Lecture 12_Network Types and Client Server Architecture PDF

Chapter 6: Computer Networks Lecture 12 :Major Types of Networks and The Client/Server Architecture  Learning Outcomes In this section: We discuss the major types networks and network topologies The Client Server Architecture  Major Types of Networks...

Chapter 6: Computer Networks Lecture 12 :Major Types of Networks and The Client/Server Architecture  Learning Outcomes In this section: We discuss the major types networks and network topologies The Client Server Architecture  Major Types of Networks Local Area Network (LAN) If the network is contained within a relatively small area, such as a classroom, school, or single building, as shown in Figure 1.6, it is commonly referred to as a local area network (LAN). This type of network has the lowest cost and least overall capability of the three geographic classifications. Because the pieces of equipment in a LAN are in relatively close proximity, LANs are inexpensive to install This network classification usually has the highest speed components and fastest communications equipment because it takes less overall investment to get the smaller network running the faster equipment. LANs, therefore, are commonly considered the building blocks for creating larger networks the building blocks for creating larger networks.  Major Types of Networks When the network spans the distance of a typical metropolitan city, as shown in Figure 1.7, it can be referred to as a Metropolitan Area Network (MAN) Although the speeds achieved in a MAN are typically as high as in a LAN, it requires high- speed connections, such as fiber optics. Increasing the distance and the technology levels increases the relative installation and operation costs of MANs.  Major Types of Networks Wide Area Network (WAN) The MAN outgrows its usefulness when the network must expand beyond the confines of the typical metropolitan area. When the network spans a larger area, as shown in Figure 1.8, it is classified as a wide area network (WAN). Because of the extensive distances over which WANs communicate, they use long-distance telecommunications networks for their connections, which increases the costs of the network. The Internet is just a giant WAN  Network Topologies A network’s topology is a description of its physical layout. How computers are connected to each other on the network and the devices that connect them are included in the physical topology There are four basic topologies: bus, ring, star, and mesh. Other topologies are usually hybrids of two or more of the main types. Choosing the physical topology type for your network is one of the first steps in planning your network. – The choice of a topology will depend on a variety of factors, such as cost, distances, security needs, which network operating system you intend to run, and whether the new network will use existing hardware, conduits, and so on.  Bus Topology A physical bus topology, also called a linear bus, consists of a single cable to which all the computers in the segment are attached (see Figure 5.3). Messages are sent down the line to all attached stations, regardless of which one is the recipient With a bus topology, when a computer sends out a signal, the signal travels the cable length in both directions from the sending computer. When the signal reaches the end of the cable length, it bounces back and returns in the direction it came from This is known as signal bounce. – Signal bounce is a problem, because if another signal is sent on the cable length at the same time, the two signals will collide and be destroyed and then must be retransmitted. For this reason, at each end of the cable there is a terminator. back and returns in the direction it came from. – A malfunction of a station or other component on the network can be difficult to isolate. Furthermore, a malfunction in the bus backbone can bring down the entire network. The bus topology is the fastest and simplest way to set up a network. It requires less hardware and cabling than other topologies, and it is easier to configure. It is good way to quickly set up a temporary network. It is also usually the best choice for small networks (e.g. those with 10 computers or less).  Ring Topology In a physical ring topology, the data line actually forms a logical ring to which all computers on the network are attached (see Figure 5.4). A computer can only send data when it has possession of the token. Because each computer on the ring is part of the circle, it is capable of retransmitting any data packets it has received that are addressed to other stations on the ring. – This regeneration keeps the signal strong, eliminating the need for repeaters. Because the ring forms a continuous loop, termination is not required A ring network topology is relatively easy to install and configure, requiring minimal hardware. A physical ring topology has a couple disadvantages. – As with a linear bus, a malfunction on one station can bring down the entire network. – It is also difficult, especially in larger networks, to maintain a logical ring. – Also, if adjustments or reconfigurations are necessary on any part of the network, you must temporarily bring down the entire network. Ring Topology  Star Topology In a star topology, all computers on the network are connected to one another using a central hub (see Figure 5.5) Each data transmission that the station sends goes directly to the hub, which then sends the packet on toward its destination Like in the bus topology, a computer on a star network can attempt to send data at any time; however, only one may actually transmit at a time. – If two stations send signals out to the hub at exactly the same time, neither transmission will be successful, and each computer will have to wait a random period of time before reattempting to access the media  Star Topology A major advantage of implementing a star topology is that, unlike on a linear bus, a malfunction of one station will not disable the entire network. – It is easier to locate cable breaks and other malfunctions in a star topology. This capability facilitates the location of cable breaks and other malfunctions. – Additionally, the star topology’s centralized hub makes it is easier to add new computers or reconfigure the network. There are several drawbacks inherent in the implementation of a star topology. – For one, this type of configuration uses more cabling than most other networks because of the separate lines required to attach each computer to the hub. – Also, the central hub handles most functions, so failure of this one piece of hardware will shut down the entire network.  