CS 3 - Kurtz 

Santa Monica College 







A system that links computers (nodes) and computer hardware in order to exchange data or share peripherals


Data Communications Model


Sending device(s)

Receiving devices(s)

Communication device(s)

Communications channel


Peer-to-peer Network 


Networking between "equals"
All nodes communicate between one another
Popular for sharing files within workgroups 
AppleTalk (Macintosh OS), Windows95, WindowsNT Client




Client-server network


All nodes connect to a central computer (file server) 
Allows file sharing
particularly useful for sharing central records 
Application programs can reside on the server. Temporary copies are loaded onto workstations as needed. 
Network or site licenses for application programs enable many users to share a server based application. Cost is similar to having local copies at each workstation (i.e., no cost saving for network installations)




Converts between digital signals (computer) and analog signals (telephone) 
Enables communication via the PSTN (Public Switched Telephone Network) 
Internal modems are typically less expensive and mount inside of the computer 
External modems connect to ports or slots available on most desktop and portable computers 
Fax modems enable computers to communicate with fax machines in addition to "normal" modem operation 
Speed is measured in bits per second (BPS) 
Works with special "modem software" or "dialing programs" that include a personal phone book called a "dialing directory" 

Connection cost is the same as a phone call

Mobile users can connect to their main office for E-mail

A different line is selected by the telephone company for each call, making connection quality difficult to control

Parameters include speed (baud or bps), data bits, stop bits, and parity, local echo, terminal type




Integrated Services Digital Network

Digital leased telephone lines

Used to transmit digital data over telephone lines

One control line (channel) is combined with multiple 56Kbps data lines.

Three channels provides 128Kbps




Digital Subscriber Line

Digital lease line


High data rate

Uses conventional twisted pair


T-carrier line


T-1, T-3

Digital transmission line


May carry multiple signals on one line

High (or very high) speed





Combines multiple data streams onto one line

Reduces equipment requirements

Increases reliability

Increases efficiency of data lines


Hub or Switch



MAU (Multistation Access Unit)

Central point for network cables

A signal sent to one port is transmitted from all others



Amplifies and retransmits received signals

 Used on long communication runs to compensate for signal loss




Connects two networks using same protocol




Connects two networks using different protocols




Special purpose computer
Connects multiple networks

Allows for multiple protocols

May determine best path for a signal

May provide filtering and security

May block protocols


Local Area Network 


LAN Typically includes one building or group of adjacent buildings 
LAN radius is almost always <1 mile 
Computers require a network interface card 
Connecting hardware is typically Coaxial cable (coax) or twisted pair lines. 
GroupWare adds E-mail, shared calendars, conferencing, etc.


Wide Area Network 


WAN Similar to a LAN, except "wider" 
May cover an entire city, county or be worldwide in the case of a multinational corporation 
May connect LANs and may use coax or twisted pair.
May also use leased telephone lines, microwave, satellite, or fiber optic links.




Working away from a company’s standard workplace using communications technology


  • Flexibility
  • Job satisfaction
  • Convenience and comfort for disabled employees
  • Combination of personal and business responsibilities
  • Reduced traffic and commutes
  • Decreased personal contact  with other staff




Uses video images and computer technology

Permits face-to-face meetings across great distance




Permits groups of people to work together and share information over a network




Global Positioning System

Determines exact location of a receiver on the earth’s surface

System of satellites in geosynchronous orbit

Satellites transmit signals

Ground receivers triangulate


Fiber Optic Cable


Thin, flexible glass strands

Transmits a modulated laser beam

Low noise susceptibility

High security

High installation cost

Maintenance and repairs are difficult


Cellular Radio


Used by cellular telephones

Used for mobile communications

Transceiver switches between fixed stations as it is moved between “cells”




High frequency radio waves used for communications

High reliability

High bandwidth

Line of site transmission, typically between fixed stations

Generally not secure


Communications Satellite


Space based repeater

High frequency radio (S-band, K-band, X-band)

Satellite transponder relays signals between ground stations

High speed

High bandwidth

Illuminates a large area

Generally not secure





Wireless transmission media

Low frequency light waves

Line of site





Special characters used in electronic communications that look like facial characters to communicate emotions

:)          happy  

:D         very happy      

:(          sad      

;)          wink    

:‑0        Oh!     

:‑P        My tongue is hanging out          

7:‑)       I saw Elvis!      






On‑line Bulletin Board System 
Character based (non‑graphical) 
Typically allows E‑mail, conferencing, advertising, and software downloads (and uploads)





Popular protocol for a high speed LAN developed by Xerox in early 70's using packet switching technology and requiring coaxial cable connections

Allows personal computers to contend for access to network

Requires a bus topology



Token Ring


Popular protocol for a high speed LAN

Requires a ring or star topology

Nodes share a special signal called a token

Only the node with the token may transmit

Helps prevent data collisions







Packet Switching


Used in many modern network protocols 
Divides data up into small blocks called "packets" with headers containing a source and destination "address"



Bus Topology

Network nodes all connect to a single cable



Ring Topology

Network nodes all connect to a single cable arranged in a loop



Star Topology

Network nodes all connect to a central computer







Print server


A computer on a LAN that regulates the use of one or more printers





A mass mailing of E‑mail for advertising purposes




Tactless message sent in a moment of anger