Communication and internet technoligies

Communication and internet technoligies

1. When data is sent from one computer to another it is important to consider how that data is sent

2. Three factors need to be considered when sending data and must be predetermined between devices already

   • Direction of data transmission
     • Simple data transmission is one direction only
     • Half duplex is both directions but one at a time
     • Full duplex is both directions at the same time
   • Method of transmission
     • Serial data transmission is when data is sent one bit at a time over a wire or channel(by any method of transmission)
       • Works well over long distances
         • Data is transmitted at a slower rate but there is no synchronisation problem
     • Parallel data transmission is where several bits of data are sent down several wires of channels simultaneously
       • Works well over shorter distance, but can become skewed(unsychronised)
         • Faster than serial data transmission
   • Method of sychronisation
     • Asynchronous data transmission refers to being transmitted in an agreed bit pattern. Data bits are grouped together and sent with a control bit so that the receiver knows where data starts and ends
       • USB is an asynchronous serial data transmission method
         • Consists of a four wire shielded cable, one for power and earth and two for data transmission
           • The computer detects the USB and loads the appropriate drivers
             • Industrial standard for data transmission
               • Devices plugged in are recognised immediately
               • Connectors only fit one way
               • Industrial standard(help available)
               • Several data transmission rate are supported and new models are backwards compatible
               • Maximum cable length of 5m
               • Less than 500mb/second transmission rate
     • Synchronous data transmission refers to a continuous stream of data accompanied by time signals which the receiver counts and determines how many bits were sent and reassembles them into bytes
       • Faster than asynchronous data but must have accurate time signals else data becomes skewed

3. Error checking

   • When we send data between computers there is a risk that the data is changed or corrupted as it is sent
     • We check for errors as computers understand everything literally and cannot comprehend incorrect data
   • Parity check
     • A parity check is when a byte of data is used and one bit is the parity bit so that the sum of the binary digits is odd or even depending on the parity used
       • Does not show where error is
     • Parity blocks are used to located an error if one exists. Multiple bytes of data are sent and the number of 1 bits totalled horizontally or vertically equal the type of parity used showing where the error lies
   • Checkdigit
     • One digit is set to be the check digit on something like a barcode(checksum character is on far right). The purpose is to verify that information has been entered correctly as the barcodes reader calculates the checksum using mathematical operations on the preceding digits. This value is compared the checksum character and if it is correct the reader emits a signal such as a beep
   • ARQ'S
     • Uses a timeout and acknowledgment to see if data has been transmitted correctly
       • Acknowledgement is a message sent by the receiver to the sender stating data was transmitted successfully
         • Timeout is the time allowed to elapse before an acknowledgment is recieved
           • If an acknowledgement is not received before the timeout occurs the data is resent automatically
   • Echo check
     • Data is sent to another device and back to the sender. Data sent is compared to data received to see if there is any difference indicating an error.
       • Not able to tell whether error was made during transmission from sender to reciever or reciever to sender

4. Internet technologies

   • All devices on the internet can communicate with each other providing a number of rules and protocols are adhered to
   • Internet service providers provide you with your internet access
   • Internet Protocol address
     • 32 BIT number written in the form 123.123.123.1
     • The IP is unique for each session and is provided by the ISP
     • The only IP addresses that remain unchanged are web servers
     • IP addresses can be used instead of a url
   • IP address gives the location of the device on the internet
   • IP is the persons address and the MAC address is the person
   • WEB browser is a software which allows the user to display a web page on their screen
     • Have a home page
     • Recent history
     • Forwards backwards
     • Favorites tab

5. HTML structure and presentation

   • HTML is a markup language
   • Difference between structure and presentation
     • Structure is essential part of the document and includes semantics(meaning) and structural mark up
     • Presentation is the style of the document
     • CSS file will contain everything to control the actual presentation of the webpage
     • Two features should be kept separate
   • HTTP
     • Hypertext transfer protocol is a set of rules that must be obeyed when transferring files across the internet