Tropology of network

Bus Topology

Bus networks (not to be confused with the system bus of a computer) use a common backbone to connect all devices. A single cable, the backbone functions as a shared communication medium that devices attach or tap into with an interface connector. A device wanting to communicate with another device on the network sends a broadcast message onto the wire that all other devices see, but only the intended recipient actually accepts and processes the message.

Ethernet bus topologies are relatively easy to install and don't require much cabling compared to the alternatives. 10Base-2 ("ThinNet") and 10Base-5 ("ThickNet") both were popular Ethernet cabling options many years ago for bus topologies. However, bus networks work best with a limited number of devices. If more than a few dozen computers are added to a network bus, performance problems will likely result. In addition, if the backbone cable fails, the entire network effectively becomes unusable.

Ring Topology

In a ring network, every device has exactly two neighbors for communication purposes. All messages travel through a ring in the same direction (either "clockwise" or "counterclockwise"). A failure in any cable or device breaks the loop and can take down the entire network.

To implement a ring network, one typically uses FDDI, SONET, or Token Ring technology. Ring topologies are found in some office buildings or school campuses.

HawkEye and GoalRef

That bad call, however, would still not have been avoided even if the technology that FIFA has approved today—through the International Football Association Board (IFAB)—had been in place.

The approved tech only affects ghost goals. Ghost goals occur when the ball passes the goal line but then jumps straight back out. Technically, it is a goal. But referees sometimes don't call it as such because they can't see it from their point of view—even while the entire world can see it clearly on their TVs.

Other times, the refs seem to have ignoble intentions—like when Lampard scored for England against Germany in South Africa '10, or Spain's Michel kicked the ball inside Brazil's goal in Mexico '86 only for it to bounce out.

The first technology is called HawkEye, which had been used in tennis for quite some time. HawkEye's cameras will be installed on the pitch's goalposts. They will track the ball and make sure it has passed the line.

For redundancy, there will be another tracking technology called GoalRef. Developed by Fraunhofer Institute for Integrated Circuits IIS, GoalRef uses a low magnetic field around the goals, creating an invisible radio curtain. When the ball fully goes through that curtain, the magnetic field changes and the system sends an alert to the referee's wristwatch.Both will be great additions that will avoid extremely bad and embarrassing decisions by the referee.

Chapter 10.1 & 10.2 summary

CALCULATING STATISTIC: 10.1

Statistic can be use to summarise sets of data, and also use them to compare sets of data.

There  are three different average:

1. Mode: most common value or numaber
2. Median: the middle value, when they listed in order
3. Mean: the sum of all the value divided by the numbers of values.

Range is not an average. it measure how spread out a set of values or numbers is.

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

EXAMPLE:
the table shows the number of breads on 200 necklaces

Number of breads: 25, 30, 35, 40, 45, 50
Frequency            : 34, 48, 61, 30, 15, 12

A. find the mode
B. Find the mean
C. Find the range

ANSWER:
A. The mode is 35
B. (25x34 + 30x48 + 35x61 + 40x30 + 45x15 + 50x12) = 6900
   
     6900 divided by 200(sum of all frequencies) = 34.5
C. 50 - 25 = 25 (this is the difference between the largest and the smallest number of bread)


USING STATISTIC: 10.2

If you want to measure how spread out a set of measurement is, the range is the most useful statistic.

Here is a 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 put in order. half the numbers are greater than the median and the half the number are less than the median
• the mean depends on every value. if you change one number you can change the mean.   

EXAMPLE:

Here are the ages, in years, of the football players in football team.
Work out the average age.
16, 17, 18, 18, 20, 20, 21, 21, 32, 41

• The mode is not good choice. There are three modes. Each has a frequency of only 2.
• The mean will be affected by 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 is the best average to use in this case. Five player are younger than the median and five are older

Logic test

1. If you saw a person that need help, you would:
    A. help
    B. call police
    C. do nothing

2. If you could be a hero, you would be:
    A. The Joker
    B. Captain America
    C. Office boy

3. If you could choose any type of weapon, you would choose:
    A. slingshot
    B. fire arm
    C. brass knuckle

4. Imagine your family are killed, you would:
    A. be a vigilante
    B. call the police
    C. call ambulance

5.Imagine you are stranded on an island with no supplies, you would:
    A. sleep
    B.try to survive
    C. swim back to where you come from





1. a=1       11-15= perfect
    b=3       less than 10= normal
    c=2       less than 6= not smart enough
2. a=2
    b=1
    c=3
3. a=2
    b=1
    c=3
4. a=2
    b=1
    c=3
5. a=2
    b=3
    c=1

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