Advantages and Disadvantages of Different Network Topologies

1. Bus Topology

http://www.ciscorouting.com/networkbasic.html

Advantages of Bus Topology

• It is easy to handle and implement. 
• It is best suited for small networks.

Disadvantages of Bus Topology

• The cable length is limited. This limits the number of stations that can be connected.
• This network topology can perform well only for a limited number of nodes.

2. Ring Topology

http://doit.ort.org/course/networks/32.htm

Advantage of Ring Topology

• The data being transmitted between two nodes passes through all the intermediate nodes. A central server is not required for the management of this topology.

Disadvantages of Ring Topology

• The failure of a single node of the network can cause the entire network to fail.
• The movement or changes made to network nodes affects the performance of the entire network.

3. Star Topology

http://doit.ort.org/course/networks/30.htm

Advantage of Star Topology

• Better performance: star topology prevents the passing of data packets through an excessive number of nodes. At most, 3 devices and 2 links are involved in any communication between any two devices. Although this topology places a huge overhead on the central hub, with adequate capacity, the hub can handle very high utilization by one device without affecting others.
• Isolation of devices: Each device is inherently isolated by the link that connects it to the hub. This makes the isolation of individual devices straightforward and amounts to disconnecting each device from the others. This isolation also prevents any non-centralized failure from affecting the network.
• Benefits from centralization: As the central hub is the bottleneck, increasing its capacity, or connecting additional devices to it, increases the size of the network very easily. Centralization also allows the inspection of traffic through the network. This facilitates analysis of the traffic and detection of suspicious behavior.

• Easy to detect faults and to remove parts.

• No disruptions to the network when connecting or removing devices.

Disadvantages of Star Topology

• High dependence of the system on the functioning of the central hub
• Failure of the central hub renders the network inoperable
• The performance and scalability of the network depends on the capabilities of the hub
• Network size is limited by the number of connections that can be made to the hub
• Other nodes may see a performance drop if traffic to another node occupies a significant portion of the central node's processing capability or throughput.
• Wiring up of the system can be very expensive.

4. Tree Topology

http://www.wifinotes.com/computer-networks/network-topology.html

Advantage of Tree Topology

• Central hub (repeater) increases the distance a signal can travel between devices.                 

Disadvantage of Tree Topology

• Entire network collapse if central Hub fails.

 5. 

Hybrid Topology

http://www.mcmcse.com/glossary/h.shtml

Advantage of Hybrid Topology

• · t   Provides a better result by it.
• ·  It can be designed in many ways for various purposes.

Disadvantage of Hybrid Topology

• Difficult to identify the problem if the entire network shuts down.

6. Mesh Topology

http://www.mcmcse.com/glossary/h.shtml

Advantage of Mesh Topology

• The arrangement of the network nodes is such that it is possible to transmit data from one node to many other nodes at the same time.

Disadvantage of Mesh Topology

• The arrangement wherein every network node is connected to every other node of the network, many of the connections serve no major purpose. This leads to the redundancy of many of the network connections.

Citation: 

1. http://wiki.answers.com/Q/Advantages_and_disadvantages_of_topologies

2. http://www.bukisa.com/articles/114886_network-topology-advantages-and-disadvantages-of-tree-  bus-and-ring-topology

3. http://en.wikipedia.org/wiki/Star_network

4. http://www.buzzle.com/articles/advantages-and-disadvantages-of-different-network-topologies.html

Systems Development Life Cycle

Model of the Systems Development Life Cycle with the Maintenance bubble highlighted.

The Systems Development Life Cycle (SDLC), or Software Development Life Cycle in systems engineering, information systems and software engineering, is the process of creating or altering systems, and the models and methodologies that people use to develop these systems. The concept generally refers to computer or information systems.

In software engineering the SDLC concept underpins many kinds of software development methodologies. These methodologies form the framework for planning and controlling the creation of an information system: the software development process.

Systems development phases

A Systems Development Life Cycle (SDLC) adheres to important phases that are essential for developers, such as planning, analysis, design, and implementation, and are explained in the section below.

 A number of system development life cycle (SDLC) models have been created: waterfall, fountain, spiral, build and fix, rapid prototyping, incremental, and synchronize and stabilize. 

The oldest of these, and the best known, is the waterfall model: a sequence of stages in which the output of each stage becomes the input for the next. These stages can be characterized and divided up in different ways, including the following:

• Project planning, feasibility study: Establishes a high-level view of the intended project and determines its goals.

• Systems analysis, requirements definition: Refines project goals into defined functions and operation of the intended application. Analyzes end-user information needs.

• Systems design: Describes desired features and operations in detail, including screen layouts, business rules, process diagrams, pseudocode and other documentation.

• Implementation: The real code is written here.

• Integration and testing: Brings all the pieces together into a special testing environment, then checks for errors, bugs and interoperability.

• Acceptance, installation, deployment: The final stage of initial development, where the software is put into production and runs actual business.

• Maintenance: What happens during the rest of the software's life: changes, correction, additions, moves to a different computing platform and more. This, the least glamorous and perhaps most important step of all, goes on seemingly forever.

