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Unformatted text preview: Connexions module: m0100 1 Wireline Channels * Don Johnson This work is produced by The Connexions Project and licensed under the Creative Commons Attribution License † Abstract The analysis and transfer characteristics of wireline channels. Wireline channels were the rst used for electrical communications in the mid-nineteenth century for the telegraph. Here, the channel is one of several wires connecting transmitter to receiver. The transmitter simply creates a voltage related to the message signal and applies it to the wire(s). We must have a circuit a closed path that supports current ow. In the case of single-wire communications, the earth is used as the current's return path. In fact, the term ground for the reference node in circuits originated in single-wire telegraphs. You can imagine that the earth's electrical characteristics are highly variable, and they are. Single-wire metallic channels cannot support high-quality signal transmission having a bandwidth beyond a few hundred Hertz over any appreciable distance. Coaxial Cable Cross-section central conductor dielectric outer conductor insulation r i r d σ σ d , ε d , μ d σ Figure 1: Coaxial cable consists of one conductor wrapped around the central conductor. This type of cable supports broader bandwidth signals than twisted pair, and nds use in cable television and Ethernet. Consequently, most wireline channels today essentially consist of pairs of conducting wires Figure 1 (Coaxial Cable Cross-section), and the transmitter applies a message-related voltage across the pair. How these pairs of wires are physically con gured greatly a ects their transmission characteristics. One example is twisted pair , wherein the wires are wrapped about each other. Telephone cables are one example of a * Version 2.29: Jul 6, 2009 5:41 pm GMT-5 † http://creativecommons.org/licenses/by/1.0 http://cnx.org/content/m0100/2.29/ Connexions module: m0100 2 twisted pair channel. Another is coaxial cable , where a concentric conductor surrounds a central wire with a dielectric material in between. Coaxial cable, fondly called "co-ax" by engineers, is what Ethernet uses as its channel. In either case, wireline channels form a dedicated circuit between transmitter and receiver. As we shall nd subsequently, several transmissions can share the circuit by amplitude modulation techniques; commercial cable TV is an example. These information-carrying circuits are designed so that interference from nearby electromagnetic sources is minimized. Thus, by the time signals arrive at the receiver, they are relatively interference- and noise-free. Both twisted pair and co-ax are examples of transmission lines , which all have the circuit model shown in Figure 2 (Circuit Model for a Transmission Line) for an in nitesimally small length. This circuit model arises from solving Maxwell's equations for the particular transmission line geometry....
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