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Candidates should be able to:
- describe the nature of the Internet as a worldwide collection of computer networks
- describe the hardware needed to connect to the Internet including modems, routers etc
- explain the need for IP addressing of resources on the Internet and how this can be facilitated by the role of DNS servers
- explain the importance of HTML and its derivatives as a standard for the creation of web pages
- describe common file standards associated with the Internet such as JPG, GIF, PDF, MP3, MPEG
- explain the importance of compressing files that are transmitted via the Internet
- describe the differences between lossy and lossless compression.
What is the Internet?
The Internet is a world-wide system of inter-connected computer networks. These connections can involve telephone lines, fibre-optic cables, satellites, microwaves and other wireless links.
Once a computer is connected to the Internet it can communicate with any other computer that is also connected and uses the same protocol.
The World Wide Web (WWW) is a system where files (web pages and other files) on computers connected to the Internet are linked by hyperlinks. Every file that can be accessed using the World Wide Web has a unique address called a Uniform Resource Locator (URL).
A web browser is used to view web pages and navigate between them using hyperlinks. A hyperlink is simply a link to the URL of a file, either another web page or some other type of file.
The World Wide Web and the Internet are NOT the same thing. The World Wide Web is just one of the many services that uses the Internet.
What hardware is needed to connect to the Internet?
The hardware needed to connect a computer to the Internet includes:
- Modem – this device allows an individual computer to connect to the Internet via a phone line. There are two main categories:
Dial-up modem – this device modulates a digital signal from a computer into analogue sound tones which can be sent along standard phone lines. At the other end the tones are demodulated back into a digital signal and passed to a computer. Since the signal is transmitted as an audible sound, the phone line cannot be used for conversation at the same time. A data transmission speed of up to 56 Kbit/s is possible.
- Broadband modem – this device also modulates a digital signal from a computer into an analogue signal, but uses a frequency band that is much higher than that used by voice communications, so the phone line can be used for conversation at the same time.
A broadband modem requires a Digital Subscriber Line (DSL), a standard phone line that is upgraded at the telephone exchange to allow it to carry the high frequency digital signal. Data transmission speeds from 256 Kbit/s up to 4 Mbit/s and higher are possible, depending on the phone line quality and the distance from the exchange.
Router – this device is used to link one network to another network. A router will therefore allow all the computers in a network (a LAN) to have a connection to the Internet.
- A typical router will combine a broadband modem with a hub or switch and often a wireless access point to allow wireless access to the LAN and therefore the Internet.
Why do Internet resources need IP addressing and what is the role of DNS servers?
Internet resources need to be uniquely identified and this is done by allocating a unique IP address to each website address (URL) such as www.teachcomputerscience.com.
A DNS (Domain Name System) Server is basically a database table of URL addresses and the corresponding IP addresses. Numerous DNS servers around the world hold regularly updated copies of this database.
When a Web browser requests an Internet resource using a URL, a DNS server is first contacted to determine the IP address of the host. This IP address is passed back to the Web browser which then uses it to request the data using the Internet Protocol. If the IP address cannot be found on the first DNS server then the request is passed onto other DNS Servers.
The advantages of using DNS servers are:
- Users do not need to remember IP addresses
- It is easily to upgradable (eg IPv4 to IPv6)
- It is easy to add new URL’s to the system as DNS Servers are constantly updated by other DNS Servers so they have the latest version of URLS and matching IP addresses
- As long as a computer can access a DNS server it can have access any website addresses in the world
Why is HTML and it’s derivatives so important?
HTML stands for HyperText Mark-up Language. It is important because it is an open and accepted standard meaning data will be displayed correctly on any computer type that uses a web browser that conforms to the HTML standard (for example Internet Explorer™, Chrome, or Firefox™).
An HTML file is a text file made up of HTML elements which are in the form of tags (an instruction, often with additional attributes, between a pair of angled brackets <>).
Tags are used to:
- Control how the text data in the web page is displayed
- enable the web browser to find the location of images and media content, then display them correctly
- enable hyperlinks to other locations within the webpage (bookmarks) or to other websites or online files address using their URL.
It is important to realise that images and other media files are downloaded separately by a web browser and are not part of the webpage file itself.
Examples of HTML elements, with and without attributes:
|HTML element||Tags used||An example||What the browser would display|
|FONT||<font> </font>||<font color=”red” size=”5″>This is in red and big</font> and this is not.||This is in red and big and this is not.|
|BOLD||<b> </b>||<b>This is in bold</b> and this is not.||This is in bold and this is not.|
What are the common file standards associated with the Internet?
Common file standards associated with the Internet include:
- JPG/JPEG – a bitmap image format commonly used for high resolution colour images such as digital photographs. JPEG files use lossy compression and the compression level can be set to allow a balance between file size (and therefore download time) and image quality.
