OSI Model

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OSI Model
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Switches

• point to point bridges
• two types
• store forward entire frame received the
  decision made, and can handle frames with errors
• cut-through starts to transmit frame as
  destination address received, cannot handle
  errored frames but are the fastest bridges
• latency data transmission delay created by
  frame or packet processing

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DATA LINK LAYER (BRIDGE)
Application
Application
Presentation
Presentation
Session
Session
transport
transport
BRIDGE
Network
Network
Data link
Data link
Data link
Data link
Physical
Physical
Physical
Physical

• EXAMPLE MAC Bridge

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Bridge Operation
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How a Bridge works!

• listens to all traffic
• checks source and destination addresses
• builds a routing table
• forwards packets on the following basis
• if destination not in routing table send to all
  segments
• if destination in routing table forward to that
  segment only
• bridges segment networks hence reducing network
  traffic in resulting segments
• Spanning Tree Algorithm (SPA) can be used to
  prevent data going in a continuous loop

Source Segment
Nodes
A D F
1, 2, 3 5, 45, 68 34, 233
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Bridge Protocol Data Units

• bridges announce themselves to each other using
  special MAC frames - BPDUs
• a bridge is elected root bridge
• they discover other bridges and paths (including
  how many hops away from each other)
• spanning tree algorithm used to disable loops
• spanning tree can be turned off and bandwidth
  used by BPDUs saved

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Multiple LANs
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Routing options

• Transparent
• Fixed
• Spanning Tree

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Bridges That Use A Spanning Tree Algorithm

• Spanning Tree Algorithm Used To Dynamically
  Discover A Loop-free Tree In Topology
• Bridges Exchange Configuration Messages To
  Calculate A Spanning Tree
• When the Topology Changes, the Spanning Tree is
  Re-calculated
• Spanning Tree Algorithm
• Frame Forwarding
• Address Learning
• Loop Resolution

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NETWORK LAYER (ROUTER)
Application
Application
Presentation
Presentation
Session
Session
ROUTER
transport
transport
Network
Network
Network
Network
Data link
Data link
Data link
Data link
Physical
Physical
Physical
Physical

• EXAMPLE IP Router

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Routers I

• operates at the Network layer of the OSI model
• interconnect networks
• meets need for best path
• can filter, isolate and connect segments
• better traffic managers than bridges and do not
  pass broadcast storms
• has interfaces on multiple networks
• can share routing information with other routers
• uses NETWORK NUMBERS to forward packets

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Routers II

• forwards packets between networks and transforms
  packets as required to meet standards for each
  network
• routers maintain a TABLE as follows to determine
  route
• all known network addresses
• how to connect to other networks
• possible paths between routes
• costs of sending data over paths
• router selects best route based costs and
  available paths

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APPLICATION LAYER GATEWAY
APPLICATION GATEWAY
Application
Application
Presentation
Presentation
Presentation
Presentation
Session
Session
Session
Session
Transport
Transport
Transport
Transport
Network
Network
Network
Network
Data link
Data link
Data link
Data link
Physical
Physical
Physical
Physical

• EXAMPLES DBMS, E-MAIL, WP