OSI

The purpose of OSI ( Open Systems Interconnection) Model to provide a set of design standards for equipment manufacturers so they could communicate with each other.

Nice explanation of various layers: OSI & TCP/IP Models 

More details on OSI layers and how different protocols and devices fit into these layers: OSI Layers, Protocols, Devices . Routers work at network layer(3) , switches works at Data Link Layer (2) and hubs and cables work at Physical layer(1).  

Network Devices : Routers, Switches, Hubs

One key difference between switch and hub is that switch switches the data frames  intelligently to the  port which is connected to the destination device, while hub sends it to all the ports. Hub tends to cause traffic congestion in the network and data frames end on devices which are not the intended destination for it and those devices have to process it unnecessarily only to figure that it is not intended for them.  Router is needed to route traffic between different networks.  Switches/Hubs are used within same network. 

OSI vs TCP/IP 

OSI model is logical and protocol independent. It explains all kinds of communication between devices.  TCP/IP model was developed  by US DoD to facilitate certain protocols ( HTTP, FTP, SMP, POP, SMTP, DNS ). It can only explain those specific protocols. TCP/IP does not take responsibility for sequencing and acknowledgement functions, leaving these to the underlying transport layer.

TCP/IP has only four layers in place of sever layers for OSI and it collapses multiple OSI layers into single layers: 

Application ( Application,Presentation,Session ) , Transport ( Transport), Internet(network), Network Access ( Data Link, Physical ) 

Fun Facts

Acronyms to remember the OSI layers: 

Bottom-Up: 

  • Please Do Not Throw Sausage Pizza Away
  • Pew! Dead Ninja Turtles Smell Particularly Awful
  • People Don’t Need To See Paula Abdul
  • Pete Doesn’t Need To Sell Pickles Anymore

Top_Down:  

  • All People Seem To Need Data Processing
  • All Pros Search Top Notch Donut Places
  • A Penguin Said That Nobody Drinks Pepsi
  • A Priest Saw Two Nuns Doing Pushups

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