week 6: Network topology

Network topology is the study of the arrangement or mapping of the elements (links, nodes, etc.) of a network, especially the physical (real) and logical (virtual) interconnections between nodes.

Logical topology :
the mapping of the flow of data between the nodes in the network determines the logical topology of the network

Physical topology
Any given node in the LAN will have one or more links to one or more other nodes in the network and the mapping of these links and nodes onto a graph results in a geometrical shape that determines the physical topology of the network

There are several basic types of topology in networks:
1. Bus topology
A linear bus topology consists of a main run of cable with a terminator at each end. All nodes (file server, workstations, and peripherals) are connected to the linear cable.Ethernet and LocalTalk networks use a linear bus topology.The bus cable carries the transmitted message along the cable. As the message arrives at each workstation, the workstation computer checks the destination address contained in the message to see if it matches it's own. If the address does not match, the workstation does nothing more. If the workstation address matches that contained in the message, the workstation processes the message. The message is transmitted along the cable and is visible to all computers connected to that cable.

Advantages of a Linear Bus Topology

• Easy to connect a computer or peripheral to a linear bus. 
• Requires less cable length than a star topology.

Disadvantages

• Entire network shuts down if there is a break in the main cable. 
• A faulty cable or workstation will take the entire LAN down 
• terminators are required at both ends of the backbone cable. 
• Difficult to identify the problem if the entire network shuts down. 
• Not meant to be used as a stand-alone solution in a large building. 

 2.Star topology 
A star topology is designed with each node (file server, workstations, and peripherals) connected directly to a central network hub or concentrator. Data on a star network passes through the hub or concentrator before continuing to its destination. The hub or concentrator manages and controls all functions of the network. It also acts as a repeater for the data flow.This configuration is common with twisted pair cable; however, it can also be used with coaxial cable or fiber optic cable.The protocols used with star configurations are usually Ethernet or LocalTalk.

Advantages

• Easy to install, and wire. 
• Easy to add new workstations 
• No disruptions to the network when connecting or removing devices. 
• Any non-centralised failure will have very little effect on the network 
• Easy to detect faults and to remove parts. 
• Centralized control Centralized network/hub monitoring

Disadvantages

• Requires more cable length than a linear topology. 
• If the hub or concentrator fails, nodes attached are disabled. 
• More expensive than linear bus topologies because of the cost of the concentrators.
• 
  

3.Star-Wired
A star-wired topology may appear (externally) to be the same as a star topology. Internally, the MAU (multistation access unit) of a star-wired ring contains wiring that allows information to pass from one device to another in a circle or ring. The Token Ring protocol uses a star-wired topology.

4. tree topology 
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. Tree topologies allow for the expansion of an existing network, and enable schools to configure a network to meet their needs.

Advantage

• Point-to-point wiring for individual segments.

Disadvantages

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

VIDEO!!!

1. A network interface controller is a computer hardware component that connects a computer to a computer network.Whereas network interface controllers were commonly implemented on expansion cards that plug into a computer bus, the low cost and ubiquity of the Ethernet standard means that most newer computers have a network interface built into the motherboard.

2. A networking operating system (NOS) is the software that runs on a server and enables the server to manage data, users, groups, security, applications, and other networking functions. The network operating system is designed to allow shared file and printer access among multiple computers in a network, typically a local area network (LAN), a private network or to other networks. The most popular network operating systems are Microsoft Windows Server 2003, Microsoft Windows Server 2008, UNIX,Linux, Mac OS X, and Novell NetWare.

3. A network switch or switching hub is a computer networking device that connects network devices. The term commonly refers to a multi-port network bridge that processes and routes data at the data link layer of the OSI model. Switches that additionally process data at the network layer and above are often referred to as layer-3 switches or multilayer switches.

4. REPEATER is a network device used to regenerate or replicate a signal. Repeaters are used in transmission systems to regenerate analog or digital signals distorted by transmission loss. Analog repeaters frequently can only amplify the signal while digital repeaters can reconstruct a signal to near its original quality. In a data network, a repeater can relay messages between sub networks that use different protocols or cable types. Hubs can operate as repeaters by relaying messages to all connected computers. A repeater cannot do the intelligent routing performed by bridges and routers.

