Local Area Networks

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Local Area Networks

• Topologies and Collision Detection

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What do we know so far?

• Data communications involves
• Exchange of digital information
• Between two or more devices
• Across a transmission medium
• How are these devices connected?

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Private Branch Exchanges (PBX)

• Special computer that handles phone calls within
  a company
• Carry both voice and data
• Can store, transfer, hold and redial calls
• Can also be used to transfer data between
  computers
• Does not require special wiring
• PCs can be plugged or unplugged anywhere in the
  building
• Supported by commercial vendors (no internal
  expertise needed)
• Geographic scope limited to several hundred feet
• Cannot handle large amounts of data

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Local Area Networks (LAN)

• Connect several buildings in close proximity
• Typically within 2000 feet
• Requires own communication lines
• Have higher transmission speeds
• Typically used to connect PCs and shared printers

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Typical LAN Components
Network Server (with network software)
LAN
Another LAN
Gateway
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Network Topologies

• In the case of LANs, the shape of the network
  defines its topology
• Star
• Bus
• Ring

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Star Network Topology
Host Computer
- Used to connect a smaller number of computers -
depends on health of host computer
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Bus Network Topology

• Central line (bus) that may be TP, Coaxial, or
  fiber
• All messages broadcast to entire network
• Software identifies which device receives message
• Bus network can only handle one message at a time
• Can slow down at peak hours
• Collisions may occur

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Ring Network Topology

• - Each computer part of a closed loop
• messages passed from one device to another
• Only passes in one direction

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Ethernet

• Designed for multiple devices sharing a single
  communication cable
• Devloped by Bob Metcalfe of Xerox in 1973
• Tried to link a Xerox Alto computer to a printer

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Ethernet Terms
Medium, Segment, Node, frames
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CSMA / CD

• An analogy
• Imagine a group of people sitting at a table
• They are having a polite conversation
• Everyone can hear others speak
• They wait for conversations directed at them
• Wait for a pause in conversation before speaking
• Two people waiting for lull speak up at same time
• Must repeat themselves

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Contention issues

• All devices on a bus or ring can send messages
• Devices keep listening to the network to check
  for messages meant for them
• What happens if messages are sent at the same
  time?
• Messages can sometimes collide and be garbled or
  lost
• LANs must have a predetermined way to deal with
  these conflicts or contentions

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CSMA/CD(Collision Sensing Multiple
Access/Collision Detection)

• This is used in traditional bus network
  topologies
• Ethernet uses bus topology with CSMA/CD
• Any device on the bus can send a message
• If the line is idle two devices may send at same
  time
• Device recognize collision and send message again
  after random period of time

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Limitations of Ethernet Networks

• Mostly relate to length of cable segments
• Electrical signals attenuate as they travel
  longer distances
• Segment must be short enough for devices to hear
  each other clearly
• Places limit on size on network
• Network diameter
• Since CSMA/CD only allows one device to
  communicate at a time, limits number of devices
  without degrading performance

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Repeaters

• Repeaters connect multiple Ethernet segments
• Any signals heard on one segment will be heard
  and repeated on all other segments connected to
  repeater
• Allows for expanding network diameter

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Bridges

• What happens if there are a large number of
  people at the table?
• Multiple simultaneous conversations
• In large networks, devices would constantly be
  interfering and sending colliding signals
• Bridges are like repeaters that echo signals, but
  can also regulate traffic

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• -The bridge aims at reducing unnecessary traffic
  on Ethernet segments
• -If signal from A is meant for B, there is no
  point echoing it on Segment 2
• If it is meant for C or D, the frame is forwarded
  to Segment 2.
• A can simultaneously transmit to B since it only
  uses Segment 1

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Token Ring

• Used in ring topology networks
• Eg IBM token rings
• A token (series of 0,1) floats along line
• A device wishing to send message on line must
  first grab the token and keep it only then can
  it send
• Once the message has been sent, device releases
  token back into the ring
• Collisions can never occur
• Token-ring networks typically transmit data at
  either 4 or 16 Mbps.

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IBM Token Ring
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Problems with bridges

• In the Ethernet design, messages are broadcast to
  every device (or node) on the network segments.
• The bridge forwards these broadcasts to all
  connected segments
• In very large networks, this can cause congestion
• Many stations on different segments broadcast at
  the same time
• Can be as bad as if all nodes were on one segment

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Routers

• Routers are advanced network components
• They divide the network into two virtual (or
  logically independent) networks
• Broadcasts cross bridges in search of their
  desired node
• They do not cross routers
• The router forms a logical boundary of the
  network

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Fujitsu GeoStream R900 Router