OSI Reference Model:

OSI reference model is a logical framework for standards for the network communication. OSI reference model is now considered as a primary standard for internetworking and inter computing. Today many network communication protocols are based on the standards of OSI model. In the OSI model the network/data communication is defined into seven layers. 

To remember the names of seven layers in order one common mnemonic used is -"All People Seem to Need Data Processing". i.e

All- Application

People-Presentation

Seem- Session

To- Transport

Need- Network

Data- Data Link

Processing- Physical.

Application Layer:

The application layer serves as the window for users and application processes to access network services. The application layer makes the interface between the program that is sending or is receiving data and the protocol stack.

Functions of Application  layer:

·         Resource sharing and device redirection.

·         Remote file access.

·         Remote printer access.

·         Inter-process communication.

·         Network management.

·         Directory services.

·         Electronic messaging (such as mail).

Presentation Layer

Presentation Layer is also called Translation layer. The presentation layer presents the data into a uniform format and masks the difference of data format between two dissimilar systems.

Functions of Presentation Layer:

·         Character code translation: for example, ASCII to EBCDIC.

·         Data conversion: bit order, CR-CR/LF, integer-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.

Session Layers:

Session layer has the primary responsibility of beginning, maintaining and ending the communication between two devices, which is called Session. It also provides for orderly communication between devices by regulating the flow of data.

The examples of session layers and the interactive logins.

Functions of Session Layer:

·          Session establishment, maintenance and termination.

·       Session support: performs the functions that allow these processes to communicate over the network, performing security, name recognition, logging and so on.

·      Dialog control: Dialog control is the function of session layer that determines which device will communicate first and the amount of data that will be sent.

·   Dialog separation or Synchronization: The session layer is also responsible for adding checkpoint or markers within the message. This process of inserting markers to the stream of data is known as dialog separation.

·    Protocols: The protocols that work on the session layer are NetBIOS, Mail Slots, Names Pipes, and RPC.

Transport Layer:

Transport layer manages end to end (source to destination) (process to process) message delivery in a network and also provides the error checking and hence guarantees that no duplication or errors are occurring in the data transfers across the network. It makes sure that all the packets of a message arrive intact and in order.

Functions of Transport Layer:

·         Segmentation of message into packet and reassembly of packets into message:

·         Message acknowledgment

·         Message traffic control

·         Session multiplexing

·         Service point addressing

·         Flow control

Network Layer:

This layer is in charge of packet addressing, converting logical addresses into physical addresses. It is responsible for the source-to-destination delivery of a packet across multiple networks (links). This layer is also in charge of setting the routing. The packets will use to arrive at their destination, based on factors like traffic and priorities. The network layer determines that how data transmits between the network devices.

Functions of Network Layer:

·        Subnet Traffic Control: Routers (network layer intermediate systems) can instruct a sending station to "throttle back" its frame transmission when the router's buffer fills up.

·         Logical-Physical Address Mapping

·         Subnet Usage Accounting

·         Internetworking

·         Logical Addressing

·         Routing

·         Packetizing

·         Fragmentation

      

Data Link Layer:

It is responsible for reliable node-to-node delivery of data. It receives the data from network layer and creates frames, add physical address to these frames and pass them to physical layer

The data link layer provides error-free 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.

Functions of Data Link Layer:

·         Link Establishment and Termination

·         Physical addressing

·         Frame Traffic Control

·         Frame Sequencing

·         Frame Acknowledgment

·         Frame Error Checking: 

·         Media Access Management

·         Flow control 

·         Error control 

·         Access control 

Physical Layer:

The physical layer, the lowest layer of the OSI model, is concerned with the transmission and reception of the unstructured raw bit stream over a physical medium. It describes the electrical/optical, mechanical, and functional interfaces to the physical medium, and carries the signals for all of the higher layers. Physical layer defines the cables, network cards and physical aspects.

Functions of Physical Layer:

·         Data Encoding: 

·         Transmission Technique:

·         Physical Medium

·         Protocols : ISDN, IEEE 802 and IEEE 802.2.

·         Bit synchronization

·         Provides physical characteristics of interfaces and medium

·         Bit rate control

·         Line configuration

·         Transmission mode

·         Physical topologies

·         Multiplexing

·         Circuit switching

Why Layering?

