7.1) Using computers to Communicate
  
  Communications  - transfer of data from the sender or source to a receiver or
    sink.
	
Analog Signals:

  Frequency: the number of times a wave repeats during a time interval.  
  measured in megahertz.
  
  Amplitude: the maximum height of a wave.  Proportional to the strength or
  loudness of a signal.
  
  Digital signals: discrete bursts (0 or 1)
  Morse Code was the first digital signal.
  
  Modem - short for modulator/demodulator.  Translates from digital (computer)
  to analog (phone wires) and back again.  Modems can be external or internal.
  They used to come in different speeds, but all new modems are 56 Kbps.  All
  new modems also adhere to the V.90 stand standard which allows all newer 
  modems to communicate with each other.
  
Communications software:

  Error correction:  Static on phone can introduce errors.  There is software
  that can compensate for that.
  
  Data compression:  compresses files to up to 1/10th their original size.
  
  Remote Control:  Allows you to drive one PC from another PC.  PCanywhere.
  
  Terminal emulation:  Mainframes and minicomputers are designed to be accessed 
  by terminals.  You can use your PC as a terminal using terminal emulation
  software.
  

Data carriers (table on pg. 7.7)

  ISDN: Integrated Services Digital Network.  Uses POTS (Plan Old Telephone
  System).  128 Kbps.
  
  ADSL: Asymmetric digital subscriber line.  Also uses POTS.  
  
  Cable modem:  Best overall value.  Transmits and receives very fast.
  
  Satellite Dish:  Receives very fast, sends slow.  Requires the use of
  another modem for sending.  (Dishes do not transmit, this is a typo in the 
  book.)
  
7.2) Communications Channels:

  Twisted Pair Wire: 2 strands of insulated copper wire twisted around each 
  other.  Used in POTS.
  
  Coaxial Cable: several insulated copper wires twisted around each other.
  Used in Cable TV and now Cable modems.
  
  Fiber Optic Cable: Transmits light, not electricity, over very long distances
  ans is extremely fast. Originally made out of glass, the newer fibers are 
  plastic.  They are more flexible, lighter weight, and cheaper, and faster,
  carrying data at an unprecedented 1 trillion bits per second.
  
  Microwave Systems: transmit data through the air.  Microwaves can not transmit
  through obstacles, that is why microwave relay stations are always on the tops
  of mountains.
  
  Other Wireless: 2 way radio, 
  GPS.  Guidestar system now available in some rental cars.
  Pagers.
  Cell Phone: Digital vs. Analog.
  Packet Radio: sends data to handheld computers.
  
  
7.3) Factors Affecting Communications

  Frequency: cycles per second.  The higher the frequency, the more quickly
  data can be transmitted.
  
  Bandwidth: the range of frequencies that are in use.  The larger the 
  bandwidth, the more data that can be transmitted.
  
Line configurations

  Point-to-point: transmits data directly from one device to another
  
  Multipoint:  Several devices are connected via one line to one computer.
  
  
Serial and Parallel

  Serial: only one stream of data at a time.
  
  Parallel: multiple streams of data at a time.  (Think of a 6 lane highway)
  
Direction of transmission

  simplex: one way communication.  ex. Television.
  
  Half-duplex: sends data both directions, but only one direction at a time.
  ex. walkie-talkie.
  
  Full duplex: sends data both directions simultaneously.  ex. Phone.
  
Transmission mode:

  Asynchronous: one byte at a time with start and stop pulses
  
  Synchronous: sends data in blocks
  
Packet switching: messages are divided into packets before they are transmitted.
Each packet is <= the maximum allowed amount of data by the system.  It also
contains information about its destination.

Multiplexing

  multiplexer: combines several lowspeed transmissions into one high speed
  transmision.  (like a funnel)
  
  Concentrator: like a multiplexor except that it contains the data in a
  temporary storage area.
  
  Front end Processors: a smaller computer that is connected to a larger 
  computer to assist with communication functions.
  
Protocols: Rules of data transmission

  protocol: a standard set of rules used by all computers that determines how
  transmitted data is packaged.  This allows all computers to "speak the same
  language" so they can communicate with each other.
  
  OSI - Open Standards Interconnection
  
7.4) Communications Networks

  WAN - Wide Area Network - a very large network that covers a large geographic
  region such as a state or country.
  
  MAN - Metropolitan Area Network - a network that covers an area the size of a 
  city or suburb
  
  LAN - Local Area network - a network that covers a single business.  Colleges
  commonly have LANs.
  
Network features:

  Host Computer - the main computer in charge of the network.
  
  Server - a smaller computer that serves the data to the nodes.
  
  Node - an end terminal by which a user can access the network.
  
Advantages of networks

  Sharing of peripheral devices
  Sharing of programs and data
  Better communications
  Security of information - data is available to select users and backed up
  Access to database
  

  
7.5) Communications Networks

Types of local Networks

  PBX: Private Branch Exchange.  ex. telephone system with operator.
  becoming obsolete because they can't handle a high volume.
  
  LAN: Local Area Network.  Computers that are linked together and can share info
  or equipment.
  
Types of LANs

  Client-Server:  Server holds the data.  Client is the end terminal or node.
  
  peer-to-peer:  All computers are at the same level and communicate with each
  other.
  
Components of a LAN

  Cabling or connections system.  Mist LANs are wired, wireless LANs are slow.
  Microcomputers and network cards.
  Network Operating System.  Windows is commonly used for peer-to-peer LANs.
  Other devices: printers, fax machines, plotters, etc...
  Bridges, routers, and gateways
    Bridge: connects to segments of a LAN
	router: connects different types of networks together
	gateway: performs all functions of bridges and routers
	
Topology of LANs

  Star network: server is in middle and has a direct connection to each client
  or device
  
  Ring network: no specific central device.  Traffic circles in one direction
  and all computers or devices are connected in series.
  
  Bus network: all devices connect to one central line, but not a central
  server
  
  Hybrid network: combination of 2 or more of the above
  
  FDDI: Fiber Distributed Data interface.  Like a Ring network, but has two
  circular paths for data to flow in both directions simultaneously.  
  Extremely fast and expensive.  Another advantage, if one of the rings fails 
  the other will continue communications, although it will be slower.
  
7.6) Networks at work

  Voice Mail
  e-mail (pros and cons)
  Videoconferencing and V-Mail
  Workgroup computing and Groupware - allows many people to work on the same
  project based on the same database
  EDI: Electronic Data Interchange - direct electronic exchange of standard
  business documents.
  Intranets and Extranets:
    Intranet: an internal mini-internet for use by one company
	Extranet: an Intranet that has been expanded to include a few select clients
	Firewalls: electronic security that prevents people for breaking in to
	privileged information.  A connection between the Intranet and the Internet.
	
  Telecommuting
  Virtual Office: office on the go.
		

Homework 7:
in Interactive computing book
EX 3.1 - 3.20 
Nothing to turn in this week.  Study for Mid-Term.