scope:

Introduction

This chapter discusses the transmission of information over various physical media. It also identifies the impact that information and communications technology (ICT) systems cabling installation methods have on the transmission of information.

In the context of this manual, transmission is the movement of information by electromagnetic energy in the form of electrical or optical signals from one point to another through a transport medium (e.g., air, balanced twisted-pairs, optical fiber strands). This chapter deals with the use of metallic (e.g., copper) conductors and optical fiber (e.g., glass) strands for the transmission of signals in analog or digital form.

Three transmission methods are:

• Simplex-transmits signals in one direction only. A public address system is an example of simplex transmission. The signal, or the speaker 's voice, is carried to a number of loudspeakers. The listener has no path to respond.

• Half-duplex-transmits signals in either direction but in only one direction at a time. This type of transmission typically requires agreement between stations and involves a push-to-talk switch arrangement on voice circuits or a signaling protocol on data circuits. A two-way radio or an intercom system is an example of half-duplex transmission.

• Full duplex-transmits signals in both directions at the same time. All telephone circuits are full duplex, allowing both parties to talk simultaneously. The choice of a specific transport medium is influenced by economics and technical considerations such as the:

• Physical construction (e.g., diameter, conductor size, pair/strand count) characteristics of the cable.

• Type of services to be provided (e.g., voice, data, video).

• Topology and size of the network.

• Transmission path distance.

• Transmission performance characteristics of the cabling.

Electromagnetic interference (EMI) conditions can affect transmission in systems using metallic media. In balanced twisted-pair cabling systems, when the adverse effects of EMI conditions cannot be remedied by shielded cabling or by increasing the physical separation between the cabling and the source of problematic EMI, it may be necessary to place unshielded cabling in a shielded pathway (e.g., metallic conduit). In some cases, the use of optical fiber cabling instead of balanced twisted-pair cabling may be necessary.

All balanced twisted-pair cable, coaxial cable, and optical fiber media feature certain transmission characteristics that limit or define their respective performance capabilities. Therefore, media selection may be based on specific network requirements when such network requirements are known.

For balanced twisted-pair cabling, the assignment of a category or classification performance rating (e.g., category 5e/class D, category 6A/class EA) provides a simple means to select cabling suitable for the applications intended to be supported.

Twisted-pair cable is called balanced cable because the physical construction of both conductors and the associated connections to equipment at the cable terminations transmit information electrically at the same potential referenced to ground. Coaxial cable is unbalanced because the two conductors and the associated connections to equipment at the cable terminations transmit information electrically at different potentials referenced to ground.