Subject: Operating System

Topic: DISTRIBUTED SYSTEMS

Difficulty: Hard

Network Operating System (NOS) is a software program that controls other software and hardware running on a network. It also allows multiple computers, known as network computers, to communicate with one central hub and each other to share resources, run applications, and send messages. Such a system can consist of a wireless network, WAN,Local Area Network (LAN), or even two or three computer networks connected together. Administrators running these networks typically have training in different network operating systems.

Structure of network operating system:

The structure over a network is of two types :

LAN(Local Area Network )

WAN(Wide Area Networks)

LAN(Local Area Network ) : A local-area network (LAN) is a computer network that spans a relatively small area. Most often, a LAN is confined to a single room, building or group of buildings, however, one LAN can be connected to other LANs over any distance via telephone lines and radio waves.

Most LANs connect workstations and personal computers. Each node (individual computer) in a LAN has its own CPU with which it executes programs, but it also is able to access data and devices anywhere on the LAN. This means that many users can share expensive devices, such as laser printers, as well as data. Users can also use the LAN to communicate with each other, by sending email or engaging in chat sessions.

LANs are capable of transmitting data at very fast rates, much faster than data can be transmitted over a telephone line; but the distances are limited and there is also a limit on the number of computers that can be attached to a single LAN.

WAN(Wide Area Networks)

A computer network that spans a relatively large geographical area. Typically, a WAN consists of two or more local-area networks (LANs).

Computers connected to a wide-area network are often connected through public networks, such as the telephone system. They can also be connected through leased lines or satellites. The largest WAN in existence is the Internet.

Enterprise WANs allow users to share access to applications, services and other centrally located resources. This eliminates the need to install the same application server, firewall or other resource in multiple locations, for example.A virtual private network (VPN) facilitates connectivity between WAN sites.

Direct fiber optic links are also used to connect sites on a WAN – and they almost always offer greater performance, reliability and security than VPNs, but they are cost-prohibitive for most enterprises to procure and operate.

Topology

Network topology is the arrangement of the various elements (links, nodes, etc.) of a communication network.Network topology is the topological[3] structure of a network and may be depicted physically or logically.

The physical topology of a network is the actual geometric layout of workstations. There are several common physical topologies, as described below and as shown in the illustration.

In the bus network topology, every workstation is connected to a main cable called the bus. Therefore, in effect, each workstation is directly connected to every other workstation in the network.

In the star network topology, there is a central computer or server to which all the workstations are directly connected. Every workstation is indirectly connected to every other through the central computer.

In the ring network topology, the workstations are connected in a closed loop configuration. Adjacent pairs of workstations are directly connected. Other pairs of workstations are indirectly connected, the data passing through one or more intermediate nodes.

If a Token Ring protocol is used in a star or ring topology, the signal travels in only one direction, carried by a so-called token from node to node.

The mesh network topology employs either of two schemes, called full mesh and partial mesh. In the full mesh topology, each workstation is connected directly to each of the others. In the partial mesh topology, some workstations are connected to all the others, and some are connected only to those other nodes with which they exchange the most data.

The tree network topology uses two or more star networks connected together. The central computers of the star networks are connected to a main bus. Thus, a tree network is a bus network of star networks.

Logical (or signal) topology refers to the nature of the paths the signals follow from node to node. In many instances, the logical topology is the same as the physical topology. But this is not always the case. For example, some networks are physically laid out in a star configuration, but they operate logically as bus or ring networks.