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OSI MODEL
- PHYSICAL (LAYER1) This layer is concerned with the transmission and
reception of the bit stream over a physical medium. Physical Layer
Carries the signals for all of the other layers. If there is a problem at the
physical layer, the rest of the layers are effected because data cannot be
transmitted. (uses bits)
- DATA (LAYER2) The data link layer provides error-transfer of data frames
from one node to another over the physical layer, allowing layers above it to
assume virtually error-free transmission over the link. (Uses Frames)
- NETWORK (LAYER3) The Network Layer Controls the operation of the subnet, deciding which
physical path the data should take based on the network conditions, priority of service, and other
factors. It Provides ; Routing; Routes frames among networks. Subnet Traffic control.
- TRANSPORT (LAYER4) Ensures that messages are delievered error-free, in sequence, and with no losses or
duplications. Seperates a large message in to small amounts. Message Acknowledgements from TCP increases
reliability over recieving/ sending.Session multiplexing: multiplexes several message streams, or sessions onto one
logical link and keeps track of which messages belong to which sessions. Typically, the transport layer can accept
relatively large messages, but there are strict message size limits imposed by the network (or lower) layer.
Consequently, the transport layer must break up the messages into smaller units, or frames, prepending a header to
each frame. (Uses segments)
- SESSION (LAYER5) The session layer allows session establishment between processes running on
different stations. It provides: Session establishment, maintenance and termination: allows two application
processes on different machines to establish, use and terminate a connection, called a session. Session
support: performs the functions that allow these processes to communicate over the network, performing
security, name recognition, logging, and so on.(Uses Data)
- PRESENTATION (LAYER6) The presentation layer provides: Character code translation: for example, ASCII to
EBCDIC. Data conversion: bit order, CR-CR/LFinteger-floating point, and so on. Data compression: reduces the
number of bits that need to be transmitted on the network. Data encryption: encrypt data for security purposes. For
example, password encryption.(Uses Data)
- APPLICATION (LAYER7)The application layer serves as the window for users and application processes to
access network services. This layer contains a variety of commonly needed functions: Resource sharing and
device redirection, Remote file access ,Remote printer access Inter-process communication ,Network management
Directory services, Electronic messaging (such as mail) Network virtual terminals
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