5. LocalTalk refers to the physical networking -- that means the built-in controller in many Apple computers, the cables and the expansion cards required on some systems. The "official" Apple cabling system typically uses a "bus topology" where each device in the network is directly connected to the next device in a daisy chain. The illustration on the Farallon book cover below gives an idea of how a bus looks.

6. Token ring local area network (LAN) technology is a local area network protocol which resides at the data link layer (DLL) of the OSI model. It uses a special three-byte frame called a token that travels around the ring. Token-possession grants the possessor permission to transmit on the medium. Token ring frames travel completely around the loop.

7. Ethernet is a family of computer networking technologies for local area networks (LANs) commercially introduced in 1980. Ethernet has largely replaced competing wired LAN technologies.

FOR THE FURTHER INFORMATION, PLZ CLICK:

http://blog.teachbook.com.au/index.php/computer-science/networking-2/network-topologies/

week 5: Networking

Hey everyone,

This week we will discuss about networking. The lec notes are cover the introduction, importance of networking, network components, methods, networks devices and software. But we just focus about the introduction, types of networking and network components. Others will discuss next week….I think so……….

1st of all, we talk about computer networking. What is computer networking? Computer networking is connecting a computer with other computers or other devices to enable them to communicate with each other such as
· Computer devices/equipments
· transmission media to send/control data/signals,
· Communication devices to transmit/send data from sources to destinations
· Software

Next, is about types of the networking. Common examples of area network types are:

LAN - Local Area Network





WAN - Wide Area Network




PAN - Personal Area Network




MAN - Metropolitan Area Network





Last, is about network components. Network components can be categories into 5. There are terminals, transmission media, network electronics, software and network architecture standards.

1. Terminals & workstation: Normally refers to data sources and destinations. Example: Personal computers, terminals, workstations, computers, Automatic teller machines (ATM)

2. Transmission media: The physical media used to transmit data in a networked environment. It is also use to transmit signal and electrical pulse through a medium. 2 types of transmission media are bounded/guided and unbounded/unguided. Examples of bounded media are twisted pair wire, coaxial cable and fiber optic cables. Examples of unbounded are AM & FM radio, TV broadcasting, satellite communication, microwave radio, infrared signals

3. Network electronic devices are devices that control data transmission from sources to destinations. It also act as interface between different transmission media or communication protocol. For examples: Bridges, concentrators, routers, Front End Processors, Private Branch, Switches Exchange (PBX), Hubs, Multiplexers, and Gateways.

4. Software: At the nodes, it involves techniques and protocols. Functions of the software are measure the data transmits to the destination.

5. Network architecture standards: Architecture is blueprint of standards for a network consisting of items such as choice of media, media interfaces, encoding methods, transmission protocols, routing protocols and so on. It is needed to ensure interoperability between various devices and equipment made by different vendors.




More information:
1. Bridges

A Bridge does just what you would expect it to do - it joins two networks together so as far as data packets are concerned it looks like one large network




A bridge is not as capable as a Router - but it is less expensive.


2. Hub
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.
However there is a problem with doing this - see 'Switches' for more details.



Typical network making use of a hub:



3. Switches
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.

4. Protocols
A 'protocol' is a rather technical word. But it simply means an agreed method of doing something.
A 'NETWORK PROTOCOL' is the agreed method of communication to be used within the network.
Each device / computer will use this protocol.

5. End nodes

Within a vast computer network, the individual computers on the periphery of the network, those that do not also connect other networks, and those that often connect transiently to one or more clouds are called end nodes. Typically, within the cloud computing construct, the individual user / customer computer that connects into one well-managed cloud is called an end node. Since these computers are a part of the network yet unmanaged by the cloud's host, they present significant risks to the entire cloud. This is called the End Node Problem. There are several means to remedy this problem but all require instilling trust in the end node computer.

6. Gateways
if your computer does not use the AOL protocol (and the chances are it won't). Then how do you get email to your friend on the AOL network?
Answer: A gateway.



