Data needs to be transferred from one location to another, this is a fundamental concept in Computing, for example:
- The data in your personal computer needs to be sent from a secondary storage device such as the hard disk drive to memory. For instance, this is what happens when you load an application, the instructions for that app and it’s data are copied from your computer’s hard disk to your computer’s RAM.
- When you download data from the world wide web, data needs to be sent from a server to your computer via the internet.
Regardless of whether the data is being sent the short distance between components inside your computer or vast distances from one geographical location to another, the basic principles of how this is achieved are the same.

We can “send” binary data through a wire. At it’s most basic level, when the wire is carrying an electrical current then we are sending a 1. When there is no current flowing through the wire, that is a 0.
Synchronous & Asynchronous transmission
If the presence of electricity in a wire represents a 1 and the absence represents 0, how can we send a pattern of 1’s and 0’s?
For instance, if we were to send 11110000, how can we do this? How does the receiver distinguish one 1 from another?
One solution is to use synchronous data transmission.
In this method of communication there are 2 lines used. One line is used to transmit data, another line is used to transmit a regular series of on and off pulses called a “clock signal”. This “clock signal” is then used to synchronise the two devices so that they write and read a 0 or 1 simultaneously.

Clearly, this method of communications requires two lines (data & clock). Another method which requires only a single line is called asynchronous transmission.
Asynchronous transmission involves sending a specified number of bits in each transmission. When data needs to be sent, a “start bit” is first sent, this “wakes” the receiver and indicates to it that it needs to listen on that line. Next, the specified number of bits (generally this would be 8 bits) is sent.
The receiving device uses it’s own internal timer to determine where the 1’s and 0’s are in the data and a margin of error is included. Finally a stop bit is sent which indicates that this particular transmission has ended. Larger amounts of data need to be broken into several transmissions.
This method has the advantage of not requiring an additional “clock line” but a disadvantage is that each transmission includes two additional bits that do not represent any actual data. As the amount of data being sent increases, the effect on performance of these ‘wasted bits’ becomes more pronounced.
Serial and Parallel Transmission
A single line can be used to send a single signal at any one time. This is called “serial transmission”
If we have several lines together then we can send several signals at once. This is called “parallel transmission”.
Problems with Parallel
Being able to send more data at a single time is advantageous but parallel lines do have some specific problems.
Crosstalk: The signal that is being transferred in one of the lines generates electromagnetic interference. That interference can affect the data in the other lines and can lead to errors. This effect becomes more pronounced as the length of the line increases.
Skew: Imagine several walkers with slightly different stride lengths all setting off and walking forward tens steps. When they are finished they will all have walked approximately the same difference.
If those walkers were to walk a thousand steps instead, the slight differences in their stride lengths would add up and they would all be in significantly different positions by the time they stopped.
The same thing can happen with data when it is being send across a parallel connection. Some of the bits might overtake others and “catch up” with a bit pattern that had been sent previously, others might lag behind and get mixed up with the data being sent slightly later. All of this will cause data errors and once again as the length of the line increases, the effect becomes more pronounced.
If we try to send data through a parallel connection more quickly, these two issues also become more pronounced to such an extent that it can be faster to send more bits through a serial connection, even though this can only handle a single signal.
Types of Transmission
The sound “plex” comes up frequently in communication. Plex refers to something that has more than one part. In communication we can have three basic modes of data transmission. These are:
- Simplex: Data is sent from a sender to receiver in one direction only. There is no way that the “receiver” can send a signal back again.
- Half duplex: Data can be sent from the sender to the receiver. After this transfer has completed it is possible for the receiver to become a sender and send data back the other way. Data can go on one direction and then in the other direction but not at the same time.
- Full duplex: Data can be sent from device A to device B while device B can send data to device A simultaneously.
We can combine these types with methods, for example we can have a “simplex-serial” which would indicate that a single data line can only be used to send data in one direction whereas “full duplex – parallel” refers to data being sent over a number of lines in both directions at once.
Multiplexing
Until now we have really only considered “turning on” or off the electricity that is flowing in a wire, essentially sending a single one or zero through that line at any one instant of time. It is possible to send multiple signals through a line at once by using a method called multiplexing.


Multiplexers can combine many digital signals into one, complex analogue signal. This can allow much greater quantities of data to be transferred through a single line. Demultiplexers restore the data back to it’s original format.
When sending data in this way the data can be sent using time division multiplexing. Each communication cycle is assigned a specific time period and this is then assigned to a specific device. If the device does not need to transmit data then this can lead to a wasted communication cycle.
Statisitical time division multiplexing helps to alleviate this problem by allocating a cycle to the next waiting device instead of leaving it empty.
Frequency division multiplexing allows several signals to be transmitted by allocating a specific frequency band to each device. These frequency bands cannot overlap.
Bandwidth
Bandwidth refers to the maximum amount of data it is possible to transmit at one particular time.
This can be measured in bits per second.
Baseband lines allow only a single signal to be transmitted at once whereas broadband allows multiple signals at once through the use of multiplexing. That word, “broadband” is what many people are referring to when they mention their internet connection.
Sometimes Baud rate is used as an indication of communication speed. Baud rate is defined as the number of signal changes per second.
In a simple system, each signal change might correspond to a single bit but where multiplexing has been used one signal change might correspond to many bits per second
The bit rate can therefore be calculated from the Baud rate using the formula:
Bit rate = Baud rate * Number of bits per signal change.
If a broadband connection encodes 8 bits per signal change and a Baud rate of 2400 then it will have
8 * 2400 = 19,200 Bits per second (or 2.4 KBytes per second (19200 ÷ 8)÷1000))
This example covers sending data as electric signals through wires but the same concepts also apply when we transmit data as radio waves or as light.