OSI Model

The OSI, or Open System Interconnection, model defines a networking framework for implementing protocols in seven layers. Control is passed from one layer to the next, starting at the application layer in one station, and proceeding to the bottom layer, over the channel to the next station and back up the hierarchy.

Application (Layer 7)	This layer supports application and end-user processes. Communication partners are identified, quality of service is identified, user authentication and privacy are considered, and any constraints on data syntax are identified. Everything at this layer is application-specific. This layer provides application services for file transfers, e-mail, and other network software services. Telnet and FTP are applications that exist entirely in the application level. Tiered application architectures are part of this layer.
Presentation (Layer 6)	This layer provides independence from differences in data representation (e.g., encryption) by translating from application to network format, and vice versa. The presentation layer works to transform data into the form that the application layer can accept. This layer formats and encrypts data to be sent across a network, providing freedom from compatibility problems. It is sometimes called the syntax layer.
Session (Layer 5)	This layer establishes, manages and terminates connections between applications. The session layer sets up, coordinates, and terminates conversations, exchanges, and dialogues between the applications at each end. It deals with session and connection coordination.
Transport (Layer 4)	This layer provides transparent transfer of data between end systems, or hosts, and is responsible for end-to-end error recovery and flow control. It ensures complete data transfer.
Network (Layer 3)	This layer provides switching and routing technologies, creating logical paths, known as virtual circuits, for transmitting data from node to node. Routing and forwarding are functions of this layer, as well as addressing, internetworking, error handling, congestion control and packet sequencing.
Data Link (Layer 2)	At this layer, data packets are encoded and decoded into bits. It furnishes transmission protocol knowledge and management and handles errors in the physical layer, flow control and frame synchronization. The data link layer is divided into two sublayers: The Media Access Control (MAC) layer and the Logical Link Control (LLC) layer. The MAC sublayer controls how a computer on the network gains access to the data and permission to transmit it. The LLC layer controls frame synchronization, flow control and error checking.
Physical (Layer 1)	This layer conveys the bit stream - electrical impulse, light or radio signal -- through the network at the electrical and mechanical level. It provides the hardware means of sending and receiving data on a carrier, including defining cables, cards and physical aspects. Fast Ethernet, RS232, and ATM are protocols with physical layer components.

Network Topology

A network topology is the pattern of links connecting pairs of nodes of a network. So It is a physical layout of the network’s computers, terminals, and links. The different network topologies are:

Ring: A ring network is a topology of computer networks where each node is connected to two other nodes, so as to create a ring.

Disadvantage:

·        if one of the nodes in the network breaks down then the entire network will break down with it as it requires a full circle in order to function

·        Inefficient when compared to Star networks because data must travel through more points before reaching its destination.

·        Communication delay is the directly proportional to number of node.

Bus: A bus network is a network architecture in which a set of clients are connected via a shared communications line, called a bus. Bus networks are the simplest way to connect multiple clients, but often have problems when two clients want to transmit at the same time on the same bus.

Advantages

• Easy to implement and extend
• Well suited for temporary networks (quick setup)
• Typically the cheapest topology to implement

Disadvantages

• Difficult to administer/troubleshoot
• Limited cable length and number of stations
• A cable break can disable the entire network
• Maintenance costs may be higher in the long run
• Low security

Star: Star networks are one of the most common computer network topologies. In its simplest form, a star network consists of one central switch, or hub computer which

acts as a router to transmit messages.

Advantages

• Easy to implement and extend, even in large networks
• Well suited for temporary networks (quick setup)
• The failure of a non-central node will not have major effects on the functionality of the network.
• No problems with collisions of Data
• Security can be implemented in the hub/switch.

Disadvantages

• Failure of the central node can disable the entire network
• Limited cable length and number of stations
• Maintenance costs may be higher in the long run

Mesh: All computers are connected to each other so it is sometimes called completely connected networks. The number of links on networks are determined by: n(n-1)/2.

Advantages:

·        Reliable: any line breakdown will affect only communication between the connected computers

·        Each node of the network need not have individual routing capacity.

·        Communication is very fast between any two nodes.

