# 6.2: Types of Communication Channels

Learning Objectives

• Maxwell's equations govern th propagation of electromagnetic signals in both wireline and wireless channels.

Note

You will hear the term tetherless networking applied to completely wireless computer networks.

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

$curl(E)=-\frac{\partial \mu H}{\partial t}$

$div(\varepsilon E)=\rho$

$curl(H)=\sigma E+\frac{\partial (\varepsilon E)}{\partial t}$

$div(\mu H)=0$

where E is the electric field, H the magnetic field, ε dielectric permittivity, μ magnetic permeability, σ electrical conductivity, and ρ is the charge density. Kirchoff'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.

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