Dip Singh published an excellent primer on communication fundamentals including:

• Waves: frequency, amplitude, wavelength, phase
• Composite signals, frequency domain and Fourier transform
• Bandwidth, fundamental and harmonic frequency
• Decibels in a nutshell
• Transmission impairments: attenuation, distortion, noise
• Principles of modern communications: Nyquist theorem, Shannon’s law, bit and baud rate
• Line encoding techniques, quadrature methods (including QPSK and QAM)

Even if you don’t care about layer-1 technologies you MUST read it to get at least a basic appreciation of why stuff you’re using to read this blog post works.