Data Transmission

Data Transmission

1. Computers connected together are networked

   • One type of network is Wide Area Network (WAN)
     • One example of this is the internet
     • Since WAN uses a public communications medium it is easier to intercept messages
   • One type of network is Local Area Network (LAN)
     • Used in places such as schools or wireless connected computers at home
   • Differences
     • LAN has all the machines close but WAN is more spread out
     • LAN is probably hard wired using cable or is connected via a wireless system. WAN usually uses a communication system that is not owned by the owners of the computer.
       • The internet would use a telecommunications company such as BT
     • LAN is more secure then WAN as long as its not connetcted to a WAN such as the internet

2. Network Interface Card (NIC)

   • Needed in each computer so that the computer can communicate with other machines and controls the communications
     • Network can be used to allow:
       • Communication
       • Sharing of files
       • Software
       • Hardware
         • E.g. Printer
     • If lots of computers are trying to access the same folder then lots of traffic will be made on the network
       • Will cause bottlenecks and slow down the network
       • Solutions
         • Change the way the network is wired together
           • May mean that extra hardware is needed
         • Store files and networks at different places on the network so that all the data is not coming from one place
           • System will be reliant on the ability of the hardware to maintain communications and a compromise is usually made between what is practicable and what speeds are acceptable
   • Will use a medium
     • By being connected to a cable or a wireless antenna so that messages can be passed,
       • and to software that can control the whole process (a Network Operating System (NOS))

3. Methods Of Transmission

   • Serial Data Transmission
     • This is when data is sent one bit at a time through a single wire (connection)
     • Very safe way to transmit data since the data is transmitted 1 by 1 however it is a lot slower than parallel transmission
   • Parallel Data Transmission
     • Data is transmitted using a number of wires. One for each of the bits in the byte
     • Faster than serial transmission but less safe
   • Simplex Data Transmission
     • This is when data can only be transmitted in one direction.
   • Duplex (Full Duplex) Transmission
     • This is when data can be transmitted in both directions
   • Half Duplex
     • This is when data can be sent both ways but is only being used to send data one way

4. Bits

   • The number of bits that can be transmitted in a given period of time is called the bit rate
     • The units its measured in is called baud and the the baud rate is the number of bits that can be transmitted per second.
     • High bit rates are used for streaming and low bit rates are used for downloads
       • When streaming if the video starts to freeze then its because the bit rate is not high enough
         • This may be because more than one device is using the network and lowering the bit rate available
   • Bits can be corrupted over transmission
     • 1's can be become 0's and vise versa

5. Procedures to prevent corruption

   • Echoing Back
     • This is when a set of data is sent from one device to another and then when the data reaches the device it is sent back and compared to what was sent to check that the data matches
   • Check Sums
     • This when a set of data is sent and the numbers are added together and the answer is sent with the data. When data reaches its destination it is added together to figure out the checksum and the checksum is compared to the answer to ensure the data has not been corrupted
   • Parity Check
     • The last bit of data in a byte s reserved as a parity bit. This then works by using the parity check as a test to check if there is an odd or even amount of 1's or 0's

6. Circuit Switching and Packet Switching

   • Packets
     • When data is sent from one machine to another it must first be gathered and put into a bundle of data called packets
       • They have a label attached saying what data file it is part of, which packet it is and where its going
     • They have a standard size
     • There are only two ways of sending packets
       • Circuit Swithing
         • A path between the two machines is first established and then all the packets are sent along the path in order
           • The path is reserved for the duration of the message and is then shut down
           • This ties up resources on the network and can be done more efficeintly but the packets will arrive at their destination in the right order
       • Packet Switching
         • The packets are sent onto the network to follow the most convenient path at that time
           • The packets will travel using diffrient routes
         • When a packet reaches a machine on a network the label is read and if its not destined for that machine then the most suitable path is chosen and the packet is launched down another route
           • When they arrive at the destination they have to be re-ordered before that file of data can be used
         • Doesnt tie up resources and is hard to intercept because all the packets travel using diffrient routes
       • Speed is not an advantage for either

7. Protocols

   • Definition: A set of rules
     • In data transmission the machines involved need to know how they should carry out the task