Add to the diagram of frequencies:
The typical frequency range of human hearing is 20 Hz to 20 KHz.
A good way to remember which wire is the "tip" and which wire is the "ring" is to remember the two R's -- Red and Ring are the same.
"Attenuation" means signal loss. It is usually measured in decibels (dB). Decibels always involve a comparison between one signal level and another. You are probably familiar with using decibels to measure noise levels. When sounds are compared in decibels, the base level is usually taken as the quietest sound that can be heard by human ears. Thus, a 70-decibel sound, that of a normal conversation, is 70 decibels above the quietest sound that an average person can hear.
For voice and data signals, though, another type of dB measurement is used. Here, the dB typically compare voltage or power at one point with the voltage or power at a point further down the data system. For now, we can simplify dB to this fact -- every 3-dB power loss means that the new power level is 1/2 the original. Thus, a 6-dB power loss means that the new power level is 1/4 times the original (1/2 x 1/2), a 9-dB power loss means that the new power level is 1/8 the original power (1/2 x 1/2 x 1/2), etc.
Voltage loss is similar, except that each 6-dB drop means that the new voltage is 1/2 the original voltage. Thus a 12-dB loss means that the new voltage is 1/4 the original voltage (1/2 x 1/2) and an 18-dB voltage loss means that the new power has become 1/8 of the original voltage (1/2 x 1/2 x 1/2).
If you are not an electronics student, these pages will probably be a mystery to you. Don't panic! You only need to understand the conclusion of this section, even if you do not follow the steps taken to get there. If yo like, skip the reading and just study the paragraph below:
Whenever two wires, such as phone lines, are separated by an insulator, they have the property of "capacitance," or the ability to store electric charge. Because of the capacitance effect, high frequencies become weaker in voice transmission across telephone lines. To counteract this effect, telephone companies put "loading coils" into the phone lines. The coils were an excellent solution for the voice problem, but there were designed and deployed decades before people used high-speed modems to transmit data along those same lines. Unfortunately, it simply isn't possible to have correct-sounding voice signals and high-speed data transmission on the same telephone lines. When you use a modem to transmit data, it is the phone line, not the modem, that limits the speed of the data transmission.
A codec is a "coder/decoder." It takes an analog input and changes it to a digital output, and does the opposite on its other end.
Compare the action of a codec with a modem, or modulator/demodulator, which starts with a digital signal from a computer and coverts it to an analog signal for transmission over phone lines, where another modem reconverts the received signal from analog to digital for use in another computer. Codecs and modems are exactly opposite in function.
Codecs can be implemented in either hardware or software. You are probably familiar with RealAudio/RealPlayer, which coverts analog music into digital format for transmission over the Internet, and then reconverts it at your computer to analog music for your ears.
Remember that your can't hear music very well on a phone line because the telephone frequencies don't go as high as music frequencies. Higher frequencies require higher sampling rates, according to Nyquist's equation. The recording industry has taken the upper limit of human hearing as 20,000 Hz and added a little extra to make 22,050 as the practical limit for use in CD's. The sampling rate is double that, or 44,100 Hz, on each channel of a CD. Good listening through good sampling!
This section on frequency division multiplexing is presented for historical purposes, since analog multiplexing is now obsolete. There is no need to understand the details of FDM, since it is no longer used.
A nanometer is a billionth of a meter (10-9 m). Most references state that human vision is in the 400-nm (violet) to 700-nm (red) range.
Wavelength multiplexing is a bit like the older FDM, since every wavelength of light has a corresponding frequency. So, just like FDM, we have different sets of data being transmitted at different frequencies. You can either say that the different signals are transmitted on different wavelengths or on different frequencies; it' all the same. It's a bit of stretch, though, to say that they are transmitted on different colors, such most data transmission takes place at ultraviolet or infrared wavelengths, which are invisible to the human eye.