OSI Layers


International standardization organization understood a need of standardizing multilevel networking which posses’ flexible structure.


Activities and requirements are detailed at each layer.

There are seven distinct layers:

1) Physical layer:

  • It is responsible for carrying individual bits.

  • There is a hidden layer within it called transmission media

  • It defines mechanical, electrical, procedural and functional aspects.

  • Mechanical aspects include cable type & medium of link.

  • Electrical include impedance, operating power, attenuation, frequency & bandwidth.

  • Functional aspects include data & control functions.

  • Procedural aspects include lowest protocol rules required for good link.

Features of physical link:

a) Data rate: defines data rate and baud rate required.

b) Synchronization: Clock must be synchronized.

c) Line configuration: Line to line ,Line to multiple.

d) Physical topology: Star, bus, ring.

e) Transmission mode: Simplex, half duplex, full duplex.

2) Data link layer:

  • It is responsible for taking datagram & move across link.

  • It is responsible for moving packets through link.

  • It specifies data format, sequence, acknowledgement & error detection.

  • Two sub layers in it is MAC(media access control) & LLC(logic link control.

  • MAC covers address management functions & responsible for network access & control.

  • LLC manages flow & error control, ARQ(automatic request for retransmit) methods, acknowledgments & handshaking functions.

3) Network layer:

  • There are several routers from host to host, it is responsible for choosing best routing path

  • Data moves in form of datagram.

  • It doesn’t provide flow control, congestion control or error control.

  • There are two virtual circuits used in network layer: connection-less & connection oriented.

  • In connection oriented, network link is always available. So system resources are always used even if no message is to be sent.

  • In connection-less, when message is finished connection is dropped.

4) Transport layer:

  • As source host gets message from application layer it encapsulates in form of user datagram and sends it.

  • TCP (transmission control protocol) is connection oriented protocol which establishes logical connection between two transport layers.

  • UDP ( user datagram protocol) transmits user datagram without creating logical connection.


a) Mapping of address onto network.

b) Error detection & recovery.

c) Segmentation of message to maximize efficiency.

d) sets QoS (quality of service) to assure integrity & sequence traffic control.

5) Session layer:

  • Concerns file management & networking functions.

  • It provides method by which station can organize, synchronize & manage transfers within themselves.

  • It can discard all nodes, without node being aware that data was ever sent.

Features :

a) Connection & disconnection of nodes.

b) Authentication of access.

c) Binding processor to network address.

d) provide user interface to network

6) Presentation layer:

  • Meets higher level of interfacing requirements.

  • Include data compression, format conversion, encryption & decryption.

  • Also include character set & code translation, format & syntax resolution.

Three protocols used here are:

a) Virtual terminal protocol, allows different terminal to use different applications.

b) Virtual file protocol, handle code conversion.

c) Job transferring & manipulating protocol, controls structure of jobs & networks.

7) Application layer:

  • Process to process communication is duty of application layer.

Various protocols used here are:

a) HTTP (hypertext type protocol) is vehicle for accessing WWW (World Wide Web).

b) FTP (file transfer protocol) used for transferring file.

c) TELNET (telecommunication network) & SSH (secure shell) used to access site remotely.


Very informative