In this unit we are going to speak about how computers use the networks to work together. There are several models (that in some cases are called

Transcription

1 In this unit we are going to speak about how computers use the networks to work together. There are several models (that in some cases are called architectures) that computers can follow when they collaborate using a network. We are going to see the most significative. We will also speak about the network operating system concept, and how it has been incorporated in modern operating systems.

2 The first model of collaboration we are going to see is the central computer with terminals. This model was developed in the early 1970s, when many mainframes developed interactive user interfaces and operated as timesharing computers, supporting hundreds of users simultaneously along with batch processing. At first these users logged with dumb local terminals connected with serial ports, which very soon became remote terminals used as modems. In this model all the computing power is in the central computer and the remote systems act only as input/output devices. It is used with mainframes and supercomputers. To access as a remote terminal with a personal computer we use terminal emulation software. A typical example of this model are the point-of-sale devices of a big department store, connected to its central computer.

3 In peer-to-peer networking, users are able to share resources and files located on their computers and access shared resources from others, all without the need for central coordination by servers or stable hosts. A peer-topeer network sets all connected computers as equal; they all share the same abilities to use the resources available on the network. Peers are both suppliers and consumers of resources. They are said to form a peer-to-peer network of nodes.

4 Peer-to-peer networking has been popularised by file sharing systems like Napster, where an algorithm in the peer-to-peer communications protocol balances load, and even peers with modest resources can help to share the load. Files are split in small pieces that are available in every node that is downloading the file. If a node becomes unavailable, its shared resources remain available as long as other peers offer them. Ideally, a peer does not need to achieve high availability because other, redundant peers make up for any resource downtime; as the availability and load capacity of peers changes, the protocol reroutes requests. Emerging collaborative P2P systems are going beyond the era of peers doing similar things while sharing resources, and are looking for diverse peers that can bring in unique resources and capabilities to a virtual community thereby empowering it to engage in greater tasks beyond those that can be accomplished by individual peers, however still beneficial to all the peers.

5 In the client/server architecture, a server provides a function or service to one or many clients, which initiate requests for such service. Client/server architecture allows the network s administrators to centralise functions and applications in one or more dedicated servers. The server is the center of the system, giving access to resources and instituting security. The server's job is to respond to service requests from clients following the policies established by the company, while the client's job is to use the service provided in response in order to perform some tasks. Client and Server are completely independent instances.

6 Servers are classified by the services they provide. For instance, a web server serves web pages, a file server serves computer files and a database server answers database queries and requests. A server can offer a service sharing any of its resources, hardware or software, from programs and data to processors and storage devices. A single computer can offer several services at the same time, it can run a web server and a file server at the same time to serve different data to clients making different kinds of requests. Whether a computer acts as a client, a server, or both, it s determined by the nature of the application that requires the services. For example, client software can communicate with server software within the same computer. Communication between servers, such as to synchronised data, is sometimes called inter-server or server-to-server communication.

7 The advantages of client/server architecture include: Centralised servers are more stable, Security policies are easily implemented in the server. New technology and hardware can be easily integrated into the system. Hardware and the operating system can be specialised, with focus on performance. Back-ups are easier to setup and update Servers are able to be accessed remotely from different locations and types of systems. The disadvantages include: Cost of buying and running a server are high Dependence on a central location for operating, what can create bottlenecks and sensible points of failure Requires regular maintenance and updates Servers are more difficult to configure In client/server architecture the computing power, memory and storage requirements of a server must be scaled appropriately to the expected work load. Load balancing and failover systems are often employed to avoid bottlenecks and minimise the effects of a server failure.

8 A network operating system can be used in order to refer to two rather different concepts: a specialised operating system for a network device such as a router, switch or firewall Or An operating system oriented to computer networking, conceived as an extension to early personal computer operating systems that where designed for a single user using a computer. These operating systems allow shared file and printer access among multiple computers in a network, enabling the sharing of data, users, groups, security, applications, and other networking functions. Network operating systems for personal computers disappeared long time ago, as these functions are now integrated in all common operating systems, so we could say that all modern personal operating systems are network operating systems. There were Peer-to-Peer operating systems as Windows for Workgroups, AppleShare or LANtastics and client/server network operating systems as Novell Netware or Banyan Vines.

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