LAN Network topologies (IT)

Network topology is the name given to the way in which the devices (called nodes) are physically connected in a network.
There are three common network topologies, called ring, line (bus) and star.
You will be expected to briefly describe the features of each one, know their advantages and draw simple line diagrams to represent then.

Ring topology

In a ring topology, the nodes are connected in a ring and data travels in one direction using a control signal called a 'token'.

Advantages:

• Not greatly affected by adding further nodes or heavy network traffic as only the node with the 'token' can transmit data so there are no data collisions.
• Relatively cheap to install and expand.

Disadvantages:

• Slower than a star topology under normal load.
• If the cable fails anywhere in the ring then the whole network will fail.
• If any node fails then the token cannot be passed around the ring any longer so the whole network fails..
• The hardest topology to troubleshoot because it can be hard to track down where in the ring the failure has occurred.
• Harder to modify or expand because to add or remove a node you must shut down the network temporarily.
• In order for the nodes to communicate with each other they must all be switched on.

Bus (line) topology:

Nodes are connected to a main (bus) cable. If data is being sent sent between nodes then other nodes cannot transmit. If too many nodes are connected then the transfer of data slows dramatically as the nodes have to wait longer for the bus to be clear.

Advantages:

• The simplest and cheapest to install and extend.
• Well suited for temporary networks with not many nodes.
• Very flexible as nodes can be attached or detached without disturbing the rest of the network.
• Failure of one node does not affect the rest of the bus network.
• Simpler than a ring topology to troubleshoot if there is a cable failure because sections can be isolated and tested independently.

Disadvantages:

• If the bus cable fails then the whole network will fail.
• Performance of the network slows down rapidly with more nodes or heavy network traffic.
• The bus cable has a limited length and must be terminated properly at both ends to prevent reflected signals. 
• Slower than a ring network as data cannot be transmitted while the bus is in use by other nodes.

Star topology:

In this type of network, a central computer (server) usually forms the main node and the subsidiary nodes are connected to it and to each other through a switch or hub.

Advantages:

• The most reliable because the failure of a node or a node cable does not affect other nodes.
• Simple to troubleshoot because only one node is affected by a cable break between the switch and the node.
• Adding further nodes does not greatly affect performance because the data does not pass through unnecessary nodes.
• Easily upgraded from a hub to a switch or with with a higher performance switch.
• Easy to install and to expand with extra nodes.

Disadvantages:

• Uses the most cable which makes it more expensive to install than the other two topologies.
• The extra hardware required such as hubs or switches further increases the cost.
• As the central computer controls the whole system, the whole system will be affected if it breaks down or if the cable link between it and the switch fails.
• If the switch, the link to the server or the server itself fails then the whole network fails.

Resources:  http://www.klbict.co.uk/gcse/theory/5_3/5_3_4_topologies.htm

Article about Computer Network (IT)

Want to know more about computer networking? We lay out the basics for nonprofits, charities, and libraries.

What Is a Computer Network?

In the broadest sense, a network is any interconnected group of people or things capable of sharing meaningful information with one another. In a technology context, network is usually short for "computer network" or "data network" and implies that computers are the things sharing the meaningful information. At a conceptual level, all data networks consist of nodes, which refers to any computer or digital device using the network and links, the physical connections (either wired or wireless) that carry messages between nodes.

Data networks are important to all contemporary organizations because they provide faster, easier access to any message or data that can be represented and stored in digital format. For example, when your colleagues and predecessors research an issue relevant to your organization and share their data and conclusions with you in a data format your computer recognizes, you can copy key information from their report into your own, saving yourself significant amounts of time (of course, you're always sure to give credit where it's due). If the colleague whose work you're relying on works in the same cubicle as you and they remember where they've stored the relevant report, a network may not offer significant advantages since you can turn to him or he and ask for the file on a CD or USB flash drive. However, in many organizations, large distances separate co-workers, and data sharing becomes a significant logistical problem in the absence of a network.

In addition to data sharing, computer networks also enable resource sharing, an important consideration in all budget-conscious charities and organizations. Rather than buying one printer for every employee and replacing them when they wear out, an organization with a network can buy a single printer, connect it to the network, and configure it in such a way that every computer user in the organization can print to it. The initial cost of a networked printer is usually more than the cost of a single desktop printer, but when considering costs on a per-user basis, the average cost of the networked printer is often much less than the cost of buying a printer for every employee. While some networked devices such as printers, scanners, and fax machines have predetermined, specialized functions, you can also network and share generic, unspecialized computing power in the form of servers. Servers are large, powerful computers that can handle resource-intensive tasks more efficiently than desktop computers. As with the networked printer, the initial outlay for a server is more than that for a desktop computer, but across the organization, it's often cheaper to run the server-based version of a program since individual users won't need expensive, high-performance desktop and laptop computers. Servers can also deploy software to other networked machines at a lower cost.

Standards and Protocols

Protocols, also known as standards, are essential parts of many modern technologies. In essence, they're consensus-based documents that describe guidelines for vendors, manufacturers, and other stakeholders in order to ensure quality and interoperability in a diverse, competitive marketplace. Most networking protocols are fairly obscure and you only need to worry about them if something goes wrong. However a few standards are so widely adopted that every decision-maker and accidental techie needs to know their names and basic functions. Discussing networking without implicit or explicit reference to Ethernet and TCP/IP protocols is like discussing American law and politics without reference to the Constitution and the Bill of Rights.

