Networking: A network is a physical connection between / among the devices (autonomous computer) that are distributed widely in different geographical location. It is the computer and communication link that permits computer to communicate with each other and to share program, facilities, data and knowledge base. It is a group of devices that are linked to one another by data communication system. In a computer network two or more computers are linked together with a medium and data communication devices for the purpose of communicating data and sharing of resources.
According to Martin “a network is a group of individuals or organizations that are interconnected. The linking must include a communication mechanism, and many networks exist for the express purpose of facilitating certain types of communication among their members. In the library world, institutions from network primarily to achieve better sharing of resources – resources consisting of bibliographic information and of collection – and better services to patrons.”
Based on the type of participants who are participated in the network, network can be divided into the following categories-
a) Private Network: These networks are usually owned by some corporation or other entity that control access and use of network to its staff.
b) Public Networks: These networks provide services to any individual or organization who becomes the member or subscriber. Eg. Telephone system.
c) Cooperative Networks: These networks are managed and supported by their user.
Based on the techniques used to transfer data and control, communication network can be classified into two categories - Switched Network and Broadcast Network. In Broadcast system, there is no intermediate switching nodes. All station share a single transmission channel. Packet transmitted by one station is received by all other station. An address field within the packet specifies the destination of the packet. Packets that are intended for other station are ignored. Eg. Satellite network, LAN, etc.
Network architecture refers to the arrangement of nodes and their interconnection communication circuit to represent the structure of the network and the significance of nodes in the network. Based on the architecture of the network, network can be of the following types-
a) Peer to Peer Architecture: In this type of network each workstation has equivalent capabilities and responsibilities. No one computer is in charge of the network.
b) Client / Server Architecture: Here the servers are dedicated to serving the client. Servers are powerful computer or processor and clients are less powerful PCs or workstation on which users run applications.
Topology refers to the way in which the end points or station of the network are interconnected. It is the structure or the arrangement of nodes for a network. It is the physical layout of the LAN. Topologies are named for the figure created by the web of wiring called data path used for data transfer. Based on the topology network can be of the following types-
a) Bus: In a bus network each computer is connected to a single communication cable via an interface and every computer can directly communicate with each other. The bus is a single pair or a bunch of wires that carry the electrical signals. The individual nodes are connected to the bus using a passive tap such that all systems are able to monitor the signal on the bus simultaneously. The bus acts as a broadcast medium. Access and control of bus network are typically maintained by a method called contention where by if a line is unused a terminal or device can transmit its message at will but if two or more terminal initiate message simultaneously they must stop and transmit again at different intervals. The connection of the node to the bus is similar to Christmas light. Each ends of the bus network is also terminated with a resistor to keep the signal that is sent by a node across the network from bouncing back when it reaches the end of the cables. Traffic travels in both directions.
The bus topology appears to be the most flexible one and suitable for LAN networks. It is able to handle the wide range of devices in terms of numbers of devices, data rates and data types. Again being passive units the failure of an interface tap in a bus topology does not affect the operation of a LAN.
Fig. Bus Topology
b) Ring Topology: Like a bus network, ring has the nodes daisy chained. The difference is that the end of the network comes back around to the first node creating a complete circuit. In a ring network several devices or computer are connected to each other in a closed loop by a single communication cable so a ring network also called as loop network. The data travels around the ring to each station in turn until they arrive at the required station.
A ring can be unidirectional or bidirectional. In a unidirectional ring, data moves in one direction only and in bidirectional ring data moves in both direction, but in only one direction at a time. Each node of the ring takes a turn sending and receiving information through the use of a token. The token along with any data is sent from the first node to the second nodes which extract the data address to it and adds any data it wishes to send. Then the second node passes the token and so on. Only the node with the token is allowed to send data, others must wait for the token as it comes to them.
The advantage of ring network is that they can travel larger distances with a very high speed link than other types of network such as bus network because each node regenerates messages as they pass through it.
In a ring network each unit serves as a controller. All messages travel in only one direction so when one unit is down the system could be down. So, it becomes necessary to consider fault tolerance techniques such as dual ring or folded ring operation for handling RIV failure in a ring topology.
