<< Chapter < Page Chapter >> Page >
Maxwell's equations govern th propagation of electromagnetic signals in both wireline and wireless channels.

Electrical communications channels are either wireline or wireless channels. Wireline channels physically connect transmitter to receiverwith a "wire" which could be a twisted pair, coaxial cable or optic fiber. Consequently, wireline channels are more privateand 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 tappedand the voltage measured. Some wireline channels operate in broadcast modes: one or more transmitter is connected to several receivers. One simple example of thissituation is cable television. Computer networks can be found that operate in point-to-point or in broadcast modes. Wirelesschannels are much more public, with a transmitter's antenna radiating a signal that can be received by any antennasufficiently close enough. In contrast to wireline channels where the receiver takes in only the transmitter's signal, thereceiver's antenna will react to electromagnetic radiation coming from any source. This feature has two faces: The smileyface says that a receiver can take in transmissions from any source, letting receiver electronics select wanted signals anddisregarding others, thereby allowing portable transmission and reception, while the frowny face says that interference andnoise are much more prevalent than in wireline situations. A noisier channel subject to interference compromises theflexibility of wireless communication.

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, andoptic fiber transmission.

E t H
E H E t E H 0 where E is the electric field, H the magnetic field, dielectric permittivity, magnetic permeability, electrical conductivity, and is the charge density. Kirchoff's Laws represent special cases of these equations for circuits. We are not going to solveMaxwell's equations here; do bear in mind that a fundamental understanding of communications channels ultimately depends onfluency 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 (andtherefore the voltages and currents) resulting from two or more sources will add .
Nonlinear electromagnetic media do exist. The equations as written here are simpler versions that apply to free-spacepropagation and conduction in metals. Nonlinear media are becoming increasingly important in optic fiber communications,which are also governed by Maxwell's equations.

Get Jobilize Job Search Mobile App in your pocket Now!

Get it on Google Play Download on the App Store Now




Source:  OpenStax, Fundamentals of electrical engineering i. OpenStax CNX. Aug 06, 2008 Download for free at http://legacy.cnx.org/content/col10040/1.9
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'Fundamentals of electrical engineering i' conversation and receive update notifications?

Ask