Data Transmission: Direction, Method, and Synchronization, Summaries of Computer Science

Download Data Transmission: Direction, Method, and Synchronization and more Summaries Computer Science in PDF only on Docsity! lOMoARcPSD|26858109 M. Ahmed, Computer science Data transmission is the movement of data (bits) between at least two digital devices or between two components of the computer. Data transmission can be over a short distance (e.g. from your computer to a plugged in device) or over longer distances (e.g. networks). When data needs to be sent from one device to another, it is important to consider how that transmission is going to occur. Factors to be considered and agreed between sender and receiver Direction of transmission (i.e. one direction or both) – Simplex, Half Duplex or Full Duplex (Duplex) Method of transmission (how many bits are sent at the same time) – Serial or Parallel Synchronisation method between the two devices – Asynchronous and Synchronous Direction of data transmission Simplex, Half Duplex and Duplex are the three possibilities for the direction of data transmission. Simplex Simplex is data transmission in one direction only. Two directional transmissions are not possible. E.g. a radio station broadcast, Microphone to computer, Keyboard to computer, computer to printer etc. Half Duplex Data transmission in both directions is possible, but not at the same time. e.g. Walkie-talkies Full Duplex Data transmission is possible in both directions and at the same time (simultaneously). e.g. Telephone call, Voice over IP, Video conferencing, Instant messaging, Broadband connection etc. Key Differences Between Simplex, Half Duplex and Full Duplex 1. In a Simplex mode of transmission, the signal can be sent only in one direction; hence, it is unidirectional. On the other hand, in half duplex, both the sender and receiver can transmit the signal but, only one at a time, whereas, in full duplex, the sender and receiver can transmit the signal simultaneously at the same time. 2. The performance of full duplex is better than half duplex and simplex because it better utilizes the bandwidth, as compared to half duplex and simplex. 3. If we take the example of keyboard and monitor, it is observed that keyboard inputs the command and monitor displays it, monitor never replies back to the keyboard; hence, it is an example of the simplex transmission mode. In a walkie-talkie, only one person can communicate at a time so; it represents an example of half duplex mode of transmission. In a telephone, both the person on the either side of a telephone can communicate in parallel at the same time; hence, it represents an example of a full-duplex mode of transmission. Page 1 of 6 Universal Serial Bus (USB) Universal Serial Bus (USB) is a type of connector commonly used for asynchronous serial data transmission between a computer and attached devices. A USB cable consists of a shielded casing containing two wires for power and earth, plus two wires for data transmission. ❖ devices are automatically detected and configured when initially attached ❖ impossible to connect device incorrectly/connector only fits one way ❖ has become the industry standard ❖ supports multiple data transmission speeds ❖ lots of support base for USB software developers ❖ supported by many operating systems ❖ backward compatible ❖ faster transmission compared to wireless ❖ After a certain cable length transmission becomes problematic Error Detection during data transmission There is always the chance that data has been corrupted (changed in some way) during transmission. This is true regardless of the distance of transmission. To avoid having corrupted data it is really important that computers check for errors and correct them wherever possible. There are several methods of error checking, with some being more effective than others. Error Detection method Parity Check An extra bit (parity bit) added to a string of binary code to ensure the number of 1-bits is either even or odd, depending upon the parity check system used. If the number of 1s is even then it is regarded as 8even9 parity and if the number of 1s is odd then it is regarded as 8odd9 parity Method • The sending and receiving computers agree the protocol to be used (even or odd) • The sending computer adds the correct parity bit to the binary data (either an extra 1 or 0) • The sending computer sends the binary data, including the parity bit • The receiving computer checks to make sure the overall parity of the data received is as agreed (an even or odd number of 1 bits) • If the parity of the data is incorrect, the receiving computer will request that the data is transmitted again Drawbacks of parity checks • It does not have the ability to pin point the bit which has changed Parity Block/Double parity: This method of error checking is similar to a traditional parity check except the data is arranged in a virtual table with parity bits added to each row and column. • A single corrupted bit can be spotted at the intersection of a corrupted row and column • Multiple corrupted bits would flaw this system • If two bits are transposed (change places) then the