Fig. Ring Topology
c) Star Topology: In the star topology single controller is there in the middle of the system. All communication takes place through the controller. All other devices are connected to the centralized device called hub / switches. In the star topology, the controller is responsible for managing the network. Other nodes share a fixed amount of the Central’s CPU’s time called a time slice. So if controller is inoperative the whole system is down for all.
The star topology is good for terminal intensive requirement because of the minimal processing works. Star LANs are generally designed in banking sector for centralized record keeping.
Fig. Star Topology
d) Tree Topology: In a tree network several devices or computer are linked in a hierarchical fashion. In constructing the network the frequently interacting nodes are placed near to each other so it is also known as hierarchical network. This type of distribution system is commonly used in the organization where headquarters communicates with regional offices and regional offices communicate with district offices and so on. The tree networks are difficult to expand once established.
Fig. Tree Topology
e) Completely Connected or Mesh Network: Completely connected network has a separate physical link for connecting each node to any other node. Thus each computer of such network has a direct dedicated link, called a point to point link with all other computers of the network. The control is distributed with each computer deciding its communication priorities.
This type of network is very reliable as any line breakdown will affect only communication between the connected computers. Again, each node of the network needs not to have individual routing capacity and communication is very fast between any two nodes.
It is most expensive system from the point of view of line cost. If there are n nodes in the network, then n(n-1)/2 links are required. Thus the cost of linking the system grows with the square of the number of nodes.
Fig. Mesh Network Topology
f) Hybrid Network: The basic network topologies can be linked together to form hybrid network of considerable complexity. For example, the star and bus topologies can be combined to form a hybrid network.
Fig. Hybrid Network Topology
There are a number of other topologies that have been studied in the content of LAN. These include hypercube, multistage store, and forward network. However, LANs based on these topologies are not widely used at present.
Again, based on utility criteria, network can be of the following types-
a) Resource Sharing Network: This type of network are intended to share the resources which includes specialized computer, software or other devices that are expensive and are not affordable by an individual user. Eg. A super computer in an institution is accessed by several station located in different department / section of the institution.
b) Data Sharing Network: This network provides access to various databases from workstation situated at distance apart. Eg. Remote access to stock exchange data or hotel and airline reservation system.
c) Communication and Data Exchange Network: This type of network allows users to exchange data graphs and document with each other using such devices as e-mail, bulletin board, etc irrespective of their location.
Classification of network based on the concept of “area” made good sense at this time because a key distinction between a LAN, MAN and WAN involves the physical distances that the network spread. As technology improves the new types of network appeared on the sense. These too become known as various types of area network for consistency’s sake (eg. Storage area network, system area network, campus area network, etc) although distances no longer proved a useful differentiation.
a) LAN: In an LAN two or more computers or node are directly linked within a small well defined areas such a room, office, building, campus or a local neighborhood with a range of 10 kilometer. Each hardware device on a LAN such as computer or a printer is called a node. Most LAN’s are privately owned, controlled and managed by a single person or organization and uses direct high speed cables to share hardware, software and data resources. LAN uses the Institute of Electrical and Electronic Engineers (IEEE-802.5, IEEE 802.3) Ethernet, IBM token ring, etc protocol where as WAN uses TCP/IP, HTTP, FTP, etc.
b) MAN: The networks which spread over a city or that connect an area longer than a LAN but smaller than WAN are known as MAN. It covers a city with dedicated or high performance hardware. Eg. DELNET, CALIBNET, MANLIBNET.
c) WAN: A Wide Area Network is a computer network that directly connects computer separated by long distances, more than a mile and as much as half the globe. WAN uses special purpose telephone wires, dedicated line, fiber optic cables, microwaves or satellites for communication.
WAN is a geographically dispersed collection of LAN and are not owned by any one organization but rather exists under collective or distributed ownership and management. The largest WAN in existence is the internet.
A WAN is composed of a number of autonomous computer that are distributed over a large geographical area often a county or continent and it is a satellite based network eg. INFLIBNET, ERNET, I-NET, INDONET.