Hybrid: Mixture of more than two of star, ring, bus, mesh, tree etc topology is known as hybrid topology.

Wireless transmission Media:

Following are different types of wireless transmission media:

1.     Microwaves

2.     Satellite

3.     Infrared

4.     Bluetooth

5.     Wifi

6.     Wimax

Microwaves: Microwaves are radio waves that provie high speed signal transmission. Microwaves transmission involves sending signals from one microvwave staion to another. It contains antenna, tranceiver and other equipment necessary for microwave communicatons. Microwaves sometimes called fixed points wireless.

Satellite: In popular usage, the term satellite normally refers to an artificial satellite, which would be a man-made object that orbits the Earth (or another body)

Satellite links uses microwaves frequencies. Satellite contains amplifier that amplify frequencies coming from earth stations.

Major drawbacks of the satellite communication is high cost of installing satellite in its orbit.Three satellite spaced at equal interval on120 angular degree can give  coverage of entire globe.

Infrared: It is the type of electromagnetic wave of large wavelength and small frequencies than visible light. Its frequencies id low. IR dat transmission is alsoemployed in short range comunicationan among computer peripherals.

It is used in optic fibre communication, wireless LAN, computre and printers & in many other electonic devices.

Bluetooth:  Bluetooth is a radio standard and communications protocol primarily designed for low power consumption, with a short range (power class dependent: 1 meter, 10 meters, 100 meters) based around low-cost transceiver microchips in each device.

Bluetooth lets these devices communicate with each other when they are in range. The devices use a radio communications system, so they do not have to be in line of sight of each other, and can even be in other rooms, so long as the received power is high enough.

Wifi:   It is an international standard describing the characteristics of a wireless local area network (WLAN). The name Wi-Fi (short for "Wireless Fidelity", sometimes incorrectly shortened to WiFi) corresponds to the name of the certification given by the Wi-Fi Alliance, formerly WECA (Wireless Ethernet Compatibility Alliance), the group which ensures compatibility between hardware devices that use the 802.11 standard. Today, due to misuse of the terms (and for marketing purposes), the name of the

standard is often confused with the name of the certification. A Wi-Fi network, in reality, is a network that complies with the 802.11 standard. Hardware devices certified by the Wi-Fi Alliance are allowed to use this logo:

With Wi-Fi, it is possible to create high-speed wireless local area networks, provided that the computer to be connected is not too far from the access point. In practice, Wi-Fi can be used to provide high-speed connections (11 Mbps or greater) to laptop computers, desktop computers, personal digital assistants (PDAs) and any other devices located within a radius of several dozen metres indoors (in general 20m-50m away) or within several hundred metres outdoors. 

Network media:

Co-axial cable: Coaxial cable is an electrical cable consisting of a round conducting wire, surrounded by an insulating spacer, surrounded by a cylindrical conducting sheath, usually surrounded by a final insulating layer.

It is used as a high-frequency transmission line to carry a high-frequency signal.

Coaxial cables may be rigid or flexible. Rigid types have a solid sheath, while flexible types have a braided sheath, both usually of thin copper wire Thin coaxial: 10base2 is an example of thin coaxial cable

Thick: Example: 10base5

Twisted Pair:  Twisted pair cabling is a common form of wiring in which two conductors are wound around each other for the purposes of canceling out electromagnetic interference which can cause crosstalk. The greater the number of twists, the more crosstalk is reduced. It can transmit data several kilometers without amplifier. It has high error rate beyond 100m

They are classified into two categories:

1.     UTP ( unshielded twisted pair)

2.     STP (shielded twisted pair)

UTP: UTP cables are not shielded. This lack of shielding results in a high degree of flexibility as well as rugged durability. UTP cables are found in many Ethernet networks and telephone systems.

STP:Twisted pair cables are often shielded to prevent electromagnetic interference. Because the shielding is made of metal, it also serves as a ground STP cabling includes metal shielding over each individual pair of copper wires. This type of shielding prevents cable from external EMI (electromagnetic interferences).

