Digital technologies 01
The Digital Technologies subject in the draft Australian Curriculum: Technologies includes content descriptors and elaborations for a variety of topics that may not be familiar to many teachers. The level of detailed knowledge that will be required to address such topics in the primary school classroom may not be great but it is likely that many current and future teachers will need to extend their knowledge of digital technologies. This module and the next address some topics, identified with the help of student responses to a survey,that appear to represent areas in need of development for at least some students.
What is digital?
When we measure something, such as the temperature, the speed at which we are travelling, our height or weight, we record the value to a suitable level of accuracy such as a degree, km per hour, cm or k. However, we understand that the real value could be anywhere along a continuous range regardless of whether we can measure with that degree of accuracy. This real world of continuous values is sometimes described as analog.
In contrast, the digital world is one of discrete values. That is, only certain values are possible and there are no possibilities between them.
The diagram at right demonstrates how an analog electrical signal with continuously varying voltage shown by the blue sine curve would be converted to a digital representation by sampling the voltage value at specified intervals of time. The digital representation would be restricted to representing the voltage at only those intervals with no values between. The actual values recorded would also be limited to a fixed set of possibilities.
Digital representations of real world data are simplified in this way. That has certain advantages for storage, transmission and processing but it may also present disadvantages in loss of accuracy and other ways.
It is conventional to distinguish between data and information and some people suggest that there is a DIKW hierarchy from data to information to knowledge to wisdom. The usual understanding is that data is recorded facts and figures without particular meaning. Information emerges at the point where data begins to have meaning, usually by relation to other data. Knowledge emerges when humans appropriate the information. Wisdom comes from some combination of knowledge and experience usually associated with increased age.
The origin of ideas about the DIKW hierarchy is uncertain but one commonly suggested source for at least part of the idea is the poet, T S Eliot (1934) in these lines from The Rock:
Where is the Life we have lost in living?
Where is the wisdom we have lost in knowledge?
Where is the knowledge we have lost in the information?
The importance of these ideas for our work in digital technologies lies in our need to go beyond merely processing data to develop understanding that will allow us to respond appropriately to human needs and wants.
Codes are used to record, transmit, and receive information that is shared. Human languages are codes that we use in direct interaction for sharing information. When we are separated by distance or time that language is encoded in other ways, including writing and specialised codes such as Morse Code and Semaphore. Well designed codes include features to reduce errors in transmission. Using a limited number of simple, unambiguous symbols is one strategy. Another is the inclusion of a degree of redundancy so that even if part of the message is lost in transmission the core of it can be reconstructed using what has been received. Traditional stories and poems often include repeated elements and multiple expressions of the same idea that can help to preserve the core even if part is forgotten. African drumming used for messaging employs similar techniques, saying the same thing in different ways to ensure that even in adverse conditions there is a greater chance of the message arriving. Digital systems often include calculated check codes that can be used to confirm (or not) that data has arrived intact and request retransmission when necessary.
|10 000 000||1 000 000||100 000||10 000||1000||100||10||1|
In daily life we use the decimal number system which is based on ten and uses ten symbols (0 1 2 3 4 5 6 7 8 9) to represent numbers using place value. The use of ten as the basis for that system probably derives from our having ten digits (fingers and thumbs) on each of two hands.
Digital computer systems use a binary system which requires just two symbols (0 1) that can be represented by the off/on condition of switches and other simple hardware components.
The table at right compares place value for decimal and binary systems.
Although some people may think that mathematics and mathematicians are dull, boring and humourless, there are mathematical jokes such as the following play on binary numbers:
There are only 10 kinds of people in the world: those who understand binary and those who don't.
The computer term, bit, derives from the contraction of binary digit. A byte is a group of 8 bits which can form 256 different patterns from 00000000 to 11111111.
Familiar prefixes (Kilo = 103, Mega = 106, Giga = 109, Tera = 1012, etc.) are used to refer to the larger collections of bytes that are usually required for the amounts of data that are now stored in computers and associated devices.
A single byte can assume 256 different patterns which is many more than required to represent all the characters of our alphabet in upper and lower case along with the numerals from 0 to 9, punctuation symbols and some additional characters. The ASCII (American Standard Code for Information Interchange) code uses just 7 bits in a byte to represent 128 symbols which is sufficient for English and some other languages. Another system, Unicode, has been devised using multiple bytes to represent up to 110 000 different characters from 100 or more different scripts adopted around the world.
Data on screens
Data is represented on screens as pixels (picture elements). Older systems relied upon a beam of electrons being switched on or off to light or not light a phosphor spot on a screen. Most screens now are liquid crystal displays in which the individual elements on the screen are controlled electronically but the principles for display are essentially the same.
For a black and white image the element representing a pixel is switched on for white and off for black (#1 at right) and the data can be represented by a single bit. Greyscale images (#2) vary the intensity with which the pixel is lit and can represent up to 256 different shades using data from a single byte. Colour is achieved by using 3 sub-pixels (red, green, blue) for each pixel (#3).
