• Definition
• Wide Area Networks
• Local Area Networks
• Metropolitan Area Networks
• Communication Channels
• Analog and Digital Signals
• Popular Connection Types

Definition

Networks are composed of computers that are connected with some communications channel in such way that all users have access to the resources found in that network.

Three types of networks are Local Area,  Wide Area, and Metropolitan Area.

Wide Area Networks

Wide Area Networks emerged in the 1960s and 1970s to connect multiple computers across large geographic distances.  WAN systems uses same technologies as the long-distance telephone systems. 

Local Area Networks

 Local Area Networks emerged and became popular in the late 1970s.  As computer manufacturers introduced smaller and less expensive mini and micro computers, many departments in large organizations were able to afford to buy several computer systems and soon needed to interconnect these computers with LANs.  Local Area Networks interconnect devices that located in close physical proximity such as the same office or building.  

Metropolitan Area Networks

These networks interconnects offices and buildings located within the same city usually using wireless radio or infrared communication.  

Communication Channels

A communication channel is a transport medium that connects two computers together.  This channel can be a wire or the air.  Five kinds of communication channels exists:

• Telephone Lines
  • Made up of twisted pair of copper wires.
  • Had been the standard transmission medium for both voice and data.
  • Now is being phased out by more technically advanced and reliable media.
      
• Coaxial Cable
  • Made up of a shielded single solid-copper wire.
  • Has over 80 times the transmission capacity of twisted pair.
      
• Fiber-Optic Cable
  • Transmits data as pulses of light through tubes of glass.
  • Has over 26,000 times the transmission capacity of twisted pair.
  • A fiber-optic tube is about half the diameter of a human hair.
  • Are immune to electronic interferences.
  • Is rapidly replacing twisted pair telephone lines.
      
• Microwave
  • Transmit data as high-frequency radio waves.
  • Radio waves travel in straight lines through the air.
  • Can only be transmitted over short distances.
  • Longer distances require signals to be relayed by means of dishes or antennas installed on towers, high buildings, or mountaintops.
      
• Satellite
  • Used to amplify and relay microwave signals from one transmitter on the ground to another.
  • Orbit about 22,000 miles above the earth.
  • Rotate at precise point and speed.
  • Can be used to send large volumes of data.
  • Bad weather can sometimes interrupt flow of data.

Analog & Digital Signals

Communication channels can transport one of two types of signals: analog or digital.

• Analog signals are continuous wave signals that usually represents a sound or a voice.  The human voice is an example of an analog signal since it is continuous. 
    
• Digital signals are discrete signals that represent the presence or absence of an electronic pulse.  Computers are examples of devices that transmit digital signals.

Since phone lines are used to transmit analog signals (i.e. the human voice) it follow that computers cannot be directly connected to phone lines because they are digital devices.  A special device instead called the modem is used to connect computers to phone lines.

The word modem stands for modulator-demodulator:

• Modulation is the process of converting digital to analog.
• Demodulation is the process of converting analog to digital.
• The modem therefore is a hardware device that converts digital signals to analog and analog signals to digital, allowing two computers to communicate over a telephone line.

Popular Connection Types

Popular user connections to communication systems include:

• Telephone Modems
  • Communication speed is measured in bits per second (bps)
  • Typical speeds are 36.6 kbps (36,600 bps) and 56 kbps (56,000 bps)
      
• Leased Lines (T1, T2, T3, T4)
  • High-speed lines usually leased by large companies from telephone companies.
  • T1 lines provides 1.5 mbps (1,500,000 bps) and cost several thousands of dollars.
      
• ISDN (Integrated Services Digital Networks)
  • Provided by many local telephone companies 
  • Primarily used by smaller organizations and individuals
  • Costs much less than T1 lines.
      
• ADSL (Asymmetric Digital Subscriber Line)
  • Uses existing telephone lines.
  • Provide high-speed connections as fast as T1 lines.
  • Presently limited to few large cities.
  • Expected to become widely available in the next few years.
      
• Cable Modems
  • Use existing television cables
  • Provide high-speed connections as fast as T1 lines.
  • Not all cable companies support cable modems.
  • 100 percent availability expected within the next few years.

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