·         Divide a task into pieces and then solve each piece independently (or nearly so).

·         Establishing a well defined interface between layers  makes porting easier.

·         Functions of each layer are independent of functions of other layers

• Thus each layer is like a module and can be developed independently

·         Each layer builds on services provided by lower layers

• Thus no need to worry about details of lower layers -- transparent to this layer

Major Advantages

n  Code Reuse

n  Eases maintenance, updating of system

TCP/IP

TCP/ IP stands for Transmission Control Protocol/ Internet Protocol. If this leads you to think that it is not just one protocol, you’re right. In fact, it is not just two protocols, either. TCP/ IP is a suite of protocols.

TCP/IP is a family of protocols. A few provide "low- level" functions needed for many applications. These include IP, TCP, and UDP. Others are protocols for doing specific tasks, e.g. transferring files between computers, sending mail, or finding out who is logged in on another computer. Initially TCP/IP was used mostly between minicomputers or mainframes. These machines had their own disks, and generally were self contained.

 

Application Layer

The application layer is provided by the program that uses TCP/IP for communication. An application is a user process cooperating with another process usually on a different host (there is also a benefit to application communication within a single host). Examples of applications include Telnet and the File Transfer Protocol (FTP).

The Process Layer contains protocols that implement user-level functions, such as mail delivery, file transfer and remote login.

Transport Layer

The transport layer provides the end-to-end data transfer by delivering data from an application to its remote peer. Multiple applications can be supported simultaneously. The most-used transport layer protocol is the Transmission Control Protocol (TCP), which provides connection-oriented reliable data delivery, duplicate data suppression, congestion control, and flow control.

Another transport layer protocol is the User Datagram Protocol It provides connectionless, unreliable, best-effort service. As a result, applications using UDP as the transport protocol have to provide their own end-to-end integrity, flow control, and congestion control, if desired. Usually, UDP is used by applications that need a fast transport mechanism and can tolerate the loss of some data.

 

Internetwork Layer

The internetwork layer also called the internet layer or the network layer, provides the “virtual network” image of an internet this layer shields the higher levels from the physical network architecture below it. Internet Protocol (IP) is the most important protocol in this layer. It is a connectionless protocol that does not assume reliability from lower layers. IP does not provide reliability, flow control, or error recovery.

 These functions must be provided at a higher level. IP provides a routing function that attempts to deliver transmitted messages to their destination. A message unit in an IP network is called an IP datagram. 

This is the basic unit of information transmitted across TCP/IP networks. Other internetwork-layer protocols are IP, ICMP, IGMP, ARP, and RARP.

 

Host to network(PL n DLL)

The network interface layer, also called the link layer or the data-link layer or Host to Network Layer, is the interface to the actual network hardware. This interface may or may not provide reliable delivery, and may be packet or stream oriented.

In fact, TCP/IP does not specify any protocol here, but can use almost any network interface available, which illustrates the flexibility of the IP layer. Examples are IEEE 802.2, X.25,ATM, FDDI, and even SNA.TCP/IP specifications do not describe or standardize any network-layer protocols, they only standardize ways of accessing those protocols from the internet work layer.

Difference between OSI and TCP model.

OSI(Open System Interconnection)	TCP/IP(Transmission Control Protocol / Internet Protocol)
1. OSI provides layer functioning and also defines functions of all the layers.	1. TCP/IP model is more based on protocols and protocols are not flexible with other layers.
2. In OSI model the transport layer guarantees the delivery of packets	2. In TCP/IP model the transport layer does not guarantees delivery of packets.
3. Follows horizontal approach	3. Follows vertical approach.
4. OSI model has a separate presentation layer	4. TCP/IP does not have a separate presentation layer
5. OSI is a general model.	5. TCP/IP model cannot be used in any other application.
6. Network layer of OSI model provide both connection oriented and connectionless service.	6. The Network layer in TCP/IP model provides connectionless service.
7. OSI model has a problem of fitting the protocols in the model	7. TCP/IP model does not fit any protocol
8. Protocols are hidden in OSI model and are easily replaced as the technology changes.	8. In TCP/IP replacing protocol is not easy.
9. OSI model defines services, interfaces and protocols very clearly and makes clear distinction between them.	9. In TCP/IP it is not clearly separated its services, interfaces and protocols.
10. It has 7 layers	10. It has 4 layers