Furthermore, adopting widely-accepted protocols (also known as de facto standards) has benefits in all aspects of technology, since it's a form of future-proofing. When you need to repair, replace, or upgrade networking equipment years from now, you can safely predict that products and services compatible with today's widely-adopted standards will still be available. Therefore, you can replace a few pieces of outdated equipment without building a new network from scratch. Also, you'll be able to find technicians and consultants who still understand these protocols.

After several decades of struggle amongst competing network protocols (each with a different corporate sponsor), three sets of network protocols emerged as de facto standards for local networks by the late 1990s. In local area networks, Ethernet over twisted pair (also known as 10 BASE T, twisted-pair Ethernet, or IEEE 802.3) is the accepted standard for wired networking while 802.11b/g/n (or WiFi) is the accepted standard for wireless networks. The TCP/IP protocol stack is a standard component of almost all modern networks, especially when those networks expect to communicate on the Internet where TCP/IP is the lingua franca. In wide area networking, there still exists a large degree of diversity and competition among networking protocols. However, at the local network level, any networking technology your organization adopts should be compatible with the three standards mentioned above.

Conclusion

Networking offers a variety of advantages for many nonprofits, charities, and libraries over standalone machines, but they're not necessarily needed for every office or work scenario. Continue reading the other parts of our Networks 101 series to better assess your needs and see what options are out there to fit them.

Resources : http://www.techsoup.org/support/articles-and-how-tos/networking-101-concepts-and-definitions

Comment :  So, we have to use it properly

10.2 Using Statistics (MATH)

Now you can work out several different statical measures.
In a real situation, you need to decide which one to use.
If you want to measure how spread out a set of measurement in, the range is the most useful statistic.
If you want to find a representative measurement, you need an average. Should it be the mode, the median or the mean? That depends on the particular situation.

Here is summary to help you decide which average to choose :

• Choose the mode if you want to know which is the most commonly occurring number.
• The median is the middle value, when the data values are put in order. Half the numbers are greater than the median and half the numbers are less than the median.
• The mean depends on every value. If you change one number you change the mean

Worked Example 10.2

Question:

Here are the ages, in years, of the players in a football team.

Work out the average age. Give a reason for your choice of average. 

Answer:

The mode is not a good choice.

   -There are three modes. Each has a frequency of only 2.

The mean will be effected by the two oldest people.

   -They are much older and will distort the value. In fact the mean is 22.1 and nine people are younger than this; only two are older.

The median is 20 and this id the best average to use in this case

   -Five players are younger than the median and five are older.

10.1 Calculating Statistics (MATH)

You can use statistics to summarise sets of data.          

You can also use them to compare different sets
of data.

You should already be able to calculate three

different averages: the mode, the media and    

the mean.  

Remember that the range is not an average. It
measures how spread out a set of values or numbers is.

For a large set of data, it is not practical to list every number
separately. instead, you can record the data in a frequency table


Worked Example 10.1 

Question :

The table shown the number of beads on 200 necklaces.
a   Find the mode.
b   Find the mean.
c   Find the range.

Answer : 

a  The mode is 35
    The mode is the number with the highest frequency.

b  6900 : 200 =34.5
    (25 x 34 + 30 x 48 + 35 x 61 + 40 x 30 + 45 x 15 + 50 x 12) : the sum of all the frequencies. This is a reasonable answer because it is near the middle of all the possible number of beads.

c  50 - 25 = 25
   this is the different between the largest and smallest number of beads.

ARE YOU A CARING PERSON? Try our questionnaire- and answer truthfully!

1.) If you saw a homeless person, what would you do?

     a.) Not do anything
     b.) Talk to her/him and give her/him food.
     c.) Yell a bad word to her/him.


2.) If you found your friend was Ill, would you take care of her/him?
 
     a.)No, I would pretend like I know nothing, because take care of a person would make me feel                                exhausted.
     b.)No, I wouldn't take care of her/him because I still have another thing to do
     c.)Yes, I would take care of her/him because she/he is my friend.
 

3.)If you found your friend swearing, what would you do?
 
    a.) Tell all teachers.
    b.) Tell/remind the person not to say  bad words.
    c.) Pretend like you hear nothing, and not do anything


4.) If you saw a rubbish around your environment, what would you do?
 
     a.) Walk past and not do anything
     b.) Pick it up and put the rubbish into a rubbish bin.
     c.)  Kick the rubbish to make it unseen.


5.)If you won a competition and received a huge amount of money, what would you do?
 
    a.) Spend all the money for your personal needs.
    b.) Share half of the money to other people that need the money.
    c.) Use all the money for celebration party.

Answers:

    1.) a: 2 b:4 c:0
    2.) a:0 b:2 c:4
    3.) a:2 b:4 c:0
    4.) a: 0 b:4 c:2
    5.) a:0 b:4 c:2

Results :

   less than 8 points: You don't really care to others. You need to show your empathy to others, if you don't,                               it means that arrogance is one of your characteristic and people will start to go away                                     from you.

13- 8 points: Sometimes you care for other people and your surrounding, but you often forget to look                               around and be busy with your own business. and it is shown that you've less empathetic to                             others.

More than 13 points: Well done- you're perfect! keep it up!