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OSI Model

  • Ops
tags: ccna

OSI Model

  • By International Organization of Standard


  • Split development/role: hide developer from lower layer
  • Quicker development


You need to remember both the name and the layer number

  • Layer7: Application
  • Layer6: Presentation
  • Layer5: Session
  • Layer4: Transport
  • Layer3: Network
  • Layer2: DataLink
  • Layer1: Physical

Trick: All People Sleeping Through Networking Don’t Pass

Network Engineer: Focus on 1, 2, 3, 4 Layers
Web Developer: Focus on 5, 6, 7 Layers


When we talk about telnet, we’re not talking about Tera Term or putty these telnet applications, we’re talking about telnet PROTOCAL.
Likewise, we’re talking about PROTOCAL on HTTP, not on chrome or firefox.

Use PROTOCAL to communicate with lower layers of OSI Models

Network processes to applications

  • provide access for users
  • provide network service to application processes
  • identity communication patrnets
  • provide USER authentication


Data Representation / syntax

  • data is READABLE by receiving system, to Machine Independent Format
  • formats data to be presented to application layer
  • structures data
  • negotiates data transfer syntax for application layer, e.g. ACHII or unicode
  • provides encryption

Ensure the data transmitted in the format that both sides understand, e.g. We can connect a .mp3 file to a website running an Apache server on Linux from your windows machine or from a mobile device such as an iPhone. Thus we can listen to it.


Interhost Communication

  • establishment, maintenance and termination of sessions between applications
  • coordinator of communication between systems: two application can establish a session
  • provide security, name recogition, logging… via sessions

Network Basic Input/Output System(NetBIOS): As an API using Point-to-point Tunneling Protocal(PPTP) to communicate between interhosts


End to End connections

  • split message to smaller unit
  • handles transportation issues between host
  • ensure data transport reliablilty

Main protocal:

  • Transmission Control Protocal (TCP):
    • establishes, matains, terminates of VIRTUAL CIRCUITS between devices
    • 3-way handshake
    • provide reliablilty
    • Flow Control: transmission management
    • Session Multiplexing: serveral message streams or sessions onto one logical link
  • User Datagram Protocal (UDP)
    • does not provide reliablilty: if package dropped, they’re lost
    • does not transmit lost pocket
    • VoIP
    • lightweight

Think TCP as a telephone call, UDP as a postal service


Data Delivery

  • routes data packets

Layer 3 switches

  • have router capabilities
  • select the best path to deliver data base on criteria(cost, hop, bandwith):
    • Open Shortest Path First (OSPF)
    • Boarder Gateway Protocal (BGP)
    • Intermediate System to Intermediate System (IS-IS)
  • path determination and logical addressing
  • routing decisions are based on logical addressing format in IPv4, IPv6
  • not concern with reliablilty, it relies on Layer 4

Access to Media

  • defines how data is formatted for transmission
  • defines how to access the network
  • provides error detection

e.g. Ethernet, the data send to the network need to formatted with Ethernet rules. May traverse WAN link that may uses Point to Point Protocal (PPP)


Binary Transmission

  • what state represents a 0 or 1
  • defines the electrical, mechanical, proceural and functional specifications for manage a physical link
  • focused with physical devices and physical cabling

Data Encapsulation and De-encapsulation

Information must go through the OSI model from layer7 down to layer1


Each layer of the OSI model only communicates with the equivalent layer on the other device.

Protocol Data Unit:

  • Layer7. Data
  • Layer6. Data
  • Layer5. Data
  • Layer4. Segments
  • Layer3. Packets
  • Layer2. Frames
  • Layer1: Bits

How remote host know which appliction to forward the traffic to at higher layers?

Layer 2 -> Layer 3: NIC will read the Type number at type field
Layer 3 -> Layer 4: Protocal number
Layer 4 -> Layer 5: Port number


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