COMMUNICATION TECHNOLOGIES
2.3 DATA TRANSER PRINCIPLES

This section discusses the way in which data is transferred from one device to another. The physical medium that carries the data and the transmission and receiving of data.

HOW DOES THE INTERNET WORK

Before looking at how data is carried it is important to understand the principles of how the internet works so you understand the need for data transmission.  The internet is a series of networks, servers and clients connected together using protocols and infrastructure to allow smooth data sharing on a global scale. Most data is currently transmitted from one device to another through the use of cables, even with your mobile phone, although the data is transmitted a short distance wirelessly the majority of the journey will be through cables. 

World Science Festival, Published on Apr 2, 2013

This principle is slowly changing and the launch of Elon Musk's Starlink satellites is speeding up further changes to how data is transferred.

PACKET SWITCHING

With packet switching there is no dedicated route the data will take and before the data is send it is broken up in to small packets of data the often take various different routes depending on the traffic on any particular route.
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✓  Has no set state
✓  Has lots of people sharing at the same time
✓ ​ Is easy to share

CIRCUIT SWITCHING

With circuit switching a dedicated circuit path is created resulting in a physical path between sender and received. This method often allows for higher and more stable transfer speeds for the session using Circuit switching however can put extra loads on the network because the same circuit cannot be as easily shared as packet switching.
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✓ Has a fixed state
✓  May be open for a set period of time
​✓ ​ Can be shared by over booking

WHAT IS BANDWIDTH

Bandwidth is a simple measure of the amount of data that can be transferred per second, it is measured in Bits per second. A common error is to refer to it as bytes per second, because data is not necessarily transferred in byte format Bandwidth is measured in Bits per second.

You would normally expect to see bandwidth measured in Megabits per second, for example 1Mbps(Megabits per second) = 1,000,000 Bps (Bits per second).

The structure of a network and the data transmission medium determines the bandwidth capacity. Therefore bandwidth is a measure of how quick data can be transferred but not necessarily is transferred at that speed.

WHAT IS BIT RATE

Bit rate is how many Bits are sent per second. Bits are represented in the form of an electrical signal or optical signal depending on the transmission medium (Copper cable, optical cable, wireless frequency. The maximum number of bits that can be sent per second is determined by many factors such as the clock speed, the transmission medium and other components on the network(routers, hubs etc) and software elements such as allocated capacity for a particular node.

WHAT IS BAUD RATE

Baud rate is how many times the signal changes per second(pulses per second), or Modulation Rate. For example to illustrate this imagine, 5 volts may represent a 1 and 0 volts may represent a 0, in this case every time the voltage changes we can either represent a 1 or a 0. The more time the signal changes per second the more frequent bits can be sent. Each time the signal changes a new bit representation can be made. How many bits that are represented by the signal change is dependent on the bit rate, but the speed of change to represent the next bit/s is known are the Baud rate.

This diagram it shows the Bit rate and the Baud rate are the same and 1 bit per second is being sent and 1 signal change per second is occurring.

In the above diagram it shows a bit rate of 4 bits per second and a Baud rate of 2 signal changes per second. It is clear to see that the greater the Bit rate and the Baud rate then the greater the data transfer will be. In this diagram more options on the voltage scale have been used to represent more bit combinations.

Note: These diagrams are just for illustration and in practice the data flow, Bit rate and Baud rate are much much higher.

PARALLEL Vs SERIAL TRANSMISSION

Parallel transmission is where bits can be sent parallel to each other through separate cables or tracks on a circuit board, it is very fast to send data via this method however it comes with problems especially when data is transmitted of any more than a short distance. Parallel transmission is mainly used within computer systems and components on a circuit board level, whereas serial transmission is mainly used between components and computers.
Early computers had 8 tracks on the circuit board to carry 8 bits in parallel, this has increased and currently 64 tracks to carry 64 bits in parallel is very popular.
The problem with parallel transmission is that each bit needs to arrive at the same time, any latency within one bit arriving causes problems, latency could be caused by any discrepancy in the properties of the cables, tracks or data carrying medium or electromagnetic interference
Serial transmission is where bits are sent one after the other, for example through a copper cable bits are sent one after the other and are represented by an electrical signal in the form of a particular voltage or change in voltage depending on the system.

