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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Distributed Processing

In a network a task is divided into multiple computers using multiple processing. Instead of a single large machine for different processes, many computers are used and following advantages can be gained by using distributed processsing.

Security/Encapsulation:
A system designer can limit the interaction of a user to an interface. for example in banks, the particular use only can see his own account not the whole bank data or accounts.

Distributed Databases:
A single system can not store the data of whole network. Therefore world wide web provide the access to information stored or available on the Internet.

Faster Problem Solving:
Different computers working on a problem can solve it earlier than a single computer solving it.

Security through Redundancy:
Multiple computers running the same program at the same time can provide security through redundancy. In space three computers runs the same application at a time so if one fails then other can override it.

Collaborative Processing:
Multiple computers and users can interact on a task. In network games each player see the action and effect of all others on the application running.

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Bus Topology

In bus topology multi point line configuration is used. One long cable acts as a backboneto link all the devices in the network.
Nodes are connected to the cable by drop lines and taps.
A drop line is a connection running between the device and main cable.
A tap is a connector that attached to the main cable to its mattalic core.
The limited number of taps can be used in bus topology because the signals become weak while treveling through wire and loose their energy in the form of heat.

Advantages:


1. Easy installation by laying the main cable along the effecient path.
2. Less cabling is required.
3. Drop line has to reach to nearest point on main cable.

Disadvantages:


1. Difficult reconfiguration and fault isolation.
2. Difficult to add more devices.
3. A fault or break in connection stops the entire transmission.

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Topology

The term topology refer to the representation of a network laid out either physically or logically.
When two or more devices are connected to a link then two or more links form a topology. It show the links of the entire network with the devcies attached for communication.

There are five basic topologies used for the communicating devices.

1. Mesh Topology
2. Star Topology
3. Tree Topology
4. Bus Topology
5. Ring Topology

There are two possible relationships for connecting the devices.


Peer-to-Peer
In peer to peer devices share the same link equally.

Primary-Secondary
In primary secondry devices one device controls the traffic and the others communicate through it.

Mesh and Ring topologies mostly use the peer to peer transmission. While Tree and Star topologies convenient for primary secondry connections. But Bus topology is equally convenient to both peer to peer and primary secondary.

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Data Communication

Data communication is the exchange of data (in the form of 0s and 1s) between two devices via some form of transmission medium(as wire cable).
Data communication is considered local if the communicating devices are in the same building or geographical area.
Data communication is considered remote if devices are farther apart.
For data communication to occur,the communicating devices must be the part of communication system. The effectiveness of data communication depends upon these three fundamental characteristics.

1.Delivery
The system must deliver the data to the correct destination. The data must be received by the device or user to whom it was sent.

2.Accuracy
The system must deliver data accurately. Data that have been altered in transmission and left uncorrected are unusable.

3.Timeliness
The system must deliver data in a timely manner. Data delivered late are useless. In real time tranmission data must be delivered without significant delay as in case of audio and video data.

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