6.5 Line-of-Sight Transmission

196 CHAPTER 6. INFORMATION COMMUNICATION

6.2 Types of Communication Channels2

Electrical communications channels are either wireline or wireless channels. Wireline channels physically connect transmitter to receiver with a “wire” which could be a twisted pair, coaxial cable or optic fiber. Consequently, wireline channels are more private and much less prone to interference. Simple wireline channels connect a single transmitter to a single receiver: a point-to-point connection as with the telephone. Listening in on a conversation requires that the wire be tapped and the voltage measured. Some wireline channels operate in broadcast modes: one or more transmitter is connected to several receivers. One simple example of this situation is cable television. Computer networks can be found that operate in point-to-point or in broadcast modes. Wireless channels are much more public, with a transmitter’s antenna radiating a signal that can be received by any antenna sufficiently close enough. In contrast to wireline channels where the receiver takes in only the transmitter’s signal, the receiver’s antenna will react to electromagnetic radiation coming from any source. This feature has two faces: The smiley face says that a receiver can take in transmissions from any source, letting receiver electronics select wanted signals and disregarding others, thereby allowing portable transmission and reception, while the frowny face says that interference and noise are much more prevalent than in wireline situations. A noisier channel sub ject to interference compromises the flexibility of wireless communication.

Maxwell’s equations3 neatly summarize the physics of all electromagnetic phenomena, including circuits, radio, and optic fiber transmission.

∇ × E = − ∂ ∂ t (µH) div ( E) = ρ

∇ × H = σE + ∂ ∂ t ( E) div (µH) = 0 (6.1)

where E is the electric field, H the magnetic field, dielectric permittivity, µ magnetic permeability, σ electrical conductivity, and ρ is the charge density. Kirchhoff ’s Laws represent special cases of these equations for circuits. We are not going to solve Maxwell’s equations here; do bear in mind that a fundamental understanding of communications channels ultimately depends on fluency with Maxwell’s equations. Perhaps the most important aspect of them is that they are linear with respect to the electrical and magnetic fields. Thus, the fields (and therefore the voltages and currents) resulting from two or more sources will add. note: Nonlinear electromagnetic media do exist. The equations as written here are simpler versions that apply to free-space propagation and conduction in metals. Nonlinear media are becoming increasingly important in optic fiber communications, which are also governed by Maxwell’s equations.

6.3 Wireline Channels4

Wireline channels were the first 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 flow. 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.

Consequently, most wireline channels today essentially consist of pairs of conducting wires (see Figure 6.1), and the transmitter applies a message-related voltage across the pair. How these pairs of wires are physically configured greatly affects their transmission characteristics. One example is twisted pair, wherein the wires

2 This content is available online at http://cnx.org/content/m0099/2.13/. 3 James Clerk Maxwell’s biography can be found at http://www- groups.dcs.st- andrews.ac.uk/~history/Biographies/ Maxwell.html. 4 This content is available online at http://cnx.org/content/m0100/2.29/.