Networking: Computer Connections

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Networking Computer Connections
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Data Communications

• Data communications systems that transmit data
  over communication lines
• Centralized processing-all the data, software and
  hardware all in one central place inconvenient
• Distributed data processing-accommodates both
  remote access and remote processing
• Network-a system that connects two or more
  computers and allows for sharing of resources

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Basic Components of a Data Communications System

• Sending device
• Communications link
• Receiving device
• Example-you send your mom an email (sending
  device) which uses the internet(communications
  link) and she receives the email on her PC
  (receiving device)

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Data Transmission

• Digital
• Sends data as distinct pulses (0s and 1s again!)
• Used to transfer computer data
• Newer technology
• Must be converted to analog to travel over analog
  lines

• Analog
• Continuous electrical signal in the form of a
  wave
• Older technology
• Can carry data but is mostly used for voice

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Conversion from Digital to Analog

• First a carrier wave is the first step in the
  conversion from digital to analog
• Amplitude (height of wave) and the Frequency
  (number of waves during a set time interval)
• Conversion from digital to analog is called
  modulation
• The opposite, from analog to digital is called
  demodulation.

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Modems

• Device that converts the analog and the digital
  signals (comes from Modulation/Demodulation)
• Different types
• External-separate from the computer
• Internal-most commonly used modem now
• Some lap top cards come on small plugin devices
  known PC cards.

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Speeds of Modems

• Standard modem which works with regular telephone
  lines now transfer data at 56,000 bits per second
  (generally analog)
• ISDN integrated services digital network is a
  faster modem (128,000 bps) and has two lines
  where you can talk on the phone and surf the net
  at the same time
• DSL-Digital Subscriber Line, uses the traditional
  phone in a digital way. Much faster than
  standard modem, is always online, and you can use
  your phone at same time. Geographical
  limitations and cost are issues.

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More Modems

• Cable modems are also always on and have speed
  rates up to 10 million bps. Uses the lines that
  sends your cable TV. There are configuration
  problems as well as security issues.
• Cellular Modems-modem for cell phone are very
  slow but new cell technology promises faster
  connections in the future

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Data Transmission

• Two ways to make sure both computers on each end
  are receiving all of the data
• Asynchronous-means the data stops and starts
  during transmission. A start signal is sent, the
  receiving computer times the transmission, and
  waits for a stop message. Then repeats.
• Synchronous-means at the same time. This sends
  the data continuously by synching the intenral
  clocks of both machines. Error checking is built
  into make sure all data was received.

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Direction of Data Flow

• Simplex-means data can only flow one way during
  the conversation (radio, tv, etc.)
• Half-duplex transmission-data flows both ways but
  only once at a time (CB, Walkie Talkie, etc)
• Full-duplex-allows data to flow both ways at the
  same time (telephone, many high speed computer
  connections)

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Communications Media

• The medium (singular or media) is the physical
  means of data transmission
• Bandwidth is the measure of the capacity of the
  link and the range of frequencies that that a
  medium can carry

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Wire Pairs

• Also known as twisted pairs
• Transmission of an electrical signal requires two
  conductors
• In a twisted pair, the two conductors are twisted
  together and sheathed in plastic to reduce
  interference
• A single cable can have multiple twisted pairs
• Are inexpensive and usually already wired in a
  building
• They are susceptible to interference or noise

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Coaxial Cables

• Stronger signal than wire pairs
• Has higher bandwidth with less noise
• Has a center conductor wire surrounded by a layer
  of insulation and then a metallic outer conductor
• Is used in things like TV cable

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Fiber Optics

• Cables made of glass or plastic that transmit
  data using light
• Higher bandwidth than coaxial cable but cheaper
  to make
• Immune to electrical noise
• More secure than coaxial and wire

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Microwave Transmission

• Line of sight transmission
• Cannot bend around the curvature of the world
  and must use relay stations (approximately 30
  miles apart)
• High speed and cost effectiveness
• Can be problematic in bad weather

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Satellite Transmission

• A different form of microwave transmission where
  a satellite acts as the relay station
• An earth station acts as the sender (uplink) to
  the satellite which has a transponder that
  receives the data, amplifies, changes the
  frequency, and send back to the receiving earth
  station (downlink)
• Good for transmitting data quickly anywhere
  around the world
• Network implementation can be costly launching or
  renting satellites

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Satellite Postions

• Communication satellites are suspended 22,300
  miles above earth to enable them to be positioned
  over the same spot on earth at all times, called
  geosynchronous
• LEO or low-earth orbit are only about 1000 miles
  above earth and make complete revolutions every
  few hours, requiring tracking information
• Generally cheaper to launch

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Wireless Transmission

• Uses infrared to transmit data over short
  distances
• IrDA transmits data a few feet between devices,
  i.e. between a computer and printer
• Bluetooth uses radio waves to connect mobile
  devices at short distances
• For distances up to 150 feet, the 802.11
  standards are emerging. 802.11a and 802.11g
  allows for transmission of 54Mbps and 802.11b up
  to 11mbs
• Called WiFi and is built into some laptops