- GIF – a bitmap image format that is commonly used for simple images such as diagrams, logos, website buttons and cartoon graphics. GIF files use lossless compression and a GIF image can have a palette of any 256 colours from a possible 16,777,216. As each image can have a maximum or 256 colours, the GIF format is unsuitable for photographic images or images with a continuous colour gradient. GIF files do however support animations and transparency.
- PDF – a document file format that allows documents to be viewed and printed without the need for the original software that created them. The PDF file includes the layout, text, fonts, bitmap images and vector graphics of the original file but can be read on any computer with a PDF reader installed. PDF files can also support hyperlinks and interactive forms.
- MP3 – an audio file format commonly used to store music and voice data. MP3 files use lossy compression.
- MPEG – a file format commonly used to store video data. MPEG files use lossy compression and the audio/video compression levels can be set to allow a balance between file size (and therefore download time) and sound/image quality.
- ZIP – a file format that stores one or more files as a single lossless compressed file. ZIP files are commonly used to reduce the file size of email attachments or downloads. Also, files that need to be archived are often stored as ZIP files to save storage space. Each compressed file is compressed individually, allowing it to be decompressed without having to decompress the whole ZIP file.
Why is file compression so important for files transmitted over the Internet?
File compression is the process of encoding information using fewer bits so that the compressed file size is smaller.
It is important for files transmitted over the Internet because if they are not compressed then there would be considerably more data to transmit. This would result in more network traffic, greater bandwidth requirements and slower download times leading to delays in viewing web pages, particularly those with multimedia content. Streaming audio and video would in many cases be impractical without file compression.
However, compressed data may be of lower quality (if using lossy compression) and must be decompressed to be used. This extra processing may slow some applications and in the case of video decompression, require dedicated hardware such as graphics cards for the video to be viewed as it is actually being decompressed.
The level of data compression for files transmitted over the Internet therefore involves a balance between:
- the greater speed and reduced bandwidth needed to transfer a compressed file
- a possible loss in quality in a compressed file
- the increased processing needed to compress and decompress a file
What alternatives are there to file compression for files transmitted over the Internet?
With a bitmap image file the alternatives to compression involve:
- Resizing the image to reduce the number of pixels overall.
- Reducing the colour depth to reduce the number of bits needed to store each pixel.
- Reducing the resolution to reduce the number of pixels per unit of length/area.
With an audio file the alternatives to compression involve:
- Re-sampling the data at a lower sample rate so some of the data is missed out.
- Converting the data to a lower bit-rate so that fewer bits are used to record each sound sample.
Video files are made up of separate audio and image data so all of the methods above can be combined to reduce the files size.
What is the difference between lossy and lossless compression?
This allows the original file to be re-created exactly from the compressed file. It works by searching for patterns in the file so, instead of repeatedly storing a block of identical data, the data is stored once and then indexed. Further occurrences are simply stored as the index number so the decompression software can simply look up the data and place it back in the correct position.
Text files compress well because certain letters and words will often appear together in the same pattern. Software files also compress well for similar reasons, they are made up of a relatively small number of different instructions, often arranged in a set pattern.
In both cases, the larger the original file, the better the compression ratio as there are more likely to be repeating patterns and each pattern will be repeated more frequently.
Image files with a limited palette such as GIF files also compress well if there are blocks of similar colour next to each other.
Files that include a lot of unique information cannot be compressed much with lossless compression because there is so little repeated data to index.
Lossy compression works differently, it removes data that is not needed, either because a drop in quality is acceptable or the difference cannot be detected by the human eye or ear. Lossy compression is therefore suited to bitmap graphics, audio and video files.
With lossy compression you do not get the original data back when it has been decompressed. This sort of compression would therefore NOT be suitable for anything that needed to be reproduced exactly such as software, databases or text.
Lossy compression on bitmap images:
This works best when there are lots of similar coloured pixels together. In such cases the compression algorithm will be able to calculate the average colour of a large number of blocks of pixels and store all those blocks as one colour, greatly reducing the file size.
For example, the two images on the right are both 300×240 pixels and when saved as uncompressed Windows™ bitmap files the file size was identical (73.08KB).
When saved as JPEG files (a lossy compression system) with a compression setting of 50% the sky image file was 3.3KB and the tree was 10.4KB. This is because the sky contains large areas with similar colours which compress well (although the banding effect can be seen).
Lossy compression on audio files:
This involves removing data from the original file so there is a drop in quality. However, to minimise this the algorithms that carry out the compression put more emphasis on removing data that the human ear cannot detect or is less sensitive to.
- Voice data can be compressed far more than music data and still be acceptable. Such voice compression is used extensively for conversations over the Internet to reduce the bandwidth required.
- MP3 files use lossy audio compression and can produce a file that is about 10 times smaller than a CD file created from the same original audio source.