Introduction

I am Ahmed Ouda Mosalam, Egyptian, Male, High school computer teacher, IGCSE teacher  and American, teaching ICT at Educational Language Complex in Ismailia town. I created my Blog to communicate with my students, parents and any human wanted to learn. It is contains lessons (syllabus), questions, answer and assignment.

I wish to make this Blog contains every thing that my students or any human needs, also let all teachers of other subjects be in our Blog.

I need your opinion about any thing in the Blog, any improvements.

Lesson 1

COMPUTER NETWORKS

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– A computer network is developed by linking computer systems together. The network includes the computer system, the connection and hardware needed to allow the communication. Networks can be limited to a building/area, short distances (Local Area Network or LAN) or worldwide such as internet, long distances (Wide Area Network or WAN). Using WANs, computers may be linked together in different countries using satellites, microwaves or telecommunication (electronics means). Wireless Local Area Network (WLAN) is similar to other wireless devices and uses radio instead of wires to transmit data back and forth between computers on the same network. All networks are encrypted (locked unless you have the code, or key), which means anyone in range can access them. Wireless clients can be mobile devices such as laptops, personal digital assistants, IP phones, or fixed devices such as desktops and workstations that are equipped with a wireless network interface.

LANs and WLANs are often connected to WANs via a special gateway.

– Network Features:

 – Topology: Physical or logical layout of cables and devices that connect the network nodes (stations).

Media: Wires and cables that carry data.

Bandwidth: Amount of data media can carry.

Protocol: The method computers use to communicate with each other.

 

– The advantages of networking:

1- Data and files can share between computers.

2- Sharing of expensive peripherals such as printers, plotters.

3- Sharing of software.

4- Easy communications between users.

5- All the users work can be stored in a central place (file server).

6- All files can be backed up from one place.

7- Network software can be purchased cheaper than buying an

individual  package for each machine.

8- Unauthorized interference can be reduced by allowing users

different levels of

9-  By using passwords your data can be kept secure.

– The disadvantages of networking:

1- The initial cost is high.

2- Viruses can easily be spread.

3- As data is shared there is a greater need for security.

4- If a network breaks down, all connected computers will not work.

5- Many computers on the network may make it go slow.

6- Network manager (experienced person) may be needed to run the

system.

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Questions

Q.1 Ring two items which are used to connect computers to the internet.

ISDN link                             Keyboard                             MIDI

Modem                                 Monitor                              RAM

Q.2 What is meant by the phrase “Computer Network”?

Q.3 Describe what is meant by LAN and a WAN.

Q.4 Name an item of hardware that a home computer must have before it can be connected using a telephone line to the internet. Give a reason why it is needed.

Answer

1- ISDN link.

2- Many computers linked together to share data and files between computers, Sharing of expensive peripherals such as printers, plotters, Sharing of software, Easy communications between users and All the users work can be stored in a central place (file server).

3-  (Local Area Network or LAN) Networks can be limited to a building/area, short distances.

(Wide Area Network or WAN) worldwide such as internet, long distances.

Lesson 2

– Analogue data And Digital data:                                     RockItnetworks

Digital or Discrete data can take fixed values within a

certain range, for example: the number of children in a

family, number of days in a week. Digital values can fed

directly into the computer, it is made up of 0‘s and

1‘s,Text, music, images etc.

Analogue or Continuous data can take any value within a certain range, for

example: the speed of a car this might be 65.3 or 65.889, also temperatures don’t jump from one degree straight to the next, there are many values in between. Almost all physical measurement are continuous data (weight, pressure, temperature, sound level, voltage, time …. etc.). Such data is often measured within an analogue device such as sensors, and then converted to a digital signal with an Analogue-to-Digital Converter (ADC) to be readby computer, since all computers process only digital values.

If an analogue signal is required, all output signals from computer can be fed into Digital-to-Analogue Converter (DAC) and then to activators.

– Modems: Telephone lines are designed to carry analogue waves or signals. In order for a computer to send data by telephone, you need to convert its signals from digital to analogue.