Optical Fiber:  It is based on the  principle of total internal reflelction of light.It is at the one end of optical fiber to accept electrical signal and convert it into light pulses. The presence of pulse indicates a 1 bit  and absence of pulse indicates 0 bits.It contains the following components:

·        Core: It is the innermost part of  fibre made up of glass.

·        Cladding: Glass cladding covers core. Its refractive index is less than core.

·        Jacket:It is an outermost PVC( polyvinoyl chloride) covering of cladding

On the basis of geographical location:

A.   LAN(Local Area Networks): A Local Area Network (LAN) is a computer network covering a small local area, like a home, office, or small group of buildings

such as a home, office, or college. Current LANs are most likely to be based on switched Ethernet or Wi-Fi technology running at 10, 100 or 1,000 Mbit/s

B.   MAN(Metropolitan Area Networks) : MANs are large computer networks usually spanning a campus or a city. They typically use wireless infrastructure or optical fiber connections to link their sites. For instance a university or college may have a MAN that joins together many of their local area networks (LANs) situated around site of a fraction of a square kilometer. Specifically, this type of MAN is known as a campus area network.

C.   WAN (Wide Area Network): A wide area network or WAN is a computer network covering a wide geographical area, involving a vast array of computers. WANs are

used to connect local area networks (LANs) together, so that users and computers in one location can communicate with users and computers in other locations. Many WANs are built for one particular organization and are private. Others, built by Internet service providers(ISP), provide connections from an organization's LAN to the Internet.

What is computer Networks?

A computer network is a system for communication among two or more computers. These networks may be fixed (cabled, permanent) or temporary (as via modems).

Computer networking is the scientific and engineering discipline concerned with communication between computer systems. Such networks involves at least two computers, which can be separated by a few centimeters (e.g. via Bluetooth) or thousands of kilometers (e.g. via the Internet)

Types of network

Types of computer Networks: Computer networks may be classified into two types:

1.     On the basis of architecture:

a.     Peer to peer

b.    Client/server

c.      Hybrid

2.     On the basis of geographical location:

a.     LAN (Local Area Networks)

b.    WAN (Wide area networks)

c.      MAN (Metropolitan area networks)

1.     On the basis of architecture:

Peer to Peer(P2P): A peer-to-peer (or P2P) computer network is a network that relies primarily on the computing power and bandwidth of the participants in the network rather than concentrating it in a relatively low number of servers.

Such networks are useful for many purposes. Sharing content files (see file sharing) containing audio, video, data or anything in digital format is very common, and realtime data, such as telephony traffic, is also passed using

P2P technology.

A pure peer-to-peer network does not have the properties of clients or servers, but only equal peer nodes that simultaneously function as both "clients" and "servers" to the other nodes on the network

• Peers act as equals, merging the roles of clients and server
• There is no central server managing the network
• There is no central router

Advantages:

·        Easy to install and configure

·        Individual users control their own shares resources

·        Inexpensive to operate

·        No additional hardware and software beyond a suitable operating system is needed

·        No dedicated administrated are required.

Disadvantages:

·        Network security applies only to a single resource at a time

·        Each machine must be backed up individually to protect all shared data.

·        There is no centralized organizational scheme to locate or control access to data

Client-Server

Client-server is a network architecture which separates the client (often a graphical user interface) from the server. Each instance of the client software can send requests to a server or application server. There are many different types of servers; some examples include: a file server, terminal server, or mail server.

The term server is most commonly applied to a complete computer system today, but it is also used occasionally to refer only to the hardware or software portions of such a system.

Server software generally, but not always, runs on powerful computers dedicated for exclusive use to running the business application. Client software on the other hand generally runs on common PCs or workstations.

Properties of a server:

• Passive (Slave)
• Waiting for requests
• On requests serves them and send a reply

Properties of a client:

• Active (Master)
• Sending requests
• Waits until reply arrives

Advantages:

·        Simplified user accounts, security, and access controls to simplify network administration

·        Single password for network login delivers access to all

·        More powerful equipment means more efficient access to network resources.

Disadvantages:

·        At worst, server failures leads to whole network failure

·        Dedicated hardware and special software (Network Operating Ssystem: NOS) add to the cost. Increases expenses, it may need system administrator to handle.