ENGINEERING ARTICLES

CSMA/CD: Carrier sense multiple access with collision detection (CSMA/CD) is one of the most popular access methods, with CSMA/CD, every host has equal access to the wire and can place data on the wire when the wire is free from traffic. If a host wishes to place data on the wire, it will “sense” the wire and determine whether there is a signal already on the wire. If there is, the host will wait to transmit the data; if the wire is free, the host will send the data, as shown in Figure 1. Figure 1: A host "sensing" the wire to see if it is free of traffic. The problem with the process just described is that, if there are two systems on the wire that “sense” the wire at the same time to see if the wire is free, they will both send data out at the same time if the wire is free. When the two pieces of data are sent out on the wire at the same time, they will collide with one another, and the data will be destroyed. If the data is destroyed in transit, the data will ne...

Networks are built using a topology of bus, star, or ring, but how the systems will be connected in the topology that you choose. Cabling is the medium for the transmission of data between hosts on the LANs. LANs can be connected together using a variety of cable types, such as unshielded twisted-pair, coax, or fiber. Each cable type has its own advantages and disadvantages. There are three primary types of cable media that can be used to connect systems to a network: Coaxial cable Twisted-pair cable Fiber-optic cable Transmission rates that can be supported on each of these physical media are measured in millions of bits per second, or megabits per second (Mbps). 1. COAXIAL CABLE: Coaxial, or coax, cable looks like the cable used to bring the cable TV signal to your television. One strand (a solid-core copper wire) runs down the middle of the cable. Around that strand is a layer of insulation, and covering that insulation is braided wire and metal foil, which shiel...

Organizations of different sizes, structures, and budgets need different types of networks. A local newspaper company has needs for its network that would be different from the needs of a multinational company. Networks can be divided into one of two categories: peer-to-peer or server-based networks. PEER-TO-PEER NETWORK: A peer-to-peer network has no dedicated servers; instead, a number of workstations are connected together for the purpose of sharing information or devices. When there is no dedicated server, all workstations are considered equal; any one of them can participate as the client or the server. Peer-to-peer networks are designed to satisfy the networking needs of home networks or of small companies that do not want to spend a lot of money on a dedicated server but still want to have the capability to share information or devices. For example, a small accounting firm with three employees that needs to access customer data from any of the three systems or print to ...

There are two different techniques that may be used to transmit the signal along the network wire which are base-band communication and broadband communication. BASEBAND Sends digital signals through the media as a single channel that uses the entire bandwidth of the media. The signal is delivered as a pulse of electricity or light, depending on the type of cabling being used. Baseband communication is also bidirectional, which means that the same channel can be used to send and receive signals. BROADBAND Sends information in the form of an analog signal, which flows as electromagnetic waves or optical waves. Each transmission is assigned to a portion of the bandwidth, so unlike with baseband communication, it is possible to have multiple transmissions at the same time, with each transmission being assigned its own channel or frequency. Broadband communication is unidirectional, so in order to send and receive, two pathways will need to be used. This can be accomplished either ...

It is typical for networks to implement a mixture of topologies to form a hybrid topology. For example, a very popular hybrid topology is a star-bus topology, in which a number of star topologies are connected by a central bus. This is a popular topology because the bus will connect hubs that are spread over distance. Another very popular hybrid topology is the star-ring topology. The star-ring topology is popular because it looks like a star but acts as a ring. For example, there is a network architecture known as Token Ring (more on this later, in the section “Network Architectures”) that uses a central “hub” type device, but the internal wiring makes a ring. Physically it looks like a star, but logically it acts as a ring topology. Figure: Hybrid Topology. Advantages of Hybrid Network Topology 1) Reliable: Unlike other networks, fault detection and troubleshooting is easy in this type of topology. The part in which fault is detected can be isolated from the rest of networ...

Basically, the DSTATCOM system is comprised of three main parts: a Voltage Source Converter (VSC), a set of coupling reactors and a controller. The basic principle of a DSTATCOM installed in a power system is the generation of a controllable ac voltage source by a voltage source inverter (VSI) connected to a dc capacitor (energy storage device). The ac voltage source, in general, appears behind a transformer leakage reactance. The active and reactive power transfer between the power system and the DSTATCOM is caused by the voltage difference across this reactance. The DSTATCOM is connected to the power networks where the voltage-quality problem is a concern. All required voltages and currents are measured and are fed into the controller to be compared with the commands. The controller then performs feedback control and outputs a set of switching signals to drive the main semiconductor switches (IGBT’s, which are used at the distribution level) of the power converter accordingly. Fi...

A network topology is the physical layout of computers, cables, and other components on a network. There are a number of different network topologies, and a network may be built using multiple topologies. The different types of network layouts are Bus topology Star topology Mesh topology Ring topology Hybrid topology Wireless topology 1) BUS TOPOLOGY A bus topology uses one cable as a main trunk to connect all of the systems together. A bus topology is very easy to set up and requires no additional hardware such as a hub. The cable is also called a trunk, a backbone, or a segment. 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...