computer could be fooled into thinking the data is correct and not corrupted • If two random bits change state then the system could also be fooled Page 4 of 6 Checksum • A value is calculated from the data following a particular algorithm • Value is transmitted with data • Value is recalculated after transmission by the receiver using the same algorithm • If the values match the data is accurate • If not then a request is sent to resend the data Automatic Repeat Request (ARQ) • uses acknowledgement (a message) and timeout (set time agreed between the parties) • when no error is found then a positive acknowledgement is sent by the receiver to the sender • when the receiver detects an error in data transmission it asks for the packet to be resent (i.e. a negative acknowledgement is sent to the sender) • if the sender does not receive anything back from the receiver before timeout occurs, the data is automatically resent. • This will keep happening until the sender receives a positive acknowledgement/until the ARQ limit is reached Echo check • the data is sent to the receiver • receiver keeps a copy of the data and sends back another copy to the sender • data is compared to see if it matches by the sender • if it does not match then error detected • difficult to know when the error actually occurred (if any) Uniform Resource Locator (URL) A uniform resource locator (URL) is the address of a resource on the Internet. A URL indicates the location of a resource as well as the protocol used to access it. Example of a URL: http://www.computerhope.com/jargon.htm Here http is the protocol www.computerhope.com is the web server9s name or domain name jargon.htm is the file name or the web page name Domain Name System (DNS) An Internet service that translates domain names into IP addresses. Because domain names are alphabetic, they are easier to remember. How the web browser uses the URL to access the webpage? • The web browser sends URL to DNS • DNS stores an index of URL and matching IP address • DNS searches for URL to obtain the IP address • IP address sent to web browser, (if found) • Web browser sends request to IP of webserver • Webserver sends web page to web browser • Web browser interprets HTML to display web page • If URL not found DNS returns error MAC address and IP address MAC address • Media Access Control address • unique number that identifies a device connected to the Internet • hardware/physical address • unique address/number associated (with network card) in a device/computer Page 5 of 6 • address is made up of manufacturer id + serial number of device • usually 48/64 bits (12/16 hex digits) • first 6/8 hex digits = manufacturer code • last 6/8 hex digits = serial number of device (NIC) IP address • Internet Protocol address • location/address of a device on the Internet • address is unique for given Internet session • address is supplied when a device connects to the Internet • address is allocated by the ISP (Internet Service Provider) • usually 32 bits in IPV4 and 128 bits in IPV6 HTML HyperText Markup Language is used when writing and developing web pages. HTML uses tags (<tags>) which are used to bracket a piece of code. HTML can be divided into two parts; the structure part and the presentation part. HTML structure and presentation • Structure is used to create layout (eg. headings, paragraphs etc.) • Presentation is used to format colour/style • In a HTML document structure and presentation are often kept separate • By keeping the presentation separate it is easier to update colour/font • Presentation is often stored in a separate file called CSS (Cascading Style Sheet). The CSS is then linked to the HTML document to implement the presentation requirements HTTP HTTP (Hypertext Transfer Protocol) is the set of rules for transferring files (text, graphic images, sound, video, and other multimedia files) on the World Wide Web. As soon as a Web user opens their Web browser, the user is indirectly making use of HTTP. HTTPS Hyper Text Transfer Protocol Secure (HTTPS) is the secure version of HTTP, the protocol over which data is sent between your browser and the website that you are connected to. The 'S' at the end of HTTPS stands for 'Secure'. It means all communications between your browser and the website are encrypted. HTTPS is often used to protect highly confidential online transactions like online banking and online shopping order forms. Web browsers such as Internet Explorer, Firefox and Chrome also display a padlock icon in the address bar to visually indicate that a HTTPS connection is in effect. Web browsers • Displays web page • Interprets/translates the HTML code from websites and show the result of the translation • Provides functions such as bookmarks and history • Identifies protocols such as https, SSL, TLS etc. Internet Service Provider (ISP) ISP stands for Internet Service Provider. An Internet Service Provider is a company that provides Internet access to organizations and home users. A monthly fee is usually charged for this service. The ISP will set up a user account which will contain a username and a password; most ISPs also give the user an email address. Page 6 of 6