Strength and Weaknesses of SDLC
Strengths	Weaknesses
Control.	Increased development time.
Monitor Large projects.	Increased development cost.
Detailed steps.	Systems must be defined up front.
Evaluate costs and completion targets.	Rigidity.
Documentation.	Hard to estimate costs, project overruns.
Well defined user input.	User input is sometimes limited.
Ease of maintenance.
Development and design standards.
Tolerates changes in MIS staffing.

For more information, you can go to

http://en.wikipedia.org/wiki/Systems_Development_Life_Cycle

Citation :

1. http://en.wikipedia.org/wiki/Systems_Development_Life_Cycle

HEAD CRASH

What is head crash???

A serious disk drive malfunction. A head crash usually means that the head has scratched or burned the disk. In a hard disk drive, the head normally hovers a few microinches from the disk. If the head becomes misaligned or if dust particles come between it and the disk, it can touch the disk.

When this happens, you usually lose much of the data on the hard disk and will need to replace both the head and the disk. For this reason, it is important to operate disk drives, particularly hard disk drives, in as clean an environment as possible. Even smoke particles can cause a head crash.

A head crash in a modern drive. Note circular mark on the platter.

For more detailed information, you can go to http://en.wikipedia.org/wiki/Head_crash

                                                                    http://en.wikipedia.org/wiki/Hard-disk_failure

A Hard Drive Crash Can Be Bad... Trying To Fix It Yourself Can Be Worse !!!

We rely on our computers more than ever, and when a hard drive crash occurs, years of data can be gone in the blink of an eye. In most cases the data is fully recoverable, but there are some common mistakes people make when attempting to do the job themselves.

If you experience a hard drive crash, it doesn't take much to make a bad situation even worse. In the event of a hard drive crash, in most instances, the data is completely recoverable at first. However, as part of human nature, we often try to avoid the high costs associated with hard drive data recovery in order to find some sort of "quick fix" to a very serious problem. In this report we will go over some of the symptoms of a hard drive crash and the things you can try in the event of a suspected hard drive crash, along with things you should avoid.

 

What Noise Is The Drive Making???

The first thing to take into account is whether or not the hard drive is making any unusual noises. Often times when there is a problem with the read/write head within the drive or a firmware issue in the logic board a "clicking" sound will be heard. If this is the case, then you should immediately power down the system and refrain from reapplying power. Same thing goes for any unusual sound, whether it's grinding, squealing, our sounds like a "laser gun". If any of these occur you should immediately power the system down, just pull the plug from the wall.

Unfortunately in the situations listed above, there is little that you can do to recover the data yourself. Actually, unless you have a clean room, fully equipped with the right tools and a trained technician there is nothing that can be done. Even hard drive manufacturers avoid this line of work, and many who do offer data recovery services are only sub-contracting the work out to an established data recovery firm.


Never Ever Open Your Hard Drive !!!
 If the data on a hard drive has even the most minimal value to you, then it is imperative that you do not open the drive. Data recovery companies often get calls or e-mails from people who felt they had the right equipment and know-how to perform something as serious as a head replacement. Once they've removed the hard drive's protective case, more often than not, the damage is irreparable.


Working with the internal components of a hard drive requires at least a Class-100 clean room. A clean room does not mean a room that you just vacuumed and dusted (believe it or not people say that to us all the time). A clean room is a special work area in which air quality is heavily controlled and it is vital to hard drives during the manufacturing or assembly process.

 The air in the room is regulated in term of air particles, temperature and humidity. A Class-100 clean room means there exists no more than 100 particles that are larger than 0.5 microns in one cubic foot of air. Opening a hard drive in air meeting anything less than the standard listed above will mean certain death for your hard drive and any data contained therein.


The Freezer Method

There has been a long running myth about putting your hard drive in the freezer when it crashes in order to revive it just long enough to pull your data off of it. ACS Data Recovery has run this test for fun on many occasions in different scenarios with junk test drives that they have in the lab.

So far they have yet to see this actually produce any positive results. In fact, in one instance they actually began to see the formation of tiny microscopic ice crystals on the platter themselves, which is a definite crash waiting to happen.
See the problem with this theory is, drives are not completely sealed.

Regardless of what you've heard or read, all hard drives have a tiny breather hole (usually marked with a label that says "do not cover"). This hole not only aids in cooling but it also helps to equalize air pressure in the drive when the platters are spinning. On the other side of this hole is a filter, which keeps dirt and debris from getting inside the drive.

However, this filter does not stop heavy amounts of moisture (especially in flood situations) or moisture vapor (such as found in a freezer). This moisture vapor has been known to settle on the platters and when a system is powered up, the read/write head would resemble an airplane flying into a mountain.

For more infomation, you can surf to tis website~ http://www.beststuff.com/computers/a-hard-drive-crash-can-be-bad-trying-to-fix-it-yourself-can-be-worse.html 

http://www.ibas.com/espana/noticias/pictures

http://www.youtube.com/watch?v=PoVBHG4kajA&feature=related

Citation :

1. http://www.howstuffworks.com/question60.htm
2. http://www.webopedia.com/TERM/H/head_crash.html
3. http://en.wikipedia.org/wiki/Head_crash
4.  http://en.wikipedia.org/wiki/Hard-disk_failure
5. http://www.beststuff.com/computers/a-hard-drive-crash-can-be-bad-trying-to-fix-it-yourself-can-be-worse.html