O.S.I model.

the O.S.I model starting from the top application levels working down to more physical aspects.

open system interconnection = O.S.I

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layer 7

layer 7 - the application layer

  • provides application support
  • defines the way in which applications interact wit hthe user. as the top layer it provides a gateway through which all the other layers are accessed.
  • type of communication: E-mail, File Transfer, Client/Server




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layer 6

layer 6 - presentation layer

  • data format conversion, data compression and encryption.
  • this layer defines the way in which data is represented. it deals with the translation of data that arrives via the network in a different format to the one used by the hostcomputer. for example, displaying UNIX style data on an MS-DOS interface.
  • encryption, data conversion:


BCD to binary, etc.


examples: .zip, conversion file formats, compressiono software

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layer 5

layer 5 - session layer

  • user identification; establishing, maintaining, and terminating a session.
  • this sharing of a connection between networked computers is refered to as a session. tje session layer handles events such as synchronisation of data transmission, permission requests, etc. this layer also sets boundaries to define the start and finish of the message.
  • starts, stops session, maintains order.


examples: SIP

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layer 4

layer 4 - transport layer.

  • end-to-end transport.
  • the transport layer is concerned with ensuring the data arrices at the destination computer(s) correctly. this may involve subdividing the data into segments, adding check sums to trap transmission errors, determining the optimum size for each segment, etc.
  • ensures delivery of entire message


examples: TCP, UDP, RTP, RTCP, check sums.

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layer 3

layer 3 - network layer

  • addressing, routing.
  • the network lsyer is primarily concerned with the movement of the data across the network. this involves defining segmentsof data as appropriately sized packets, addressing the packets, etc. it also defines the use of routers, switches, etc. to create an optimum route for the data.
  • routes data to differemt LANs and WANs based on network address.


examples: IP, IPSec, VoIP, IPX, NetBEUI, AppleTalk, ICMP

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layer 2

layer 2 - data link layer

  • medium access control error detection, retransmission.
  • the data link layer supervises the actual transmission of data, confirming checksums and creating duplicates of data packets. the data link layer holds the duplicate packet until the point on the route confirms that is has received a correct copy.
  • transmits packets from node to node based on station address.


examples: Ethernet, Wi-Fi, HomePNA, HomePlug, PPP, PPTP, L2TP, ATM, Frame Relay, Token Ring, FDDI.

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layer 1

layer 1 - physical layer

  • electrical/optical signaling, cabling, connector pin assignment,
  • the physical layer is concerned with the physical aspects of network transmission, the cabling, network cards, wireless network devices, etc. it specifies how the data will actually travel along these media, what translation will need to happen.
  • electrical signals and cabling.


examples: RF, UTP, STP, Coax, Fibre Optic, Connectors, Signalling, Voltages.

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basic overview

  • layering is the process of each successive layer building upon its predecessor.
  • each layer is independant of other layers.
  • each layer provides services/functionality to its predecessor.
  • layering is seperating the components into physical and logical.

benefits of O.S.I

  • allows standardisation of any make of device and operating system to be able to communicate to each other.
  • allows devices/software of any make to communicate.
  • precise identification of errors/error report identifies cause of error/where error has occured.
  • easy to update (the protocol) because only relevant layers need to be updated.
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good but a few typos!

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