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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Categories of Networks

There are three primary types of networks including local area networks, metropolitan area networks and wide area networks. Here we differentiate them by their size, ownership, distance and structure.

Local Area Network (LAN) :
A local area network is a private network having links of devices within a campus,building or office. A LAN can have two or more systems connected or devices i.e printers in a office. It also depend on the technology used in the networking.
In LAN we basically share data or sometimes  hardware. The restrictions can be used on some particular software or hardware for the usage, by the number of  users. It also depends which topology and transmission medium is used.
LANs have data rate from 4 to 6 Mbps range. Now a days its increasing and may be 100 Mbps depending on the speed of systems.

Metropolitan Area Network (MAN) :
A metropolitan area network is more widely can be used. It can be used in a entire city. It may be a single network or it can be combination of some LANs. A MAN can be used for a company having many offices in a city.
A MAN is owned by a company. Some service provider can also provide the MAN to a company by switched  multi megabit data services (SMDS).

Wide Area Network (WAN) :
A wide area network provides a long distance transmission of data. It may be overr large geographical areas thay may be acountry, a continent or whole world.
A WAN can use public or private communication devices or usually in combination. It can be on unlimited number of miles.

Internetworks:
When two or more networks are connected, they become an internetwork or internet. The networks are connected by internetworking devices i.e routers,gateways.
The term internet is a generic term used to mean an interconnection of networks.
The Internet is the name of a specific worldwide network.

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Transmission Mode

The direction of a signal between two devices is defined by a transmission mode.
Or
The transmission mode defines the direction of a signal between two devices.There are three types of transmission mode given as below;

Simplex Mode:
In the simplex mode the communication is unidirectional. Only one can send data and other can receive. No any other possibility is available. A remote is an example of simplex mode.

Half Duplex Mode:
In half duplex  mode both devices can send and receive data, but not at the same time. At a time one device send data and other receives. Walkie talkie is an example of half duplex mode.

Full Duplex Mode:
In full duplex mode both devices can send and receive data at the same time. It has two way traffic at the same time. The telephone network is an example of full duplex mode.

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Network Criteria

For an effecient and effective networking the following criteria is most important.

Performance:
The performance depends on many factor some are discussed below:

Number of  Users:
A large number of users on a network can slow its reponse time if it is designed for heavy traffic loads. In peak times on a certain network can slow its performance.

Transmission Medium:
The transmission medium used for the network also a main factor for its performance. As data has to travel on it. It can be fibre optics for a faster transfer of data.

Hardware:
The hardware used in networking affect the speed and capacity of transmission. A high computer with big storage will perform better.

Software:
The software used for running applications and data processig at sender reciever end and intermediary nodes also effects the performance of the network.

Reliability:


Rliability of a network on the followings;

Frequency of Failure:
All networks fail occasionally. But a network that fails often have little value to a user.

Recovery Time:
The time taken to restore itself after the failure is recovery time. The network quickly restores is more useful than the other takes much time.

Catastrophe:
The network must be protected by fire,earthquake or theft. There may be a system for back up of network software.

Security:
Network security isues include unauthrized access and viruses.

Unauthorized Access:
On a network there may be sensitive data, therfore it must be protected by an unauthorized access. It may be user identification and password or at higher level it can be encryption techniques.

Viruses:
As network can be accessed at many points, it may have computer virus. A virus is a code that damages the system. A good network must have the virus protection by hardware or software based.

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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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