Computer Science : S2 : L2 : Transmission Methods

Data transmission can occur over very short distances, from one component to another. Short distances from a computer to a local computer or over a long distance, from one computer to another.

Three factors need to be considered when transmitting data, which have to be agreed by both sender and the receiver to work without error. 

1. Direction of data transmission

2. Method of transferring

3. Method of synchronisation between the two devices.

Simplex data transmission is one direction only; this might be used to send data from a computer to a monitor as traffic only needs to move in one direction.

Half duplex transmission can travel in both directions using a single cable but not at the same time. This occurs when using parallel printer cables , a file is sent to the printer and the printer can send back error codes; however the printer needs to wait until the computer has finished .

In full duplex transmission data can travel in both directions at the same time with two communication channels.  

Serial Data : individual bits of data are sent one at a time via a single cable:

Serial transmission uses a single wire to transfer data bits one after the other in a single stream. A single wire is cheap to build and can transmit data over long distances however data is transmitted at a slower rate than parallel data transmission. The bits are transmitted sequentially, one bit at a time. There is a set time interval between sending each bit, this interval depends on the speed of the transmitting and receiving devices.  An example is USB.

Parallel Data : Individual bits of data are sent simultaneously via a series of cables

Parallel data transmission sends several bits of data down several wires or channels at the same time, one wire or channel is used to transmit each bit. Bits can be transmitted as simplex, half duplex or full duplex. This method of data transmission works very well over short distances, (over longer distances the bits can become skewed meaning they will no longer be synchronised.) It is however a faster method of data transmission than serial but is prone to more interference.  (Examples are printer cables and other internal devices.)

Universal Serial Bus : USB is a serial method of transmitting data.USB allows for both half and full duplex data transmission.

Essentially the USB cable consist of four wire shielded cable:

• Two wires are for power (red and black)

• Two wires are for data transmission (white and green)

When a device is plugged into a computer using one of the USB ports:

• The computer automatically detects that a device is present (this is due to the small change in voltage level on the data signal wires in the cable)

• The device is automatically recognised and the appropriate driver is loaded so the computer and device can communicate

• If a new device is detected the computer will look for the device driver which matches the device, if it is not available the user is prompted to download the software.