Because much computer terminology originates in the USA which still uses the British Imperial measurement system, the densities of pixels displayed on screens and dots printed on paper are usually given in pixels per inch (ppi) or dots per inch (dpi). Current computer screens typically display about 100 ppi (or 40 ppcm) though screens on modern mobile devices can exceed 300 ppi (retina screens). Printers vary from 300 to 600 dpi for ink jet to 600 to 1200 dpi or more for laser printers. These differences in density affect the amount of data needed to store images of acceptable quality.
Colour on screens and projectors is produced using the RGB additive model in which separate sources provide red, green, and blue light that is mixed and interpreted as colour in our eyes. Colour printed on paper or other surfaces is produced by illumination with white light from which the ink or dye subtracts some light and reflects the balance to the eye. The most common set of colours used for inks is cyan, yellow, and magenta (CYM subtractive model) black added to avoid the 'muddy' black produced by mixing inks (CYMK model).
Images in digital computer systems fall into two major categories.
The most common and best known is the bitmap image in which data is stored for each dot or pixel. Bitmap images are most commonly seen as photographs or as images produced using painting programs. Among their disadvantages are the pixelation that occurs when they are magnified and the difficulty of separating elements from the image. The quality of output depends on the stored data.
The second type of image is a vector or object image which, instead of data about each pixel, stores instructions for drawing the image. Storing the mathematical description of a shape along with its colour requires less data than storing details of each pixel so such images can require less storage, allow elements to be separated, and be magnified smoothly. The quality of output depends on the output device.
Storing bitmap images - photographs
For most of us the images we are most likely to store in any number will be photographs, which are bitmap images. Most often they are stored in JPEG (Joint Photographic Expert Group) format (.jpg extension) which is a compressed format that sacrifices some image quality for reduced storage requirements. There are different approaches to compressing images but one that is relatively easily understood is run length encoding in which, if several pixels of the same colour appear in sequence, the system stores just the code for the colour and the number of times it occurs rather than the full 3 bytes for each pixel.
Black and white images need just one bit per pixel to record the on/off state. Greyscale images can use a byte (8 bits) per pixel to store 256 shades of grey. Colour images can use a byte for each of the three colour channels (red, green, blue) allowing 256 levels for each colour and 256 x 256 x 256 = 16 777 216 possible shades to be represented. Although that seems like a lot of shades of colour it is not sufficient to represent all the colours visible to a human eye. If transparency is required then a fourth byte can be used to represent 256 levels of transparency.
Knowing that images are represented as data in this way it is not difficult to calculate the storage requirements for typical photographs (or other bitmap images). A common aspect ratio for photographs is 4 x 3. That would represent the proportions of a 4 x 3 inch or 8 x 6 inch print and is also equivalent to common screen ratios - 640 x 480, 800 x 600, or 1024 x 768. In a digital camera a 4000 x 3000 pixel photograph is equivalent to a 12 megapixel image which is typical of a moderately capable camera. If each pixel were represented by 3 bytes (RGB) it would require 36 MB to store a 12 Mpx image but most cameras immediately apply JPEG compression to produce a file of between 2 and 5 MB depending on the complexity of the image.
A common current screen size of 1920 x 1080 is equivalent to about 2 Mpx, which means that very few computers are capable of directly displaying all the content of a 12 Mpx photograph. Displaying a 4 x 3 inch photograph on a 100 ppi screen will use 400 x 300 = 120 000 px. At 3 bytes per pixel that will amount to just 360 kB without compression. If photographs are to be sent by email and/or viewed on a screen in a webpage or otherwise there is little point in sending a full resolution 12 Mpx image. Appropriate cropping, scaling, and compression will facilitate faster transfer without appreciable loss of quality for viewing on screen.
On the other hand, if a photograph is to be printed at 300 dpi or better more data will be required for acceptable quality. A 4 x 3 inch photograph printed at 300 dpi will use 1200 x 900 = 1 080 000 px. At 3 bytes per pixel that will amount to 3.2 MB without compression. In that case it will be advisable to retain most of the data in the original photograph after some judicious cropping.
The widespread adoption of mobile devices like smartphones equipped with cameras and Internet connectivity has led to the development of new approaches to presenting information. In augmented reality a user views the world through the camera in the device while software uses the GPS location and direction data to access and overlay information on the image.
Applications include tourist guides, games, and education.
Audio and video data
Sound is an analog phenomenon from the real world. It is converted to digital format by sampling the amplitude (size) of the sound wave at regular intervals. The higher the frequency (more often) it is sampled then the more accurately it can be reproduced and converted back to analog form for listening via speakers. The frequency of sampling is measured in kHz (Hz = per second). Music on CDs is sampled at 44.1 kHz which is close to the limit beyond which we cannot hear any improvement in quality. Telephone sound is equivalent to 8 kHz and is of noticeably lower quality. The size of audio files can be reduced by storing mono rather than stereo (half the data) if stereo is not required or by reducing the sampling rate where lower quality is acceptable such as for voice-only recordings.
