Chapter 8 Flashcards
Main Types of Transmission Errors
Some problems are inherent in physics of the universe
Some problems result from devices that fail or from equipment that does not meet engineering standards
Three Categories of Transmission Errors
- Interference
- Distortion
- Attenuation
Interference (also known as noise)
electromagnetic radiation emitted from devices, background cosmic radiation
Distortion
All physical systems distort signals
as a pulse travels along an optic fiber, the pulse disperses
placing a wire near a large metal object can change the set of frequencies that can pass through the wire
metal objects can block some frequencies, while passing others
as a signal passes across a medium, the signal becomes weaker
Attenuation
as a signal passes across a medium the signal becomes weaker
Single Bit Error
a single bit in a block of bits is changed (often results from very short-duration interference)
Burst Error
Multiple bits in a block of bits are changed (often results from a longer-duration interference)
Erasure (Ambiguity)
The signal that arrives at a receiver is ambiguous (does not clearly correspond to either a logical 1 or a logical 0)
Burst Size (or Burst Length)
number of bits from the start of the corruption to the end of the corruption (range, including those not affected)
Channel Coding
variety of mathematical techniques to handle errors
The techniques can be divided into two broad categories: FEC and ARQ
Forward Error Correction (FEC)
add additional info to data that allows receiver to verify that data arrives correctly and to correct errors (if possible)
allow receiver to know exactly which bits have been changed and to compute correct values
Automatic Request reQuest
requires the cooperation of a sender; a sender and a receiver exchange messages to ensure that all data arrives correctly
Parity Bit
On the sending side, encoder adds an extra bit, to each byte before transmission
Receiver uses parity bit to check whether bits in the byte are correct