A modem (short for modulator/demodulator) is the device that coverts telephone signals to digital signals and vice versa. Typical speed of modem is 56,000 bps (bits per second), about one page of text every second.

A modem installed inside the CPU is called an internal modem.

A modem that is a separate unit is called an external modem.

– Digital phone line:

1- ISDN (Integrated Services Digital Network): It

provides digital access that is faster than analogue.

2- DSL (Digital Subscriber Line) Broadband: It

another type of digital telephone service that allows greater bandwidth.

– Advantages of digital phone line to a modem:

1- Data is transmitted much faster.

2- Data can compress and sent faster.

3- Provides good video conferencing.

Lesson 3

How WiFi Works                                                              More Information

If you’ve been in an airport, coffee shop, library or hotel recently, chances are you’ve been right in the middle of a wireless network. Many people also use wireless networking, also called WiFi or 802.11 networking, to connect their computers at home, and some cities are trying to use the technology to provide free or low-cost Internet access to residents. In the near future, wireless networking may become so widespread that you can access the Internet just about anywhere at any time, without using wires.

WiFi has a lot of advantages. Wireless networks are easy to set up and inexpensive. They’re also unobtrusive — unless you’re on the lookout for a place to use your laptop, you may not even notice when you’re in a hotspot. In this article, we’ll look at the technology that allows information to travel over the air. We’ll also review what it takes to create a wireless network in your home.

What’s in a name?

You may be wondering why people refer to WiFi as 802.11 networking. The 802.11 designation comes from the IEEE. The IEEE sets standards for a range of technological protocols, and it uses a numbering system to classify these standards.

One wireless router can allow multiple devices to connect to the Internet.

A wireless network uses radio waves, just like cell phones, televisions and radios do. In fact, communication across a wireless network is a lot like two-way radio communication. Here’s what happens:

1-  A computer’s wireless adapter translates data into a radio signal and transmits it using an antenna.

2-  A wireless router receives the signal and decodes it. The router sends the information to the Internet using a physical, wired Ethernet connection.

3-  The process also works in reverse, with the router receiving information from the Internet, translating it into a radio signal and sending it to the computer’s wireless adapter.

– The radios used for WiFi communication are very similar to the radios used for walkie-talkies, cell phones and other devices. They can transmit and receive radio waves, and they can convert 1s and 0s into radio waves and convert the radio waves back into 1s and 0s. But WiFi radios have a few notable differences from other radios: They transmit at frequencies of 2.4 GHz or 5 GHz. This frequency is considerably higher than the frequencies used for cell phones, walkie-talkies and televisions. The higher frequency allows the signal to carry more data.

 

Bluetooth

­If you were to randomly pick up a piece of electronics equipment in your house, there’s a reasonable chance that it has Bluetooth capabilities, especially if the gadget in question is fairly new. Whether it’s a cell phone, smartphone, laptop, printer or keyboard, Bluetooth wireless technology has made life easier for those of us with too many electronics on our hands. Bluetooth devices get rid of frustrating wires and expensive adapters by using short-range radio signals to connect devices to each other and send information back and forth.

Bluetooth is especially common in mobile phones, which make up more than 60 percent of the Bluetooth market. Bluetooth headsets, for example, transmit calls from your phone to the headset in your ear — this allows you to keep your phone in your pocket, backpack or handbag while walking around. It’s also helpful to drivers wanting to cruise around hands-free.

Bluetooth devices use the free, 2.4-gigahertz radio band known as ISM, which stands for industrial, scientific and medical devices. The band is unlicensed for low-power use, so headsets and other Bluetooth accessories use very little in the way of batteries. While any two Bluetooth devices can share ­data at a range between 10 and 100 meters (33 and 328 feet), phones usually operate at the former, laptops at the latter. Any Bluetooth device can communicate with up to sevenother devices at a time.

You can select whether or not your phone or laptop is visible to others within the area.

If several Bluetooth devices are set on discoverable mode, they all have the ability to search for and locate each other, so long as they remain within range. Every device has its own address, a unique 48-bit identifier with six bytes of information that might look like this: 01:23:45:67:89.10. The first three bytes (01:23:45) are assigned to the specific manufacturer of the device, while the last three bytes (67:89:10) are assigned by the manufacturer. These make each device entirely unique.

