Please enable JavaScript.
Coggle requires JavaScript to display documents.
Digital Data transmission (Data transmission is communication in the form…
Digital Data transmission
sequence of discrete, discontinuous voltage pulses
Each pulse is a signal element
Binary data is transmitted by converting each bit into a signal element.
Encoder takes a input stream, either digital or analogue, g(t), and converts it into digital, x(t). A decoder takes x(t) and converts it back to g(t)
Bits and Bytes
Bit is the smallest possible unit of binary information
number of permutations given
by a binary signal is 2^n
where n is the number of bits
8 bits has 256 permutations
Byte is 8 bits
prefix is normally added, ie. kilo, mega, giga,terra
mega byte is 1024 bytes, or 8194 bits
America Standards Code for Information interchange (ASCII)
Used to encode alphanumeric characters on a keyboard. (upper/lower case letter, numbers and special characters)
Data transmission is communication in the form of electro magnetic waves.
Guided Media
Twisted Pair
Co-axial cable
Optical fibre
Unguided Media (Wireless)
Propagation through air, vaccum and seawater
Direct link
No intermediate systems, other than amplifiers or repeaters which boost the signal
Point to Point
direct link between 2 devices.
They are the only two devices sharing the transmission medium
Multi-point
More than 2 devices share the same medium
Tranmission Terminology
Simplex
transmissions are sent in one direction only.
One device is the transmitter, the other the reciever
Half Duplex
Both devices are capable of transmitting and recieving.
Only one device transmits at a time
Full Duplex
Both devices are capable of transmitting and receiving.
Devices can transmit simultaneously
Serial transmission
data is transmitted over a single signal path, rather than in a parallel set of lines.
The receiver must reorganise the beginning and end of a block of bits
The reciever needs to know the duration of each bit (bit rate) so that it can sample the line with proper timing
the answer seems simple. Set the transmitter and receiver to the same clock rate.
However the clocks will vary, or drift, over time.
Meaning the bits will be sampled at the wrong time instance
If TX and RX are running at slightly different rates we say that the clock is skewed. Or there is a clock skew.
Protocol Data Unit (PDU)
Layer 2 - Frames
Encoding Schemes
Line coding - bitstream encoding
Mandatory
Unipolar
uses simple voltage level to represent data. I.e. 5v for 1 0v for 0.
Sometimes called unipolar non return to zero (as the voltage doesn't return to zero in the middle of the bit)
adds a DC bias to the line
As the voltage tends to +ve
Wastes power
polar
Manchester coding
bipolar
Block coding - operates on a block of data bits
Optional - used for error correction
redundant bits added to each block of bits to ensure synchronisation and error detection
data is split into blocks and then encoded by block coding
Layer 3 - Packets
Layer 4 - segment