What is a communication?

• Transmit information from Point A to Point B
• HOW?
  • Create the message
  • Describe the message using a set of symbols
  • Encode the symbols
  • Transmit the symbols
  • Decode the symbols
  • Re-create the message from the symbols

ASCII Table

• American Standard Code for Information Interchangec(ASCII)
• ASCII has 128 symbols, 0-127 but has 8 bits (256 values)
• The remaining 128-255 are for other characters: $, £, Å, etc.

Unicode

UNICODE, started with 65536 characters, 16 bits, but now has 110,187 characters.​

Transmitting

Regardless of the data, it is all...

• 0s and 1s

• off or on

...as electrical signals. ​

Analogue Signal

It's a continuous signal that represents physical quantities with continuous values.

Wavelength's Relationship To a Dish (Parabola)

Sky Dish
Diameter = 0.45m - 0.60m
Frequency = 500MHz - 666MHz

The Equation

Wavelength Equation:

What is the of 2.4GH, in meters?

Now how about, the period? It refers to the time it takes for one complete cycle of the waveform to occur. i.e., It's the duration of time for the waveform to repeat itself.

Frequency and Wavelength

Frequency AND Wavelength have a relationship (inverse). ​

Remember is the speed of light or

Analogue

and Digital

Waveforms

Analogue is related to the (unit) circle, so signals are all based on an angle

Analogue and Digital Signals

Modulation

ADC & DAC

The opposite of digital transmission is analogue transmission, in which information is transmitted as a continuously varying quantity. ​

ADCs use a method called “quantisation” to convert a varying AC voltage to a stepped digital one.​

Digital Transmission - anatomy

Digital Transmission is the transmission of signals that vary discretely with time between two values of some physical quantity, one value representing the binary number 0 and the other representing 1.​

Digital Representation

Transmission of signals that vary discretely with time between two values of some physical quantity, one value representing the binary number 0 and the other representing 1.​

Data Transmission

Digital signals use discrete values for the transmission of binary information over a communication medium such as a network cable or a telecommunications link. On a serial transmission line, a digital signal is transmitted 1 bit at a time.​

Transmission - 8 Bit chunks

When data is sent or received using serial data transmission, the data bits are organized in a specific order, since they can only be sent one after another. ​

The order of the data bits is important as it dictates how the transmission is organized when it is received. ​

It is viewed as a reliable data transmission method because a data bit is only sent if the previous data bit has already been received.​

FYI: LSB and MSB

Asynchronous

Asynchronous Transmission is a mode of serial transmission for modems and other telecommunication devices in which the data is transmitted as a continuous stream of bytes separated by start and stop bits (could be 1 or 0).

80% of transmitted bits actually contains data, the other 20% is signalling information.

Synchronous

This is in contrast to Asynchronous transmission in which some timer or clocking mechanism is used to ensure a steady flow of data between the devices.

• There are no gaps between characters being transmitted.
• Timing is supplied by modems or other devices at each end of the connection.
• Special syn characters precede the data being transmitted.
• The syn characters are used between blocks of data for timing purposes.

Parallel Communication

Received Signals

Received signals are never as 'clean' as the transmitted signals because they are attenuated and distorted by the transmission medium ​
​​​
There are occasions when the receiver cannot distinguish between transmitted binary 1 and binary 0 signals​

• This results in an error​

Forward Error Correction (FEC)

This was quite wasteful as it needed 3 times more bits to send the same information.​

Even and Odd Parity

Another way is to use parity checking…​

Even/odd parity:
basic method for detecting if an odd number of bits has been switched by accident. ​

Odd parity:

The number of 1-bits must add up to an odd number​

Even parity: ​
The number of 1-bits must add up to an even number​

Even and Odd Parity

The computer knows which parity it is using ​

If it uses an even parity: ​

• If the number of 1-bits add up to an odd number then it knows there was an error: ​

If it uses an odd:​

• If the number of 1-bits add up to an even number then it knows there was an error: ​

However, if an even number of 1-bits is flipped the parity will still be the same. But an error occurs​

• The even/parity can’t this detect this error: ​

Even and Odd Parity

A single bit is appended to each data chunk makes the number of 1 bits even/odd.​

Even:

Odd:

Bit Rate and Baud Rate

Bit rate refers to the number of bits transmitted per unit of time (second). Bit rate indicates the speed at which data is transmitted. E.g., a bit rate of , it means that one million bits of data are transmitted every second.

Baud rate on the other hand, refers to the number of signal events (e.g., voltage or frequency changes) per second. Baud rate represents the number of times the signal changes state per second.

Example

Working from the previous example, of 9600bps, and assumming there is 1 frequency changes/ bit per /division, how long does it take for one bit to be transmitted?

Calculating Data Transfer Time

Lets calculate how long it would take to transmit 20GB file over at 10Mbps (10M baud rate)​

Transmitting the symbol !

So how long for 10 bits travelling at 9600bps? Assumming 1 frequency changes/ bit per unit

How long would it take for the word Linux?

Calculate the Baud Rate(transmission speed)

• To calculate(estimate) the BR of the signal (e.g. displayed on the oscilloscope):

• Where is the number of times the signal changes state per second or bits per division

• Then match with list of baudrates above and pick the closet baudrate.

Calculate the Baud Rate: Example

Calculate the Baud Rate: Example

Therefore the closet baudrate is which is

Lab

• Access lab via this.