3.1.5 - Data Transmission

3.1.5 - Data Transmission

1. 3.1.5a - Networks

   • One computer on its own is called a "Standalone"
     • So, one PC that is simply left to 'stand on its own' without any sort of connection to a network is a Standalone.
     • As soon as you connect two or more computers together it forms a network
       • Networks are categorized into LAN's and WAN's
       • Advantages of Networks:
         • Allows resource sharing
         • Allows sharing of data held on disk drives
         • Easier to back up data from a file server than from many individual machines
         • Use of Email
         • Allows for the connection of many different types of computers
       • Disadvantages of Networks:
         • Dependancy forms upon them
         • If the network breaks, many resources cannot be accessed
         • Hackers become a problem
         • As traffic increases, performance decreases
       • LAN: a method of connecting computers together over a small area
         • In order for a PC to connect to a network, it needs a NIC. It converts data from the computer into a transmitable form over the network and converts received data into a readable form
         • A Network OS can also be used. This can achieve transparency; where the user is unaware of the actions being performed
       • WAN: used for connecting computers over a large area
         • This can include a range of methods:
           • Public Telephone Network
           • Dedicated Lease Line
           • Radio Waves
           • Fibre Optic Cables
         • There exist several characteristics of a WAN:
           • Communication Facility - This can save long distance phone calls and overcome the time lag from oversea communication.
           • Remote Data Entry - From any location, data can be entered into the WAN; whether its updating, entering or querying it.
           • Centralised Information - Despite being spread over a large area, the most important data is stored within a central site.

2. 3.1.5b - Network Hardware and Software

   • There exist specific hardware and software that can be used when setting up a LAN or WAN.
     • Repeaters: Devices used to 'repeat' a signal and send it further. These devices do however repeat noise on the line.
     • Switches: Smart Devices which only pass data intended for the recipient.
     • Hubs: Devices which broadcast messages to all devices attached to them.
     • Different Types of caballing media are also used between computers:
       • Twisted Pair: Copper cable; used for much of the telephone network
       • Coaxial cable: High quality and well insulated cable
       • Fibre Optic: Uses pulses of light to send data
     • For computers to work as a network, a Network OS is needed. It is specialized server side information for controlling communication between differenet devices
       • A WAN requires a Browser, FTP clients and Protocals

3. 3.1.5c - Types of transmission

   • Serial: sends bits for a character, one after another, on the same line
   • Parallel: sends bits for a character simultaneously along separate lines
   • Duplex: Data that can be transmitted in both directions at the same time
   • Half Duplex: Data that can only travel in one direction at a time
   • Simplex: Data that can only travel in one direction
   • Baseband: Single stream of data using the whole bandwidth
   • Broadband: Several signals sent simultaneously where the bandwidth is shared

4. 3.1.5d - Bit Rate

   • The Bit Rate is the number of bits that are transferred between devices in a specified amount of time.
     • This is measured in units called 'Baud Rate', which is the number of bits that are transmitted per second.
       • Things such as live-streaming requires a high bit rate in order for all the data to arrive.
       • These types of files/ transmission are said to be 'Time Sensitive', because they lose their value in a particular time period.

5. 3.1.5e - Error Checking Methods

   • With the complexity of computer and the large amount of bits being sent along caballing to represent even a text document, errors can occur
     • Errors are formed by interference on a network, which usually formed electronic noise, but the end result is the bits get corrupted.
     • There exist different types of error detection and correcting methods:
       • Parity Checks
         • Computers either use even or odd parity
         • Computers send data in bytes, or 8 bits.
           • Normal ASCII characters do not make use of the most significant bit. This in then used as the parity bit
         • If the computer was checking even, the data being received must have its 'on' bits equal an even number, and visa versa
       • Echoes: A feature of data transmission which involves sending the data received back to the point of origin for comparison
         • However, a paradox could occur if the report sent back has an error in it.
       • Check-sum: Extra words of data are added to a block of data that are then worked out form the value of the data in the block.
         • At later stages the checksum is worked out again, and compared with the original, to ensure corruption hasnt occurred.

6. 3.1.5f, g - Circuit and Packet Switching

   • Circuit: Where a circuit is established between two nodes and all the data follows the path.
   • Packet: The data is split into smaller packets, which is sent across many different routes to their destination

7. 3.1.5h - Protocals

   • In order for a computer to be able to represent data, it requires protocals
     • Protocol: A set of rules relating to communication between devices
     • For equipment to be networked, a standard set of protocols have to be used to cover:
       • Physical Connections
       • Cabling
       • Mode of Transport
       • Speed
       • Data format
       • Error Detection
       • Correction
     • Any equipment that uses the same protocol can be linked together
     • If data is translated using a different set of protocols, it will appear differently

8. 3.1.5i, j - Protocol Stacks

   • In 1982, the International Standard organisation developed the Open System Interconnection model, aka ISO-7 Layer Model
   • All computers that are connected to a network must have a protocol stack made up of layers.
     • Each layer has its own function; software/ data can be designed for a specific later.
       • That is then passed up or down a layer with the rules of that layer being applied.
         • As such, it might pass through certain conditions that configure a device to work with the computer.
         • This is how computers and devices can communicate.
     • A Protocol has physical and logical parts:
       • Physical: The rules that are used to govern the methods used to set up the communication medium.
         • Hardwired or Wireless?
         • Frequencies used in transmissions
         • Serial or Parallel Transmission?
         • Duplex/ Half Duplex/ Simplex
     • Logical: The rules which the data is made to follow
       • Baud Rate?
       • Error Checking Options and Algorithm (Type of Parity, Error method)
       • Type of addressing (IP or MAC)
       • Character set in use
       • Type of Switching (Circuit or Packet)
     • The specific layers within a protocol stack:
       • 1) Physical Layer: Defines electrical and mechanical aspects. Includes drivers for each communication device.
       • 2) Data Link Layer: Establihses an error free communication, frames data for transmission and manages access to and use of the media
       • 3) Network Layer: Addresses messages, sets up paths between nodes and controls the flow of messages between nodes.
       • 4) Transport Layer: Provides end to end control of a communication session once a path is established.
       • 5) Session Layer: Establishes and controls system dependent aspects and allows or disallows load balancing.
       • 6) Presentation Layer: Translates and converts data for the displaying to applications and printers.
       • 7) Application Layer: Provides services to the user, including file transfer, remote file access and data base management
     • Commom protocols include: IP, TCP, FTP, HTTP and SMTP
     • Handshaking is used for:
       • Ensuring both devices are ready for data transmission
       • Ensure both devices are using the same protocol
       • Ensures signal synchronisation
       • Agrees the method of error checking