cs chapter 2

• Packet


  A small part of a message/data that is transmitted over a network; after transmission all the data packets are reassembled to form the original message/data
• Parts of a data packet

  • Packet header
  • Payload
  • Trailer
• Packet Header


  Contains the IP addresses of the sender and receiver, and includes the packet number which allows reassembly of the data packets
• Packet Trailer


  Indicates the end of the data packet and some form of error checking to ensure the packet arrives error free
• Payload


  The actual data being carried in a data packet
• Packet Switching


  1. Data is broken down into packets

  2. Each packet could take a different route

  3. A router controls the route a packet takes

  4. Packets may arrive out of order

  5. Once the last packet has arrived, packets are reordered
• Packet Switching


  • There is no need to tie up a single communication line
  • It is possible to overcome failed, busy or faulty lines by simply re-routing packets
  • It is relatively easy to expand package usage
  • A high data transmission rate is possible
• Packet Switching


  • Packets can be lost and need to be re-sent
  • The method is more prone to errors with real-time streaming (for example, a live sporting event being transmitted over the internet)
  • There is a delay at the destination whilst the packets are being re-ordered
• Serial Transmission


  Transfer of the data bits sequentially using a single wire
• Serial Transmission


  • More reliable over long distances
  • Reduced costs as only single wire needed
  • Less interference as only single wire is needed
  • Safer transmission as it is easier to accurately collate the bits together as they are sent one at a time // less chance of the data being skewed
  • Slower transmission as data bits are only transmitted one bit at a time (sequentially) over a single wire
  • Additional data may need to be sent to indicate the receiving device when the data transmission has started and stopped. These are called a start bit and a stop bit
• Parallel Transmission


  Transfer of data bits simultaneously using several/multiple wires
• Parallel Transmission


  • Suitable for short distances. Parallel cables are not manufactured above approx. 5m
  • Increased costs as multiple wires needed
  • More interference as multiple wires are needed that can create crosstalk
  • Less safe transmission as bits are sent simultaneously and errors can occur in collating them together at the receiver's side // Bits may arrive skewed
  • Quicker transmission as data bits are sent simultaneously over multiple wires
• USB


  • Universal Serial Bus
  • Data transmission method (busses)
  • It uses serial transmission where all bits of data are sent one at a time using a single wire
  • Universal standard // Common Interface
• Uses of USB


  • Connect electronic devices such as mobile phones and tablets to computers
  • Connect mouse and keyboard to a computer
  • Use it as a storage device using USB flash drive
• Benefits of USB


  • Devices plugged into the computer are automatically detected and device drivers are automatically downloaded/installed so no need to install them manually
  • The connectors can only fit one way, this prevents incorrect connections being made
  • USB has become the industry standard; this means that considerable support is available to users (USB is a universal standard so it is likely to be compatible with the computer)
  • Newer USB standards are backward compatible with older USB standards so no additional technology is needed
  • Several different data transmission rates are supported
• Drawbacks of USB


  • The maximum cable length is presently about 5 meters
  • The transmission speed is relatively high for a USB connection but it isn't as high as other types of connection, such as ethernet
• USB uses serial transmission for sending data externally (to and from peripherals / between devices)
• IC uses parallel transmission for sending data internally
• Simplex Transmission


  Data is sent in one direction only
• Full-Duplex Transmission


  Data is sent in both directions at the same time
• Half-Duplex Transmission


  Data is sent in both directions but only one direction at a time
• Different methods of data transmission can be combined
• Parity Check

  • Odd or even parity is set/agreed for the data
  • At the transmitter, a parity bit (1 or 0) is added (to the parity byte) as an extra bit to make the sum of the bits in a byte odd or even
  • At the receiver, a check is performed when data is received to check the number of 1's is even or odd
  • If parity is incorrect, error is detected
• Checksum


  • At the transmitter, the checksum value for the bytes is calculated and transmitted with the block of data
  • At the receiver, the checksum is recalculated, from the block of data received
  • The calculated value is compared to the checksum transmitted; if the values are different then an error has been found
• Automatic Repeat Request (ARQ)


  • A check is performed on receiving data // error is detected by e.g. parity check, check sum
  • If error detected, a request is sent to the transmitter to resend the data. (negative acknowledgment)
  • This resend request will be sent repeatedly until the packet is received error free or a limited amount of resend requests is reached or requests timeout
  • The receiver sends a positive acknowledgement when data is received correctly
• Check Digit


  • A digit is calculated from the data and is appended/added to the data
  • Digit is recalculated when data is entered/input
  • Digits are compared to check for error
  • Check digit is a validation method used to check data entry
• Echo Check


  • Data is send from the sender/Transmitter to the receiver
  • Copy of data is sent back to the sender
  • Data is compared to see if it matches
  • If it doesn't match; error is detected
• The Internet is a worldwide collection of interconnected networks and devices. Internet is the infrastructure.
• Internet


  • Allows online chatting (via text, audio and video)
  • Users can send and receive emails
  • Makes use of transmission protocols (TCP) and internet protocols (IP)
• World Wide Web (WWW)


  • It is a collection of multimedia web pages and other information on websites
  • Web resources are accessed by web browsers
  • Uses the internet to access information from web servers
  • HTTP(S) protocols are written using hypertext mark-up language (HTML)
  • Uniform resource locators (URLs) are used to specify the location of web pages
• Browser


  • A software that enables users to access/view webpage, documents and other resources on the internet
  • Purpose: Render HTML and display webpages
  • Functions: Storing bookmarks and favourites, Recording user history, Allowing use of multiple tabs, Storing cookies, Providing navigation tools, Providing an address bar, Identifies protocols such as https and SSL // Manages SSL/TLS certificate process
• Web Server


  A computer that responds to requests to provide information and services over the internet
• Uniform Resource Locator (URL)

  • A text-based address for a web page; it can contain the protocol, the domain name and the web page/file name
  • The URL is translated by a special type of webserver, known as DNS (domain name server), into the 32-bit binary IP address
  • Advantage: A URL is far easier for a user to remember than a 32-bit binary. It allows an organization to personalize its web address with the organization's name. An IP address can be used instead of typing in the full URL.
• Web browser


  • Use of multiple tabs
  • Storing cookies
  • Providing navigation tools
  • Providing an address bar
• Uniform resource locator (URL) is a text-based address for a web page; it can contain the protocol, the domain name and the web page/file name
• URL is translated by a special type of webserver, known as DNS (domain name server), into the 32-bit binary IP address
• Advantage of URL


  A URL is far easier for a user to remember than a 32-bit binary. It allows an organization to personalize its web address with the organization's name
• Web browser breaks up the URL


  protocol 2) web server name 3) file name
• Protocol


  The protocol used (i.e http)
• Web server name


  The web server's name