2.1 Types and methods of data transmission

How computers represent text sounds and images

Transmission of data

  • Before data is sent it is broken up into packets
  • data packets of a file can be sent across different routes(reduces congestion)
  • Packets need to be reassembled when they reach their destination

Composition of a data packet:

  • Packet header Contains: ip address of sender, ip address of receiver, number of packets, the sequence of packets
  • Payload Contains the data of the packet
  • trailer Contains error checking method, and a method of identifying the end of the packet

Cyclic Redundancy Check

  • Used to check data packets in the Packet Trailer:
  • Involves sending computer adding up all the 1-bits in the payload and stores it as a hex value in the trailer before it is sent.
  • Once the packet arrives, the receiving computer re-calculates the number of 1-bits in the payload.
  • If the two values match, then no transmission errors have occurred, otherwise the packet needs to be re-sent.

Explain how a file is transmitted using packet switching

  • Data is broken into packets
  • Each packet can take a different route
  • A router controls which path the packet will take
  • …selecting the fastest available path
  • Packets may arrive out of order
  • Once the last packet has arrived, all packets are reordered
  • If a packet is missing, it is requested again

Advantages of Packet switching

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

Disadvantages of Packet switching

  • Packets can be lost and need to be re-sent.
  • The method doesn’t work well with real-time streaming
  • There is a delay at the destination while waiting for all of the packets to arrive and then to reassemble them in the correct order.

Data Transmission

Method:

  • Serial: data is transmitted in 1 bit at a time, single wire is used
  • Parallel: several bits of data can be sent at the same time, several wires are used

Direction:

  • Simplex: transmission in 1 direction
  • Half duplex: transmission in both directions, not at the same time
  • Full duplex: transmission in both directions simultaneously

Advantages & Disadvantages of Serial

  • Works well for long distances
  • cheaper as it uses less wires
  • less likely for data to be skewed
  • Data arrives in the correct order
  • slower data transmission compared to parallel (1 bit at a time)

Advantages & Disadvantages of Parallel

  • Works well for short distances
  • Faster method of transmission (several bits at a time)
  • Used for integrated circuits
  • Data can be skewed over long distances
  • Data arrives out of order

USB

USB features:

  • Universal serial bus
  • Serial transmission
  • Universal standard
  • Data transmission method
  • Consists of 4 wires (2 for power 2 for transmission)

USB advantages/disadvantages

  • Device drivers are automatically detected
  • ...no need to manually search for them online/install them manually
  • Can charge the device
  • ..so no additional source of power is needed
  • Industry standard
  • ..so the device is more likely to be compatible with the computer
  • Can only fit in 1 way
  • ..so there is less chance of connecting the device incorrectly
  • It is backwards compatible
  • ..so no additional technology is needed /will work with older computers or devices
  • Transfer rates are not as fast as other types of connections (eg Ethernet)
  • Maximum length supported is 5m

What happens when a USB device is plugged into a computer?

  • The computer automatically detects a device is being connected
  • Appropriate device driver is loaded
  • If it is the first time the device is being connected the computer will search for appropriate drivers so that the device will operate correctly
  • Device will begin charging or power on