TCP/IP Protocol

The TCP/IP protocol suite is used in internet was developed prior to OSI model. Therefore the layers used in Transmission control protocol/Inter networking protocol are different from layers in OSI model.
The TCP/IP protocol is made of five layers; physical,data link, network, transport and application layer. The first four layers provide the physical standards, network interface, inter-networking and transport functions that correspond to first four layers of OSI model.

In TCP/IP protocol the top three layers are shown by single application layer. The different modules in TCP/IP suite are relatively independent and changes as per the need of system. The term hierarchical means top layer is supported by below layers.
At transport layer TCP/IP defines the transmission control protocol(TCP) and user datagram protocol(UDP)
At network layer the inter-networking protocol is defined(IP). Some data movement protocols are also supported on this layer.
                                               

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Application Layer

The application layer enables the user to access the network. It provides user interface and support foe services such as mail,remote file access and transfer, shared database management. It relates between presentation layer and user.
The services provided by application layer are :

• Network Virtual terminal; It is a software version of a physical terminal and allows a user to log on to a remote host.
• File transfer,Access and Management; It provides a user to access and transfer files in a remote computer.
• Mail Services; it provides the email forwarding and storage.
• Directory service; It provides distributed database sources and access for global information.

                                            

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Presentation Layer

The presentation layer is concerned with the syntax and semantics of information exchanged between two system.
The presentation layer has following responsibilities;

•  Translation; The process of changing information in two systems is usually in the form of character strings,numbers and so on. The information is changed into a bit stream by encoding on presentation layer.
• Compression; Data compression reduces the number f bits to be transmitted. It is important mostly in the transmission of multimedia such as text,audio and video.

                                           

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Session Layer

The session layer is te network dialog controller. It establish,maintain and synchronize the interaction between communicating systems.
The responsibilities of session layer are:

• The session layer allows two systems to enter into a dialog. It allows the communication between the two in half or full duplex. It called dialog control.
• It make synchronization checkpoints into a stream of data. The checkpoints can be specified by time limit or data limit.

                                          

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Transport Layer

The transport layer is responsible for source to destination delivery of the entire message. The transport layer ensures that the whole message arrive intact and in order and check all the packets relation to each other. Also check the error control and flow control at source to destination level.
For additional security, network layer may create the connection between two end ports. It involves the etablishment, data transfer and connection release.
Transport layer have following responsibilities;

• Service point addressing; As compters run many programs at the same time. So here source to destination delivery is not from one computer to other but it is froma specific program on one computer to a specific program on the other. Therefore transport layer header must include a service point address or port address.The transport layer gets the entire message to the correct program running on computer.
• A mesasge is divided into transmittable segmentrs, each segment contains a sequence number. These number used in reassembling.
• The transport layer can be connectionless or connection oriented. In connectionless it simply deliver packet to network layer at destination but in connection oriented it makes connection with the transport layer and then deliver packets.
• Like data link layer it is responsible for flow control. Here flow control is end to end not on a single link.
• Like data link layer it is responsible for error control. It makes sure that the entire message is delivered correctly at the destination without any error.

                                                          

                                   

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Network Layer

The network layer is responsible for the source to destination delivery of a packet possibly across multiple networks. It also determines the delivery of packet from origin to destination in a network.
If two systems connected to same network then network layer is not needed. But for different network from source to destination delivery it is needed.
Some responsibilties of network layer are below:

• For different networks the network layer adds a header to the packet coming from upper layer including logical address of sender and receiver.
• When the independent networks are connected together to create an internetwork, the connecting devices ( routers and gateways ) route the packets to their destination. 

                                            

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Data Link Layer

The data link layer transforms the physical layer a a raw transmission facility to a reliable link and is resposible for nod eto node delivery. It makes the physical layer error free for the network layer.
This layer includes the following responsibilities :

• It divides the stream of bits received from network layer into manageable data units called frames.
• If frames are to be distributed to different systems on network, it adds a header to the frame to define the physical address of the sender(source address) and/or receiver(destination address) of frame.
• It also control that the data rate absorbed by reveiver must match the data rate at which is sent by sender called flow control.
• It also control the retransmitting of data if any frame is lost or damaged and the duplication of frames called error control.
• If two or more devices are connected to a same link. Then data link layer determines that which device has control over the link at any given time.

                                           

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Physical Layer

The physical layer coordinate the functions required for the transmission of a bit stream over a physical medium. It deals with the interface and transmission medium. It is concerned with the following:

• The physical characteristics of interface between medium and devices.
• At physical layer data must be in 0 and 1 (bit form). So it define the encoding.
• Tha data rate is defined that how many bits per second are sent.
• The sender and receiver must be synchoronised at bit level.
• The line configuration is also concerend with physical layer. It may be point to point or multipoint.
• The topology used for the network is also concerned with the physical layer.e.g; star,mesh,ring etc.
• The transmission mode is also concerned with physical layer. It may be simplex,half or full duplex.

                                        

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The OSI Model

The OSI model is a model for understanding and designing a network architecture that is flexible. It is a layered famework which allow communication across all types of computer systems. It consist of seven separate layers which define a segment of process of moving information across a network. The OSI model provides the basis ofor exploration of data communication.

The seven ordered layers of OSI model are :
1. Physical Layer
2. Data Link Player
3. Network Layer
4. Transport Layer
5. Session Layer
6. Presentation Layer
7. Application Layer

When a message travels from one device to other, it travels through different intermediary nodes. These intermediary nodes usually involves the first three layers of OSI model.
At first layer the entire package is converted to a form that can be transferred to the receiving machine. At the receiving machine, the message is unwrapped layer by layer,with each process receiving and removing the data meant for it.
The passing of data anf information through the layer is made possible by an interface between each pair of adjacent layers.

The seven layers further divided in three subgroups.

Layers 1,2 & 3 physical,data link and network layer are network support layers. They deal with the physical aspect of moving data from one device to another.

Layers 5,6 & 7 session,presentation and application layer are user support layers. They allow the interoperability among the softwares.

Layer 4 transport layer ensures the end to end reliable data transmission whereas the data link layer ensures reliable transmission on a single link.

The upper OSI layers are used in softwares.
The lower layers are combination of software and hardware, except physical layer which is mostly hardware.

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