Mesh Topology The mesh topology connects each computer on the network to the others (see Figure 5.6). Meshes use a significantly larger amount of network cabling than do the other network topologies, which makes it more expensive. Additionally, these networks are much more difficult to install than the other topologies. Major advantage of Mesh Topology? The answer is fault tolerance. – Fault tolerance is the ability of a system to work around a failure. – On a network with a broken segment, that means going around it. Every computer has multiple possible connection paths to the other computers on the network, so a single cable break will not stop network communications between any two computers Mesh Topology  Distributed Processing With centralized processing, all processing occurs in a single location or facility. This approach offers the highest degree of control because a single centrally managed computer performs all data processing With decentralized processing, processing devices are placed at various remote locations. Each computer system is isolated and does not communicate with another system Decentralized systems are suitable for companies that have independent operating divisions With distributed processing, computers are placed at remote locations but connected to each other via telecommunications devices. One benefit of distributed processing is that managers can allocate data to the locations that can process it most efficiently Distributed processing can also minimize the consequences of a catastrophic event at one location and ensure uninterrupted systems availability  Client/Server Systems In client/server architecture computers called servers are dedicated to special functions. For example: – An application server holds the programs and data files for a particular application. – An e-mail server sends and receives e-mails. – A web server sends out web pages Each server is accessible by all computers on the network Servers can be computers of all sizes A client is any computer (often a user’s personal computer) that sends messages requesting services from the servers on the network Client/Server Systems  Figure 5.2 Components of a client/server system As shown in Figure 5.2, the clients are connected to a more powerful PC or server computer via a local-area network within one site of a company, or a wide-area network connecting different sites and/or companies. The network is made up of both telecommunications processors to help route the information and the channels and media which carry the information.  Client/server continued The server is a more powerful computer that is usually used to store the application and the data shared by the users. When a user wants to run a program on a PC in a client/server system, the applications, such as a word processor, will usually be stored on the hard disk of the server and then loaded into the memory of the client PC, running or ‘executing’ on the processor of the client. The document the user creates would be saved back to the hard disk of the server. This is only one alternative.  Client/server continued To summarise, the main components of a client/server system shown in Figure 5.2 can be defined as follows: Client software is the interface by which the end-user accesses the software. It includes both the operating system, such as Windows 10, and the applications software, such as word processors. Increasingly, web- based browsers are being used as clients on a company intranet. Server software is used to store information, administer the system and provide links to other company systems. Again, this may be a web server or a database server. The application development environment provides interactive programming tools to develop applications through the application programming interface (API) of the package. The infrastructure or plumbing of the system. This is based on local- and wide-area networking techniques and consists of the telecommunications processors and media  Why Use Client/Server? One of the benefits of client/server is that there are many choices for sharing the workload between resources. Cost savings were originally used to drive the introduction of client/server. – PC-based servers were much cheaper than mainframes, although the client PCs were more expensive than dumb terminals. The overall savings were dramatic. Distributed approach:The system designers can decide to distribute data and processing across both servers and client computers Faster execution:Rather than all the tasks involved in program execution (other than display) occurring on the mainframe, client/server gives the opportunity for them to be shared between a central server and clients. This gives the potential for faster execution, as processing is distributed across many clients. Customisation and flexibility: Customization of the client is also possible – the end-users are empowered through being able to develop their own applications and view data to their preference. With queries occurring on the back end, this reduces the amount of network traffic that is required.  