A gateway converts the data passing between dissimilar networks so that each side can communicate with each other. i.e converts data into the correct network protocol.
The gateway is a mixture of hardware components and software.
This is unlike a standard 'Bridge' which simply joins two networks together that share the same protocol.

6. Routers
A Router is a device that transfers data from one network to another in an intelligent way. It has the task of forwarding data packets to their destination by the most efficient route.

In order to do this, the router has a micro computer inside it. This holds a table in memory that contains a list of all the networks it is connected to, along with the latest information on how busy each path in the network is, at that moment. This is called the 'routing table'

7. Bluetooth
Bluetooth is a proprietary open wireless technology standard for exchanging data over short distances (using short-wavelength radio transmissions in the ISM band from 2400–2480 MHz) from fixed and mobile devices, creating personal area networks (PANs) with high levels of security. It can connect several devices, overcoming problems of synchronization. The devices can switch roles, by agreement, and the slave can become the master. At any given time, data can be transferred between the master and one other device.

8. Infra-Red

This is a very familiar method of transferring data if you are at all aware of your remote control ! The television remote control makes use of an infra-red link. Personal organisers often make use of an infra-red link to synchronise calendars and 'to-do' lists.

Take a look!!!

week 4: Data communication

hello, everyone.
today we will talk about data communication.

Data Communications

what is data communication? it is the transfer of data or information between a source and a receiver. The source transmits the data and the receiver receives it. The actual generation of the information is not part of Data Communications nor is the resulting action of the information at the receiver. Data Communication is interested in the transfer of data, the method of transfer and the preservation of the data during the transfer process.

MoDem (types of signal)

Modem, device that converts between analog and digital signals. Digital signals, which are used by computers, are made up of separate units, usually represented by a series of 1's and 0's. Analog signals vary continuously; an example of an analog signal is a sound wave.

Modems are often used to enable computers to communicate with each other across telephone lines. A modem converts the digital signals of the sending computer to analog signals that can be transmitted through telephone lines. When the signal reaches its destination, another modem reconstructs the original digital signal, which is processed by the receiving computer.

To convert a digital signal to an analog one, the modem generates a carrier wave and modulates it according to the digital signal. The process of receiving the analog signal and converting it back to a digital signal is called demodulation. The word "modem" is a contraction of its two basic functions: modulation and demodulation.

Amplitude, period, frequency

Amplitude (A): how high the peaks are or how low the troughs are, in meters.

The displacement is how far the wave vibrates / oscillates about its equilibrium (center) position.

Amplitude is correlated with the total energy of the system in periodic motion. Larger amplitude = greater energy.

Period (T): the time it takes for one cycle, in seconds.

Frequency (f): the rate, or how many cycles per second, in Hertz (cycles per second). 

Coaxial Cable

Coaxial Cable consists of 2 conductors. The inner conductor is held inside an insulator with the other conductor woven around it providing a shield. An insulating protective coating called a jacket covers the outer conductor.

The outer shield protects the inner conductor from outside electrical signals. The distance between the outer conductor (shield) and inner conductor plus the type of material used for insulating the inner conductor determine the cable properties or impedance. 

Optical Fibre
Optical Fibre consists of thin glass fibres that can carry information at frequencies in the visible light spectrum and beyond. It made of glass or plastic & transmits signals in the form of light
Advantages-

• Noise resistance. Light is not affected by electrical or magnetic field. 
• Less signal attenuation. Transmission distance is further b4 regenerated 
• Higher bandwidth. Higher data rates 

Disadvantages

• Fibre-optic cable is expecsive 
• Fragility. Glass fibre is easy to broken than wire. Making it less useful for applications where handware portability is required.

Data transmissions

The physical connection determines how many bits (1's or 0's) can be transmitted at a single instance of time. If only 1 bit of information can be transmitted over the data transmission medium at a time then it is considered a Serial Communication.

If more than 1 bit of information is transmitted over the data transmission medium at a time then it is considered a Parallel Communication. 

Data Flow

Data flow is the flow of data between 2 points. The direction of the data flow can be described as:

Simplex: data flows in only one direction on the data communication line (medium). Examples are Radio and Television broadcasts. They go from the TV station to your home television.

