Notes, comments, and amplification for Chapter 2


about frequency

The author often describes the "frequency" of the tones used in the telephone system. Sounds are made by the compression and expansion of air, and a sound's frequency is the number of times per second that the air compresses and expands. The unit of frequency is the Hertz (Hz), or one such compression-expansion cycle per second.

The lowest-pitched sound most humans can hear is about 20 Hertz, and the highest-pitched sound is about 20,000 Hertz, or 20 kiloHertz (kHz). Frequencies on most telephone systems run from about 300 Hertz to about 3000 Hertz. You can't hear the very low tones or very high tones on the telephone because the additional cost of adding them is too expensive for what we gain. The telephone system is designed for normal conversation, not for listening to music, which requires lower and higher frequencies.

page 16 -- RBOC

In the trade press, you might see this term written as "arbock," just the way the acronym sounds. Our local RBOC is Verizon, formerly Bell Atlantic. Others you might recognize are BellSouth and NyNex.

page 16 -- tip and ring

The tip and ring connections are similar to the ones you see on the chargers for CD players, cell phones, etc.

page 18 -- wire gauges

The larger the number of the wire gauge, the smaller the wire is. Telephone wire is actually quite small -- remember that if you look at it, you will see an outer insulation covering four individual insulated wires inside. Here are some typical AWG numbers -- a lamp cord is probably 16 gauge, house wiring is usually 14 or 12 gauge, and a heavy-duty wire for a stove or clothes drier might be a 10.

page 21, 22 -- voltage formula

Since that's a negative voltage coming from the central office, you should put parentheses around the "-48". That way, you know that you're multiplying numbers, not subtracting. If you're not comfortable with equations, be sure to perform each calculation within the parentheses, rewrite the equation and continue. Remember that calculators have an "order of operations" that may mess up your results if you do not use the parentheses.

page 23 -- about grounds

Your car, for example, uses a 12-volt DC system. The red connection is at +12V, and the black connection is at ground (0V). This is the current standard, but it wasn't always that way. Up until the mid-1950's, Fords used a 6-volt DC systems with a "positive ground" similar to the ones used in the telephone system. Volkswagens used this system until the mid-1960's. In those days, giving one car a jump-start from the wrong type of car could produce big trouble.

Electronics students should read and understand the section on why the telephone system uses -48V instead of 48V. Everyone else only needs to understand that it is -48V, and that the reason for this is corrosion prevention.

page 24 -- local loops over three miles

When doing our homework assignments, we will assume that your local loop does not contain any loop extenders, coils, or repeaters.

pages 25, 26 -- Telephone Function 1: Providing a Signal .

Non-electronics types should skip the Ohm's Law current calculation.

page 31 -- ring signal

The 20-Hertz signal mentioned is just below the range of hearing for most people; you can feel the vibration, but not hear it. In very old telephones, this signal vibrated an actual bell. The tone has been kept as a standard, but different manufacturers do different things with it, which is why all telephones do not have the same ringing sound.

page 36 --Preferred Interchange Carrier

Now you know the origin of those annoying "10-10-ATT" television advertisements.