Client/Server Benefits Other benefits: – Centralisation – centralised control and resource sharing – Security – Enforced access control – Scalable – No of clients or servers can be increased with minimal disruptions – Easier File Management – Sharing of information  Client/server Disadvantages High cost of ownership – generally higher TCO: Although the purchase price for a PC is relatively low, the extra potential for running different applications and modifications by end-users means that there is much more that can go wrong in comparison with a dumb terminal. – More support staff are required to solve problems resulting from the complex hardware and software. Instability caused by clashes with applications: Client/server technology is often complex and involves integrating different hardware and software components from many different companies. High cost of ownership – generally higher TCO: Although the purchase price for a PC is relatively low, the extra potential for running different applications and modifications by end-users means that there is much more that can go wrong in comparison with a dumb terminal. – More support staff are required to solve problems resulting from the complex hardware and software.  Client/server Disadvantages Performance: For some mission-critical applications, a smaller server cannot Client/server deliver Benefits the power required. In a travel agency business, for example, this will give rise to longer queues and poorer customer service. – For this reason, many banks and travel agents have retained their mainframe- based systems where performance is critical. – The use of a PC can also cause delays at the client end, as the screen takes a long time to redraw graphics compared to a teletext terminal. Lack of worker focus. Although PCs can potentially empower end-users, the freedom of choice can also lead to non-productive time-wasting, as users rearrange the colours and wallpaper on their desktop rather than perform their regular tasks!  Client/server Disadvantages Other disadvantages: – Time lost by staff configuration – Instability caused by clashes with applications – Maintenance – done by specialists,& must be done immediately – Performance – can be compromised due to resource sharing – Cost – generally higher TCO. – Traffic congestion – server can crash under pressure of too many requests, – Central single point of failure can disrupt the whole network – Resources – Not all resources on the server are acquirable Despite these difficulties, the compelling arguments of ease of use and flexibility of client/server still remain. The empowerment of end-users to develop their own applications and to use and share the data as they see fit is now considered to be the main benefit of client/server.  Servers Server: A server is a powerful computer used to control the management of a network. It may have a specific function such as storing user files or a database or managing a printer. – they regulate the flow of information around the network in the way that a heart controls the flow of blood around the body. Network servers run the network operating system (NOS), the software that is used to manage the network, and are often used to store large volumes of data. The server and NOS together perform the following functions: – Maintain security: Access to information in files is restricted according to the username and password issued to users of the network. – Sharing of peripheral devices connected to the network, such as printers and tape drives. These are often attached directly to the server. – Sharing of applications such as word processors, which do not then need to be stored on the hard drive of the end-user’s computer. The cost of buying applications can be reduced through buying a ‘site licence.’ – Sharing of information: Access to these data are maintained by the NOS and they are stored within the hard drive of a server as files or as part of a database. Table 5.2: Types of Server  Severs continued When creating an information system, there are a number of critical functions which must be designed into the server. These are important requirements which must be checked with server vendors, database vendors and operating systems vendors: Performance. The server should be fast enough to handle all the requests from users attached to the network. A margin should be built in to accommodate future growth in users and network traffic. This means specifying a suitable amount of memory, a fast hard disk and, less importantly, a fast processor. Capacity. When initially specified, the hard disk capacity should be large enough that it will not need to be upgraded in the near future. Resilience/fault tolerance. If there is a problem affecting the hardware, such as a power surge or a problem with the hard disk, it is important that the whole network does not ‘crash’ because of this. Preventive measures should be taken, such as installing an uninterruptible power supply or running two disks in parallel (disk mirroring or RAID – Redundant Array of Inexpensive Disks). Clustering. This is used to spread the load across different servers, so improving reliability and performance. It involves linking several servers together via a high-speed link such as fibre-optic cabling. This can enable parallel processing, where tasks are shared between processors, and also storage mirroring, where duplicate copies of data are stored on different servers to improve performance and reduce the risk of one server failing.  End-user Computers or Terminals The access points for users of a network are known variously as clients, nodes, workstations or, most commonly, just PCs. It is best to use the term ‘client PC’, as this helps distinguish clients from servers which may also be PC-based. To work on the network each client must have networking software such as Open Enterprise Server (OES) or TCP/IP installed (see later section). Of course, a physical connection to the network is also required. – For a PC on an office LAN, this is provided by a network interface card in one of the PC’s slots. The card is then attached to the network cabling. – For a PC at home which is linked to the Internet, the network card is replaced by a modem.  Communications Software How does an application program request data from a disk drive on the network? Through a Network Operating System (NOS) The NOS is systems software that controls the computer systems and devices on a network and allows them to communicate with each other The NOS performs the same types of functions for the network as operating system software does for a computer, such as memory and task management and coordination of hardware