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Whats on a network

• Most networks will combine technologies within
  one network
• For example, a network may use fiber optic to
  connect to the internet, twisted pair to connect
  all of the PCs and possibly wireless
  transmission or satellite transmission to meet
  specific needs

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Protocols

• A set of rules for the exchange of data between
  two points (computers usually)
• A language that both devices on each end
  understand that consists of statements such as
  Are you ready to receive a message or Im done
  sending data now
• Standards are necessary so people can develop
  software and hardware that allows devices to talk
  to each other
• One of the most common protocols called TCP/IP
  (transmission control protocol/internet protocol)
  allows any computer to communicate with the
  internet

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Network Topology

• Topology is another word for the physical layout
  of a network
• Each item or location on the network is called a
  node (i.e., each computer, printer, etc.)
• Three common types
• Star
• Ring
• Bus

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Star Network

• A central hub that manages the network. All
  traffic goes through the network first.
• Collisions are controlled by the central hub.
• If the hub goes down, the network fails.

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Ring Network

• All nodes are linked together in a circular chain
• Collisions are avoided by traffic only being able
  to travel in one direction
• If one node fails in the circle, the network fails

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Bus Network

• All nodes are connected to a single line called
  the bus
• Very little control over collisions, nodes just
  retransmit if a collision is detected
• Nodes can be attached and detached without
  affecting the network
• If one node fails, the network continues to
  operate

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Wide Area Networks (WANs)

• Can link computers across town or around the
  world
• Some WANs which link computers in a metropolitan
  area are called MANs
• Uses common carriers which are companies
  regulated by the FCC to provide WAN services

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WANs continued

• Services provided by the common carriers include
  two types
• Switched services or dial up services-usually use
  POTS (plain old telephone service) and the
  connection is broken when a call is ended
• Dedicated services provide permanent connections
  between nodes. Can be leased telephone lines,
  ISDN, or DSL lines.

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Larger Capacity Dedicated Lines

• Many companies need more network power and lease
  dedicated lines called T1 or T3
• T1 carries 1.54 mbps and the T3 carries the
  capacity of 28 T1 lines or 43mps

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WAN Hardware and Software

• Usually controlled by one mainframe called the
  host computer
• The host computer connects to the WAN through the
  front-end processor (FEP) and performs some
  network maintenance
• Multiplexers combine data streams from more than
  one device to send out over a larger capacity
  line
• Terminal Emulation Software is sometimes used to
  allow PCs to talk to the mainframe

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File Transfer

• If a PC acts as a terminal, file transfer
  software allows users to put and retrieve files
  from the PC
• Download means to get files from another computer
• Upload means to put files on another computer

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Local Area Networks (LANs)

• Usually a network of PCs
• LANs hooks computers together to share resources
• Cover short distances such as a campus or one
  business location

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Local Area Network Components

• Network cables generally connect the nodes on the
  LAN. Can be cheaper twisted pairs or coaxial or
  fiber optic that is faster but more expensive.
  Also can be wireless
• Network interface card or NIC connects the PC to
  the network
• A bridge connects two networks that are similar
  and passes messages between nodes on the
  different networks

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LAN components continued

• A router is a special computer that directs
  traffic when several networks are connected
• Some routers have been replaced by IP switches
  which are faster and less expensive
• A gateway is a hardware/software combo that lets
  a node communicate with a node on a network that
  is not similar
• Wireless access stations or base stations
  transmit data between the network and the
  wireless device

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Types of LANs

• Client/Server consists of a server which controls
  the network and shares files to the clients, the
  workstations on the network
• A thin client is a workstation that doesnt have
  storage capacity and acts as a machine that sends
  and receives data from the server

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Peer to Peer networks

• Another LAN configuration which has nodes of
  equal status. There is no server which controls
  the network
• Common in small computing environments such as
  small businesses
• Some famous Peer to Peer networks include Napster
  and Kazaa

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LAN Protocols

• Remember a protocol is a set of rules that access
  the network and control data flow
• Two common kinds
• Ethernet
• Token Ring

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Ethernet

• Most common LAN protocol using either a bus or
  star configuration
• Inexpensive and simple to set up
• Uses CSMA/CD or carrier sense multiple access
  with collision detection
• Means that nodes listen to see if the line is
  free and then transmit
• If a collision occurs, the data is destroyed and
  the node detects the problem and resends the data

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Token Ring Protocol

• Tied closely to IBM
• Used on ring networks
• The token circulates among the computers on the
  network, called token passing
• A computer waits for an empty token to come by
  and attaches its data to the token
• Larger networks allow multiple tokens for more
  than one message

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What Networks Do

• Email
• Facsimile Technology or faxing with a fax modem
• Groupware
• Teleconferencing
• Electronic Data Exchange (EDI)
• Electronic Fund Transfer (EFT)
• Telecommuting
• The Internet