OSI MODEL
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:
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It is responsible for carrying individual bits.
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There is a hidden layer within it called transmission media
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It defines mechanical, electrical, procedural and functional aspects.
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Mechanical aspects include cable type & medium of link.
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Electrical include impedance, operating power, attenuation, frequency & bandwidth.
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Functional aspects include data & control functions.
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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:
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It is responsible for taking datagram & move across link.
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It is responsible for moving packets through link.
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It specifies data format, sequence, acknowledgement & error detection.
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Two sub layers in it is MAC(media access control) & LLC(logic link control.
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MAC covers address management functions & responsible for network access & control.
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LLC manages flow & error control, ARQ(automatic request for retransmit) methods, acknowledgments & handshaking functions.
3) Network layer:
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There are several routers from host to host, it is responsible for choosing best routing path
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Data moves in form of datagram.
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It doesn’t provide flow control, congestion control or error control.
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There are two virtual circuits used in network layer: connection-less & connection oriented.
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In connection oriented, network link is always available. So system resources are always used even if no message is to be sent.
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In connection-less, when message is finished connection is dropped.
4) Transport layer:
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As source host gets message from application layer it encapsulates in form of user datagram and sends it.
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TCP (transmission control protocol) is connection oriented protocol which establishes logical connection between two transport layers.
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UDP ( user datagram protocol) transmits user datagram without creating logical connection.
Features:
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:
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Concerns file management & networking functions.
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It provides method by which station can organize, synchronize & manage transfers within themselves.
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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:
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Meets higher level of interfacing requirements.
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Include data compression, format conversion, encryption & decryption.
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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.