(Latest Revision: 09/10/2002)
Notes On Chapter Six
-- Long-Distance Communication (Carriers, Modulation, And Modems)
- 6.1 Introduction
- We need to use a carrier signals for long distance networking.
- We use a modem to modulate/demodulate the carriers. This is how
information is transmitted.
- 6.2 Sending Signals Across Long Distances
- The voltage placed on a line at a source drops off with distance
and is not detectable from far away. This phenomenon of signal
loss or attenuation explains why simple voltage-changing
standards such as RS-232 are not suitable for long-haul
communication.
- Fortunately, a carrier, a continuously oscillating wave form such
as a sine wave will propagate much further than signals such as
those generated by RS-232. This kind of signal reinforces
itself. The reinforcement is a form form of resonance.
- The sender modifies (modulates) the carrier slightly to get the
effect of transmitting data. The receiver extracts the
information by decoding (demodulating) the modulation.
- Amplitude modulation (am) modifies the amplitude of the carrier
to convey meaning. Frequency modulation (fm) modifies the
frequency. Phase shift modulation performs a translation
(sliding) of the wave form. It is easiser to get higher data
rates with phase shift modulation -- you don't need to wait for a
whole cycle of the wave form to pass through, so phase shift
modulation is very prevalent in data networking.
- Several different frequencies (channels) of carrier can be used
simultaneously and receivers can "tune" to whatever frequency
they wish to receive from. If the carrier frequencies are spaced
sufficiently far apart and certain "harmonics" are avoided, the
carriers will not interfere with each other.
- 6.3 Modem Hardware Used For Modulation And Demodulation
- A modem functions as a modulator in one direction: it inputs
digital information on one side and uses the digital information
to modulate a carrier. The modulated carrier transmits
information across a long-distance line.
- A modem also functions as a de-modulator in one direction: it
inputs a modulated carrier signal on one side, extracts the
digital information that was used to modulate the carrier, and
transmits the digital information to a local receiver such as a
computer connected to the modem.
- Modems for long distance communication often have a four-wire
configuration: a modulator-to-demodulator connection in both
directions, plus two grounds.
- 6.4 Leased Analog Data Circuits
- Laws forbid private companies to string wires across public
property. To extend computer networks across public lands,
companies usually lease wires from public utilities like phone
companies. Leased lines are extra wires that are installed in
telephone networks alongside the wires that carry voice
communications. For use as network links, the leased lines are
connected to special modems at each end.
- You only get point-to-point connectivity with leased lines -- a
connection between two specific points. You lease (rent) the
leased line. You pay the same rate no matter how little you use
it.
- A packet-switched network can share a set of leased lines among a
large group of users.
- 6.5 Optical, Radio Frequency, And Dialup Modems
- Modem technology exists for radio and light. Today's "wireless"
networks make use of radio-frequency (RF) modems.
- Dialup modems are special. They have to be used on telephone
lines so they need to perform the functions of a telephone like
pick-up, dial number, and answer phone. Also they use an audible
carrier because the line is designed to handle sound. Dialup
modems typically have to arbitrate which end is allowed to send
in order to get full duplex communication -- they do not use the
4-wire configuration.
- 6.6 Carrier Frequencies And Multiplexing
- One can put several frequencies of carriers on a wire without
interference -- different channels. One cable can simultaneously
carry several channels of television or network communication
transmission. This is called frequency division multiplexing
(FDM).
- Actually there are circumstances under which interference can
occur between carriers of differernt frequencies. Therefore
there has to be a certain minimum separation between frequencies
and also harmonic frequencies have to be avoided.
- FDM is practical only on "high-bandwidth transmission systems" --
ones that can transmit a wide spectrum of frequencies.
- 6.7 Baseband And Broadband Technologies
- It is possible to run two or more independent wireless computer
networks on equipment located in the same area. All that is
needed is for different networks to use different channels.
- Networks that use a wide spectrum of frequencies are
broadband .
- Networks that use only one frequency or a narrow range are
baseband .
- 6.8 Wave Division Multiplexing
- FDM is called wave division multiplexing when used with light.
In the case of light, an optical prism can be used to separate
frequencies.
- 6.9 Spread Spectrum
- Spread spectrum sends the same information over several carrier
frequencies. This increases the reliability of communication.
The receiver checks all the carriers and uses one that is working
well.
- 6.10 Time Division Multiplexing
- When devices take turns using a communication medium, that is
time division multiplexing. Networks use time division
multiplexing in addition to frequency division multiplexing.
- 6.11 Summary