy through a transmission medium, some of the transmitted signal will be absorbed of some of the material. This absorption converts some of the transmitted signal into heat.

As signals transfer through a transmission system, some of the energy may leak out. This leakage (egress) results in a decrease the transmitted signal energy as it propagates from end to end.

Transmission systems have a limit on the frequency range that can be transferred through the transmission channel. Signals that are applied to the transmission medium that are above or below the maximum frequency range are rejected, severely attenuated, or leaked out of the transmission medium.

Copper Wire

Twisted pair copper wire is the most utilized telecommunications medium and thus has the largest installed base in the worldwide telecommunications infrastructure by far. It is relatively inexpensive, easy to install, and, locally available in quantities. Practically every residential telephone worldwide connects to the local telephone company via a twisted pair jack or block located on an inside wall or baseboard in the residence. Because of these factors, intensive research and development (R&D) funding continues to be allocated for the purpose of extending the usefulness of twisted pair copper wire. By enhancing its ability to carry information faster and farther, R&D will continue as long as the resulting enhancements meet, or exceed, many of the requirements placed on the industry by customer demands. Indications are that twisted pair copper wire will remain a logical, cost-effective medium for providing many commercial services for some time to come.

Figure below illustrates the transmission of electrical energy (electricity) via copper wire. Note that the electricity is conducted via the outer surface of the wire, not the inner. Consequently the greater the outer surface the more electricity that can be conducted. In other words, the larger the wire diameter the more outer surface there is to conduct electricity.

Electrical Transmission through Copper Wire

In the telecommunications industry copper wire is normally referred to as twisted pair. Through twisting wire into pairs electrical radiation (eddy fields or cross talk) is reduced. This reduction is both radiation off the pair and the susceptibility to radiation from other pairs and sources. It has been found that the tighter the twist (this is still in R&D) the less interference (cross talk) and the higher the speed at given error rate.

Twisted pair cables come in a variety of sizes and jackets (outer covering). Twisted pair cables are jacketed and may contain from 2-pair to several hundred pairs. Twisted pair wire comes jacketed in PVC or plenum-rated. “Plenum” is the name given to the non-toxic PVC-like jacket that is authorized by local fire ordinances for use in ceilings and walls considered to be “air-return”.

Twisted pair cables are produced as either shielded twisted pair (STP) or unshielded twisted pair (UTP). When a twisted pair installation is to occur in an area that has abnormally high levels of electromagnetic energy, STP is recommended. In most office settings UTP is the standard; however, even in such seemingly neutral environments there can be problems such as fluorescent lighting fixtures and parallel runs with electrical wiring. A good installation plan prepared by a certified wiring engineer reduces the likelihood of such problems and also enhances the probability that the final infrastructure will operate at the desired performance (to allow the desired data transmission speed).

In office or campus environments UTP and/or STP provide the wiring infrastructure for LAN’s, some host-based data applications, video, and voice. Central office lines are normally delivered as twisted pairs to the client telephone or computer systems. Such lines range from single station analog lines up to and including T-1 (US, 1.544Mbps) or E-1 (Europe, 2.048Mbps). Of course this includes the intermediate digital service known as digital subscriber line (DSL).

In addition, twisted pair cable that is to be directly buried in the ground has a special construction to prevent water seepage. Cables to be run overhead outside are constructed with an extra steel cable known as a strength member. The strength member supports the weight of the cable between the poles from which the cable is suspended. This prevents the cable from sagging and ultimately rendering it useless. Finally, the US government contracts for “tap-proof” cable that has a special outer conductor, located under the outer covering, that is capable of conducting as much as 2,000 volts.

Figure below lists the types of twisted pair cables and their rated capacities and maximum links. Copper wire cable is “Category” rated. This table shows that the category varies based on the design of the cable (shielded or unshielded), size (gauge) of the wire, and the types of insulation material used.

Copper Wire Pair Information Capacity

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