If several Bluetooth-enabled receivers are strategically placed to cover a large area, they can track the positions of any discoverable device, recording and sending any data back to a single address. Each Bluetooth receiver acts like any regular Bluetooth device: It searches for every device within range. If a person walked down a 100-meter-long (328-foot-long) street and each Bluetooth receiver had a range of 10 meters, five receivers with a radius of 20 meters (66 feet) would be needed to track that person’s movement. As he walked toward the street, the first receiver would track him for the length of the first 20 meters, the second for the next 20 meters, and so on for the length of the street.

Lesson 4

IGCSE_ICT_Glossary

Network Devices:

In order for a computer to operate on a network, there are a range of different components that are required.

1- Switches:Forwards data packets to computer and printers.

A switch has a number of ports and it stores the addresses of all devices that are directly or indirectly connected to it on each port.  As a data packet comes into the switch, its destination address is examined and a direct connection is made between the two machines. A network cable can only have one data packet in it at any instant. So if two or more computers want to place a data packet on to the network at exactly the same time, then a ‘data collision’ will take place.

 But imagine what happens when a hundred PCs are sharing the same network and they all want to send their data packets. This will most likely result in thousands of data collisions per second – each one costing a small amount of time. You will certainly notice the network ‘slowing down’.

A concentrator is a device that provides a central connection point for cables from workstations, servers, and peripherals. In a star topology, twisted-pair wire is run from each workstation to a central switch/hub. Most switches are active, that is they electrically amplify the signal as it moves from one device to another. Switches no longer broadcast network packets as hubs did in the past, they memorize addressing of computers and send the information to the correct location directly. Switches are:

  • Usually configured with 8, 12, or 24 RJ-45 ports.
  • Often used in a star or star-wired ring topology.
  • Sold with specialized software for port management.
  • Also called hubs.

Usually installed in a standardized metal rack that also may store netmodems, bridges, or routers.

2- Hubs:A hardware device that contains multiple independent but connected modules of network and internetwork equipment. Hubs can be active (where they repeat signals sent through them) or passive (where they do not repeat but merely split signals sent through them).

A hub does not manage any of the data traffic throught it.

There are many network topologies available:

  1. Star – uses a hub.
  2. Tree – uses a hub.
  3. Bus – does not use a hub.
  4. Ring – does not use a hub.

To allow the Star and Tree network topologies to work properly, each computer must be able to send data packets to any other computer on the network. The network ‘Hub’ allows computers to share data packets within a network. Each computer will be connected to a single ‘port’ on the hub. So if you purchase an ‘8 port hub’, you will be able to connect up to eight computers together.

You can also ‘daisy chain’ hubs to allow even more computers to join the network.

Lesson 5

3- Bridges: Usually only has two ports.

A bridge is a device that allows you to segment a large network into two smaller, more efficient networks. If you are adding to an older wiring scheme and want the new network to be up-to-date, a bridge can connect the two.

A bridge monitors the information traffic on both sides of the network so that it can pass packets of information to the correct location. Most bridges can “listen” to the network and automatically figure out the address of each computer on both sides of the bridge. The bridge can inspect each message and, if necessary, broadcast it on the other side of the network.

The bridge manages the traffic to maintain optimum performance on both sides of the network. You might say that the bridge is like a traffic cop at a busy intersection during rush hour. It keeps information flowing on both sides of the network, but it does not allow unnecessary traffic through. Bridges can be used to connect different types of cabling, or physical topologies. They must, however, be used between networks with the same protocol.

 

4- Routers:

A router translates information from one network to another; it is similar to a superintelligent bridge. Routers select the best path to route a message, based on the destination address and origin. The router can direct traffic to prevent head-on collisions, and is smart enough to know when to direct traffic along back roads and shortcuts. A router links the two networks and connects both networks to the Internet

While bridges know the addresses of all computers on each side of the network, routers know the addresses of computers, bridges, and other routers on the network. Routers can even “listen” to the entire network to determine which sections are busiest — they can then redirect data around those sections until they clear up.