MULTIPLEXING, SIMPLEX,  HALF-DUPLEX AND DUPLEX

Multiplexing is the process of managing various signals between devices, three common methods of doing this are 'simplex', 'half-duplex' and 'duplex'

SIMPLEX
Simplex is when data can only be sent from its source in one direction. For example a radio station simply send out the signal, the signal cannot be received by the radio transmission hardware

HALF-DUPLEX
Half-Duplex is where a signal can be both sent and received but not both at the same time. An example of this is a walkie-talkie, only one person can talk at any time, the handheld units cannot both send and receive a signal at the same time.

DUPLEX
Duplex is where data can be both send and received at the same time. An example of this could be using the telephone where you can both talk and listen at the same timeCables frequently carry signals in both directions and as long as they are out of phase the signals will not interfere with each other

SYNCHRONOUS Vs ASYNCHRONOUS TRANSMISSION

Synchronous can be thought of as s the sender and receiver being synchronized. When the system clock of the two devices communicating are running at the same speed. When two devices are synchronized data can be sent as one continuous packet with no additional bits or information to align communication​.
 
​With many synchronous protocols the most significant bit is sent first then the data is converted back to the correct direction at the receiving end for example the letter a might be as follows:
Sender sends: 01100001
Data is inverted and transmitted: 10000110
Receiver inverts the data again: 01100001

SYNCHRONOUS DATA TRANSMISSION

Asynchronous is when the two communicating devices are not required to require permanent synchronisation of both clock speeds. To do this data is sent with additional information which includes start bits, stop bits, frames and an additional pre-assemble series of 1s and 0s before for start bit to align the communication. This process is known as framing.

ASYNCHRONOUS DATA TRANSMISSION

LATENCY

Latency is a term for how data is delayed by various components of a computer system. Latency can occur at any stage, there is even latency from when you press a key on the keyboard to when that character appears on the screen, albeit so small of a delay we do not notice.
Some causes of delay are as follows:

• Transmission Latency – This is on the level of the data travelling through its communication medium such as cables. Optical cables have less latency than copper cables.
• Propagation Latency – This is on a circuit level and is caused by the time I takes for components such as logic gates within a circuit.
• Processing Latency – This is on the level of how many processing stages there are, the more servers, ISPs, or devices the data needs to travel through the greater the latency. 

Check out this neat interactive visualisation of change in latency over time. ​Numbers Every Programmer Should Know By Year (colin-scott.github.io)

CROSS TALK

Crosstalk is interference from one cable to another. Because cables often run in the same cable trunking and run parallel to each other crosstalk is able to happen. Using twisted pairs of wires and shielded twisted pairs of wires helps minimise this effect. Wireless technologies can also interfere with each other and produce crosstalk, using various frequencies can help minimise this.

BIT STREAMING

BIT Streaming is a continuous flow of data (BITS) passing through a communication medium. Bit streaming can be broken into two categories, Real-time and On-demand.

Real-Time BIT streaming is used for data that is being transmitted from a live source in real time, such as watching a football game live of playing multiplayer online games.

On-Demand BIT streaming is when the data to be streamed is stored on a server and when requested the data is streamed to the destination. Examples of this could be a Netflix movie which is stored on Netflix server then streamed t the customer on request. 

1: Give an example of a technology that uses simplex transmission
2: Why is bandwidth often represented in BITs rather than Bytes?
3: If copper cables were the causes of a delay in data transfer, what type of latency would this be?
4: Give two reasons why is parallel data transmission not used when data is transferred over a long distance.
5: Describe why packet switching is used when a webpage has been requested.

NEXT: DATA TRANSFER MEDIUM