OSI Model (Open Systems Intercommunication)


7 Layers

·        7. Application               Away

·        6. Presentation              Pizza

·        5. Session                     Sausage

·        4. Transport                 Throw

·        3. Network                  Not

·        2. Data Link                 Do

·        1. Physical                    Please


7. Application

·        Handles denial network access, flow control, & errors

·        Allows application processes to access network services & manages communication b/t applications.

·        Directly represents services that support user applications such as software that transfers information such as email & file transfers


6. Presentation

·        Determines format used to exchange data (“Network translator”)

·        Responsible for protocol conversion, encrypting data, & managing data compression (to reduce # of bits transmitted)

·        Contains software called “Redirector Functions” that accepts I/O requests


5. Session

·        Provides synchronization b/t user tasks by placing checkpoints in data stream

·        Implements dialog control b/t communicating processes by regulating which side transmits, when, for how long, and so on.

·        Allows 2 applications on different computers to establish, use, & end connection called a session

·        Performs name recognition and functions, such as security, needed to allow 2 applications to communicate on the network


4. Transport

·        Provides flow control error handling, and is involved in solving problems concerned with the transmission & reception of packets

·        Ensures packets are delivered error free in sequence with no losses/duplications

·        Un/Repackages messages split into multiple packets into one/multiple package(s)

·        Typically sends acknowledgment of receipt.


3. Network

·        Addresses messages & translates addresses from logical into physical addresses.

·        Determines route (source to destination computer) – based on network conditions

·        Manages traffic problems on network such as packet switching & routing

·        If receiving computer can’t take data chunks as large as source computer is sending then network layer on source computer breaks down more

·        (Receiving end) Reassembles data into the original large chunk


2. Data Link

·        Sends data frames from Network layer to Physical layer

·        (Receiving end) Packages raw bits into data frames (logical structure for data)

o       Frame is often referred to as a packet, but there’s a difference

o       Frame has beginning & ending delimiter, while Network layer packets & packets encapsulated @ higher levels have only headers


1. Physical

·        Addresses transmission of unstructured raw bitstream over network cable

·        Defines …

o       how cable is attached to network adapter card

o       transmission technique to be used

o       how long each bit last

o       how each bit is translated into appropriate elec. or optical impulse


Transmitting Data

·        When data is sent, each layer adds a header with information then passes on

·        Data Link layer also adds a trailer

·        Physical layer (with all headers & trailer) passes to physical network


      __Data____                                              _____+_____+_+__Data____


·        When receiving, each layer removes appropriate header & trailer (if included), and reads info before sending data on to next layer


      _____+_____+_+__Data____                                                __Data__


·        This is repeated for every frame sent b/t the computers.