Half-Duplex: data flows in both directions but only one direction at a time on the data communication line. Ex. Conversation on walkie-talkies is a half-duplex data flow. Each person takes turns talking. If both talk at once - nothing occurs!

Full-Duplex: data flows in both directions simultaneously. Modems are configured to flow data in both directions. 

Modes of Transmitting Data

• Asynchronous Transmission sends only 1 character at a time. A character being a letter of the alphabet or number or control character. Preceding each character is a Start bit and ending each character is 1 or more Stop bits. For example: for every byte of data, add 1 Start Bit and 2 Stop Bits. 11 bits are required to send 8 bits! Asynchronous is used in slow transfer rates typically up to 56 kbps.

• Synchronous Transmission sends packets of characters at a time. Each packet is preceded by a Start Frame which is used to tell the receiving station that a new packet of characters is arriving and to synchronize the receiving station's internal clock. The packets also have End Frames to indicate the end of the packet. The packet can contain up to 64,000 bits. Both Start and End Frames have a special bit sequence that the receiving station recognizes to indicate the start and end of a packet. The Start and End frames may be only 2 bytes each

Multiplexing

Multiplexing is the transmission of multiple data communication sessions over a common wire or medium. Multiplexing reduces the number of wires or cable required to connect multiple sessions. A session is considered to be data communication between two devices: computer to computer, terminal to computer,

for further information, you can also view this website
Intro to data communication

Week 3: Web browser

 Web browser is a software application that enables a user to display and interact with text, images, and other information typically located on a web page at a website on the World Wide Web or a local area network.













For examples:

• Microsoft Internet Explorer 
• Mozilla Firefox, 
• Apple Safari 
• Netscape 
• Opera 

Metasearch Engine



A metasearch engine is a search tool that sends user requests to several other search engines and/or databases and aggregates the results into a single list or displays them according to their source. Metasearch engines operate on the premise that the Web is too large for any one search engine to index it all and that more comprehensive search results can be obtained by combining the results from several search engines. This also may save the user from having to use multiple search engines separately.


Bookmark (World Wide Web)
In the context of the World Wide Web, a bookmark is a locally stored Uniform Resource Identifier (URI). All modern web browsers include bookmark features. Bookmarks are called favorites or Internet shortcuts in Internet Explorer. Bookmarks are normally accessed through a menu in the user's web browser, and folders are commonly used for organization. In addition to bookmarking methods within most browsers, many external applications offer bookmark management.



Clear History
Clear History is free and powerfull windows desktop application to clean search history from your computer. It is full featured program that greatly surpasses most commercial programs out there.



Clear cache (video)
Cache files are nothing but regular files from web pages you visited, pictures, videos etc. Those files are used by browser instead of downloading again so your pages load faster. But, cache files are an evidence of where you have been and what you were doing on internet. So if you don't want your internet habits to be discovered you should clear cache on regular basis. To clear cache in your web browser is very straightforward - follow the instructions bellow.

To clear cache from your computer do the following:
     In Firefox:

• Go to the Tools menu
• Select Clear Private Data
• Make sure Cache is checked. Uncheck others as you wish.
• Click on Clear Private Data Now and your cache is cleared.

Clear URL History
If you don't clear URL history files, your web browser automatically keeps a detailed record of all web sites you've visited usually for more than forty days. The history feature enables you to have a more exact internet surfing experience. However if you want to eliminate only specific private searches from your URL history because there are other users of your PC who would like their URLs logged, you can selectively clear URL history files.


Multitasking


The ability to execute more than one task at the same time, a task being a program.
In multitasking, only one CPU is involved, but it switches from one program to another so quickly that it gives the appearance of executing all of the programs at the same time.

Video of Multitasking 

Plug-in


In computing, a plug-in (or plugin) is a set of software components that adds specific abilities to a larger software application. If supported, plug-ins enable customizing the functionality of an application. For example, plug-ins are commonly used in web browsers to play video, scan for viruses, and display new file types. Well-known plug-ins examples include Adobe Flash Player, QuickTime, and Microsoft Silverlight.