Data Transmission (CIE IGCSE Computer Science)

Wired connections

• Wires can be:
  • Serial
    • One bit is sent at a time across a single wire

Figure 1: A sender sends a stream of bits in sequence, one after the one across a single wire

• • Parallel
    • Multiple bits are sent at a time across several wires.
    • Transmission is asynchronous as some bits may arrive quicker than others. This is known as skewing or skewed data.
      • Asynchronous transmission means data does not always arrive at the same time

Figure 2: A sender sends a stream of bits in sequence, one after the other across multiple wires at the same time

• • Simplex
    • Simplex transmissions are unidirectional and travel in only one direction
  • Half-duplex
    • Half-duplex transmissions are bidirectional i.e. can travel in both directions, but not simultaneously!
  • Full-duplex
    • Full-duplex transmissions are bidirectional but can transmit signals in both directions at the same time
• Wires can be combinations of serial, parallel, simplex, half-duplex and full-duplex

	Simplex	Half-duplex	Full-duplex
Serial	Serial-Simplex	Serial-Half-duplex	Serial-Full-duplex
Parallel	Parallel-Simplex	Parallel-Half-duplex	Parallel-Full-duplex

Figure 3: Wire types can be combined between serial/parallel and simplex/half-duplex/full-duplex

• • Serial-Simplex
    • Data is transmitted one bit at a time in a single direction on one wire
  • Serial-Half-duplex
    • Data can be transmitted in both directions on a single wire but only one bit at a time can be transmitted in one direction at a time
  • Serial-Full-duplex
    • Data can be transmitted in both directions at the same time on a single wire one bit at a time 
  • Parallel-Simplex
    • Multiple wires transmit one bit at a time in one direction
  • Parallel-Half-duplex
    • Multiple wires send multiple bits of data in both directions but only one direction at a time
  • Parallel-Full-duplex
    • Multiple wires send multiple bits of data in both directions at the same time

Advantages and disadvantages of each method

	Advantages	Disadvantages
Serial	The data will arrive in the order it is sent It is less likely to have errors Serial transmission is cheap over short and long distances as the cost of wire is fairly inexpensive	Data transmission is slow, especially over long distances as only small quantities of data can be transmitted at a time Serial transmission is expensive over very long distances as the cost of wire dramatically increases
Parallel	Parallel transmission is fast as large quantities of data can be transmitted at any one time	Parallel transmission is expensive over short distances as multiple wires need to be purchased. Transmission is very expensive over long distances as the cost of wires dramatically increases with the distance Delays can be caused if data arrives asynchronously as the receiver has to wait for all of the bits before accepting new data. This is especially true over longer distances Buffers may be used to store data temporarily while waiting for all bits to arrive
Simplex	Simplex wires are cheap as only one wire is used	Data transmission is slow as data still travels one bit at a time in only one direction at a time Simplex transmission requires two sets of wires for bidirectional transmission meaning it can become expensive
Half-duplex	Half-duplex transmission is cheaper than simplex for bidirectional transmission as it requires fewer wires	Transmission is still slow as data travels one bit at a time in only one direction at a time
Full-duplex	Full-duplex transmission is faster as data can travel in both directions simultaneously. The receiver does not have to wait for the sender to stop before they can start transmitting their data	Full-duplex is expensive as the wire technology to transmit in both directions is more difficult to implement

Example scenarios of using each method

• Serial
  • Connecting an external hard drive to a computer
  • Transmitting data over a telephone line
• Parallel
  • Transmitting data from a computer to a printer using a multi-wire connector
• Simplex
  • Transmitting data from a computer to a printer. The printer doesn’t need to send data back to the computer
    • Modern versions of devices such as printers may send acknowledgement signals to confirm they have received the data. This may require half-duplex rather than simplex connections
• Half-duplex
  • Phone conversations where only one person needs to speak at a time
  • A walkie-talkie is a two way radio with a push to speak button. The receiver is turned off while the transmitter is turned on. This prevents you from hearing the other person while you speak
• Full-duplex
  • Broadband connections to the internet. Data must be sent and received at the same time. Accessing information on the internet is known as downloading information. Putting information onto the internet for others to access is known as uploading
  • Phone conversations where both people can talk and be heard at the same time allowing them to interrupt each other