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.
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.