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.
    
    
  • What's worse is that different pure frequencies attenuate at different rates, so a composite signal suffers distortion as it attenuates.
    
    
  • 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 much or 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