Explain OSI Model

OSI Model

Initial computer networks had their own set of standards and conventions that were hardware dependent. Each manufacturer used todevelop its own communication protocols for its networks. For example IBM launched SNA (Sstems Network Architecture) in 1974. Similarly DEC (Digital Equipment Corporation ) launched DNA (Digital Network Architecture) in 1980 for use on the DEC range computers.Hence daa communication protocols of one network were not compatible with any other network.

International Organization for Standardization (ISO) recognized this problem and established a subcommittee to develop an international standard for network architechtecture. The result of this subcommittee’s recommendations was the Open System Interconnection (OSI) model. Hence, standardized OSI protocols made it possible for any two heterogenous computer systems, located anywhere in the world, to communicate easily with each other.

OSI  model provides following services:

1. Provides peer to peer logical services with layer physical implementation
2. Provides standards for communication between system.
3. Define point of interconnection for the exchange of information between systems.
4. Each layer should perform a well defined function.
5. Narrows the options in order to increase the ability to communicate without expansive conversions and translation between products.

The OSI model is designed in a highly structured way. A separate set of protocols is defined for each layer in the seven layer architecture. Roles of these seven layers are described below.

In actual implementation of these seven layers, first three layers are likely to be hardware, next two layers in operating system, presentation layer in library subroutines in user’s address space, and application layer in user’s program.

1. Physical Layer

Physical  layer  is responsible  for transmitting   raw bit streams  between  two nodes.  That  is, it converts  sequence  of binary  digits  into electric  signals,  light  signals, or electromagnetic   signals,  depending   on whether  the two nodes are on a ,cable circuit fiber-optic  circuit,  or microwave/radio   circuit,  respectively.  Even electrical details, such as how many volts to use for 0 and 1, how many bits can be send persecond, whether transmission can take place in one direction or both directions simultaneously are decided by physical layer protocols. In addition physical layer also deals with  mechanical details such as size and shape of connection plugs, number of pins in plugs and function of each pin. RS232-C is a popular physical layer standard for serial communication lines.

2. Data link Layer

Physical   layer  simply transmits          data  from   sender’s   node  to  receiver’s    node  as  raw  bits.  Data-link   layer   is responsible  for detecting and correcting             any error in transmitted   data.  Since physical  layer is concerned  only with a raw it stream ,  data-link  layer partitions   it into frames,  so that  error detection   and correction  can be performed independentaly  for each  frame.  Data-link   layer  also performs  flow  control  of frames       between  two sites  to ensure that a sender does not flood a receiver with data by sending  frames  at a rate faster than the receiver  can process.

3. Network Layer

The  network layer is responsible for delivery of packets from the source to destination. A transmitting network layer accepts amessage from the transport layer. During reception this layers accepts the data stream from data link layer and physical layer. A network layer interface can accept the data from a router. The message to the translport layer at the receiver’s end transmits themessage after the appropriate changes have been madein accordance with the transport protocal. The message intended for transmission may be commicated in parts. Before communicating a message to the next data link layer or router, the data byted of the message fragmenet into packets.

A packet is of specific totallength that a particular network permits. Each packet has a header. This has the additional bits for the packet’s source and destination addresses, sequence and the acknowledge managent, error control, flow control etc. The network layer has another important role. It allows an interconnection between the dissimilar networks. Two popular network layer protocols are the X.25 Protocol and the Internet Protocol (called IP).

4. Transport Layer

Transport  layer accepts  messages of arbitrary  length from  session  layer, segments  them  into packets,  submits  them to network   layer for trannsmission,  and finally reassembles packets    at destination.  Some packets  may be lost on the way from   sender  to receiver,  and depending  on routing  algorithms   used  in network  layer  packets  may arrive  at destination in a sequence   that  is different   from  the  order  in  which  they  were  sent.  Transport   layer  protocols include  mechanisms  for handling  lost and out-of-sequence   packets.  For this, the transport  layer assigns  a sequence number  to each packet,  and uses sequence  numbers  for detecting  lost packets  and for ensuring  that  messages  are reconstructed     in  correct  sequence.   Two  most  popular  transport   layer  protocols  are  Transport   Control  Protocol (TCP)  and User Datagram  Protocol  (UDP).

5. Session Layer

The session layer is network dialog controller i.e. it establishes and synchronizes the interaction between communication system. It allows communicating   parties  to authenticate  each other  before establishing   a dialog  session  between them, it   specifies  dialog  type  –  one-way,  two-way  alternate,   or two-way  simultaneous   –  and  initiates  a dialog session,  if message  is a connection   request    It also provides  priority  management   service that is useful for giving  priority  to important  and time-bound     messages  over normal less important messages.

7. Presentation Layer

Presentation   layer deals with syntax and semantics of the information being exchanged. It provides   facilities   to  covert  message  data  into  a  form  that  is meaningful   to  communicating application   layer   entities,   For  this,   it  may   perform   such   transformations    as  encoding   and  decoding,   code conversion,      compression    and   decompression,     encryption    and   decryption,   on  message  data   depending   on application’s   requirements.

8. Application Layer

Application layer is responsible for accessing the network by user. It provides user interfaces and other supporting services such as e-mail, remote, file access, file transfer, sharing database, message handling (X.400), directory service (X.500).