The router is the only device that sees every message sent by any computer on either of the company’s networks. Therouter ensures that information doesn’t go where it’s not needed. The router makes sure that information does make it to the intended destination. a router is extremely useful in dealing with two separate computer networks. It joins the two networks, passing information from one to the other and, in some cases, performing translations of various protocols between the two networks. It also protects the networks from one another, preventing the traffic on one from unnecessarily spilling over to the other.

If you have a school LAN that you want to connect to the Internet, you will need to purchase a router. In this case, the router serves as the translator between the information on your LAN and the Internet. It also determines the best route to send the data over the Internet. Routers can:

5- Repeaters:

All signals fade as they travel from one place to another. Each type of network cable has a maximum useable length. If you go beyond that length, the signal will be too weak to be useful.

Of course, computers on a real network can easily be more than 200 metres apart. Therefore the network cable is split up into segments. Each segment is less than the maximum length allowed. Joining the segments together is a device known as a ‘Repeater’.

A Repeater boosts the signal back to its correct level.

6- The Proxy Server: Can be a major component of a firewall.

This machine has the two main tasks:

  1. Supply authorised internal users with web pages.
  2. Supply external users with authorised information and services.

Quite often staff tend to use the same web sites over and over again. So to speed up access and reduce bandwidth costs, the Proxy server will keep a local copy of the web pages and serve these to the user instead.

Of course rules running on the proxy server will determine how often these local pages need to be updated.

The proxy server also acts as a filter:

  • Prevents unauthorised users from accessing external networks such as the internet.
  • Prevents unauthorised web sites to be accessed (stops time wasting!).
  • Provides a web service to external clients, but does not allow them through to the internal network itself. For example a bank web site would be running a proxy service.

In a way, the proxy server is the outward facing aspect of the company linked to the internet.

More about Networks

Lesson 6

– Network Topology: Is the name given to the way in which the computers (called terminals) are connected in the network. There are three common topologies called ring, line (bus) and star.

1- Ring topology: “Tokens” of data are passed round the ring (in one direction only) and collected by the receiving computer.

– Advantages:

a- A faster system because the data only flows in

one direction.

b- More secure.

c- No data collisions.

d- Extra computers can be added easily with little effect on performance – although you have to shut down the network to be able to do this.

– Disadvantages:

a- Must be planned in advance.

b- If one station goes down, the reminder will be affected.

 

2- Line (Bus) Topology:

– Advantages:

a- Easier to extend line as only single cable needed for each new station.

b- Useful where network cannot be planned in advance.

c- Easy to install.

d- Easy to add extra workstations.

e- Uses less cable than a Star network.

f-  Best choice for temporary networks.

– Disadvantages:

a- Data sent in both directions therefore slower than ring.

b- If main cable is down, the network will stop.

c- Performance of the network slows down with more users.

d- Low security – every workstation can see all of the data in the network.

e- Limited cable length and a maximum number of workstations.

3- Star topology:All computers are connected to a central computer (file server) which manages them.

– Advantages:

a- communication is fast because there is a direct path from the central controller to each terminal.

b- A cable failure does not affect other users.

c-Very few data collisions as each workstation has its own cable to the server.

d- Good security – no workstation can interact with another without going through the server first

– Disadvantages:

a- If file server breaks down then all the computers are affected.

b-  Uses a lot of cables which is expensive.

c- Requires a “hub” box at the file server to control all the cables.

d- The most expensive network layout to install because of the amount of cables needed.

e- Installing the network usually needs experts to set it up.

4- Hybrid (Tree): A tree topology combines characteristics of linear bus and star topologies. It consists of groups of star-configured workstations connected to a linear bus backbone cable (See fig. 4). Tree topologies allow for the expansion of an existing network, and enable schools to configure a network to meet their needs.

Advantages:

  1. Point-to-point wiring for individual segments.
  2. Supported by several hardware and software venders.

Disadvantages:

  1. Overall length of each segment is limited by the type of cabling used.
  2. If the backbone line breaks, the entire segment goes down.
  3. More difficult to configure and wire than other topologies.