Network, is the logical and structural layout of the network consisting of transmission equipment, software and communication protocols and infrastructure (wired or wireless) transmission of data and connectivity between components. Various hardware and software architectures exist that are usually used for distributed computing. At a lower level, it is necessary to interconnect multiple CPUs with some sort of network, regardless of that network being printed onto a circuit board or made up of several loosely-coupled devices and cables. At a higher level, it is necessary to interconnect processes running on those CPUs with some sort of communication system. Most networks can be classified into one of five different types. These include Wide Area Networks (WAN), Local Area Networks (LAN), Virtual Private Networks (VPN), client/server networks, network computing, and peer-to-peer network (Wikipedia, 2009).
LAN (Local Area Network) is network architecture where the computers can be connected up to limited area. LANs employ a single communication link and special routing. Approximately, this network can be configured well up to 100 meters distance. The computers are connected through devices like switches. These kinds of networks are limited to a building or campus. The users can share the network resource and securities can be implemented only within the network. LANs are traditionally used to connect a group of people who are in the same local area. However, the working groups are becoming more geographically distributed in today's working environment. There, virtual LAN (VLAN) technologies are defined for people in different places to share the same networking resource. LAN protocols are mostly at data link layer. The Institute of Electrical and Electronics Engineers (IEEE) is the leading organization defining most of the LAN protocols (Edraw Soft, 2009). "Until recently, few organizations used wireless LANs because they cost too much, their data rates were too low, they posed occupational safety problems because of concerns about the health effects of electromagnetic radiation, and the spectrum used required a license. Today, these problems have largely diminished, and wireless LAN popularity is skyrocketing. Wireless LANs must meet requirements typical of any LAN. They must also meet requirements specific to their intended environment" (Stallings, 2001).
Advantages of connecting computers in a LAN:
Workstations can share peripheral devices like printers.
Workstations do not necessarily need their own hard disk or CD-ROM drives which make them cheaper to buy than stand-alone PCs.
User can save their work centrally on the network's file server. This means that they can retrieve their work from any workstation on the network.
Users can communicate with each other and transfer data between workstations very easily.
One copy of each application package such as a word processor, spreadsheet etc. can be loaded onto the file and shared by all users.
Disadvantages of connecting computers in a LAN:
Special security measures are needed to stop users from using programs and data that they should not have access to;
Networks are difficult to set up and need to be maintained by skilled technicians.
If the file server develops a serious fault, all the users are affected, rather than just one user in the case of a stand-alone machine.
Wide Area Network
WAN (Wide Area Network) connects two or more LANs and other networks so that computers at one location can communicate to the computers at far away some other locations. The Internet is certainly the largest WAN, spanning the entire earth and beyond. Some WANs are built by private companies and only works for them and other are built by some Internet Service Providers. WANs are usually built through leased lines and routers are used to connect the network at one place to the network at other place. For example, a company has a network in UK. The company wants to open its office in USA. In order to connect the two offices in UK and USA, we have to install routers at both ends for communication.
We can have unprecedented benefits from WAN. By sitting at one city or country the user can share the resources of the network in other city or country. For example, the user can print the document or send an instant message by just one click. The users can use online softwares for communication and can work at the same time on the server database. The benefits are unlimited and it is up to the organization to what extent they need it for them. We use WAN in those cases where LAN cannot be used. When more than two computers are involved in the application, a switched communication facility is used to enable all the computers to communicate with one other at different times.
You can simply define WAN as any form of network (public, private) that covers a wide geographical area. Type of WAN used depends on the nature of network. For example, there are 100 computers in a network and all computers belong to the same site. We want to send data between sites. So, one method is to lease transmission lines from the public carriers. We will also install a private switching system at each site. One of the advantages of a WAN is that we can use this network to connect computers distributed over a large area. As LAN is limited to an office or home and does not provide services for long distance. For exchanging data between sites, services provided by wide area network are sufficient and up to the mark.
Advantages of a WAN:
Covers a large geographical area so long distance businesses can connect on the one network
Shares software and resources with connecting workstations
Disadvantages of a WAN:
Are expensive and generally slow
Need a good firewall to restrict outsiders from entering and disrupting the network
Virtual Private Network
A virtual private network (VPN) is a network that uses a public telecommunication infrastructure, such as the Internet, to provide remote offices or individual users with secure access to their organization's network. A VPN can be contrasted with an expensive system of owned or leased lines that can only be used by one organization. The goal of a VPN is to provide the organization with the same capabilities, but at a much lower cost (Search Enterprise WAN, 2009). A VPN works by using the shared public infrastructure while maintaining privacy through security procedures and tunneling protocols such as the Layer Two Tunneling Protocol (L2TP). In effect, the protocols, by encrypting data at the sending end and decrypting it at the receiving end, send the data through a "tunnel" that cannot be "entered" by data that is not properly encrypted. An additional level of security involves encrypting not only the data, but also the originating and receiving network addresses. Compared to other technologies, VPNs offers several advantages, particularly benefits for wireless local area networking.