In a ring topology, all computers are connected via a cable that loops in a ring or circle. As shown in Figure 1, a ring topology is a circle that has no start and no end. Because there are no ends, terminators are not necessary in a ring topology. Signals travel in one direction on a ring while they are passed from one computer to the next, with each computer regenerating the signal so that it may travel the distance required. Figure 1: Ring Topology ADVANTAGES OF RING TOPOLOGY: A major advantage of a ring topology is that signal degeneration is low because each workstation is responsible for regenerating or boosting the signal. With the other topologies, as the signal travels the wire, it gets weaker and weaker as a result of outside interference: eventually, it becomes unreadable if the destination system is too far away. Because each workstation in a ring topology regenerates the signal, the signal is stronger when it reaches its destination and seldom needs to be retrans...

A mesh topology is not very common in computer networking today. In a mesh topology, every workstation has a connection to every other component of the network, as illustrated in Figure. Figure: Mesh Topology ADVANTAGES OF MESH TOPOLOGY: The biggest advantage of a mesh topology is fault tolerance, meaning that, if there is a break in a cable segment, traffic can be rerouted through a different pathway because there are multiple pathways to send data from one system to another. This fault tolerance means that it is almost impossible for the network to go down due to a cable fault. DISADVANTAGES OF MESH TOPOLOGY: A disadvantage of a mesh topology is the cost of the additional cabling and network interfaces to create the multiple pathways between each system. A mesh topology is very hard to administer and manage because of the numerous connections.

In a star topology, all computers are connected through one central device known as a hub or a switch, as illustrated in Figure 1. Each workstation has a cable that goes from the network card to the hub device. One of the major benefits of a star topology is that a break in the cable causes only the workstation that is connected to the cable to go down, not the entire network, as with a bus topology. Star topologies are very popular topologies in today’s networking environments. Figure 1: Star Topology ADVANTAGES OF A STAR TOPOLOGY: One advantage of a star topology is scalability and ease of adding another system to the network. If you need to add another workstation to the network with a star topology, you simply connect that system to an unused port on the hub. Another benefit is the fact that if there is a break in the cable it affects only the system that is connected to that cable. Figure 1-7 shows a hub with a few ports available. Centralizing network components...

A bus topology uses one cable as a main trunk to connect all of the systems together (shown in Figure.1). A bus topology is very easy to set up and requires no additional hardware such as a hub. The cable is also called a trunk, a backbone, or a segment. 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 re transmitted. For this reason, at each end of the cable there is a terminator. The terminator is designed to absorb the signal when the signal reaches the end, preventing signal bounce. If there is no termination, the entire network fails because of signal bounce, which also means that if there...

Internet, intranet, and extranet are three terms that describe “Internet-type” applications that are used by an organization. INTERNET: If you wish to expose information to everyone in the world, then you would build an Internet type application. An Internet-type application uses Internet protocols such as HTTP, FTP, or SMTP and is available to persons anywhere on the Internet. We use the Internet and web applications as ways to extend who the application can reach. For example, I no longer need to go to the bank to transfer funds. Because the bank has built a web site on the Internet, I can do that from the comfort of my own home. Figure 1: Internet INTRANET: An application is considered to be on the company’s intranet if it is using Internet-type protocols such as HTTP or FTP but the application is available only within the company. The information on a company’s intranet would not be accessible to persons on the Internet because it is not for public use. For exam...

Communication between two devices can be simplex, half-duplex, or full-duplex. SIMPLEX : In simplex mode, the communication is unidirectional, as on a one-way street. Only one of the two devices on a link can transmit; the other can only receive. Keyboards and traditional monitors are examples of simplex devices. The keyboard can only introduce input; the monitor can only accept output. The simplex mode can use the entire capacity of the channel to send data in one direction. HALF-DUPLEX : In half-duplex mode, each station can both transmit and receive, but not at the same time. When one device is sending, the other can only receive, and vice versa The half-duplex mode is like a one-lane road with traffic allowed in both directions. When cars are traveling in one direction, cars going the other way must wait. In a half-duplex transmission, the entire capacity of a channel is taken over by whichever of the two devices is transmitting at the time. Walkie-talkies and CB (...

When we communicate, we are sharing information. This sharing can be local or remote. Between individuals, local communication usually occurs face to face, while remote communication takes place over distance. Data communications are the exchange of data between two devices via some form of transmission medium such as a wire cable. For data communications to occur, the communicating devices must be part of a communication system made up of a combination of hardware (physical equipment) and software (programs). The effectiveness of a data communications system depends on four fundamental characteristics: delivery, accuracy, timeliness, and jitter. 1. Delivery: The system must deliver data to the correct destination. Data must be received by the intended device or user and only by that device or user. 2. Accuracy: The system must deliver the data accurately. Data that have been altered in transmission and left uncorrected are unusable. 3. Timeliness : The system must delive...

A typical network involves having users sit at workstations, running such applications as word processors or spreadsheet programs. The workstation also is known as a client, which is just a basic computer running a client operating system such as Windows XP or Linux. These users typically store their files on a central server so that they can share the files with other users on the network. The server is a special computer that contains more disk space and memory than are found on client workstations. The server has special software installed that allows it to function as a server. This special software can provide file and print services (to allow sharing of files and printers), provide web pages to clients, or provide e-mail functionality to the company. The term host refers to any computer or device that is connected to a network and sends or receives information on that network. A host can be a server, a workstation, a printer with its own network card, or a device such as ...