Video data combines audio data as described in the previous paragraph with a stream of images. The image stream is encoded with varying degrees of compression using techniques such as recording periodic key frames and differences over time so that the images can be reconstructed by computer. The process of compressing and decompressing video using codecs (compression-decompression methods) is complex and computationally intensive so, even with powerful computers, appreciable time is required. Systems are often designed to be asymmetric so that compression, which is done just once, requires more time and computing power than decompression, which must be done every time the video is viewed. The size of video files can be reduced by reducing the frame size (dimensions of the image) and frame rate (frames per second). The associated audio may also be reduced in size using the techniques described above.
The operation of a conventional landline telephone system depends on a series of switches establishing an electrical circuit that directly connects one telephone handset to another across the street or around the world. The Internet and similar computer networks do not depend upon the creation of such private circuits. They operate as packet switching networks in which the data is bundled up into small packets prefixed with address data that will be used to direct the packet through the network. Successive packets from the one larger block of data, such as an email message or photograph, may be sent by different routes depending on congestion and other characteristics of the network. When packets arrive at their destination they are reassembled to replicate the original block of data.
Transmission of data through the network is governed by protocols (sets of rules) that specify the size and other characteristics of packets and how they should be exchanged between computer systems. Commonly used protocols include:
- TCP/IP (Transmission Control Protocol/Internet Protocol) for the basic operation of the Internet
- HTTP (HyperText Transfer Protocol) for the World Wide Web
- FTP (File Transfer Protocol) an older but still used standard for transferring files
- SMTP (Simple Mail Transfer Protocol) for email.
Various devices are used to control networks and ensure that data moves efficiently to is destination. Many, if not most, homes, schools, and businesses now have small networks that use such devices so it is useful to know something about them.
- A hub is a simple device that accepts data packets from one port (connection point) and distributes them to all other ports (typically 4 or 5) on the hub and thence to all devices on the connected network. Each device is then responsible for reading each packet to determine which are intended for it. Those that are not addressed to a device are discarded. Hubs are simple and work for small networks with limited traffic but generate much wasted traffic and busy networks can be congested.
- A bridge has just two ports and is used to connect two sections of network. Data packets arriving at one port are inspected and sent out via the other port only if they are addressed to a device on that section of the network.
- A switch is like a hub in that it accepts data packets from one port and has multiple outgoing ports that may have devices connected. It differs from a hub by inspecting packets and sending them out on only the port to which the addressed device is connected. It will generate less wasted network traffic than a hub.
- A router is similar to a switch but uses IP addresses rather than a hardware address and can connect across different network protocols. Routers that connect to the Internet and have both wired connections for a nearby computer or other device and wireless capability are now commonplace in homes, schools, and businesses.
The existing landline telephone network is built using copper wire. It has limitations on both the speed at which it can carry digital data and the distance from the exchange that is practicable for a connection. Advances in technology continue to improve performance but at a cost and there are limits that will be reached.
Wireless networks use radio transmitters and receivers for local WiFi and for cellular data connections using the 3G or 4G mobile telephone networks. Both have limitations on range and speed and may be subject to interference.
Optical fibre is expensive to install but offers greater range and data speed and is not affected by electrical or radio interference.
There are alternative proposals for a National Broadband Network using FTTP (fibre to the premises) or FTTN (fibre to the node) with existing copper wiring for the final part. An NBN Simulation suggests that there will be an appreciable difference in performance.
A firewall is intended to control the traffic allowed in and out of a network. It can be implemented using hardware or software. Many routers used for connecting a home network to the Internet include a firewall function. Modern operating systems for personal computers also include firewalls and it is possible to install additional software that improves the interface and adds functionality.
A proxy acts as a gateway between networks. It accepts requests for outside data, such as a webpage, and fetches and delivers it, thereby protecting the computer requesting the data from direct links to the Internet. In some cases the proxy will cache data from the web for a period and subsequent requests will be served from the cache rather than being fetched again. That arrangement saves time and the cost of downloading duplicate data.
A virtual private network (VPN) establishes a point-to-point connection between a computer at a distant point on the Internet and a private network such as that operated by a business or educational institution. The connection is secure and the remote computer can access facilities on the private network as though it were physically present on the network.
Securing your stuff on networks
Access to the Internet, whether from a home network, cellular phone network, or free WiFi offered by business entails security risks.
Packet sniffers can access data passing through wired or wireless networks and read unsecured packets in search of data that might allow access to financial or other information you would wish to keep private. Ensure that all transactions with your financial institutions and other important providers are conducted over a secure link. Your web browser should indicate when a link is secure. Achieving that condition will usually require the use of a login with password.
Brute force attacks on computer systems use powerful computers, often in networks, to try logins using a large number of passwords and user IDs in rapid succession. Their attempts will include dictionary words and commonly used passwords. Avoid those easily cracked passwords.
Phishing attacks arrive by email with an invitation to visit some link that requires entry of an ID and password for email, a bank account, or some other system that should be secure. Avoid clicking on links in email messages.
Password strength is enhanced by using longer passwords (more than 8 characters) with complex combinations that include a mix of alphabet letters, digits, upper and lower case, and punctuation and other symbols. Strong passwords may be difficult to remember.
Resist the temptation to use the same password in multiple systems. If it is cracked in one place you will be vulnerable in others.
Consider using a password manager such as 1Password (there are alternatives and some are free). It can store passwords securely and can generate more secure passwords.