Advantages of a VPN (Mitchell, 2009):
A VPN offers two main advantages over alternative technologies: cost savings, and network scalability. To the clients accessing these networks, VPNs also bring some benefits of ease of use.
With a VPN, you use public network infrastructure including the Internet to make these connections and tap into that virtual network through much cheaper local leased lines or even just broadband connections to a nearby Internet Service Provider (ISP).
A VPN also can replace remote access servers and long-distance dialup network connections commonly used in the past by business travelers needing to access to their company intranet.
VPN technology also works well with Wi-Fi local area networking. Some organizations use VPNs to secure wireless connections to their local access points when working inside the office. These solutions provide strong protection without affecting performance excessively.
Advantages of a VPN (Mitchell, 2009):
VPNs require detailed understanding of network security issues and careful installation / configuration to ensure sufficient protection on a public network like the Internet.
The reliability and performance of an Internet-based VPN is not under an organization's direct control. Instead, the solution relies on an ISP and their quality of service.
Historically, VPN products and solutions from different vendors have not always been compatible due to issues with VPN technology standards. Attempting to mix and match equipment may cause technical problems, and using equipment from one provider may not give as great a cost savings.
Client/Server Network
Client/server networks allow the network to centralize functions and applications in one or more dedicated file servers. The file servers become the heart of the system, providing access to resources and providing security. Individual workstations (clients) have access to the resources available on the file servers. The network operating system provides the mechanism to integrate all the components of the network and allow multiple users to simultaneously share the same resources irrespective of physical location. Novell Netware and Windows 2000 Server are examples of client/server network operating systems.
Advantages of a Client/Server Network:
Centralized - Resources and data security are controlled through the server.
Scalability - Any or all elements can be replaced individually as needs increase.
Flexibility - New technology can be easily integrated into system.
Interoperability - All components (client/network/server) work together.
Accessibility - Server can be accessed remotely and across multiple platforms.
Disadvantages of a Client/Server Network:
Expense - Requires initial investment in dedicated server.
Maintenance - Large networks will require a staff to ensure efficient operation.
Dependence - When server goes down, operations will cease across the network.
Peer-To-Peer Network
A peer-to-peer network allows two or more PCs to pool their resources together. Individual resources like disk drives, CD-ROM drives, and even printers are transformed into shared, collective resources that are accessible from every PC. Unlike client-server networks, where network information is stored on a centralized file server PC and made available to tens, hundreds, or thousands client PCs, the information stored across peer-to-peer networks is uniquely decentralized. Because peer-to-peer PCs have their own hard disk drives that are accessible by all computers, each PC acts as both a client (information requestor) and a server (information provider). Because drives can be easily shared between peer-to-peer PCs, applications only need to be installed on one computer. If users have one copy of Microsoft Word, for example, it can be installed on user A's computer and still used by user B (Free PC Tech, 2009).
The advantages of peer-to-peer over client-server include (FCIT, 2009):
No need for a network administrator
Network is fast/inexpensive to setup & maintain
Each PC can make backup copies of its data to other PCs for security.
Easiest type of network to build, peer-to-peer is perfect for both home and office use.
Less initial expense - No need for a dedicated server.
Setup - An operating system (such as Windows XP) already in place may only need to be reconfigured for peer-to-peer operations.
Disadvantages of a peer-to-peer network (FCIT, 2009):
Decentralized - No central repository for files and applications.
Security - Does not provide the security available on a client/server network.
Conclusion
There are really a lot of challenges to overcome when designing a distributed system successfully. These challenges or complexity is due to problems that are common to systems exhibiting distribution behavior: The whole is greater than the sum of its parts. Nonetheless a system that runs on this kind of computer arrangement is more fault-tolerant and more powerful than many combinations of standalone systems, because distributed system architecture is more resilient, secured and has better performance.
Distributed systems offer many advantages that are practically impossible to get from standalone systems. The flexibility of distributed system architecture allows it to grow, accept new members, interact with them and modify its parts efficiently. System reliability (availability, security and fault tolerance) is also better because if a computer fails, another one takes its place to complete the specific task without affecting the overall performance. But the single most salient characteristic of distributed systems is that as more and more computers become interconnected we can create whole new types of applications that expand not only many geographical points but even different industries and government agencies.
